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Title: A Monograph on the Sub-class Cirripedia (Volume 2 of 2) - The Balanidæ, (or Sessile Cirripedes); the Verrucidæ, etc., etc.
Author: Darwin, Charles
Language: English
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Transcriber's Note

Obvious misprints have been fixed. Archaic and unusual words, spellings
and styling have been maintained. Details of the changes are in the
Detailed Transcriber's Notes at the end of the book.

This book was published in two volumes, of which this is the second.

Transcriber Added

Table of Contents.


                                                                  Page

    Dedication                                                      v
    Preface                                                       vii
  _Monograph on the Cirripedia_                                     1
    Introduction                                                    1
            On the Names given to the different parts of Cirripedes 3
    Class Crustacea, Sub-Class Cirripedia                           9
            On the Sexual Relation of Cirripedes                   23
    Order I.--Thoracica                                             30
      Family Balanidæ                                              33
            Table of Contents                                      33
            Structure of Shell                                     34
            Structure of the Individual Compartments               43
            Structure of the Radii                                 45
            Structure of the Alæ                                   47
            Structure of the Sheath                                48
            Structure of the Basis                                 49
            Structure of the Opercular Valves (Scuta and Terga)    51
            Growth of the Whole Shell, and Its Microscopical Structure
                                                                   54
            Muscles of Sack                                        61
            Branchiæ                                               63
            Thorax and Body                                        65
            Muscular System                                        68
            Movements and Muscles of the Cirri                     71
            Mouth                                                  74
            Cirri                                                  81
            Caudal Appendages                                      85
            Alimentary Canal                                       85
            Circulatory System                                     87
            Nervous System                                         88
            Eyes and Vision                                        93
            Acoustic Organs                                        95
            Olfactory Sacks                                        97
            Male Organs of Generation                              97
            Female Organs of Generation                           100
            Metamorphoses and Homologies                          102
            Larva, First Stage                                    103
            Larva, Second Stage                                   109
            Larva in the Last or Pupal Stage                      110
            Act of Metamorphosis                                  126
            On the Homologies of the Carapace and Shelly Valves   131
            Cementing Apparatus                                   133
            Affinities, Classification, Variation                 152
            Rate of Growth, Exuviation, Powers of Repairing Injuries
                                                                  156
            Geographical Range and Habits                         159
            Geological History                                    172
      Sub-Family Balaninæ                                         175
        1. Genus Balanus                                          177
            Sections of the Genus                                 193
        Section A                                                 194
          1. Balanus tintinnabulum                                194
            Var. communis, vesiculosus, validus, zebra, crispatus,
            spinosus, coccopoma, concinnus, intermedius, occator,
            d'Orbignii
            Varieties                                             201
          2. Balanus tulipiformis                                 204
          3. Balanus psittacus                                    206
          4. Balanus Capensis                                     209
          5. Balanus Nigrescens                                   210
          6. Balanus decorus                                      212
          7. Balanus vinaceus                                     213
          8. Balanus Ajax                                         214
        Section B                                                 216
          9. Balanus stultus                                      216
          10. Balanus calceolus                                   218
          11. Balanus galeatus                                    220
          12. Balanus cymbiformis                                 221
          13. Balanus navicula                                    221
        Section C                                                 223
          14. Balanus trigonus                                    223
          15. Balanus spongicola                                  225
          16. Balanus lævis                                       227
            Var. nitidus, Coquimbensis
          17. Balanus perforatus                                  231
            Var. angustus, Cranchii, fistulosus, mirabilis
          18. Balanus concavus                                    235
          19. Balanus amphitrite                                  240
            Var. communis, venustus, pallidus, niveus, modestus,
            Stutsburi, obscurus, variegatus, cirratus
            Varieties                                             245
          20. Balanus pœcilus                                     245
          21. Balanus eburneus                                    248
          22. Balanus improvisus                                  250
            Var. assimilis
          23. Balanus nubilus                                     253
          24. Balanus corrugatus                                  254
        Section D                                                 256
          25. Balanus porcatus                                    256
          26. Balanus patellaris                                  259
          27. Balanus crenatus                                    261
          28. Balanus glandula                                    265
        Section E                                                 267
          29. Balanus balanoides                                  267
            Varieties                                             270
          30. Balanus cariosus                                    273
          31. Balanus declivis                                    275
        Section F                                                 277
          32. Balanus Hameri                                      277
          33. Balanus amaryllis                                   279
          34. Balanus allium                                      281
          35. Balanus cepa                                        283
          36. Balanus quadrivittatus                              284
          37. Balanus terebratus                                  285
          38. Balanus vestitus                                    286
          39. Balanus Imperator                                   288
          40. Balanus flosculus                                   290
            Var. sordidus
          41. Balanus bisulcatus                                  293
            Var. plicatus
          42. Balanus dolosus                                     295
          43. Balanus unguiformis                                 296
            Var. erisma
          44. Balanus varians                                     298
          45. Balanus inclusus                                    299
        2. Sub-Genus Acasta                                       302
          1. Acasta spongites                                     308
          2. Acasta sulcata                                       310
          3. Acasta cyathus                                       312
          4. Acasta undulata                                      313
          5. Acasta glans                                         314
          6. Acasta lævigata                                      315
          7. Acasta fenestrata                                    316
          8. Acasta purpurata                                     318
          9. Acasta sporillus                                     319
        3. Genus Tetraclita                                       321
          1. Tetraclita porosa                                    329
            Var. communis, nigrescens, viridis, rubescens, elegans,
            communis (young), patellaris
          2. Tetraclita serrata                                   334
          3. Tetraclita rosea                                     335
          4. Tetraclita purpurascens                              337
          5. Tetraclita costata                                   339
          6. Tetraclita vitiata                                   340
          7. Tetraclita cœrulescens                               342
          8. Tetraclita radiata                                   343
        4. Genus Elminius                                         345
          1. Elminius Kingii                                      348
          2. Elminius modestus                                    350
          3. Elminius plicatus                                    351
          4. Elminius simplex                                     353
        5. Genus Pyrgoma                                          355
          1. Pyrgoma anglicum                                     360
          2. Pyrgoma Stokesii                                     361
          3. Pyrgoma cancellatum                                  362
          4. Pyrgoma conjugatum                                   364
          5. Pyrgoma grande                                       365
          6. Pyrgoma milleporæ                                    367
          7. Pyrgoma dentatum                                     369
          8. Pyrgoma crenatum                                     370
          9. Pyrgoma monticulariæ                                 372
          Species dubiæ                                           374
        6. Sub-Genus Creusia                                      375
          1. Creusia spinulosa                                    376
            Varieties with the Scuta and Terga calcified together 380
        7. Genus Chelonobia                                       382
          1. Chelonobia testudinaria                              392
          2. Chelonobia caretta                                   394
          3. Chelonobia patula                                    396
      Second Section of the Sub-Family of Balaninæ                397
        8. Genus Coronula                                         397
          1. Coronula balænaris                                   415
          2. Coronula diadema                                     417
          3. Coronula reginæ                                      419
          4. Coronula barbara                                     421
          Species Dubiæ                                           423
        9. Genus Platylepas                                       424
          1. Platylepas bissexlobata                              428
          2. Platylepas decorata                                  429
          Species Dubiæ                                           430
        10. Genus Tubicinella                                     430
          1. Tubicinella trachealis                               431
        11. Genus Xenobalanus                                     438
          1. Xenobalanus globicipitis                             440
      Sub-Family Chthamalinæ                                      446
        12. Genus Chthamalus                                      447
          1. Chthamalus stellatus                                 455
            Var. communis, fragilis, depressus
          2. Chthamalus antennatus                                460
          3. Chthamalus cirratus                                  461
          4. Chthamalus fissus                                    462
          5. Chthamalus dentatus                                  463
          6. Chthamalus Hembeli                                   465
          7. Chthamalus intertextus                               467
          8. Chthamalus scabrosus                                 468
        13. Nov. Genus Chamæsipho                                 470
          1. Chamæsipho columna                                   470
          2. Chamæsipho scutelliformis                            472
        14. Nov. Genus Pachylasma                                 475
          1. Pachylasma giganteum                                 477
          2. Pachylasma aurantiacum                               480
        15. Genus Octomeris                                       482
          1. Octomeris angulosa                                   483
          2. Octomeris brunnea                                    484
        16. Genus Catophragmus                                    485
          1. Catophragmus polymerus                               487
          2. Catophragmus imbricatus                              490
    Remarks on Bronn's List of Fossil Balaninæ and Chthamalinæ    492
      Family Verrucidæ                                            495
        Genus Verruca                                             496
            Powers of Excavation                                  512
          1. Verruca Strömia                                      518
          2. Verruca lævigata                                     520
          3. Verruca Spengleri                                    521
          4. Verruca nexa                                         522
          5. Verruca prisca                                       525
      Family Lepadidæ                                             526
        Genus Alcippe                                             529
          Alcippe lampas                                          530
            Female                                                530
            Male                                                  555
    Order II.--Abdominalia                                         563
          Cryptophialus minutus                                   566
            Female                                                566
            Male                                                  584
    Order III.--Apoda                                              587
          Proteolepas bivincta                                    589
    Synopsis et Index Systematicus                                606
    Synopsis et Index Systematicus Specierum, et recentium, et fossilum
                                                                  611
    Description of plates                                         641
      Plate 1. Balanus tintinnabulum                              641
      Plate 2. Genus Balanus                                      641
      Plate 3. Genus Balanus                                      642
      Plate 4. Genus Balanus                                      642
      Plate 5. Genus Balanus                                      642
      Plate 6. Genus Balanus                                      643
      Plate 7. Genus Balanus                                      643
      Plate 8. Genus Balanus                                      644
      Plate 9. Sub-Genus Acasta                                   644
      Plate 10. Genus Tetraclita                                  645
      Plate 11. Genera Tetraclita and Elminius                    645
      Plate 12. Genera Elminius and Pyrgoma                       646
      Plate 13. Genera Pyrgoma and Creusia                        646
      Plate 14. Genera Creusia and Chelonobia                     647
      Plate 15. Genera Chelonobia and Coronula                    648
      Plate 16. Genus Coronula                                    649
      Plate 17. Genera Platylepas, Tubicinella, and Xenobalanus   651
      Plate 18. Genus Chthamalus                                  652
      Plate 19. Genera Chthamalus, Chamæsipho, and Pachylasma     652
      Plate 20. Genera Pachylasma, Octomeris, and Catophragmus    653
      Plate 21. Genus Verruca                                     654
      Plate 22. Alcippe lampas                                    655
      Plate 23. Genera Alcippe and Cryptophialus                  658
      Plate 24. Genera Cryptophialus and Proteolepas              660
      Plate 25. Genera Proteolepas and Balanus                    662
      Plate 26. Structure of the Mouth and Thorax                 664
      Plate 27. Nervous System and Senses                         666
      Plate 28. Cementing Apparatus                               667
      Plate 29. Cirri and Larvæ, first stages                     669
      Plate 30. Larvæ of Lepas: second and last stages of development
                                                                  671
    Errata                                                        674
    Index                                                         675
    Plates                                                        685



  THE

  RAY SOCIETY.

  INSTITUTED MDCCCXLIV.

  [Illustration]

  LONDON.

  MDCCCLIV.



  A MONOGRAPH

  ON THE SUB-CLASS

  CIRRIPEDIA,

  WITH

  FIGURES OF ALL THE SPECIES.


  BY

  CHARLES DARWIN, F.R.S., F.G.S.


  THE BALANIDÆ,
  (OR SESSILE CIRRIPEDES);

  THE VERRUCIDÆ,

  ETC., ETC., ETC.


  LONDON:

  PRINTED FOR THE RAY SOCIETY.

  MDCCCLIV.


Reprinted with the permission of the Ray Society

JOHNSON REPRINT CORPORATION
111 Fifth Avenue, New York, N. Y. 10003

JOHNSON REPRINT COMPANY LTD.
Berkeley Square House, London, W. 1


First reprinting, 1968, Johnson Reprint Corporation
Printed in the United States of America



  TO

  PROFESSOR H. MILNE EDWARDS,

  DEAN OF THE FACULTY OF SCIENCES OF PARIS;
  PROFESSOR AT THE MUSEUM OF NATURAL HISTORY;
  MEMBER OF THE INSTITUTE OF FRANCE;
  FOREIGN MEMBER OF THE ROYAL SOCIETY OF LONDON,
  OF THE ACADEMIES OF BERLIN, STOCKHOLM, ST. PETERSBURG, VIENNA,
  KONIGSBERG, MOSCOW, BRUSSELS, HAARLEM, BOSTON, PHILADELPHIA, ETC.

  THIS WORK IS DEDICATED,

  WITH THE MOST SINCERE RESPECT,

  AS THE ONLY, THOUGH VERY INADEQUATE ACKNOWLEDGMENT
  WHICH THE AUTHOR CAN MAKE OF HIS GREAT AND
  CONTINUED OBLIGATIONS TO THE

  'HISTOIRE NATURELLE DES CRUSTACÉS,'

  AND TO

  THE OTHER MEMOIRS AND WORKS ON NATURAL HISTORY PUBLISHED BY

  THIS ILLUSTRIOUS NATURALIST.



PREFACE.


Having so lately, in my volumes on the Recent and Fossil Lepadidæ,
expressed as strongly as I could, and with the utmost sincerity, the
obligations under which I lie to very many naturalists, I will not
here repeat my thanks, and will only say that the assistance formerly
rendered me from so many quarters has been most kindly continued
without intermission. The references under the Habitats, in which I
may remark the names of Mr. Cuming and of Mr. Stutchbury, and of the
British Museum, so often recur, show my deep obligations to these
gentlemen and to Dr. Gray, and indeed to most of the British and
several Foreign[1] collectors of recent and fossil shells. At the
period when the Introduction to this volume was printed, I stated that
I did not know whether the Palæontographical Society would publish
the few British fossil Balanidæ; the Council has now honoured me by
determining on this publication, so that these species will hereafter
be more fully illustrated than they could be in the present volume. I
cannot conclude this short preface, without again tendering my most
grateful thanks to the Council of the Ray Society for the publication
of my two volumes, and for the very kind manner in which they have
acceded to all my requests.

DOWN, KENT; _July, 1854_.

    [1] I feel under special obligation to Mr. Dana for several very
    interesting communications connected with the present subject, and
    for information derived from his magnificent work on the Crustacea,
    collected during the United States Exploring Expedition. Also
    to M. Bosquet, of Maestricht, for the loan and gift of several
    interesting fossils, described and illustrated with the utmost
    fidelity, in his beautiful "Monographie des Crustacés fossiles du
    terrain Crétacé du D. de Limbourg."



MONOGRAPH

ON

THE CIRRIPEDIA.



INTRODUCTION.


My former volume, published by the Ray Society, treated only of the
Lepadidæ, one family of the Cirripedia: I was induced to print it
from having the materials ready, though this partial publication has
been in some respects inconvenient. The Palæontographical Society has
done me the honour to publish, with ample illustrations, the fossil
species of this same family of Lepadidæ. This present volume completes
my work on the sub-class Cirripedia.[2] I had originally intended to
have published a small volume on my anatomical observations; but the
full abstract given in my former volume, which will be illustrated to a
certain extent in the plates appended to this volume, together with the
observations here given under the Balanidæ, appear to me sufficient,
and I am unwilling to spend more time on the subject. In the volume on
the Lepadidæ, I gave the specific or diagnostic characters in English
and Latin: I have here left out the latter, inasmuch as I have appended
at the end of this volume a Latin Synopsis of all the species, recent
and fossil, of the whole class. To each species is added a reference
to the pages and plates of my three volumes, so that the Synopsis
will serve as a systematic index to the three: an alphabetical index
to the present volume is also given. In the Lepadidæ, I gave an
additional specific character, derived from the softer parts of the
animal's body: in the Balanidæ, these parts are more alike in the
different species, and I have found it impossible to give a diagnostic
character thus derived. In those cases in which a Family contains but
one genus, or a Genus but one species, I have assigned my reasons for
the institution of such groups, but have given, as heretofore, only a
single description in full: it would have been easy to have separated,
by analogy, this description into one for the species, another for
the genus or for the family; but as I believe such separation and
subordination of the characters would have been largely conjectural,
I have thought it best to act as I have done, and give, thus saving
useless repetitions, only a single description, and leave it for my
successors, when more genera or species are known, to separate, with
such certainty as is ever possible, the generic from the specific
characters.

    [2] The number of the British fossil species of the Balanidæ and
    Verrucidæ in a recognisable state is so small, that I do not
    know whether it will be considered worth while to publish in the
    Palæontographical series more detailed illustrations than are given
    in this volume.

In nomenclature, I have endeavoured rigorously to follow the rules
of the British Association, and have never, at least intentionally,
broken through the great law of priority. In accordance with the rules,
I have rejected, that is, as compulsory, all names given before the
introduction of the binomial system in 1758. I have given much fewer
synonyms than is usual in conchological works; for it is impossible to
recognise with any approach to certainty, several even of the common
European forms, in the short descriptions given by most authors; this
holds good in many cases in which figures, in appearance excellent,
have been added. I assert this the more confidently, from having had
the advantage of having gone over some of the Linnean synonyms with Mr.
S. Hanley. I may further venture to express my conviction, that giving
references to works, in which there is not any original matter, or in
which the plates are not of a high order of excellence, is absolutely
injurious to the progress of natural history.

NOMENCLATURE OF THE SHELL OF A SESSILE CIRRIPEDE.

[Illustration: SHELL. Fig. 1.

_Orifice_ of shell, surrounded by the _sheath_. _Sheath_ formed by the
_alæ_ (_a_--_a_.) and by portions of the upper and inner surfaces of
the _parietes_ (_p_--_p_.)

N.B. In Balanus, and many other genera, the Rostrum and Rostro-lateral
compartments are confluent, and hence the Rostrum has the structure of
Fig. 2.]

COMPARTMENTS.

[Illustration: Fig. 2.

Fig. 2. Compartment with two radii, serving either as a
Rostrum or Rostro-lateral compartment.]

[Illustration: Fig. 3.

Fig. 3. serves as a Lateral and Carino-lateral
Compartment.]

[Illustration: Fig. 4.

Fig. 4. serves as a Carina or Rostrum.]

OPERCULAR VALVES.

[Illustration: Fig. 5. SCUTUM (internal view of).]

[Illustration: Fig. 6. TERGUM (external view).]

[Illustration: Fig. 7. TERGUM (internal view).]

Sessile Cirripedes, partly from being attached to surfaces having
very different characters, partly from undergoing a varying amount of
disintegration, and partly from unknown innate causes, are extremely
variable. Under the head of _Variation_, in the Family Balanidæ and
under the Genus Balanus, I have enlarged on this subject, and have
shown that there is scarcely a single _external_ character which is not
highly variable in most of the species. As whole groups of specimens
often vary in exactly the same manner, it is not easy to exaggerate the
difficulty of discriminating species and varieties. It is absolutely
necessary in most cases, for mere identification, that the valves of at
least one specimen in a group should be disarticulated and well cleaned
(for which purpose caustic potash is most useful), so that the internal
characters may be examined. Whoever attempts to make out from external
characters alone, without disarticulating the valves, the species,
(even those inhabiting one very confined region, for instance the
shores of Great Britain,) will almost certainly fall into many errors:
hence it is, and can thus only be accounted for, that I have not seen
one collection of British specimens with _all_ the species, though so
few in number, rightly discriminated; and in the large majority of
cases, either two or three species, certainly distinct, were confounded
together, or two or three varieties, as certainly not distinct, were
separated from each other.


_On the Names Given to the Different Parts of Cirripedes._

In my former volume I have stated that I found it indispensable, in
part owing to the extreme confusion of the nomenclature previously
used, to attach new names to several of the external parts of
Cirripedes. Almost all these names are applicable to the Balanidæ,
or sessile Cirripedes, and to the Verrucidæ; but a few additional
names are requisite, which, together with the old names, will, I
hope, be rendered clear by the accompanying woodcuts. In sessile
Cirripedes, the whole of that which is externally visible, may for
convenience sake be divided into the _operculum_ or _opercular valves_
(_valvæ operculares_), and the _shell_ (_testa_), though these parts
homologically present no real difference. The operculum is seated
generally some little way down within the _orifice_ of the shell; but
in very young specimens and in Verruca, the operculum is attached
to the summit of the shell, and the shell, without the operculum being
removed, can hardly be said to have any orifice; though, of course, the
opercular valves themselves have an aperture for the protrusion of the
cirri.

The shell consists of the _basis_ (called the support by some authors),
which is membranous or shelly, and flat or cup-formed, and of the
_compartments_ (_testæ valvæ_), which vary from eight to four in
number, and occasionally are all calcified together.

The compartment, at that end of the shell where the cirri are exserted
through the aperture or lips of the operculum, is called the _carina_
(fig. 1); the compartment opposite to it is the _rostrum_,--these two
lying at the ends of the longitudinal axis of the shell. Those on the
sides are the _lateral compartments_; that nearest the carina, being
the _carino-lateral_ (_testæ valva carino-lateralis_), that nearest the
rostrum, the _rostro-lateral_, and the middle one, simply the _lateral_
compartment; but these three compartments are rarely present together.
The _rostro-lateral_ compartment, which always resembles fig. 2, and
may be always known by having radii on both sides, is often absent; and
not rarely the lateral and carino-lateral compartments are confounded
together, or one is absent; in such cases the compartment that is left
is simply called the lateral one. The compartments are separated from
each other by _sutures_, which are often so fine and close as to be
distinguished with difficulty. The edge of a compartment, which can
only be seen when disarticulated from its neighbour, I have called the
sutural edge (_acies suturalis_).

Each separate _compartment_ consists of a _wall_ (_paries_), or
_parietal portion_ (_pp_ in woodcuts), which always grows downwards,
and forms the basal margin; and is furnished on the two sides either
with _alæ_ (fig. 4), or with _radii_ (fig. 2), or with an ala on one
side and a radius (fig. 3) on the other.

The _radius_[3] (adopting the name used by Bruguière, Lamarck, and
others) differs remarkably in appearance (though not in essence) from
the walls or parietal portion, owing to the direction of the lines
of growth and the state of its usually depressed surface. In the
upper part the radii overlie the alæ of the adjoining compartments:
in outline (_r_, fig. 1, 2, 3), they are wedge-formed, with their
points downwards; their summits (and this is often a useful specific
character) are either parallel to the basis or as in fig. 1 and 2,
oblique. The radii are sometimes not developed.

    [3] The radii have been called by Ranzani and De Blainville
    "areæ depressæ" (the parietal portions of the compartments being
    the "areæ prominentes"); by Poli, "areæ interjectæ;" by Gray,
    "sutures;" by Coldstream, "compartments of the second order," (the
    parietal portions being those of the first order); by some authors
    as "intersticia." I may here add that the scuta are the "ventral
    valves" of Gray, the "anterior" of Ranzani, and the "inferior
    opercular" of De Blainville: the terga are the "posterior valves"
    of Gray and Ranzani, but the "superior opercular" of De Blainville:
    the rostrum, on the other hand, is the "anterior valve" of Ferussac
    and the "ventral" of De Blainville; the carina being the "dorsal
    valve" of the latter author.

The _alæ_ (so called by Dr. Gray) are overlapped by the radii and by
part of the walls; they usually extend only about half way down the
compartment (_a_ fig. 3, 4, 1); their summits are either parallel to
the basis or oblique. The alæ of the several compartments, together
with the internal, upper, thickened surfaces of the walls, against a
shoulder of which the sutural edges of the alæ abut, have been called
(by Dr. Gray) the _sheath_ (_vagina_). The upper and greater portion of
the sheath is marked by transverse lines, caused by the exuviation of
the _opercular membrane_, as that membrane may be called, which unites
the operculum all round to the sheath, or upper internal surface of the
shell.

The _carina_ has always two _alæ_, as in fig. 4.

The _carino-lateral_ and _lateral compartments_ have always an _ala_ on
one (the rostral) side, and a _radius_ on the other (the carinal) side,
as in fig. 3.

The _rostro-lateral compartment_ (when present) has always _radii_ on
both sides, as in fig. 2.

The _rostrum_ has normally _alæ_ on both sides, as in fig. 4, but very
often from fusion with the rostro-lateral compartments on both sides,
it has _radii_ on both sides, as in fig. 2.

The walls of the shell, the basis, and the radii, are in very many
cases composed of an _outer_ and _inner lamina_, united together by
_longitudinal septa_; a set of tubes or pores being thus formed. The
points of the longitudinal septa generally project beyond the laminæ,
and are denticulated on both sides (see woodcut, further on;) the septa
are sometimes branched, several irregular rows of pores between the two
laminæ being then formed (see Pl. 7, fig. 3 _b_, and Pl. 10, fig. 1
_g_, 1 _h_).

_Operculum_, or _opercular valves_.--These consist of a pair of scuta
and a pair of terga. They are joined to the sheath of the shell by the
_opercular membrane_.

_Scutum_ (woodcut 5): this valve is generally sub-triangular, and its
three margins are the _basal_, the _tergal_, so called from being
articulated with the tergum, and the _occludent_, so called from
opening and shutting against the opposed valve. The angles are called
from the adjoining margins, as _basi-tergal_, &c.; the upper angle
being the apex. The scutum is ordinarily articulated to the tergum by
an _articular ridge_ (_crista articularis_), running up to the apex
of the valve, and by an _articular furrow_, which latter receives the
scutal margin of the tergum. The articular ridge, instead of projecting
straight up from the valve, when laid flat on its external surface,
often bends over to the tergal side, and is then said to be _reflexed_.
On the internal surface of the valve, there is almost always an
_adductor pit_ or _cavity_ (_fossa adductoris_), for the attachment of
the adductor scutorum muscle: this pit is often bounded on its tergal
and basal sides, by a ridge, called the _adductor ridge_ (_crista
adductoris_), which, in its upper part, is often confluent with the
articular ridge. Beneath the adductor ridge, in the basi-tergal corner
of the valve, there is often a _lateral-depressor pit_ (_fossa musculi
lateralis depressoris_), for the attachment of the so-called muscle;
and this pit is sometimes furnished with crests.

_Tergum_, (woodcut 6 and 7):--this valve, also, has three margins,
the _scutal_, _basal_, and _carinal_; its upper end, or _apex_, is
sometimes _beaked_; on the basal margin a _spur_ (_calcar_) depends;
the outer surface of the valve is depressed or longitudinally
_furrowed_ (_sulcus longitudinalis_) in the line of the spur. The
part called the spur is often so broad, that the name becomes not very
appropriate. The angles are denominated, from the adjoining margins,
as _basi-carinal_, or _basi-scutal_ angle, &c. On the under side, in
the upper part, there is an _articular ridge_, and on its scutal side,
an _articular furrow_, receiving the articular ridge of the scutum. In
the basi-carinal corner of the valve there are often crests for the
attachment of the tergal depressor muscle.

_Sack_, _Body_, _Cirri_, _Mouth_.--A slit-like orifice between the
opercular valves leads into the _sack_, in which the body is lodged.
The body consists of the six (perhaps the seven) posterior thoracic
segments of the archetype Crustacean; the first of these six segments
(or first two, if there be seven segments) is developed on its dorsal
aspect into a part, which I have called the _prosoma_[4] (see fig. 1,
_c_, Pl. 25). There is no abdomen. The thoracic segments support six
pairs of _cirri_. Each cirrus consists of a two-jointed _pedicel_,
carrying two multiarticulated _rami_. Rarely there are articulated
_caudal appendages_ (_appendices caudales_) on each side of the anus.
The prominent mouth consists of a _labrum_, _palpi_, _mandibles_,
_maxillæ_, and _outer maxillæ_, the latter resembling a lower lip:
these organs may be conveniently spoken of, after Milne Edwards, as
_gnathites_. Within the sack, attached to its carino-lateral end, a
folded membrane forms the _branchiæ_. The sheets of ova lying within
the sack are called the _ovigerous lamellæ_.

    [4] A discussion on the homologies of the different parts is given
    under the head of the Metamorphoses of the Balanidæ.

I have often found it convenient to designate the membrane investing
the body, lining the sack, &c., by its proper chemical name of
_chitine_, instead of by horny, or other such equivalents; but when
covering parts of the shell, for brevity's sake I have often spoken of
it as an _epidermis_, but I do not believe that such is its nature.
When this membrane sends into the body of the animal rigid projections
or crests, for the attachment of muscles or any other purpose, I call
them, after Audouin, _apodemes_. For the underlying true skin, I use
the term _corium_.

_Relative position of parts._--The centre of the generally flat
basis, which is cemented to the supporting surface, is properly the
_anterior_ end, and the tips of the terga, often hidden within the
shell, are properly the _posterior_ end of the external covering; but
I have found it more convenient to speak of the _upper_ and _basal_
surfaces and aspects, which hardly admit of any mistake. A line drawn
from the centre of the basis, along the middle of the rostrum to the
tips of the scuta, shows the strictly _medio-ventral_ surface of the
shell; and another line drawn from the centre of the basis, along the
carina, to the tips of the terga, shows the strictly _medio-dorsal_
line; but from the crooked course of these lines, I have found it far
more convenient to speak of the _rostral_ and _carinal_ end or aspect
of the different parts of the shell; this is the more necessary with
respect to the internal parts of the animal, owing to their remarkable
changes of position during the metamorphosis, whence it comes that the
dorsal surface of the thorax faces partly dorsally, partly anteriorly
or downwards, and partly even ventrally; and the ventral surface of the
whole posterior part of the thorax faces upwards or posteriorly; but
when we refer these parts to the _rostral_, _carinal_, _basal_, and
_upper_ ends of the shell, there can be no mistake. There has moreover
been great confusion in these relative terms, as applied by different
authors.

When a sessile Cirripede is held in the position in which they have
generally been figured, namely with the basis downwards and the
scuta towards the beholder, then the _right_ and _left_ sides of the
Cirripede correspond with those of the holder.

I have followed the example of Botanists, and added the interjection
(!) to synonyms, when I have seen an authentic specimen bearing the
name in question.

Every locality, under each species, is given from specimens ticketed
in a manner and under circumstances appearing to me worthy of
confidence,--the specific determination being in each case made by
myself.



CLASS--CRUSTACEA.

Sub-Class--CIRRIPEDIA.

_Crustacea attached by the anterior end of the head, by cement
proceeding from a modified portion of the ovaria; archetype composed
of seventeen segments, with the three first of large size, and almost
always developed into a carapace, not wholly exuviated, and capable of
various movements; antennæ none; eyes rudimentary; mouth prominent,
formed by the partial confluence of the labrum, palpi, mandibles, and
two pairs of maxillæ; thorax attached to the internal sternal surface
of the carapace, generally bearing six pairs of captorial, biramous,
multiarticulated limbs; abdomen generally rudimentary; branchiæ,
when present, attached to the under sides of the carapace; generally
bisexual, when unisexual, males epizoic on the female; penis single,
generally probosciformed, seated at the posterior end of the abdomen;
oviducts none; metamorphoses complex._


Within the memory of many living naturalists, Cirripedes were
universally looked on as belonging to the Molluscous kingdom; nor was
this surprising, considering the fixed condition of their shells, and
the degree of external resemblance between, on the one hand, Lepas and
Teredo, and on the other hand, between Balanus and a Mollusc compounded
of a patella and chiton. It is remarkable that this external false
appearance overbore, even in the mind of Cuvier, his knowledge of their
internal structure, namely, their lateral jaws, articulated appendages,
and regular ganglionic nervous system, which now strike us as such
conclusive evidence of their position in the great Articulate kingdom.
Straus[5] was, I believe, the first who, in 1819, maintained that
Cirripedes were most closely allied to Crustacea. But this view was
disregarded, until J. Vaughan Thompson's[6] capital discovery, in 1830,
of their metamorphoses, since which time, Cirripedes have been almost
universally admitted amongst the Crustaceans. It is well known, that
it is hardly possible to give a definition of this great class, which
shall include every member of it; nevertheless, even if the mature
Cirripede alone be considered, the following characters, viz. the
slight separation of the head and thorax, the latter generally bearing
six pairs of appendages, and the being enclosed in a carapace--together
with the periodical exuviation of the greater part of the external
membranes, would, perhaps, suffice to show that it should be classed
amongst Crustacea.

    [5] Mémoires du Muséum d'Histoire Nat., tom. v, p. 381.

    [6] Zoological Researches and Illustrations.

But it still remains undecided what rank in this class Cirripedes
should hold. Before briefly discussing this point, it is indispensable
to indicate their essential characters, which I will immediately
attempt. For as long as it remained doubtful which was their anterior
extremity, which the ventral or dorsal surface; as long as the peduncle
was thought by one naturalist to be the legs, by another the abdomen,
in a modified condition, it was hopeless to compare Cirripedes with
ordinary Crustaceans, and assign to them their due rank.

In the larva in the first stage, an eye and two pairs of antennæ are
in process of formation or are developed; here, then, according to
the analogy of all Crustaceans, we have evidence of the existence of
the first three cephalic segments. The mouth always consists of three
pairs of gnathites, and hence again, from analogy, this part may be
inferred to be formed of, and supported on, three other segments;
making thus far six segments. In two Orders out of the three into
which Cirripedes may be divided, namely, in the Abdominalia and Apoda,
eight quite distinct segments succeed the mouth; of these the first
differs slightly from the seven succeeding segments, and may, I think,
be safely considered as forming the seventh (cephalic) segment. The
next seven segments resemble each other in all essential respects, and
are no doubt the normal, seven thoracic segments. These, in both the
above orders, are succeeded by three smaller segments, which differ
in structure from the thoracic segments, and must be abdominal. Hence
we here have, altogether, seventeen segments. It should, however, be
observed that in the two orders just referred to, each includes only
a single species; but I know of no good reason why, on this account,
their structure should be valued the less. In the third order, the
Thoracica, which includes all common Cirripedes, two segments with
their appendages are missing out of the eight that should succeed
the mouth; from the open interval in the pupa, between the mouth and
first pair of natatory legs, and from some other reasons, I believe
that the two missing segments are the seventh and eighth, or last
cephalic and first thoracic segments, and that they have coalesced
close posteriorly to the mouth.[7] In the order Thoracica, the abdomen
is quite rudimentary, though often still bearing caudal appendages;
in the pupa, however, of this order, as in the mature animal of the
two other orders, it is formed of three segments. Hence I conclude
that, notwithstanding the absence of the above two segments with their
appendages in the Thoracica, the archetype Cirripede may be safely said
to be composed of seventeen segments.

    [7] This question and the whole subject of the homologies of the
    several parts of a Cirripede, will be discussed under the head of
    the Metamorphoses of the Balanidæ.

In the classification of Crustacea, the relation and number of the
segments of the different parts of the body, are viewed both by Prof.
Milne Edwards[8] and Mr. Dana,[9] as of the highest importance.
I may premise that both these authors divide the Crustacea into
Podophthalmia, Edriophthalmia, and Entomostraca; Milne Edwards making
a fourth legion, the Branchiopoda, and another division, including
Limulus, of equal value to the above four legions altogether; whereas
Dana sinks Limulus and the Branchiopoda under his Entomostraca. As
far as concerns our present discussion on Cirripedes, the first
three divisions, as valued by Dana, will best serve as standards
of comparison; but it is not unimportant to our present purpose,
as showing how difficult it is to weigh the value of the higher
divisions of a Class, to observe the wide difference in opinion of two
naturalists, so eminent for their knowledge of the class in question
and for their high abilities.

    [8] Annales des Sciences Nat., tom. xviii, p. 120, 1852.

    [9] Crustacea: 'United States Exploring Expedition,' p. 1395, 1852.

In the order Thoracica, including all common Cirripedes, the
cephalic and thoracic segments are as much confounded together (but
with coalescence and abortion of two middle segments) as in most
Podophthalmia; but in the two other orders, the cephalic and thoracic
segments are as distinct as in the Edriophthalmia. The number of the
segments, however, which _strictly_ appertain to the anterior part of
the head and mouth, being only six, is an Entomostracan character; on
the other hand, the first pair of cirri in the Thoracica, has some
claim from their position, apparent functions, and separation from the
succeeding pairs, to be said to belong to the mouth; on which view, the
first nine segments would, in function, be cephalic, as in the highest
Crustaceans. The fewness of the segments of the abdomen, and their not
bearing in two of the orders appendages, is an Entomostracan character.

Cirripedes are permanently attached, even before their final
metamorphosis, by a tissue or cement, first debouching through the
second pair of antennæ, and, subsequently, in most cases, through
special orifices, penetrating the anterior part of the head; this
cement proceeds from glands, which certainly are modified portions of
the ovarian system. This fact I consider of the highest classificatory
importance, for it is absolutely the one single character common to all
Cirripedes, besides such as show only that these animals belong to the
articulated kingdom, and are Crustaceans. No structure of this kind has
hitherto been observed in any other member of the class or kingdom.
Even in the Suctorial Entomostracans, which become immoveably attached
to the fish on which they prey, the males are free; and the means of
attachment, as far as known, are quite different.

Both the first and second pairs of antennæ are absent in the mature
animal; for the three terminal segments of the antennæ of the pupa,
which may always be found cemented under the centre of the surface of
attachment, are functionless, after maturity. The eyes are rudimentary,
and are singular from being seated far from the anterior extremity of
the head. In their rudimentary state, and in the absence of antennæ, we
have characters common with certain Suctorial Entomostracans; and this
similarity apparently arises from the fixed condition of the animals
of both groups.

The carapace, which covers the dorsal surface of the larva in the first
stage, in the last larval or pupal stage is developed so as to enclose,
like a bivalve shell, the whole body. In the mature Cirripede, the
whole external covering, whether shell and operculum, or capitulum and
peduncle, can be conclusively shown to be the carapace of the pupa,
modified. In thus enclosing the mouth and whole body, the modified
carapace resembles that of several Entomostracans; but in being
apparently formed (as I hope hereafter to show) by the development
of the third segment of the head, and in consisting generally of
distinct sclerodermic plates, arranged in an imbricated order, there
is, I think, a closer resemblance to the same part in some of the
Podophthalmia. The carapace, or portions of the carapace, being capable
of other movements, besides merely opening and shutting, differs, I
believe, from that of all other Crustaceans; as it likewise does[10] in
the greater part not being periodically moulted.

    [10] The carapace, however, of the Isaura, a Branchiopod, according
    to M. Joly ('Annales des Sc. Nat.,' 2 ser. vol. xvii, p. 293), is
    not moulted.

Moreover, in all Cirripedes there is another striking peculiarity
connected with these parts, namely, the exclusive attachment of the
whole thorax or included body to the internal ventral or sternal
surface of the carapace and head. In the pupa, the thorax, as in all
Crustaceans, opens into, and is continuously united with, the large
anterior part of the head; but from the singular fact that the thorax
of the young Cirripede is developed not within the thorax, but within
the head of the pupa (Pl. 30, fig. 2), with its longitudinal axis
placed at right angles to that which it held in the pupal condition
(the mouth and the whole exterior being developed conformably with
that of the pupa), it comes to pass after the metamorphosis, that the
Cirripede is, as it were, internally cut in twain (compare Pl. 25, fig.
1, and Pl. 30, figs. 2 and 3). Thus it is, as will hereafter be more
fully explained, that the sack originates, and thus the body becomes
attached to the internal ventral surface of the carapace and front of
head.

The thorax in two of the Orders bears no appendages, but in all common
Cirripedes it is furnished with six pairs of biramous, multiarticulated
cirri, which have a peculiar character, different from the limbs of
other Crustaceans, not being natatory, ambulatory, or branchial,
but "captorial" or fitted for sweeping the water, and thus catching
prey.[11] The cirri, at least the anterior pairs, can, besides other
movements, lengthen and shorten themselves; and this Milne Edwards[12]
states is the case with the Podophthalmia, and is considered by him
as an important character. The cirri of the first pair are attached
on each side close to the bases of the mandibles, and, as already
remarked, have some claim to be considered as maxillipeds or mouth
organs. The three or the four posterior pairs of cirri in the Balanidæ,
form a series somewhat distinct from the two or three anterior pairs,
thus recalling a characteristic feature in the Edriophthalmia.

    [11] M. A. Hancock, in 'Annals and Magazine of Natural History,' 2d
    series, 1849, p. 312, speaks of the cirri acting like a prehensile
    net.

    [12] 'Annales des Sciences Nat.,' tom. xviii, p. 121, 1852.

The mouth is prominent, and is formed by the partial confluence of the
labrum, palpi, and lower segments of the mandibles, and of two pairs of
maxillæ; it is capable of movement as a whole; in this respect we are
reminded of the Suctorial Entomostracans; but I believe the above type
of structure of the mouth is peculiar to Cirripedes.

The alimentary canal is simple, but can be distinctly divided
into--(1st) an œsophagus, singular from the bell-shaped expansion
of its lower end; (2d) the stomach, which is directed forwards and
then doubled back; and (3d) the rectum. There is no distinct liver.
The circulation is lacunal. In one family there are well-developed
branchiæ, which differ entirely in their homologies and position
from these organs in all other Crustaceans. In the nervous system,
the sub-œsophageal ganglions vary in concentration from that degree
observed in the lower Macroura, to that in the highest Brachyoura; but
the supra-œsophageal ganglions are always much less concentrated, and
are even embryonic in condition; presenting a difference not observed
in other Crustaceans. On the under side of the sub-œsophageal ganglion,
two nerves, apparently splanchnic, arise, and run almost parallel and
under the collar surrounding the œsophagus; they are very remarkable
from their great size, and from forming a plexus together with a
large branch, arising on each side from the collar close behind the
supra-œsophageal ganglion,--a structure unlike anything observed in
other Crustaceans. The eyes, as already remarked, are rudimentary, and
singular from being imbedded at a distance from the anterior end of the
animal. In the basal confluent segments of the outer maxillæ there are
two orifices, leading into little sacks, which I believe are olfactory
organs: again there are two other orifices on each side of the thorax,
beneath the first pair of cirri, leading into sacks, with a curious
elastic vesicle suspended within them; and these I can hardly doubt are
acoustic organs. Of these orifices and organs, there is no trace in the
same relative positions in any known Crustacean.

Cirripedes are ordinarily bisexual, in which they differ from all
Crustaceans: when the sexes are separate, the males are minute,
rudimentary in structure, and permanently epizoic on the females; to
these latter facts we have a partial analogy in some of the Suctorial
Entomostracans; but a far closer analogy in certain Rotifers, which are
considered by many naturalists as Crustaceans; but to the above subject
I shall almost immediately have to recur.

The male excretory organ is probosciformed and capable of the most
varied movements; it is single and medial; it is seated (in the one
instance in which this point can be safely judged of) at the extremity
of the abdomen, and therefore near the normal position of the anus;
in all these respects there is a very great difference from other
Crustaceans, in which the male organs are laterally double, and are not
seated at the extremity of the abdomen. In regard to the female organs,
the ovarian tubes and cæca inosculate together: there are no oviducts;
the ova, connected together by membrane, and so forming the "ovigerous
lamellæ," become exposed by the exuviation of the lining tunic of the
carapace or sack, and by the formation of a new tunic on the under side
of these lamellæ; a process, I believe, unknown in other Crustaceans.

The metamorphoses are highly complex. The larva in its first stage
bears a very close general resemblance, in having three pairs of
natatory appendages, the first being uniramous and the two others
biramous, and in having a single eye on its broad anterior front, to
the larvæ of most Entomostracans; but I cannot avoid the belief, that
this resemblance is only apparent, and not essential; and of false
resemblances, how many instances occur in the animal kingdom! In the
larva, when first freed from the egg, both pairs of antennæ are in
process of formation within envelopes: the mouth is probosciformed
and capable of movement, but is destitute of gnathites; it occupies a
position between the three pairs of natatory limbs; and these limbs I
must believe, for reasons hereafter to be assigned, answer (improbable
as I am well aware it must at first appear) to the second, third,
and fourth thoracic legs of the archetype Crustacean: the two hinder
pairs of limbs apparently soon become captorial, or fitted to secure
prey. Now, I cannot find in the published accounts of the larvæ of
Entomostracans, any that answer to this description.

The larva in the last stage might be included in the vast class of
Entomostracans: the attachment of the eyes to the singular apodemes
produced inwards from the basal segment of the great prehensile
antennæ, and the development of only the posterior six pairs of
thoracic limbs, are its chief peculiarities: but its rudimentary mouth,
owing to its transitional or pupal condition, renders the assignment of
its proper rank difficult.

       *       *       *       *       *

Having now given this short comparative sketch of the structure of
a Cirripede, I may venture to express strongly my opinion, that the
group is formed on a distinct type; as different from the other
three or four main Crustacean groups, namely, the Podophthalmia,
Edriophthalmia, Branchiopoda, and Entomostraca, as these differ from
each other; the differences, moreover, being of the kind considered
by the highest authorities on this subject, as the most important. It
should be observed that there is no special blending at either end of
the Cirripedial series, towards any one of the other main groups of
Crustacea; it is hardly possible to take some one Cirripede, and say
that it leads, more plainly than some other Cirripede, into ordinary
Crustaceans. Moreover, a great range of structure, as we shall soon
briefly show, is included within the group: I can adduce three or four
undoubted Cirripedes, very considerably more different from each other,
than any two members within the sub-class Podophthalmia, or within the
Edriophthalmia, or the Branchiopoda, and quite as different as within
the Entomostraca.

The opinion here expressed, that Cirripedes form a sub-class of equal
value with the other main Crustacean groups, I am well pleased to find,
accords with Mr. Dana's[13] view, who remarks that this sub-class
"has so many peculiarities of structure, that it should be regarded
as a distinct type, rather than a subordinate division of the third
(or Entomostracan) type." M. Milne Edwards,[14] after dividing all
Crustacea into two groups, divides one of them into four legions;
and of one of these, the Entomostraca, he makes the Cirripedes a
sub-group. I feel so entire a deference for any opinion on affinities
or classification expressed by Milne Edwards, that I differ from
him with the greatest hesitation. He does not give his reasons for
assigning so subordinate a rank to Cirripedes, but I imagine it is from
the nature of their metamorphoses: but if this be the case, I cannot
understand why he should assign to his Branchiopods a rank equal to his
Entomostracans. Moreover, I must repeat, that I do not believe that
the larvæ do resemble the larvæ of Entomostracans and Branchiopods
nearly so closely as at first appears to be the case. I may add, that
Burmeister[15] has assigned to the Cirripedes a place amongst the
Crustacea, almost equally subordinate to that given to them by Milne
Edwards.

    [13] 'Crustacea: United States Exploring Expedition,' p. 1407, 1852.

    [14] 'Annales des Sciences Nat.,' tom. xviii, p. 120, 1852.

    [15] 'Beiträge zur Naturgesehichte der Rankenfüsser,' 1834.

That Cirripedes have some special affinity to the Entomostraca, may
be inferred from the fewness of the cephalic appendages, the biramous
legs, the state of the abdomen, and the form of the carapace. Perhaps
in the peculiar state of confluence of the lower segments of the
gnathites, in the aborted antennæ, the rudimentary eyes, and in the
minute parasitic males (when such exist), there is a more direct
relation to the Suctorial division of the Entomostraca; but some of
these resemblances are probably only analogical, resulting from the
fixed condition of both groups. It should not be overlooked, that
out of the three orders into which Cirripedes may be divided, in
the two latter, the mature animal presents hardly any resemblance
to an Entomostracan. From the distinct presence in either pupa or
mature animal of the fourteen segments of the cephalo-thorax; from
the apparent composition of the carapace, as will be subsequently
explained; and from the concentrated condition of the nervous system,
one is led to glance at the higher Crustacea; and here we shall find
amongst the Podophthalmia, one aberrant group of low organisation,
namely, that including Phyllosoma, Amphion, &c., in which more points
of resemblance to Cirripedes may be detected, than, as I believe, in
any other group whatever; for we here see that remarkable elongation
of the head in front of the mouth, so eminently characteristic of
Cirripedes; we have a carapace overlapping the thorax, which is
sometimes free beneath; we have the abdomen sometimes almost obsolete;
we have biramous legs: and especially we have the posterior cephalic
and the first thoracic appendages more or less rudimentary and
obsolete; and this, I infer from Mr. Dana, is a very rare phenomenon,
though characteristic of all _ordinary_ Cirripedes, in which the
seventh and eighth segments with their appendages have disappeared.
In the order including Phyllosoma, &c., namely, in the Macroura, the
ganglions which give nerves to the five posterior thoracic limbs, are
distinct from the great sub-œsophageal ganglion which supplies the
several anterior appendages; this is the case with those Cirripedes in
which all the infra-œsophageal ganglions are not concentrated into one.
In the Macroura and Brachyoura, the first pair of legs almost always
differs in structure from the others, so does the homologous or second
cirrus in Cirripedes differ from the four succeeding pairs; in some
few Macroura, the second leg is antenniformed, so in some few cases is
the homologous (or third) cirrus; J. Vaughan Thompson was even struck
by the resemblance in the curious, doubly pectinated spines on the
anterior limbs of Mysis (allied to Phyllosoma[16]), and on those of
many Cirripedes: these several latter resemblances may be small, but
certainly I do not believe that they are accidental. Now the little
group of Crustaceans, which includes Phyllosoma, &c., has lately
been placed, by Milne Edwards, as a satellite amongst the Macrourous
Podophthalmia; it leads into the Stomopoda, and likewise, as has been
noticed by many authors, into the sub-class Branchiopoda, which latter
sub-class is considered by Mr. Dana as only a part of the Entomostraca;
this group, therefore, exhibits affinities radiating in several
directions, and amongst these lines of relationship, one more must, I
believe, be added, plainly directed towards the Cirripedia.

    [16] M. Martin St. Ange ('Mémoire sur l'Organ. des Cirripèdes,'
    1835, extrait des 'Savans Etrangers,' tom. vi) has compared the
    mouth of Lepas with that of Phyllosoma, and has given comparative
    figures; but the resemblance is founded, I believe, on quite false
    homologies.

One naturally wishes to ascertain how far Cirripedia are highly or
lowly organised and developed; but in all cases this, as it seems to
me, is a very obscure enquiry. Mr. Dana considers that, in Crustacea,
the greater or less centralisation of all the appendages round the
mouth is the main sign of high development; on this view, the anterior
part of a Cirripede, from being so much elongated, must be considered
as very low in the scale; the whole posterior part of the body, on the
other hand, is, in ordinary Cirripedes, brought close to the mouth; but
this is effected by the abortion of the seventh and eighth segments of
the cephalo-thorax and of the whole abdomen, and so, I presume, would
not, in Mr. Dana's estimation, raise the class much in the scale. Von
Baer[17] considers that the perfection of the type of any animal is in
relation to the amount of "morphological differentiation" which it has
undergone; on this view, Cirripedes ought to stand high in the scale,
for they differ much morphologically from the type of the class to
which they belong; as indeed is shown by the long time that elapsed
before their true position, namely amongst the Crustacea, was even
suspected; but something more must, I think, be added to Von Baer's
definition; for, to take as an example the eyes of a Cirripede,--as
seen in the first larval stage, there is only one eye, and that most
simple; in the pupa there are two, both compound, and furnished with
complicated muscles; lastly, in the mature animal there are still
two, but of very minute size, often almost confluent, and of the
simplest structure; hence, then, there has been much morphological
differentiation, but it is almost a contradiction in terms to speak, in
relation to such a case, of _perfection_ of type; and what has happened
to one organ, might happen to other organs, and so to the whole animal.
Lastly, under a physiological point of view, and taking the Balanidæ as
the most perfect type of the class, the sub-œsophageal portion of the
nervous system is highly concentrated; the organs of sense, excepting
the eyes, seem more largely developed than in ordinary Crustaceans; the
circulating system is of the simplest kind, being only lacunal; special
Branchiæ, however, are developed by the metamorphosis of, as I believe,
a special organ, occurring only in the Lepadidæ; the digestive organs
are very simple, from not having any distinct liver; the generative
system is very low, for both sexes are generally united in the same
individual; and the testes and ovaria closely resemble each other.
On the other hand, the thoracic limbs are, to a considerable extent,
specialised in their structure and functions; only the three posterior
pairs strictly resembling each other. Lastly, the dermal and muscular
systems are complicated, and not, to use Professor Owen's term, by mere
vegetative repetition, as will be obvious to any one who will study the
beautifully constructed and modified carapace--that is the operculum,
shell and basis--of a Balanus. On the whole, I look at a Cirripede
as a being of a low type, which has undergone much morphological
differentiation, and which has, in some few lines of structure, arrived
at considerable perfection,--meaning, by the terms perfection and
lowness, some vague resemblance to animals universally considered of a
higher rank.

    [17] English Translation, in 'Scientific Memoirs,' 1853, vol. i, p.
    228.

       *       *       *       *       *

It has been seen that I divide the Cirripedia into three orders,--the
Thoracica, Abdominalia, and Apoda; between which the fundamental
difference consists in the limbs or cirri being thoracic in the first,
abdominal in the second; and entirely absent in the third. For the sake
of showing the range of character in Cirripedes, to which allusion has
been made, I will briefly indicate the leading differences in each
order. In the Thoracica, three families are included,--the Balanidæ,
or sessile Cirripedes, the Verrucidæ, remarkable from their quite
asymmetrical shell, and the Lepadidæ, or pedunculated Cirripedes. The
great difference in external appearance between these three families is
known to all naturalists. Even within the one family of Lepadidæ there
are great differences in external appearance, as will be admitted on
comparison of Lepas, Pollicipes, Conchoderma, &c.; but we have also
important internal differences, as in the case of Anelasma, in which
the cirri are barely articulated, and are not capable of seizing prey,
whilst the mouth is almost probosciformed, with the outer maxillæ
and palpi rudimentary: still more important are the differences in
Alcippe, in which the cirri of the first pair act as brushes; the
second, third, and fourth pairs being quite aborted; and the fifth and
sixth pairs consist only of four segments, with one of the two normal
rami converted into a crenated, button-like projection, for the sake
apparently of triturating food; Alcippe, also, is very remarkable in
being destitute of a rectum and anus. In this same genus Alcippe, in
Ibla and Scalpellum, there are either separate males or Complemental
males, some of which are so utterly abnormal in their characters, that
by no definition which I could frame, could they be included even in
their proper Order, much less in their proper Family.

In the second order of Abdominalia (Pl. 23 and 24) the seventh or last
cephalic segment is quite distinct, and bears rudimentary organs,
answering to the first pair of maxillipeds of ordinary Crustaceans, of
which organs, and of the segment supporting them, there is no trace in
the Thoracica: the seven succeeding thoracic segments are destitute
of any appendages; but the three segments of the abdomen bear three
pairs of cirri. The mouth is peculiar in the labrum being developed
into very large, moveable, lancet-formed organ; and the lower end of
the œsophagus is armed with beautiful discs of teeth, and brushes of
hairs,--a structure confined to this order. The male resembles the
male of Alcippe; and the latter genus seems to be the connecting link
between the Thoracica and Abdominalia. But the most important character
of this latter order, in which it differs from Alcippe, and all other
known Cirripedes, is in its metamorphoses; all the first changes are
merely indicated by changes in form in an egg-like larva, without the
development of distinct organs; and the last, or pupal condition, which
is attained within the sack of the parent, is very peculiar, by the
entire absence of natatory limbs.

The third order of Apoda is the most peculiar of all; it contains,
like the last, only one known species: the most acute naturalist, I
am convinced, if he had not made the class his special study, would
never even have suspected that this animal was a Cirripede. We see
much magnified in Pl. 25, fig. 7 a naked, plainly-articulated animal,
resembling the larva or maggot of a fly, attached by two threads;
and these threads, on analysis, can be clearly shown to be the last
rudiment of the carapace, specially modified. The last cephalic, the
seven thoracic, and the three abdominal segments, are all equally
destitute of appendages. The mouth is suctorial, and constructed on a
plan unlike, I believe, anything known in the articulate kingdom; for
the mandibles and maxillæ have rotated on their axes, and stand back
to back; they can act only by tearing open a slit, and this action is
performed in a hood, formed by the confluence of the broad palpi and
labrum. Although the œsophagus is distinct, there is no stomach or
anus. Lastly, owing to there being no carapace, the ova are developed,
differently from in all other Cirripedes, within the thorax.

I will close this preliminary discussion on the confines and type of
the sub-class, by recalling attention, now that a sketch has been given
of the three Orders, to the remark before made, that a wide range of
structure is included within it, and by reurging that the Cirripedia
should be ranked, not as one of the subordinate groups, but as one of
the main divisions of the Crustacea.


_On the Sexual Relation of Cirripedes._

Cirripedes are commonly bisexual or hermaphrodite, but in Ibla,
Scalpellum, and Alcippe, members of the Lepadidæ in the order
Thoracica, and in Cryptophialus in the order Abdominalia, the sexes
are separate. As two of these genera were described in my former
volume, and two others (Alcippe and Cryptophialus) are described in
this volume, I may as well here give a brief summary of the facts as
yet known on this very curious subject. The Males, in the above four
genera, present a wonderful range of structure; they are attached in
the usual way by cement proceeding from the not-moulted antennæ of the
pupa, to different parts, in the different species, of the female.
These males are minute, often exceedingly minute, and consequently
generally more than one is attached to a single female; and I have
seen as many as fourteen adhering on one female! In several species
the males are short-lived, for they cannot feed, being destitute of a
mouth and stomach. As the females are longer lived, successive crops
of males, at each period of propagation, become attached to her.
It is the females in the above genera which retain the characters
of the genus, family, and order to which they belong; the males
often departing widely from the normal type. Some of the males are
rudimentary to a degree, which I believe can hardly be equalled in the
whole animal kingdom; they may, in fact, be said to exist as mere bags
of spermatozoa. So widely do some of them depart in every character
from their class, that twice it has happened to me to examine specimens
with a little care, and not even to _suspect_, until a long period
afterwards, that these males were Cirripedes.[18]

    [18] In my volume on the Lepadidæ (p. 200) in searching for
    analogies for the permanently epizoic and rudimentary condition
    of the male Cirripedes, I quoted two cases, which I believe are
    now known not to be analogous; namely, the _Syngamus trachealis_
    of Von Siebold, and the worm-like Hectocotyle, which latter was
    quite lately supposed to be a male Cephalopod, but has now been
    ascertained to be only one of the arms of the male wonderfully
    adapted and organised as a sperm-receptacle. The Asplanchna, the
    mouthless male of a Rotifer, (p. 292) alone remains for me.

In _Scalpellum Peronii_, and _villosum_, the males are but little
abnormal, for if classified independently of their sexual relations,
they would be considered as immature specimens of a new genus, standing
next to Scalpellum; in _Scalpellum rostratum_, the male would form
another and rather more distinct genus. The males, in the latter, are
attached to the other sex, between the basal edge of the labrum and
the adductor scutorum muscle; but in _S. Peronii_ and _villosum_ they
are attached lower down, in the furrow between the two scuta, and are
thus protected: in these three species, the internal parts of the male
present nothing particular. In Ibla, the males are attached low down
within the sack of the female; they may be said to consist of a mouth
surmounted on a long peduncle, for there is no capitulum or general
covering, and the whole thorax is in a rudimentary condition, the cirri
being reduced to two distorted pairs. As these males certainly moult
several times and grow a little, they must feed; and as they have no
cirri fit for action, they must seize their food by the contortions of
their peduncle, which we know homologically consists of the three first
segments of the head. The movements of the peduncle must, also, supply
those of the probosciformed penis, almost invariably present with other
Cirripedes, but here absent. If compelled to class these males without
regard to the female, great difficulty would be experienced; we could
hardly place in the family of the Lepadidæ, a Cirripede without a
capitulum, and without cirri, those very organs which give their name
to the class, and with a thorax reduced to the dimensions of a lower
lip; yet, if the presence of a peduncle did determine the classifier to
place these males amongst the Lepadidæ, then undoubtedly the character
of the mouth, &c. would fix their position next to Ibla.

The males of _Scalpellum vulgare_, _ornatum_, and _rutilum_, resemble
each other in all essential points, and differ wonderfully in
appearance and structure from all ordinary Cirripedes. They consist of
a minute flattened bag with a small orifice at the summit, and at the
lower end attached by the cemented pupal antennæ. On each side of the
orifice, there is a pair of calcareous beads, representing the two
scuta and two terga of ordinary Cirripedes; and between the scuta a
minute black eye is generally conspicuous. In _S. ornatum_ the beads, I
may remark, on the two sides are not equal; those either on the right
or on the left side, being larger than those on the opposite side, so
that the animal externally is asymmetrical. Inside, within a tubular
sack, the thorax is lodged, supporting four (instead of six) pairs
of limbs; and these, instead of forming biramous, multiarticulated,
captorial cirri, are reduced to almost a rudiment, supporting a few
long sharp spines, which apparently act only as defensive organs. At
the end of the thorax there is seated a large abdominal lobe, which
does not occur in the other sex. Hence the thorax, though rudimental,
has been specially modified. Of the mouth and stomach there is not a
vestige. Constructed as these males are, assuredly they have no claim
to be ranked amongst the Lepadidæ or pedunculated Cirripedes; nor is it
possible to class them in any group whatever of ordinary Cirripedes.
In _S. vulgare_ the males are attached, often several together, to the
extreme edges of the two scuta, and therefore immediately over the
orifice leading into the sack; in _S. rutilum_ and _ornatum_, they are
attached in concavities on the under side of both scuta, just above the
depression for the adductor scutorum muscle. In the former of these
species, the pit for the reception of the male is formed by shelly
matter not having been deposited over a certain space on the under side
of the valve; and the pit is converted by a covering of membrane into
a pouch. As there are two scuta so there are two pouches, in each of
which a male is lodged; hence, according to the Linnean nomenclature,
_Scalpellum ornatum_ may be said to belong to Diandria monogynia. As
these males, from being mouthless, soon die, they are succeeded by
successive pairs; the pupa being led by a wonderful instinct to crawl
into the pouch, and there undergo its metamorphosis.

Lastly, the males of Alcippe and Cryptophialus (Pl. 23, fig. 19, and
Pl. 24, fig. 19) are remarkable for their similarity to each other,
considering the essential dissimilarity of the two females. The females
live in cavities which they excavate in the shells of Molluscs, and
within which they are attached by a horny disc; this disc is the only
part of the outer integument which is not frequently moulted, and,
apparently in consequence, the males are attached to its edges. It
results from this position, that the males are protected by being
enclosed within the cavity excavated by the female; and it further
results, that the males are attached at a considerable distance from
the orifice of the sack of the female, into which the spermatozoa
have to be conveyed; and to effect this, the probosciformed penis is
wonderfully developed, so that in Cryptophialus, when fully extended,
it must equal between eight and nine times the entire length of the
animal! These males, like those last mentioned of Scalpellum, consist
of a mere bag, lined by a few muscles, enclosing an eye, and attached
at the lower end by the pupal antennæ; it has an orifice at its upper
end, and within it there lies coiled up, like a great worm, the
probosciformed penis, and beneath it a single testis, with a single
vesicula seminalis. These organs complete the whole organisation of the
male; for there is no mouth, no stomach, no thorax, no abdomen, and
no appendages or limbs of any kind. Yet all these parts are present
in the female. I know of no other instance in the animal kingdom
of such an amount of abortion. The whole exterior of these males
evidently is composed, as in all ordinary Cirripedes, of the three
first cephalic segments; of the fourteen succeeding segments of the
archetype Cirripede we have not a vestige, excepting the probosciformed
penis, which, from analogy, should arise from the ventral apex of
the seventeenth segment, the first three segments of the head being
counted in the seventeen. Here, then, fourteen out of seventeen
segments have aborted, the tip of the seventeenth having coalesced with
the third cephalic segment! I am tempted just to notice the case of
Proteolepas, in the order Apoda, as showing, within the limits of the
same sub-class, a wonderful amount and diversity in abortion; for in
Proteolepas, the three anterior cephalic segments are reduced to the
merest rudiment, encasing the cement-ducts, the fourteen succeeding
segments being unusually well developed; whereas in the above described
males, we have just seen the three anterior segments fully developed,
whilst the fourteen succeeding segments are lost or have coalesced
with the others; so that within the same sub-class all seventeen
segments of the archetype have almost disappeared.

It may be asked how I know that the several above described rudimentary
epizoons are really the males of the Cirripedes to which they are
attached. Even if the whole course of the metamorphoses had not been
known in three of the cases, the mere fact of these epizoons being
cemented by the three terminal segments of their peculiar, pupal
antennæ, would have been sufficient to have shown that they belonged
to the class of Cirripedes. In nearly every case, I was able to
demonstrate, and not in one or two but in many specimens, that these
epizoons were males; and as in several instances the spermatozoa
were developed, and as, notwithstanding, in no instance was there a
vestige of ova or ovaria, it may safely be concluded that they were not
hermaphrodites, and therefore required females of some kind. If these
epizoic Cirripedes had been independent animals, as they all belong
to the same sub-class, and all have such peculiar habits, it might
have been expected that they would have shown some special affinity
towards each other; but this is not the case; the epizoon of Ibla is
more nearly related to Ibla, and the epizoon of Scalpellum more nearly
related to Scalpellum, than are these epizoons to each other. If the
several epizoons were classed by themselves, they would be grouped in
divisions, corresponding with those of the Cirripedes on which they are
attached, which is just what might have been expected if these latter
were their females. There are, also, many special relations between
the male epizoons and the Cirripedes to which they are attached; thus,
the mouth of the epizoon of Ibla, is so like the mouth of Ibla, which
is peculiar in several respects, that I should easily have recognised
it as belonging to a member of that genus. _Scalpellum villosum_ is
remarkable as one out of only two or three members of the whole Family,
which is destitute of caudal appendages, so is its male epizoon; again,
_S. villosum_ is unusually spinose, so is its male epizoon; on the
other hand, _Scalpellum ornatum_ is remarkably smooth, so is its male
epizoon; and I could give other similar instances. Will it be believed
that these coincidences are accidental, and that the epizoons have no
special or sexual relation to the Cirripedes to which they are attached?

One other instance of coincident structure is so important, that it
must, even in this sketch, be noticed; the prehensile antennæ of
the pupa are most important and complicated organs, and differ in
the different genera of the same family; they are preserved in a
functionless condition throughout life, and in two instances I was able
accurately to compare these organs in the epizoon and in the Cirripede
to which it was attached, and they were identical in every particular.
The full force of the excessive improbability of this resemblance,
and of the above coincidences in structure, on the supposition of the
epizoon and its support not being sexually related, will hardly be
perceived without referring to the facts given in detail in my former
volume.

Lastly, in the case of Cryptophialus (and indirectly in that of
Alcippe) the nature of the male epizoon is, I think, actually
demonstrated; for I traced both it and the female or ordinary form of
Cryptophialus, through the same several larval stages, from the egg,
enclosed within the sack of the female, to the pupa and mature animal.
Moreover, if the male nature and sexual relation to the supporting
Cirripede, be admitted in any one of these epizoons, then so close
is the agreement in habits, and to a certain extent in structure,
in all the foregoing epizoons, that probably no one admitting one
instance would dispute the others, and further evidence would even be
superfluous. Indeed, had it not been for the following facts, I should
not have brought forward, either here in abstract, or in other places
in detail, so many arguments and so much evidence.

I have as yet not entered in detail on the sex of the supporting
Cirripede: in Cryptophialus, Alcippe, and in one species of Ibla, I
was able to demonstrate in many specimens, that all the male organs,
internal and external, were entirely absent; and consequently that
these Cirripedes were _exclusively_ female. In _Scalpellum ornatum_,
also, there is no trace of external male organs (the state of the four
dried specimens not allowing the internal organs to be examined), and
there cannot be any reasonable doubt that this species likewise is
exclusively female. It should be borne in mind that the male organs,
external and internal, are most easily discovered, and that in the
above cases I had an abundant supply of excellent specimens. On the
other hand, in _Ibla Cumingii_, and in four species of Scalpellum, I
was able to demonstrate in the supporting Cirripede the presence of
all the male organs, as well as of the female; and in the vesiculæ
seminales of several specimens, both in the Ibla and in _Scalpellum
vulgare_, spermatozoa were contained; the male organs, however, not
being very amply developed. These species, consequently, are not
exclusively female, but are hermaphrodite, though having male epizoons
attached to them. This statement, I am well aware, is enough, at first,
to cast a doubt on all that I have said; but let any one reflect on
the evidence, of which I have here given a summary, and which has
been elsewhere given in full, and I think he must admit that at least
those epizoons which are exclusively male, and which are attached
to Cirripedes exclusively female, are sexually related and form one
species; but if he admit this, he cannot possibly escape from the
conclusion that some of the other epizoons, for instance that of _Ibla
quadrivalvis_, are the males of the hermaphrodites to which they are
attached,--these epizoons not exclusively impregnating the ova of a
female, but aiding the self-impregnation of an hermaphrodite. Hence I
have called these males _Complemental Males_, to show that they do not
pair with a female, but with a bisexual individual. Nothing strictly
analogous is known in the animal kingdom, but amongst plants, in the
Linnean class, Polygamia, closely similar instances abound.

In the series of facts now given, we have one curious illustration
more to the many already known, how gradually nature changes from one
condition to the other,--in this case from bisexuality to unisexuality.
Finally, in the four genera so often named, we meet the following
several cases, some of them even the most diverse, occurring in closely
allied species. (1st), a female, with a single male (rarely with two)
permanently attached to her, protected by her, and capable of seizing,
by the movements of its pedunculated body, any minute animals or
substances found within her sack; (2d), a female with successive pairs
of short-lived, mouthless males, inhabiting pouches on each side under
her scutal valves; (3d), a female with many, in one instance fourteen,
short-lived males, destitute of mouth, thorax, and appendages, but
furnished with a stupendously long male organ, attached to a thickened
portion of her outer integuments, but lying within the cavity which
she has excavated; (4th), an hermaphrodite with a male attached within
the sack, capable of feeding itself, as in the first case; (5th), an
hermaphrodite with from one to three males, organised like ordinary
Cirripedes, and apparently capable of seizing prey in the common way;
and attached between the scuta, and thus protected; (6th and lastly),
an hermaphrodite with from one or two up to five or six, short-lived,
mouthless males, like those in the second case, attached in one
particular spot, on each side of the orifice leading into the sack.



ORDER I.--THORACICA.

_Cirripedia having a carapace, consisting either of a capitulum on a
peduncle, or of an operculated shell with a basis. Body formed of six
thoracic segments, generally furnished with six pairs of cirri; abdomen
rudimentary, but often bearing caudal appendages; mouth with the labrum
not capable of independent movements; larva firstly uniocular, with
three pairs of legs, lastly, binocular, with six pairs of thoracic
legs._


In the sketch of the three Orders given in the Introduction, it will
have been seen that the differences in their structure are so great,
that it would have been hardly possible to have given a single blended
account of the whole Class. But as all common Cirripedes are included
in the present Order, here would have been the natural place for a
full description of their external and internal structure. Having,
however, been necessarily, yet perhaps unfortunately, led to give, in
my former volume, a description of this kind of the Lepadidæ; and as it
is necessary to give a similar account of the other great family of the
Order, namely, the Balanidæ, I have found it more convenient to make
this latter account comparative and supplemental to the former one on
the Lepadidæ, and so serve for the Order, rather than attempt to give
a separate description in full of it. For this latter plan would have
involved much useless repetition, as, on account of the many exceptions
and limitations necessary to almost every statement, there is little
choice between a description of great length and a mere diagnostic
character of the Order, such as I have given above.

The Thoracica may be divided into three very natural Families, of
nearly equal value; firstly, the Balanidæ, or sessile Cirripedes,
which may be subdivided into two sub-families, also very natural, the
Balaninæ and Chthamalinæ; secondly, the Verrucidæ, containing only
one genus; and thirdly, the Lepadidæ, or pedunculated Cirripedes.
These three families differ from each other, besides in mere external
appearance, almost exclusively in the relation of the different
portions of their external covering or carapace, and of the muscles
moving such portions. In the Balanidæ, the four opercular valves
surrounding the orifice leading into the sack, are capable of other
movements, besides being opened and shut; whereas all the other valves
are immoveably united together. In the Lepadidæ, the valves answering
to the opercular valves, are furnished with a muscle only for shutting
them; whereas the peduncle answering to the basis in the Balanidæ is
capable of various movements. In the Verrucidæ the shell is singularly
asymmetrical; only half of the operculum (either the right or the left
side, this varying even in the same species) being moveable; the other
half being immoveably united to the remaining valves; and the whole
shell has only one muscle serving to shut the moveable half of the
operculum. All the internal parts and organs are very similar in the
above three Families. If, however, the internal structure of one of the
two sub-families, into which the Balanidæ may be divided, namely, of
the Balaninæ, be compared with that of the Lepadidæ, several important
differences may be detected;--on the one hand, in the Balaninæ, the
presence of branchiæ, the extremely complicated cementing apparatus,
the difference in structure between the third and succeeding pairs of
cirri, the large palpi, the notched labrum, and the laterally double
teeth of the mandibles;--and on the other hand, in the Lepadidæ, the
presence of ovigerous fræna, caudal appendages, bullate labrum, and
often prominent olfactory orifices. But if the Lepadidæ be compared in
these several respects with the other sub-family, or Chthamalinæ, which
cannot possibly be removed out of the family of Balanidæ, many of these
differences break down and disappear, in some or all of the species.

The Lepadidæ include, as has previously been noticed, a much greater
range of difference than the Balanidæ; and this is what might have
been expected, for it is the most ancient family, and extinction has
done its work, separating genera, which, in accordance to analogy, we
may suppose were once more nearly connected by intermediate forms. The
Lepadidæ, in one sense, may be taken as the type of their order; for
they have undergone less "morphological differentiation;" that is, they
differ the least from the last larval stage, and seem to give the most
_general_ idea of a Thoracic Cirripede. On the other hand, if we mean,
as some authors do, by the word type, that form which, in the group in
question, has been most modified, and illustrates every peculiarity of
its class in the strongest manner, then we must look to the Balaninæ,
and to its typical genus, Balanus, for the most Cirripedial form. In
this genus the different portions of the carapace differ most, and
subserve to a certain extent different ends, and in minute structure
are most complicated; here the cementing apparatus, which offers the
main characteristic of the whole sub-class, is most complex; here the
several pairs of cirri differ most from each other in structure and
action; here the peculiar branchiæ (organs apparently derived from the
modification of another organ, itself confined to Cirripedes, viz., the
ovigerous fræna) are best developed; here the nervous system is most
highly concentrated; and, lastly, here we meet with the largest and
most massive species of the whole group.



1. Family BALANIDÆ, (or Sessile Cirripedes).

_Cirripedia without a peduncle; scuta and terga furnished with
depressor muscles; other valves united immoveably together._


TABLE OF CONTENTS.

                                                           Page

  Structure of shell                                        34
     "  of the individual compartments                      43
     "  of the radii                                        45
     "  of the alæ                                          47
     "  of the sheath                                       48
     "  of the basis                                        49
     "  of the opercular valves (scuta and terga)           51
  Growth of whole shell and microscopical structure         54
  Muscles of sack                                           61
  Branchiæ                                                  63
  Thorax and body                                           65
  Muscular system                                           68
  Movements and muscles of the cirri                        71
  Mouth                                                     74
  Cirri                                                     81
  Caudal appendages                                         85
  Alimentary canal                                          85
  Circulatory system                                        87
  Nervous system                                            88
  Eyes and vision                                           93
  Acoustic organs                                           95
  Olfactory sacks                                           97
  Male organs of generation                                 97
  Female organs of generation                              100
  Metamorphoses and homologies                             102
  Larva, first stage                                       103
  Larva, second stage                                      109
  Larva, last or pupal stage                               110
  Act of metamorphosis                                     126
  On the homologies of the carapace                        131
  Cementing apparatus                                      133
  Affinities, classification, variation                    152
  Rate of growth, exuviation, &c.                          156
  Geographical range and habitats                          159
  Geological history                                       172


Almost every one who has walked over a rocky shore knows that a
barnacle or acorn-shell is an irregular cone, formed generally of six
compartments, with an orifice at the top, closed by a neatly-fitted,
moveable lid, or operculum.[19] Within this shell the animal's body is
lodged; and through a slit in the lid, it has the power of protruding
six pairs of articulated cirri or legs, and of securing by their
means any prey brought by the waters within their reach. The basis is
firmly cemented to the surface of attachment. The whole shell, basis,
and operculum consists, as we have already seen, of the first three
segments of the head, modified into a singularly constructed carapace,
which encloses the mouth and rest of the body. The anterior extremity
of the shell is situated in the centre of the basis, where indeed,
by due care, the antennæ of the pupa may be always detected; and the
posterior extremity is directed vertically upwards.

    [19] The best published description of the structure of the
    shell of a sessile Cirripede, is given by Dr. Coldstream, in the
    'Encyclopædia of Anatomy and Physiology,' article 'Cirrhopoda.'


_Structure of Shell._

When the shell of a sessile Cirripede or barnacle, for instance,
of a Balanus, is first examined, the structure appears extremely
complicated; but this can hardly be considered as really the case. The
structure will, I think, be best understood by recalling to mind that
of Pollicipes,--the oldest known genus, from which, in one sense, all
ordinary Cirripedes, both sessile and pedunculated, seem to radiate.
I must premise, and the fact in itself deserves early notice, that
the homologies of the several parts in the pedunculated and sessile
Cirripedes admits of no doubt,--that is, if amongst the pedunculated,
the genus Pollicipes, or certain species of Scalpellum, be taken as a
standard of comparison.[20] The peduncle corresponds with the basis,
as may be clearly seen, if a Pollicipes with a short peduncle, and a
Balanus, with a deep cup-formed or cylindrical basis be compared, for
the contained parts are similar, and both grow at their upper edges
upwards and outwards. Secondly, the valves round the lower part of
the capitulum of a Pollicipes, though generally much more numerous,
and forming more than one whorl or circle, and not so closely packed
together, answer to the compartments forming the shell of a sessile
Cirripede; this is shown by their lateral and downward growth, by their
upper ends generally projecting freely above the cavity in which the
animal's body is lodged; and in the case _Pollicipes mitella_, by an
actual resemblance in outline, some being triangular, some broad at
the upper end, and some sub-rhomboidal, and, lastly, in the manner in
which they slightly overlap and indent each other: moreover further
resemblances in the relative position and even in the size of the
several valves, will hereafter be pointed out between certain sessile
genera amongst the Chthamalinæ and certain genera of the Lepadidæ.
Thirdly, the scuta and terga in Pollicipes, so strikingly resemble
in manner of growth in position relatively to the animal's body--in
shape--and even in being articulated together, the valves which form
the operculum or lid of sessile Cirripedes, that their identity is at
once obvious.

    [20] Dr. J. E. Gray long ago observed these homologies. If Lepas be
    taken, the comparison is not quite so simple, owing to the growth
    of all the valves in that genus being upwards; but in several
    species of Scalpellum we may see the intermediate steps between
    the normal downward growth of the valves in Pollicipes, and the
    abnormal upward growth in Lepas.

It may be well here further to premise, that apparently none of the
sutures in the shells of Cirripedes correspond with the articulations
between the three archetype cephalic segments, of which the whole shell
is formed; or with the eight elemental pieces, of which each separate
segment in the archetype crustacean is known to consist. But, as I
believe, the several valves in the shell of a Cirripede are homologous,
or at least analogous, with the sclerodermic plates,[21] of which
the carapace of the Podophthalmia is formed; with this difference,
that in the latter they become, after their first formation, united
together into a single piece, and are thus moulted as a whole; whereas
in Cirripedes, the valves or sclerodermic plates are not moulted, but
continue to be added to throughout life.

    [21] Milne Edwards, 'Annales des Sciences Naturelles,' tom. xviii,
    (1852), p. 236.

In Pollicipes, there is no difficulty in understanding the growth of
the lower valves of the capitulum, especially if a species be taken
in which these valves stand a little way apart: at each period of
growth, they are added to at their basal edges and a little way up
both sides; at the same time, a new membrane connecting them together
is formed, the old membrane disintegrating, or being left hanging in
tatters to the last zone of growth. Now if we look at the shell of
a sessile Cirripede, there is no essential difference in the growth
of the compartments or valves; all grow downwards and laterally; but
they overlap each other much more laterally than in Pollicipes, and
the connecting membrane is in most parts reduced to a mere film jammed
in between the valves; but, in the case of the opercular membrane, it
still remains wide, and is periodically moulted.

In the annexed woodcut (fig. 1), of the rostrum of _Balanus Hameri_,
the downward growth and the lateral growth on both sides is plain.
The modified sides (_rr_) for convenience sake, have been called the
_radii_; they invariably overlap the adjoining compartments. The middle
part (_p_), has been called the wall, or parietal portion: in the
specimen figured, the walls and radii are distinctly separated, but in
some cases, especially amongst the Chthamalinæ, the lines of growth are
absolutely continuous from one to the other. In fig. 2 of a Lateral
compartment of the same Balanus, we have the same essential structure;
but the left side (_a_) is more protuberant, and is hollowed out in its
lower half; it is, also, more distinctly separated from the parietal
portion: this side has for convenience been called the _ala_; it is
invariably overlapped by the adjoining compartment: in some few cases,
as in Pachylasma, the ala is not hollowed out in its lower part, and
from being added to in a straight line along its whole edge, with the
lines of growth continuous with those on the wall, it differs hardly at
all in appearance from a radius. Lastly, in fig. 3 of the carina, or
compartment facing the rostrum, we have alæ (_aa_) on both sides; these
being, as in all cases, overlapped by the adjoining compartments.

[Illustration: Fig. 1.

Fig. 2.

Fig. 3.

_p_, _p_, Parietes; _r_, _r_, Radii; _a_, _a_, Alæ.

Fig. 1, Rostrum with two radii, serving in the Chthamalinæ for
rostro-lateral compartments.

Fig. 2, always serving for lateral and carino-lateral compartments.

Fig. 3, Carina, serving in the Chthamalinæ, also, as a rostrum.]

Now, the compartments in the shell of every sessile Cirripede, are
without exception constructed on the above three simple patterns. In
number, they are 8, 6 or 4, or all confluent together.

Considering this simplicity in growth and form of the separated
compartments, it seems at first surprising that the construction
and enlargement of the whole shell in Balanus, should long have been
viewed as a difficulty. But the radii, from growing against rectangular
indentations, or rather furrows, in the opposed compartments, come to
be set a little inwards; and their external surfaces assume a very
different appearance from the wall-portions of the compartments,
which grow against the surface of attachment. In different species,
the summits of the radii (and of the alæ) grow either very much more
obliquely than in the species figured, or more squarely--that is,
they extend from tip to tip of the adjoining compartments, parallel
to the basis. In this latter case, and when the surfaces of the radii
differ considerably in appearance from the walls, as in _Balanus
tintinnabulum_ (Plate 1), I am not at all surprised that the radii
should have been described as separate elements, and called "areæ
interjectæ," or "compartments of the second order:" for the shell of
this Balanus seems to be composed of six wedges with their points
upwards, namely, the parietal portions of the compartments, and of
six other narrower wedges, the radii, with their points downwards;
and the fact that these latter wedges consist simply of the sides
of the parietal portions, modified by growing against the adjoining
compartments, is completely masked. I should add, that sometimes the
radii are not developed, which simply means that the overlapping
lateral edges of the compartments have not been added to during growth.

The alæ are originally developed at the period of the metamorphosis,
as slight lateral protuberances in the upper part of the compartments;
from being overlapped, and therefore not exposed to external
influences, and from growing (as in the case of the radii) against
rectangular indentations or furrows in the adjoining compartments, they
generally assume an extremely different appearance from the parietes,
and might naturally be thought to have a very different nature. But
the alæ in all cases (as is obvious in Pachylasma) are nothing but
the protuberant lateral edges of the compartments, rendered thin and
modified during growth. In order that the margins of the alæ should be
received in an indentation, the upper internal surfaces of the walls
of the recipient compartments are thickened all round, excepting where
they receive the alæ. This thickened, upper, internal portion of the
walls or shell, together with the alæ themselves, form the part called
the sheath. The sheath sometimes blends insensibly into the lower parts
of the compartments, and then perhaps it would not be thought to be a
distinct element; but often it is abruptly separated by an overhanging
edge (see Pl. 9, fig. 5 _b_, 9 _b_; Pl. 20, fig. 1 _b_; Pl. 25, fig. 1,
K′) from the lower part, and then the sheath greatly complicates the
internal appearance, but not the essential structure of the shell. The
sheath acts beautifully, like an internal hoop, in strengthening the
shell round the orifice, where it is naturally weaker than at the lower
or basal end, where it adheres to the surface of attachment: in the
upper part of the shell, moreover, the sutures between the compartments
do not go straight through, but owing to the alæ projecting and being
overlapped, are extremely oblique; or the joints, in the language of
carpenters, may be said to be broken.

There is one other point of structure in the shells of the Balanidæ,
more especially of species like _Balanus tintinnabulum_, which adds to
their apparent complexity, namely, that the rim or orifice of the shell
formed by the upper ends of the compartments, projects considerably
above the opercular valves. In a young Balanus, immediately after the
metamorphosis, the operculum is attached by the opercular membrane all
round to the summits of the compartments, and there cannot be said to
be any orifice to the shell itself, but only an orifice or slit between
the opercular valves; but during growth, as the compartments are added
to at their basal edges, their upper ends are deserted, and cease to
enclose the sack, within which lies the animal's body. Hence the upper
ends come to project freely, either quite separately as in some species
of Pollicipes, where they cannot be said to form an orifice; or more or
less united into a ring so as to form an orifice, as in the different
species of Balanidæ. It follows, that to understand the real shape of
a Balanus, or rather of the cavity enclosing the animal's body, all
that part of the shell which projects above the opercular membrane,
may, in imagination, be removed as something extraneous, like so many
spines; not that I mean to say that these points of shell are dead; on
the contrary, they are often porose and penetrated by numerous threads
of corium. This upper part of the shell, thus produced so as to form
an orifice, no doubt serves to protect the less strong and moveable
operculum.

[Illustration: Fig. 4. Octomeris.

Fig. 5. Chthamalus.

Fig. 6. Chamæsipho.

Fig. 7. Balanus.

Fig. 8. Tetraclita.

_a_, Rostrum; _b_, Rostro-lateral, _c_, Lateral, _d_, Carino-lateral
compartment; _e_, Carina.

Horizontal sections through the Shells of the principal genera of
Balanidæ, showing the arrangement of the Compartments. Genera 4, 5, and
6 belong to the Chthamalinæ; 7 and 8 to the Balaninæ.]

_Number and Arrangement of the Compartments._--I have already stated
that the shell, in every one of the Balanidæ, consists of eight, six,
or four compartments, or of all fused together into a single piece;
and that the compartments themselves are all constructed on the three
simple patterns of which woodcuts (figs. 1, 2, 3) have been given.
They are arranged in a certain definite order. The type arrangement is
found amongst the Chthamalinæ, as might have been expected, inasmuch
as this sub-family is so closely related to the ancient genera
Pollicipes and Scalpellum, whence all the Thoracic Cirripedia may be
said to radiate. In Octomeris (fig. 4) the type-arrangement of the
compartments, eight in number, is well shown; the rostrum and carina
resemble each other, and have alæ on both sides, and therefore are
overlapped on both sides: the rostro-lateral compartments have radii
on both sides, and therefore overlap the adjoining compartments on both
sides; the lateral and carino-lateral compartments have radii on their
carinal, and alæ on their rostral sides; and therefore overlap on one
side, and are overlapped on the other side. Now the shell of every
other sessile Cirripede differs, I believe, from that of Octomeris,
only in the fusion together or abortion of some of the eight normal
compartments: in one genus, however, Catophragmus, outer whorls of
small compartments, arranged like the lower valves in the capitulum
of Pollicipes, are superadded. The genus Chthamalus (fig. 5) differs
from Octomeris only in the carino-lateral compartments being aborted,
(as will presently be discussed), and hence has six compartments.
Chamæsipho (fig. 6) differs from Chthamalus only in the rostro-lateral
and lateral compartments being fused together; and hence has only four
compartments. In Balanus (fig. 7) and the whole sub-family of the
Balaninæ, the rostrum is compounded of the true rostrum, as seen in the
type Octomeris, and of the two rostro-lateral compartments; hence this
compounded rostrum has radii instead of alæ on both sides, and there
are only six compartments. Tetraclita (fig. 8) and Elminius differ
from Balanus only in having the carino-lateral compartments absent, and
probably aborted; hence there are only four compartments. Lastly, in
Pyrgoma, all the compartments are blended together into a single piece.

In Pollicipes, the old type-form of the whole order, and in Scalpellum,
we have four valves, (answering to the operculum), surrounding the
aperture leading into the sack, and the valves below are arranged in
successive whorls, with a strong tendency to alternation. For, the
rostrum alternates with, that is faces the interval between, the two
scuta; the carina alternates with the two terga; and the upper lateral
valves alternate with the scutum and tergum on each side. These four
valves, namely, the carina and rostrum, which resemble each other
in structure, and the pair of upper latera, which are larger than
the other lateral valves, together form the uppermost whorl, or that
beneath the scuta and terga. In the next whorl we have the rostro-
and carino-lateral valves, alternating with those above them; and
beneath them there are generally other valves, which decrease in size
and display the same tendency to alternation. The valves here just
specified, namely, the rostrum, carina, and three pairs of lateral
valves, in the Lepadidæ, are so much larger, and are so much more
commonly present, than the other valves of the capitulum, that to
them alone I affixed special names. Now if amongst sessile Cirripedes
we look to that genus, viz., Catophragmus, which comes in its whole
structure the nearest to Pollicipes, one of the Lepadidæ, we find (as
in fig. 4), firstly, a rostrum and carina resembling each other, and
a pair of lateral compartments, larger than the other lateral pairs;
these four valves alternating with the opercular valves: and, secondly,
we find, but forming part of the same whorl, a pair of rostro-lateral
and a pair of carino-lateral compartments, which, just as in
Pollicipes, are larger than the exterior and lower valves. These lower
little valves, I may remark, decrease in size in the successive whorls,
and tend to alternate in position, just as in Pollicipes. Observing
these several striking points of correspondence in the valves, (and
indeed in the whole structure), of Catophragmus and Pollicipes, one
is strongly inclined to suspect that in Catophragmus, and therefore
in Octomeris and other sessile Cirripedes, although the rostro- and
carino-lateral compartments appear to lie in the same whorl with the
rostrum, carina, and large lateral compartments, yet that they really
belong, as in Pollicipes and Scalpellum, to a lower whorl. Now if a
very young shell of Balanus, immediately after the metamorphosis, be
examined, the carino-lateral compartments will be found not to have
been developed; they first appear after two or three zones of growth
have been added to the other compartments; bearing in mind that in
Pollicipes and in Catophragmus the lower whorls are added successively
during growth, we find in this fact strong confirmation of the view
that the carino-lateral compartments normally belong to a whorl beneath
that including the rostrum, carina, and lateral compartments. Whether
the rostro-lateral, like the carino-lateral compartments, are developed
subsequently to the others, I have had no opportunity of ascertaining,
and therefore cannot confirm the above analogical conclusion, namely,
that they, also, belong to a lower whorl.

In the sub-family Balaninæ, which includes Balanus (woodcut 7), and
Tetraclita (woodcut 8), the shell is characterised by not having
rostro-lateral compartments, and by the rostrum having radii: now in
_Pachylasma giganteum_, which undoubtedly belongs to the sub-family
Chthamalinæ, at a very early age the rostro-lateral compartments become
blended with the true rostrum, making a compound rostrum, exactly like
the rostrum in the Balaninæ; distinct evidence of a similar fusion
is retained throughout life (Pl. 15, fig. 1) in all three species of
Chelonobia, which is undoubtedly a member of the Balaninæ. Hence, I
think, we may conclude that in all the genera of the Balaninæ the
rostro-lateral compartments are probably not aborted, but are blended
with a normal rostrum (resembling that in woodcuts 4, 5, 6), making
together a compound rostrum furnished with radii: it must, however, be
observed that I could not detect any actual evidence of this fusion
in Balanus, even immediately after the metamorphosis. In Chamæsipho
(woodcut 6), either the rostro-lateral compartments attain a most
unusual breadth, or, as is more probable, they have become confluent
with the lateral compartments, which in the Lepadidæ seem to be the
most persistent of all the lateral valves. In such genera as Tetraclita
and Chthamalus, in which the carino-lateral compartments are absent,
they may be fused with the lateral compartments or with the carina; but
seeing that they are normally developed later than the other valves,
it appears to be the simplest theory to assume, until the contrary
be proved, that they are aborted. Finally, the somewhat unexpected
conclusion that the shell (not including the operculum) of sessile
Cirripedes normally consists of eight valves, four belonging to an
upper whorl, and four to a lower whorl, all forced into a single ring,
and often more or less fused together, though not strictly proved,
is rendered highly probable. I will only further add, that the Basis
perhaps represents several whorls of the small valves or scales on the
peduncle of Pollicipes, fused together; the comparison of the basis
with the calcareous cup, forming the lowest portion of the peduncle in
Lithotrya, which has been made by some authors, I do not think is very
accurate, as the cup in Lithotrya seems to have a special relation to
the boring habits of that genus.

[Illustration: Fig. 9.

Basal edge of wall of compartment in _Balanus tintinnabulum_; _a_, _a_,
outer lamina; _b_, _b_, inner lamina; _c_, _c_, longitudinal septa
uniting the inner and outer laminæ with their ends denticulated.]


_Structure of the Individual Compartments._

If the basal margin of a compartment, for instance, of _Balanus
tintinnabulum_, be examined, it appears sufficiently complicated,
being composed of an outer and inner lamina, separated by longitudinal
septa, which are denticulated at their bases; and the tubes formed
by these longitudinal septa are crossed by transverse septa. On the
other hand, in some cases, as in the genera Chthamalus and Elminius,
each compartment consists of a simple shelly layer. These two extreme
states graduate into each other: we have, firstly, on the internal
surface, quite irregular points and ridges; these become regular,
causing the internal surface to be longitudinally ribbed; then these
ribs themselves become finely furrowed on their sides and at their
lower ends, producing sharp, minute ridges, the ends of which I have
called the denticuli; and, lastly, some of the denticuli on the
adjoining longitudinal septa become united into a solid layer, forming
the internal lamina of the wall. These denticuli do not generally
cover the whole surface of the longitudinal ribs, but leave a portion
near the outer lamina of the compartment smooth. The denticulated ends
of the longitudinal septa project beyond the basal edge of both the
outer and inner laminæ, and enter the mouths of the tubes (where such
occur) in the basis, and thus strengthen the shell. The whole of the
internal lamina generally is more or less striated longitudinally,
thus displaying its origin from the union of the inner edges of the
longitudinal septa. I need only further remark that on the internal
surface of the outer lamina, between the main longitudinal septa, there
are generally (as in the woodcut) smaller longitudinal ridges, which do
not reach the inner lamina, and on this account alone are not called
septa.

Tubes are formed by the longitudinal septa, between the outer and
inner laminæ. These tubes are almost square, and are occupied by
threads of corium, which enter at pores left open between the edge of
the compartment and that of the basis on which it rests. The tubes
extend high up the compartments; but in the uppermost part they are
generally cut off by thin, transverse, calcareous septa, deposited by
the ends of the threads of corium; a cancellated structure being thus
produced. Or the uppermost part of each tube becomes filled up solidly
with compact shelly layers, which are always first thrown down on the
side of the tube facing the outside, and thus greatly strengthen the
shell: in several instances, as in _Balanus perforatus_ and _Tetraclita
porosa_, in which the disintegration of the upper part of the shell is
a necessary element in its growth for the enlargement of the orifice,
these filled up tubes become exposed. In Coronula and Tubicinella, the
tubes in their upper parts are, I believe, crossed only by transverse
membranous septa.

_Anomalies and exceptions._--In Tetraclita (Pl. 10, fig. 1 _g_, 1 _h_)
from the branching of the longitudinal septa, several irregular rows
of tubes are formed. In certain varieties of _Balanus balanoides_
(Pl. 7, fig. 2 _b_), and in _B. cariosus_ (Pl. 7, fig. 3 _b_), slight
branching ridges on the internal surface of the walls, seem to
answer to the longitudinal septa, and produce, during the downward
growth of the shell, extremely irregular cells, and short tubes. In
_Balanus vinaceus_ (Pl. 2, fig. 7 _d_), the internal lamina, instead
of being solid, as in every other species, is left cancellated, and
thus betrays, much more plainly than usual, its origin in the united
denticuli of the adjoining longitudinal septa. In _Balanus porcatus_,
between the main longitudinal septa, there are (Pl. 6, fig. 4 _e_)
what may be called rudimentary and disconnected longitudinal septa. In
Coronula and its allies (Pl. 16, fig. 6, and Pl. 17, fig. 4 _c_) it
is the outer lamina of the compartment which is anomalous; for in the
two or three lower zones of growth, it forms only a ledge on each side
of the longitudinal septa; which ledges, higher up, become confluent,
and so form an ordinary outer lamina. In Coronula, also, the wall of
each compartment (see transverse section, Pl. 16, figs. 5, 7) is very
remarkable from being deeply folded, the folds being on their internal
faces firmly calcified together, and on their external faces closely
pressed together (often with a neatly serrated suture), so that the
whole nature of the shell might be, as has happened, easily quite
misunderstood; and the walls be considered as very thick, instead of
being, as is really the case, very thin. In Chelonobia (Pl. 15, fig.
1), however, the walls are truly of such great thickness, that the
nature of the relative parts might likewise be misunderstood; in this
genus the ovarian tubes enter the walls, extending up between the
longitudinal septa, or, as they may here be more naturally called,
the radiating septa. I will specify a few more peculiarities worthy
of remark:--in some species of the sub-genus Acasta, clefts are left,
covered only by membrane, on the lines of suture (Pl. 9, figs. 7 _a_,
8 _a_), between the compartments, just above the basis; and in other
species the basis is perforated by numerous membrane-covered, minute
orifices. In Platylepas, each compartment has one deep inward fold (Pl.
17, fig. 1), somewhat analogous to the three folds in Coronula; this
fold is produced into an internal midrib, supporting and rendering
convex the membranous basis; in this genus, also, the rostrum, owing
to its midrib, is generally thrust a little on one side, and the shell
thus rendered asymmetrical. In _Chamæsipho scutelliformis_ the shell
is symmetrically perforated (Pl. 19, fig. 4 _a_) by four apertures.
Lastly, in _Chthamalus Hembeli_ and _intertextus_, after a certain age,
the basal edges of the walls become inflected, and continue to grow
inwards till they entirely take the place of the true membranous basis.


_Structure of the Radii._

_Radius._--This term, as we have seen, is applied to that side of the
compartment, the growth of which is modified, by abutting against and
overlapping the adjoining compartment. Hence the structure of the
radius is essentially the same with that of the parietal portion of the
compartment. When best developed, as in _Balanus tintinnabulum_, the
radius consists of an outer and inner lamina, separated by denticulated
septa, extending in horizontal lines parallel to the basis, and is
consequently perforated by minute tubes or pores. The tubes become
filled up solidly much more commonly than do the parietal tubes; and
the inner lamina, in such cases, is hardly distinct from the outer
lamina. The denticuli often fail, or are present only on the lower
sides of the septa; and very frequently the edge of the radius can only
be said to be crenated. Notwithstanding these frequent anomalies, if a
series of species and genera be taken, it is certain that there is, as
might have been expected, a close relationship in internal structure,
between the radii and the parietes. The edge of the radius is received
in a slight furrow (generally marked like a seal, with the impression
of the denticulated septa) in the opposed compartment: sometimes the
outer edge or lid of the recipient furrow, is so broad as to give the
false appearance of a radius having been developed, at least in the
lower part of the shell, on both sides of the suture. A crest of corium
runs into each suture between the edge of the radius and the furrow in
the opposed compartment; and when the radius is permeated by pores (as
in woodcut 10), threads of corium branch off this crest, and enter the
pores. In the lower part of the shell, these crests of corium project
from the corium forming and surrounding the sack; but in the upper
part of the shell, above the opercular membrane, and therefore above
the sack, the corium is produced up each line of suture as a separate
ribbon. In proportion as these ribbons extend more or less near to the
summit of the shell, so do the edges of the radii continue to be added
to, to a greater or less height from the basis; and consequently their
summits become less or more oblique.

[Illustration: Fig. 10.

Edge of the radius of _Balanus tintinnabulum_. _a_, outer lamina; _b_,
inner lamina; _c_, denticulated septa, uniting the two laminæ.]

_Peculiarities in the Structure of the Radii._--In some of the species
of Tetraclita, in which genus the walls consist of several rows of
tubes, the radii are likewise perforated by several rows; and in some
of the other species (Pl. 10, fig. 1 _h_), the edge, or disarticulated
surface of the radius, is marked by irregularly branching ridges; and
these evidently correspond with the branching septa or ridges of the
wall. In Chelonobia, the outer lamina of the radius, as well as of
its recipient furrow, is of extraordinary thickness; and this lamina,
in _C. testudinaria_ (Pl. 14, fig. 1 _a_, 5, _b_, and Pl. 15, fig.
1, _f_), is modelled into sharp transverse ridges and valleys. In
the Chthamalinæ, the radii, like the parietes, are simply solid; and
apparently in consequence, for the sake of strengthening the sutures,
the edges of the radii, and of the recipient furrow in Octomeris
(Pl. 20, fig. 3 _a_) and in _Chthamalus dentatus_ and _Hembeli_ (Pl.
18, fig. 3 _b_, 5 _a_), are neatly dentated. In some other species
of Chthamalus (Pl. 19, fig. 1 _a_), the radii present a slight
modification of this structure, the sutures being formed by oblique
interfolding laminæ. In the radii of Coronula and Tubicinella, there
is a peculiarity, in apparent connection with the fact, that in these
genera the parietal tubes are not crossed by transverse calcareous
septa, namely, that the pores by which the radii are permeated keep
unclosed throughout their length, and open into a special longitudinal
tube (Pl. 16, fig. 7, _d′_), which runs along that margin of the wall,
whence the radius arises. In Coronula the wall is of extreme thinness,
and in conformity so is the true radius, but that the shell might not
thus be rendered very weak, complementary or pseudo-radii are developed
on their inner sides (Pl. 16, fig. 7, adjoining the true radii A _d_,
C _d_, and shaded by distant convex lines). Even in the allied genus
Xenobalanus, in which the whole shell tends to become rudimentary,
traces of these pseudo-radii (Pl. 17, fig. 4 _b_, _d_) can be detected.
In Coronula, though the radii (Pl. 16, fig. 7, A _d_, C _d_) are, by
the above special means, rendered thick, and though the alæ also are
thick (C _a′_, D _a′_), yet together they do not equal in thickness the
folded walls; and consequently, there is left between the radii and alæ
square chambers (_v_), occupied by the branching ovarian tubes.


_Structure of the Alæ._

These project, generally abruptly, from the sides of the upper part of
the compartments; they appear from the first growth of the shell; they
are overlapped by the radius and by part of the wall of the adjoining
compartment; they are thinner, and have, owing apparently to being
overlapped, a very different aspect from the parietal portion; but
they do not differ from it in essential nature. They are solid, that
is, they are never permeated by pores; but their edges are generally
crenated, and there is, in some cases, as in Chelonobia, sufficient
evidence that these crenations answer to the horizontal septa on
the edges of the radii (also often reduced to mere crenations), and
consequently, likewise, to the longitudinal septa of the parietes.
In Coronula the edge of each ala consists of a medial ridge, sending
off denticulated septa on _both_ sides, and is therefore anomalous as
compared with the alæ in other genera, but corresponds in structure
with the similarly anomalous radius of Coronula. In order to allow of
the growth of the edge of the ala, a fine thread of corium runs up the
narrow furrow in which the edge is lodged, proceeding from the corium
of the sack. In proportion as this thread runs up higher or lower,
so are the summits of the alæ rendered, during growth, less or more
oblique.


_Structure of the Sheath._

As the compartments overlap each other, the edges of the alæ would have
projected, and the inner surface of the orifice of the shell would not
have been smooth and rounded, had not that part of each wall, which
does not overlie an ala, been thickened so as to allow of the formation
of a shoulder or indentation, against which the edge of the ala fits
and abuts. The thickened portions, and the alæ themselves, together
form the sheath, of which the use seems to be to strengthen, like a
broad internal hoop, the upper part of the shell round the orifice,
where naturally it is weak. The sheath is composed of successive, fine,
shelly layers, which extend, as the shell is added to at the basal
margin, lower and lower down on the inner surface of the walls. The
lower edge of the sheath either simply projects a little inwards, or
more commonly is formed into a sharp depending ridge, as represented
in fig. 1, K′, Pl. 25. In some species of Pyrgoma (Pl. 13, fig. 2
_b_), the sheath reaches nearly to the bases of the compartments;
and in Chelonobia (Pl. 14, fig. 4 _e_ _c_ _e_), the inner layer of
shell surrounding the sack, which seems to correspond more nearly
to the sheath than to the inner lamina of the walls, actually rests
on the basal membrane. The opercular membrane is generally, but not
invariably, attached only a little way above the lower edge of the
sheath: at each exuviation, a new opercular membrane is formed, and is
attached to the next lower zone of the sheath; the old membrane being
cast off, but a circular slip of it is left, investing the last zone.
Hence the whole upper part of the sheath above the opercular membrane,
comes to be thus invested; and is marked by circular lines, one above
the other, caused by the successive exuviations. This investing
membrane often supports rows of minute bristles, directed upwards.
Generally, a film of shell is deposited, at the period of the formation
of each new opercular membrane, on that part of the sheath which lies
immediately beneath. This innermost film or thin layer of shell, on
the lines of suture between the compartments, breaks joint, at least
in some cases, with the underlying shelly layers,--that is, the suture
in this last-formed film does not lie exactly over the suture in the
subjacent layers of the sheath. In Tubicinella, the sheath extends down
close to the basis; and what is unique in this one genus, the opercular
membrane, gradually thinning out downwards, closely adheres to the
whole inner surface of the shell. In Tubicinella and in Xenobalanus
(Pl. 17, fig. 4 _b_), the sheath separates easily into separate
successive rims of shell; and this structure evidently is for the sake
of facilitating the breakage of the upper end of the shell, which, as
we shall presently see, is necessary to allow of the increase in size
of its orifice.


_Structure of the Basis._

This, in several genera and species, is composed of simple membrane,
and consists of successive, concentric, circular slips, added round the
outside, at each period of growth. In some species of Tetraclita and
Balanus the basis is calcareous, but diaphanous, very thin, smooth, or
somewhat granulated. In other cases it consists of a single calcareous
lamina, either smooth, or with ridges radiating from its centre; it
is formed of two laminæ, (as is most usual in Balanus,) separated by
radiating septa. These septa, as well as the radiating ridges in the
case of the single lamina, are homologous with the longitudinal septa
of the parietes. The denticulated ends of the latter enter the mouths
of the tubes formed by the radiating septa of the basis: threads of
corium pass between the denticuli of the parietal septa, and thus
enter the basal tubes. The ends of these threads of corium generally
deposit transverse calcareous septa, exactly as within the parietal
tubes. When the basis is thick the septa themselves (_ccc_) between
the proper basal tubes, become porose, (or rather cancellated,) and
they sometimes expand into a very thick, cancellated layer, separating
the outer lamina (_a_) of the basis from the proper basal tubes, which
always lie close under the inner lamina (_b_). This structure differs
only slightly from that seen in the parietes of Tetraclita, in which
the branching of the longitudinal parietal septa, produces thick walls,
formed of several rows of tubes or pores. With respect to peculiarities
in structure of the basis, _Balanus lævis_ offers the most remarkable
case; for here, in specimens which have grown crowded together, the
whole interior appears sometimes to have become too much elongated or
too deep for the animal's body, and consequently the lower part of
the deeply-concave basis has been filled up (Pl. 4, fig. 2 _a_) by
thin, irregular, calcareous diaphragms. In elongated specimens, also,
of _Balanus balanoides_, the shell sometimes appears to have grown
too long for the animal's body; but in this case the membranous basis
becomes extremely convex inwards; it still reaches the basal edges of
the parietes all round, but in the middle it is raised high above the
surface of attachment; yet sometimes threads of the cementing tissue
depend from the middle part to the surface of attachment. In _Balanus
terebratus_ (Pl. 8, fig. 2 _a_, 2 _b_), and in some species of Acasta,
the basis is riddled, as previously stated, by numerous, minute,
membrane-covered orifices. In _B. declivis_ the membranous basis is
always extremely oblique, owing to the rostral end of the shell being
twice as high as the carinal and opposite end.

[Illustration: Fig. 11.

Portion of edge of basis of _Balanus tintinnabulum_, _a_, _a_, outer
lamina; _b_, _b_, inner or upper lamina; _c_, _c_, _c_, porose or
cancellated radiating septa.]

Regarding the very remarkable means by which the basis of sessile
Cirripedes is cemented to the surface of attachment, it will be
convenient to defer for a little the description, on account of its
necessary length.


_Structure of the Opercular Valves (Scuta and Terga)._

These are situated on each side of the slit or orifice leading into the
sack; from their shape, their powers of movement, their separation by
flexible membrane from the shell, to which they serve as a lid, they
appear at first as if they constituted an element very distinct from
the shell itself, but this is not the case. They are, together with the
opercular membrane, as essentially as the whole of what is externally
visible, a part of the modified carapace, of which they occupy the
upper or posterior extremity: from tracing the metamorphoses, or
even by comparison of a Balanus with Pollicipes, there can be no
doubt of the truth of this conclusion. The opercular valves are four
in number,--a pair of scuta and a pair of terga; but the latter in
_Coronula diadema_ and _reginæ_, are either aborted or represented by
a mere rudiment; and in Xenobalanus both scuta and terga are quite
absent. In several cases, more especially in the genus Pyrgoma (Pl.
13, fig. 1 _b_), the scutum and tergum on each side are calcified
together, so that sometimes not even a trace of the line of junction
can be discovered. In most cases the scutum is firmly united, being
articulated in a manner presently to be described, to the tergum; but
in Coronula, Tubicinella (Pl. 17, fig. 3 _c_), and Platylepas, the ends
of these valves are simply approximated.

_Scuta._--These valves are important, inasmuch as the animal's body
is attached to them; in Pl. 25, fig. 1, the broken line, surrounding
_a_, _b_, shows where the body has been cut, in removing the scutum
on the near side, the other scutum, S, being left articulated to the
tergum, T. In shape the scuta are generally sub-triangular; but in some
species of Pyrgoma and in Chelonobia, &c. they are much elongated. The
lines of growth are usually prominent; and along the occludent margins
the alternate, or sometimes every third or fourth line, is developed
into a knob, which produces a serrated edge, serving to lock the two
opposed valves together; there is, however, no trace of this structure
in Coronula and Tubicinella. In some species of Pyrgoma, a ledge of
considerable breadth (Pl. 13, fig. 3 _e_, &c.) is developed along
the occludent margins of the two scuta, as well as of the two terga,
giving them an anomalous structure. The _Terga_ differ considerably
in outline in the different genera and species: their shape approaches
more nearly to a triangle than to any other regular form; but there is
generally a projection or spur on the basal margin, on the side towards
the scutum. In some species of Pyrgoma, the tergum is of so irregular a
shape as to defy description. In most cases, a longitudinal depression
or furrow runs down the valve, from the apex to the extremity of the
spur; and it not rarely happens that the sides of this furrow become
folded inwards and almost closed. The spur probably answers to the
basal point of the usually sub-rhomboidal tergum in Pollicipes and
Scalpellum.[22] The tips of the terga in some species of Balanus, &c.,
are specially modified into sharp points or beaks (Pl. 2, fig. 3 _b_,
3 _d_), bowed a little inwards, and projecting considerably above the
tips of the scuta; this is effected by the medial, uppermost part
of the valve being internally thickened and hardened, and then, by
the disintegration of the two margins and the external surface, the
internal modified portion becomes exposed. The whole valve, also, at
least in such cases as in _Balanus psittacus_, appears to be forced
slowly upwards in the articular furrow of the scutum. I am assured, by
a competent observer, that the beaks of the terga in _B. porcatus_ can
give an object placed within the orifice of the shell a sharp tap.

    [22] In comparing the Tergum of one of the Balanidæ with that
    of a typical member of the Lepadidæ, for instance, that of
    Balanus with that of Pollicipes, apex corresponds with apex:
    the extremity of the spur in Balanus corresponds with the basal
    point of the whole valve in Pollicipes: the scutal margin, (which
    in Balanus homologically extends down to the extremity of the
    spur), corresponds with the scutal margin of Pollicipes: the
    carinal margin in Balanus corresponds with the _upper_ carinal
    margin in Pollicipes: the basal margin of Balanus on the carinal
    side of the spur, corresponds with the _lower_ carinal margin in
    Pollicipes: lastly, (and this is the chief difference), in Balanus
    there is no appreciable occludent margin, the apex of the valve
    being brought close to the upper angle of the scutal margin; in
    Chthamalus, however, there is yet left some remnant of an occludent
    margin,--which margin in Pollicipes is conspicuous.

The scutum and tergum, with the few exceptions above stated, are
articulated together at a large or open angle. The articulation (see
Pl. 11, fig. 5 _b_, _c_, _d_, and fig. 6 _b_, _c_) is effected by the
margin of the tergum being a little inflected, and lodged in a furrow
in the margin of the scutum. This furrow in the scutum has its further
border generally prominent and often reflexed or curved over; I have
called it the articular ridge; it, also, is lodged in a furrow in the
upper part of the tergum, which again is bordered by a ridge, viz., the
articular tergal ridge. So that in both scutum and tergum there is an
articular furrow, bordered in each case, on one side by the margin of
the valve, and on the other side by the so-called articular ridge. In
Chelonobia (Pl. 14, fig. 1 _b_) the articular ridge of the scutum is
horny. When, as often happens, the scuta and terga have been much worn,
the manner of their articulation (Pl. 18, fig. 1 _a_) is pretty well
shown even from the outside; in this case their external appearance
is very different from what it is in those individuals (fig. 1 _c_)
of the same species, which have not suffered disintegration. This
articulation of the scuta and terga is prefigured amongst the Lepadidæ,
in _Pollicipes mitella_, and in Lithotrya.

The scuta are brought together by a short, strong, straight, adductor
muscle (Pl. 25, fig. 1, _a_); its attachment leaves (with very few
exceptions, as in Tubicinella) a rounded impression, or even pit, on
the under side of the valve in its upper part. This pit is frequently
bounded, on its lower side, by a sharp ridge, which, though not in
actual connexion with the adductor muscle, I have, for convenience
sake, called the adductor ridge; it serves apparently to give support
to the animal's body; in some few cases (as in _B. psittacus_, Pl. 2,
fig. 3 _c_) it is confluent at its upper end with the articular ridge,
and converts the whole basi-tergal corner of the valve into a deep
cavity. In some of the species of Pyrgoma (Pl. 12, fig. 5 _c_, 7 _b_),
and in some varieties of Creusia, this adductor ridge is enormously
developed, so as to depend far beneath the true basal margin, or
that to which the opercular membrane is attached. At the basi-tergal
corner of the valve, there is generally a small pit or impression, and
sometimes distinct crests, for the attachment of the lateral depressor
muscle. At the rostral end there is, also, a small cavity formed by the
overfolding of the occludent margin (rarely furnished with crests) for
the attachment of the rostral depressor muscle. In the Terga, at the
basi-carinal corner, there are usually crests, though sometimes feebly
developed, for the attachment of the tergal depressor muscle. But in
Chelonobia, Coronula, Tubicinella, Platylepas, and in some other cases,
there are no crests. The crests, when well developed, are furnished
with rectangular sub-crests or denticuli on both sides; in fact they
resemble, and are probably homologous with, the denticulated ribs or
septa in the parietes, radii, and basis. Altogether the scuta and terga
are attached, as far as muscles are concerned, to the shell and sack,
by three longitudinal pairs.


_Growth of the Whole Shell, and Its Microscopical Structure._

The opercular valves are added to along their basal margins alone;[23]
the animal's body, together with the several muscles, becoming attached
at each period of growth lower and lower down to the valves; this
no doubt is effected by the absorption of the upper surfaces of the
muscles, and the formation of new fasciæ on their lower surfaces. The
opercular membrane, which, though thin and flexible, forms part of
the general outer surface of the animal as much as does any portion
of the rigid shell, with which indeed it is strictly homologous, is
periodically moulted, together with the integuments of the whole
included animal. The new opercular membrane is of course each time
formed a little larger than the old one. In Coronula and Tubicinella,
however, several successive opercular membranes are preserved one
over the other, and the outside membrane gradually disintegrates; in
these cases the undermost opercular membrane is formed wrinkled and
considerably too large, so as to allow of being stretched, before it
is finally cast off. In Tubicinella, the opercular membrane runs down,
adhering to the inner surface of the shell, to nearly the basis, and
hence during the diametric growth of the shell, it is longitudinally
split, and is repaired by slips of new membrane, which resemble the
radii in form and in direction of the lines of growth.

    [23] In some species of Pyrgoma, the ledge (_limbus occludens_)
    which is added along the occludent margin of both scuta and terga,
    and in some species of Balanus a narrow rim, or slight protuberance
    which is added along the carinal margin of the terga, offer
    unimportant exceptions to the rule, that the opercular valves grow
    only at their basal margins.

The basis is added to only round the circumference, and hence
increases in diameter, and, when concave, in depth. The compartments
grow at their basal margins, where they are in contact with the basis;
hence the shell is added to in height, and, owing to the outward
inclination of the compartments, also, in basal diameter; but the
compartments likewise, in most cases, grow along both lateral margins,
that is, on the edges of the radii and alæ; and hence the upper part
of the shell, also, increases in diameter. The orifice of the shell,
moreover, thus becomes enlarged. In some cases the shell is destitute
of radii, only sutures being present, that is, the compartments do not
grow laterally; and sometimes, as in the whole genus Pyrgoma, there
are not even sutures, the compartments having been fused together:
in both these cases, the shell can increase in diameter only at the
base; and the orifice, it might have been thought, would necessarily
have remained, to the destruction of the animal, of the same minute
size, as when first formed after the metamorphosis: this certainly
would have been the case had not the upper ends of the compartments,
surrounding and forming the orifice, been nicely adapted always to
yield, in a certain limited degree, to the disintegrating influences
to which every shell is exposed, but which most Cirripedes can resist;
and the disintegration of the narrow end of a conical tube, of course
increases the diameter of its orifice. In Tubicinella, in which the
shell is furnished with narrow radii, and does increase in diameter
from top to bottom, the increase is not sufficient in proportion to
the continued elongation of the shell; to compensate for this, the
orifice is enlarged at short intervals by the breakage of the upper
end of the shell, for which purpose (as explained under the genus) it
is evidently constructed. Hence we see that, in certain Cirripedes,
decay or disintegration, and breakage, are necessary elements in their
growth! It is a remarkable fact, which I cannot explain, that in
some species in which the orifice of the shell is usually increased
by disintegration, if individuals are so situated that they are not
exposed to sufficiently energetic disintegrating influences, as may be
inferred from the well-preserved condition of the whole surface of the
shell, then the radii become developed, and the orifice is increased in
size by the diametric growth of the upper part of the shell: I have
seen instances of this in _Tetraclita porosa_, and _purpurascens_, and
in _Balanus perforatus_: it appeared, but of course erroneously, as if
the lateral growth of the compartments had been subjected to the will
of the animal.

Considering the strength of the shell of sessile Cirripedes, the
separation of their compartments one from another and from the basis,
during growth, has justly been thought a surprising circumstance. In
most Chthamalinæ and in some species of Balanus, however, if the shell
be boiled in caustic potash, the compartments fall apart with a touch;
this shows that their union is due to animal and probably to organised
matter, and the growth of such matter between the opposed edges of
the compartments, and their consequent gradual separation, offers
no particular difficulty. But in many Balani, boiling in potash for
hours does not seem even to weaken, in the least degree, the sutures,
which are wonderfully strong--the shell often breaking rather than
yield on these lines; if, however, the shell be dissolved in acid,
the animalised tissue which is left easily separates on the lines of
suture, and if this tissue be boiled in potash, the remnants of the
compartments fall quite separate. These facts seem to me to show, that
the compartments in such cases are joined along the lines of suture by
tissue, which must be in a calcified state, but which, nevertheless,
continues to grow by intersusception; in other words, I believe that
the tips of the complicated ridges and points interlocking on the lines
of suture, are not separated from each other by films of corium or
simple animal matter, but are actually united by corium in a calcified,
yet still growing condition.

In ordinary Crustaceans, the growth is periodical and sudden; a new
and larger carapace, for instance, is formed under the old one, and
after the exuviation of the latter, the new one soon hardens, and does
not subsequently increase in size; so it is in the case of Cirripedes,
with the membranes of the body, and even with certain parts, as
the opercular membrane, of the external covering. But a Cirripede
cannot, like a crab, crawl into some crevice and remain protected
till its shell becomes hardened; hence, probably, it is that the
shell is never[24] wholly moulted. Even if the margins of the opposed
compartments and of the basis were to grow rapidly, the shell would
necessarily be much weakened on the lines of suture, and unable to
withstand the heavy breakers, to which so many species of sessile
Cirripedes are exposed. On the other hand, although the margins are
thus compelled to grow slowly, they do not grow continuously, as may
be seen in the zones of increment on all the valves, corresponding, I
believe, with the periods of exuviation of the membranes of the body. A
layer of shell, often very thin, seems to be generally deposited over
the whole internal surface of the several valves, at the same time that
the marginal zones are added; so that the only essential difference
in the growth of the external covering, in Cirripedes as compared
with ordinary Crustaceans, is that the old shell is not cast off, but
adheres to the outside of the new shell, and that the margins are
added to (in certain definite directions) slowly yet not continuously,
instead of the whole being formed at a single period.

    [24] In the genus Alcippe, and in Cryptophialus, the whole of
    the external membranes are moulted, excepting the surface of
    attachment; but then these Cirripedes live in cavities which
    they form for themselves, and are thus protected. In Lithotrya
    the membrane of the peduncle, with its little valves or scales,
    is moulted, but here, again, this very part is protected by the
    tubular cavity, which the animal forms and inhabits. Neither of
    these three genera belong to the Balanidæ, or sessile Cirripedes,
    which we are now more especially describing.

If, now, a section of one of the shelly zones of growth be carefully
examined, it can in some cases be distinctly seen to be formed of
successive, excessively fine laminæ; but the animalised tissue (which
differs much in amount in different Cirripedes) left after the shell
has been dissolved in acid, exhibits, in most cases, neither laminæ nor
any other structure whatever. The shell seems to be the actual pulpy
corium, or true skin, in a calcified condition, but generally with its
cellular structure modified and much reduced: I have taken a bit of
recently-formed shell of Tetraclita and of Coronula, with the corium
still adherent to its under surface, and after dissolution in acid, I
could not distinguish the part, which had just before existed as shell,
from the corium itself. In the case of Coronula, immediately prior
to the period of moulting and growth, I found the unaltered corium so
charged, as to effervesce, with carbonate of lime, either in a state of
dissolution, or in granules too minute to be visible under the highest
powers.

The sutures between the several compartments and the basis are covered
by thin membrane, which is continually splitting during the growth
of the opposed edges of the underlying shell; but previously to each
splitting, a new slip of membrane is, I believe, already formed under
the old one; so that the corium is not even momentarily exposed. Owing
to this manner of growth, the slips of membrane consist of successive
rims united together; in most cases, these soon become abraded from the
older parts of the shell, but are sometimes preserved. The last-formed
slip of membrane over a suture is homologous with the opercular
membrane; and both are strictly analogous with the ring of flexible
membrane, forming the joint of the leg of a crab. In the latter case,
the flexible membrane and hardened crust are both moulted together:
in the opercular membrane, there is a double line of splitting, one
close round the opercular valves, and the other at the basal edge of
the sheath, and the intermediate portion is moulted, but with a zone of
membrane left adherent to the non-moulted valves and sheath: lastly,
in the slips of membrane covering the sutures, there is only a single
line of splitting, and no portion, I believe, is moulted; the rims of
membrane on each side remaining adherent on the compartments and basis,
until worn away.

The opercular membrane, when closely examined, exhibits no structure,
except that it can sometimes be plainly seen to be composed of
successive, numerous, excessively thin laminæ. Occasionally, however,
it presents the false appearance of being permeated by parallel and
anastomosing vessels: this appearance is clue to one or more of the
component laminæ having been wrinkled before a succeeding lamina was
thrown down and attached to its under side. If a small piece of an
opercular valve of Tubicinella, with the opercular membrane adhering
to it, and with the corium under both, be dissolved in acid, it may
be clearly made out that the corium under the valve has gone on
being converted into shell, whereas under the opercular membrane it
has been converted and condensed into fine constituent laminæ of
chitine. Inasmuch as the successive layers of shell, during each period
of growth, go on encroaching on those of the membrane, the line of
junction between the shell and chitine becomes oblique or bevelled. The
membrane on this bevelled line of junction assumes a slightly different
aspect to what it has elsewhere; it becomes yellowish or brown, thicker
and very much tougher. In many genera it is also furnished with a row
of small bristles. At the period of exuviation the opercular membrane
separates just outside this modified portion, leaving the latter
adherent, as a rim or slip, on the valves. If, however, the opercular
membrane be rudely torn off before its proper period of exuviation,
it carries with it the as yet continuous, but already modified, slip.
A slightly indented line may sometimes be traced before the period of
exuviation, showing where the separation will take place: what produces
this line I know not. The coloured, thickened, and modified slips of
opercular membrane, which are thus retained adhering to the valves,
and which together form an investing membrane, have been considered
by most authors as the epidermis; but they have no more right to be
thus called than has any other part of the opercular membrane. Exactly
similar slips of membrane are left investing the sheath. So, again, the
membrane which, when well preserved, invests the walls of the shell,
is made up, as already stated, of successively adherent slips, which
originally covered the lines of suture.[25]

    [25] In the case of Coronula there is a peculiarity, described in
    the last section of this Introduction, (under the head of Cementing
    Apparatus), namely, that the two or three last-formed, exterior
    zones of the Basal membrane continue for a period to increase in
    width; being, as I believe, dragged one from over the other, with
    fresh laminæ of membrane continually thrown down. In this same
    genus, and in Tubicinella, the walls of the shell are invested
    by membrane, which is doubled inwards under their basal edges;
    and as the latter grow, the investing parietal membrane splits
    and separates from the basal membrane, and is pulled outwards and
    downwards. This inflected, often broad border of membrane, seems to
    me more strictly comparable with the opercular membrane, than with
    those narrow, thickened rims of yellowish membrane which in other
    Cirripedes cover the suture between the basal edges of the walls
    and the basis.

The little bristles above alluded to, which arise from the slips of
membrane left adherent on the opercular valves, sheath, and walls,
stand in rows; a row corresponding to each period of exuviation of
the opercular membrane. The bristles are generally largest on the
opercular valves and sheath; in _Balanus tintinnabulum_, they are
from 1 to 2/1000ths of an inch in length, but they are longer in some
other species. I may here mention, as showing the connexion of these
bristles with the opercular membrane, that similar bristles occur in
_B. perforatus_, scattered over the surface of that membrane, and
are necessarily moulted with it. In the imbedded genera Coronula and
Tubicinella, none of these bristles exist. When a portion of valve or
shell, furnished with bristles, is dissolved in acid, tough, sinuous,
and apparently hollow, threads are seen to run from their bulb-like
bases, into and up the corresponding layer, which, before dissolution,
existed as shell; and they terminate internally in very fine points,
which I believe are united to the underlying corium. These threads, or
_tubuli_,[26] as I have called them in my volume on the Lepadidæ, are,
in _Tetraclita porosa_, about 1/5000ths of an inch in diameter, but
only half that size in _B. tintinnabulum_. On parts of the shell where
there are no bristles, similar tubuli penetrate the shelly layers, and
come to the surface. The tubuli running to the lowest and last-formed
row of bristles, just after a period of exuviation, are so delicate as
hardly, or not at all, to be distinguished; in the row above, they are
plain and longer, and for the next two or three upper rows they are, in
some cases, as in _Tetraclita porosa_, longer and longer, having been
added to during each successive thickening of the valve. These tubuli
consist of chitine, and no doubt first existed as threads of corium;
they are so tough that they must serve to strengthen the successive
layers of shell, but I imagine their chief function is to keep up the
vitality of the newly-formed layers of shell. May we not, also, venture
to suppose that by their means, some degree of sensibility is given
to the bristles? I need only further remark, that in some species of
Balanus and of Chthamalus, the under side of the shell is penetrated
by irregular pores, large enough to be visible to the naked eye, into
which threads of corium penetrate; but these can hardly be said to
appertain to the microscopical structure; and are more nearly related
to those pores and furrows, formed by the greater or less development
of the longitudinal septa, and in which the threads of corium deposit,
or rather become changed into, transverse septa, or solid shelly
matter, as previously described.

    [26] I regret that I have used this term "tubuli"; for the threads
    thus designated, I believe, are not the same with the _tubuli_ of
    Dr. Carpenter, which are not left after dissolution in acid. I have
    seen tubuli, as called by me, in the shell from the leg of a crab,
    after having been placed in acid.


_Muscles of Sack._

In the pupa, the thorax, as we shall hereafter more fully see, is
continuous with, and opens into the large anterior end or front part of
the head; but during the metamorphosis (Pl. 30, fig. 2), the thorax of
the Cirripede becomes, owing to the almost transverse position occupied
by the young animal within the pupa, to a great extent internally
separated from the anterior end,--which anterior end forms, as we know,
either the peduncle or the basis. Hence it comes to pass that the
body or Thorax (Pl. 25, fig. 1) is lodged within a sack (_f_) within
the shell. The chitine membrane lining this sack is excessively thin
and transparent, but less so in Xenobalanus and Tubicinella; it is
obviously continuous with that investing the body of the animal; it is
also essentially continuous with the opercular valves and membrane,
and consequently with the whole shell. It is periodically moulted. It
is lined by corium, as is likewise the surrounding shell; hence the
corium is double round the sack, as indeed might have been expected
from the shell and opercular valves (at least their upper parts) being
formed by the prolongation, as is obvious in the pupa, of the posterior
edges of the carapace. Between the two folds of corium, which are
united together by transverse ligamentous fibres, branching out at
both extremities, like the roots and branches of a tree, we have the
longitudinal muscles, which go to the opercular valves; and likewise a
layer-like mass of branching ovarian tubes (Pl. 25, fig. 1, _g_): the
ovarian tubes, however, are often confined to the base of the sack.
In Xenobalanus, the two folds of corium are united by longitudinal
membranous septa, making a series of quite peculiar, square tubes.

The above-mentioned muscles are attached at their upper ends to the
opercular valves, and at their lower ends to the basis. There are, in
fact, three pairs, but the pair attached to the basi-carinal angles of
the two terga (Pl. 25, fig. 1, _i_), are almost invariably confluent,
forming one great bundle; the second pair is attached to the lateral
or basi-tergal corners of the two scuta, and are hidden in the figure;
the third pair (_h_) is attached also to the scuta, to their rostral
angles. These muscles can only act as depressores; they are often
extremely powerful; they belong to the voluntary class, for they
are transversely striped. By their action, the opercular valves are
capable of varied slight movements, within the limit allowed by the
width of the flexible opercular membrane. By the action of the lateral
scutal depressores, the orifice leading into the sack is opened, the
movement being generally aided by the protrusion of the cirri. By the
sudden contraction of the rostral scutal depressores, the blows which
are sometimes given by the beaked terga at the opposite end of the
operculum, are probably effected. By the contraction of all three pairs
of muscles, the opercular valves are held down with quite surprising
force. The valves can be raised only by the action of the animal's body
against the basis.

In Coronula these muscles are more spread out, and do not extend
down to the basis; their lower portions, as is likewise the case in
Tubicinella, do not exhibit transverse striæ, and hence tend to pass
either into the involuntary class, or into ligament. This condition of
the muscles, in the above two genera, accords with the little-developed
state of their opercular valves. In Xenobalanus, there is no longer
any evidence of the muscles being collected into five or six bundles,
for they are thinly and almost uniformly spread out, and show in no
part transverse striæ. I may add that in much elongated specimens
of _Balanus balanoides_, these muscles become in their lower part
ligamentous, and destitute of striæ.


_Branchiæ._

In the Balaninæ, a pair of Branchiæ is always present: they lie on
each side, in a somewhat curved position, in the angle between the
sides of the shell and the basis. In Pl. 25, fig. 1, they are exactly
covered, on the further side, by the body of the animal. They are
attached near each other at the carinal end of the sack in a vertical
line, and likewise on each side in a transverse line, extending from
close beneath the spur of the tergum towards the point of attachment
of the body to the scutum. In Balanus, as in the figure (Pl. 25, fig.
3) of _B. tintinnabulum_, each branchia consists of a medial fold of
skin, a little curved conformably with the sack, and slightly tapering
towards its rostral and free extremity; but this fold is almost hidden
by the vertical sub-folds or membranous ridges, themselves plicated and
sub-plicated, which project on both sides: these vertical folds are
free at their tips: at their lower attached ends, they are thickest.
On the side nearest the wall of the shell, the whole branchia has a
bilobed appearance, owing to a very deep indentation caused by the
projection of the scutal lateral depressor muscle; the sub-folds on
this side are also more plicated. The branchia essentially is an inward
plicated fold of the membranes of the sack; for its outer, very thin
tunic is continuous with and moulted with that lining the sack; and
within it we have two layers of delicate, pulpy, transparent corium,
united together (as is best seen in Coronula) by ligamentous fibres,
branched at their two ends, all exactly as in the corium surrounding
the sack. There are here no distinct vessels, any more than in
other parts of the body, but a fluid could easily circulate in the
interspaces of the corium. From the large size of this organ, and its
simplicity of internal structure, being adapted exclusively to expose
a great surface of skin to the water, I do not doubt that it has been
correctly considered as a respiratory organ. By the voluntary movements
of the opercular valves (_i. e._ part of the carapace) the water is
constantly being pumped in and out of the sack; the movement, indeed,
may be almost compared to the heaving of a man's chest. Moreover,
the branchiæ on each side are attached so closely to the spur of the
tergum, that each time the latter is moved, the whole branchia must, I
think, be agitated, and the folds opened, as by the action of a lever.

In our two commonest, tidal, sessile Cirripedes, viz. _Balanus
balanoides_ and _Chthamalus stellatus_, I have observed that, when left
uncovered by water, they kept the orifice of their operculums a little
open, with a bubble of air within their sacks, so that the orifice
was in fact closed by a thin septum of water, with air beneath; when
disturbed, they closed their operculums with force, and expelled the
bubble of air with a clicking noise, which has been noticed by Dr.
Coldstream,[27] and has been thought to be made by the movement of the
operculum itself. _Bal. crenatus_, a deep-water species, when out of
water, keeps its operculum closed.

    [27] 'Encyclopædia of Anatomy and Physiology;' article Cirrhopoda.

In Coronula, Platylepas, Tubicinella, and Xenobalanus, each
branchia[28] consists of two unequal folds, both plicated on both
sides: in the two latter genera, they extend far down the deep and
elongated sacks, and hence the area of surface altogether gained is
extremely great. In most of the species of Chthamalus, the branchia
consist of a small fillet barely plicated: in the allied _Chamæsipho
columna_, they are rudimentary, forming a smooth little pouch only
1/100th of an inch in length: in _Chthamalus scabrosus_ they are quite
aborted, being perhaps represented by a slight hairy ridge; but in
_Chthamalus dentatus_, and therefore within the limits of the same
genus, the branchiæ (and this seems to me a singular fact) are large,
each being composed of two plicated folds, as in Coronula. Tapering
filaments situated near the bases of the cirri, such as those occurring
in several species of the Lepadidæ, are not found in any sessile
Cirripede; but I have observed nearly similar filaments, projecting
upwards and inwards at the base of the sack, in several species of
Balanus and in Coronula; those which I examined were simply occupied
by delicate corium, and no doubt must aid in exposing a greater surface
of corium to the circumambient water.

    [28] Burmeister has given a good figure (Tab. 2, fig. 10) of the
    branchiæ of Coronula, (but the two folds are shaded too unequally),
    in his 'Beiträge zur Naturgeschichte der Rankenfüsser.'

In my former volume on the Lepadidæ, I have described the _ovigerous
fræna_ occurring on the two sides of the sack, to which the
_ovigerous lamellæ_ are attached by a peculiar glandular secretion:
in the Balanidæ there are no ovigerous fræna, but the branchiæ just
described are identical with the fræna in essential structure and
in position; differing only in being placed a little nearer to the
carinal end of the sack, and in being generally (but not always)
larger and more plicated: seeing this, and that in _Alcippe lampas_,
and in some species of Pollicipes,--the genus which comes nearest to
the Balanidæ,--the ovigerous fræna are large and are destitute of
glands, and have therefore lost their normal function of supporting
the ovigerous lamellæ, I can hardly doubt that the _branchiæ_ in the
Balanidæ are the _ovigerous fræna_ of the Lepadidæ in a modified
condition; a transformation of function not greater than that of the
swimming bladder of a fish into the lungs of the higher Vertebrata.[29]

    [29] There is, I conceive, no foundation for the belief of some
    authors that the branchiæ of the Balanidæ are in any way connected
    with the ovaria.


_Thorax and Body._

_Parts of the body included within the shell or carapace._--These
parts (Pl. 25, fig. 1) consist of the prominent mouth, and of the
thorax (_c′_), with its largely developed portion, called the prosoma
(_c_), and with its appendages. The abdomen is quite rudimentary,
being represented merely by a small portion of membrane surrounding
the anus, and sometimes inserted like a wedge between the inwardly
inflected posterior thoracic segments; in only two genera (Catophragmus
and Pachylasma), its nature is rendered somewhat plainer by supporting
caudal appendages. The probosciformed penis lies folded under the
thorax; and I believe (from what is seen in the anomalous genus
Proteolepas), that it normally arises from the ventral surface of
the terminal point of the rudimentary abdomen.[30] The thorax is
laterally compressed, the ventral surface being very narrow, with
the bases of the cirri placed closely together. It consists, in
appearance, of two very different portions; one a soft, more or less
rounded bag, which I have called the prosoma; and the other, which
supports the five posterior pairs of cirri, is narrower, invested
with stiffer membrane, and is more or less distinctly composed of
five segments. These segments (Pl. 26, fig. 8) on their dorsal and
dorso-lateral surfaces, are generally driven like wedges one into the
other, with their points directed anteriorly: on the ventral surface
the articulations are transverse. The prolongation (_e_) of the thin
membrane (_a_) surrounding the anus (_b_), that is, the rudiment of
the abdomen, which sometimes carries caudal appendages, almost divides
(in appearance, whether really I know not) the hindermost thoracic
segment along the medio-dorsal line, into two parts. I have given the
above drawing of these segments, but with the dorsal surface much
flattened, in _Coronula diadema_; in most species of Balanus, however,
the wedges formed by one segment being driven into another, are much
sharper; on the other hand, in Xenobalanus they are nearly straight and
transverse. The three posterior segments are always the most distinct;
the two next segments are also distinct laterally, but along the dorsal
surface they become, excepting in Xenobalanus and some few other
cases, completely confluent. The greater distinctness of the posterior
segments is conformable to what takes place in the higher Crustacea.
The articulations between the segments are folded inwards, and are
formed of thin membrane, which in some cases, as in _Coronula diadema_,
forms a marked contrast with the much thicker, stiffer, and yellowish
membrane of the segments themselves; in _Balanus tintinnabulum_,
however, the whole membrane of the five thoracic segments is very
thin, excepting small wedge-shaped portions along the medio-dorsal
line. The infolded articulations between the segments supporting the
three anterior pairs of cirri (at least in the Balaninæ), are much
wider than those between the three posterior segments; the former
segments, with their cirri, being consequently capable of being moved
further apart from each other. Could there have been any doubt as to
the distinctness and reality of the five thoracic segments, it would
have been set aside by the arrangement of the muscles attached to
them, as will presently be described. I need only add, that in many
genera there are shield-like swellings at the exterior bases of the
pedicels of the posterior cirri, which I for some time thought were
the epimeral elements of the thoracic segments; but I now believe them
to be parts of the pedicels of the cirri. The basi-exterior margin,
moreover, of the pedicel of the third pair of cirri, in many species of
the Balaninæ (Pl. 25, fig. 1), is produced as a plate, thickly fringed
with fine hairs, half across the dorsal surface of the thorax; serving,
apparently, as a brush to clean the sack, or to prevent the ingress of
any intruding substance.

    [30] Von Siebold and Stannius, in their 'Anatomie Comparée,' tom.
    i, p. 473, and p. 440, (foot-note), consider the articulated
    probosciformed penis as an elongated abdomen; a view which, at the
    commencement of my examination, I was tempted to admit; but the
    position of the caudal appendages on the dorsal basis of the penis,
    suffice, I think, to show that this view is not correct; for these
    caudal appendages evidently correspond with those borne on the
    very extremity of the abdomen in the pupa. Nor, indeed, does the
    position of the anus accord well with such a view.

The soft, rounded, bag-like portion of the body, which I have called
the prosoma, is usually separated by a notch from the five posterior
thoracic segments; at its upper end it may be said to carry the mouth
and first pair of cirri. The prosoma includes the main part of the
stomach and the broad ends of the vesiculæ seminales. It is always
clothed by very thin membrane, which in _Chthamalus dentatus_, is
hairy. In Tubicinella and Xenobalanus, the prosoma is much elongated,
being produced far down the deep sack. That the prosoma is mainly
formed by a great development of that segment (homologically the second
thoracic segment) which carries the first pair of cirri, is certain,
and I should not have hesitated to have said that it was exclusively
so formed, had not the first thoracic segment in the anomalous genus
Cryptophialus been developed as a distinct and free segment, not
attached to the carapace; showing that possibly in other Cirripedes,
the dorsal half of this first thoracic segment may be concerned in the
formation of the free prosoma.


_Muscular System._

_Attachment of the Body to the Shell._--The prosoma which carries the
posterior thoracic segments, and in appearance the mouth, is the only
part of the body which is attached to the general covering (Pl. 25,
fig. 1), namely, to the opercular valves. Except through the continuity
of the lining membranes of the sack, the body lies free within the
walls of the shell. The area of attachment (shown by a sinuous broken
line round _a_ and _b_) extends from about the middle of the two scuta
down to their basal margins. As these valves lie obliquely across the
orifice of the shell, the animal's body comes to be suspended almost
in the middle of the sack. The two scuta, as we have seen, have the
power of opening and shutting a little; and are brought together by
the adductor scutorum muscle (_a_), which is generally very powerful.
The body is attached to these valves, round and beneath the adductor,
so as to hide it until one of the valves be removed. The attachment
is chiefly effected by three pairs of widely expanded, superficial
muscles, two pairs of which are spread over the flanks of the prosoma,
and the third pair over its rounded (properly dorsal) surface, which
lies close to the rostral compartment (A, fig. 1) of the shell. I
should have stated, that my chief examination of the attachment of
the body to the scutal valves, has been made on _Coronula balænaris_,
and less closely on _Balanus tintinnabulum_. Within these three pairs
of superficial muscles, there are (besides the adductor) no less than
five other pairs; of these one long pair is attached at one end to
the basal margin of the labrum (_e_), and at the other end, to the
under side, near to the basal margin of the scuta: two other, shorter,
parallel pairs of muscles are attached at one end to the interspace of
membrane between the basal edge of the labrum and the adductor scutorum
muscle, and at the other end, to the under side of the scuta, above the
attachment of the first pair: the fourth and shortest pair curls close
under the adductor, and is there attached at both ends beneath it. The
action of these four pairs of muscles must be to draw back, from the
orifice of the shell, the mouth, and that interspace of body between
the basal margin of the labrum and the adductor muscle. This movement I
saw in living specimens. The last and fifth pair of muscles is small,
but of considerable length; it is a diverging pair, attached at the
converging end, above and exteriorly to the adductor muscle; and at
the diverging end, low down on the under side of the scuta; I am very
doubtful regarding the function of this pair. Altogether we have seen
that round and within the fleshy pedicel, by which the body is attached
to the scuta, there are no less than eight pairs of muscles. The
central space between these muscles is hollow, and here many lacunal
channels seem to converge. These muscles receive nerves from the
supra-œsophageal ganglions. The interspace above alluded to, between
the basal edge of the labrum and the adductor scutorum muscle, occupies
a very different position according as the animal's body is protruded
as far as it can be, or is retracted. It is homologically part of
the third cephalic segment; and consequently the mouth ought to have
stood posteriorly (_i. e._ above, in the position figured in Pl. 25,
fig. 1) to this interspace; yet, in fact, when the animal is retracted
within its shell, the mouth usually lies almost directly beneath this
interspace and the adductor scutorum muscle.

Besides these muscles of attachment, the prosoma is furnished with
several other muscles. There are superficial muscles running up
towards the basal margin of the sides of the mouth; and other deeper
muscles, to which, I presume, the movements of the mouth, as a whole,
are due. The muscles moving the gnathites do not, as far as I could
make out, extend beneath the basal edge of the mouth. There are, also,
powerful muscles giving movement to the basal segments of the pedicel
of the first pair of cirri. Again, there are superficial muscles
running to the next succeeding thoracic segment; the anterior ends of
which are separated by a clear interspace from the lower ends of the
above-described superficial muscles, by which the prosoma is attached
to the scuta. On each flank, moreover, but more deeply imbedded, are
the long flexor and extensor muscles, presently to be described,
running to the five posterior thoracic segments. The last muscle which
I need here mention, is a deep-seated diverging pair, attached near
the upper end of the stomach, on its ventral surface, and diverging
from this point to the sides of the prosoma high up beneath the mouth.
The probable action of this pair, as well as of the three superficial
pairs of muscles by which the body is attached to the scuta, is to
draw up the whole prosoma towards or from the orifice; and likewise to
contract it firmly, so as to serve as a fulcrum for the movements of
the five posterior thoracic segments, together with the cirri, which
they carry.

The muscles of these five thoracic segments are numerous and powerful;
they are also complicated, chiefly owing to the segments on their
dorsal and dorso-lateral surfaces being driven, like wedges, one into
the other. As far as I could make out, there are on each side three,
superficial, dorso-lateral and lateral muscles (generally, if not
always, destitute of striæ), which do not cross the articulations,
but extend merely from articulation to articulation; and of which
the function can be only to contract each separate segment, and
consequently to open out the intermediate infolded articulations;
the effect of this would be to separate slightly the cirri from each
other,--more especially those borne on the two or three anterior
segments, between which the infolded articulations are deeper or
broader. There are other more deeply imbedded, powerful, long,
dorso-lateral extensor, and ventri-lateral flexor muscles, attached
at one end within the flanks of the prosoma, and at the other end to
the successive segments of the thorax. The action of the former is to
straighten and stretch out the thorax; of the latter, or ventri-lateral
muscles, to retract it. In tracing these muscles, a fascia could be
seen to become attached to a segment, and then this same fascia would
run on to the next succeeding segment: the effect of this must be, that
each segment can be retracted and protracted either from the prosoma
as a fulcrum, or from the antecedent segment as a fulcrum: we have,
also, seen that each segment can, by the agency of the superficial,
non-striated muscles, contract itself. Hence these thoracic segments
are capable of diverse movements, as was very evident when the shell
of a living specimen was opened. By one movement in common, the
whole five posterior segments could be drawn back, so as to become
even partly imbedded in the prosoma: lateral, twisting or wriggling
movements were also quite distinct: the three posterior segments
seemed to be capable of less independent movements than the anterior
segments; and I observed that the more powerful flexor and extensor
muscles did not run into these three posterior segments. The cirri, of
course, partake of the movements of the thorax; and in watching, in an
uninjured specimen, the alternate, protruding, gracefully sweeping and
retracting movements of the posterior pairs of cirri, it was evident
that the thorax was the chief agent in their movement. Besides the
muscles now mentioned, there are some immediately to be noticed, which
extend from within the thoracic segments to within the pedicels of the
cirri.


_Movements and Muscles of the Cirri._

Although the cirri have not been described, it will be most convenient
here to treat shortly of their muscles. Each cirrus consists of a
pedicel, having a long basal and a short upper segment, supporting two
multiarticulate rami. The lower segment of the pedicel can be drawn
forward by an adductor muscle, attached low down within this segment,
and crossing at right angles (at least in the case of the anterior
cirri) the corresponding muscle of the opposed cirrus, on the central,
ventral surface of the thorax. This segment can also be drawn back
by a muscle springing from the dorso-lateral surface of the thorax,
and running only a little way within the segment: I am far from sure
that the lower segment does not possess other muscles. The short upper
segment of the pedicel can be moved backwards and forwards, as I saw
in living specimens, independently of the lower segment; this movement
being best seen in the anterior cirri, which are much more often moved
independently of each other than are the posterior cirri. The rami
are capable, I believe, of being moved backwards and forwards _as a
whole_, by the movement of the few lower segments, which are generally
more or less confluent. They can, also, be curled up and uncurled by
the combined movement of each separate segment. The uncurling seems
to separate the two rami a little laterally. Each ramus, at least in
the two or three anterior pairs, can be moved to a certain extent,
independently of the other ramus of the same cirrus; and the few
terminal segments, either of both rami or of one ramus, are often a
little moved and curled (and this is especially the case with the long
anterior ramus of the first pair), without the lower segments or the
pedicel being moved.

The flexor and extensor muscles, which, as I believe, move the upper
segment of the pedicel (_a_ and _b_, Pl. 29, fig. 1), are attached at
their upper ends to its basal margin, and are thus enabled to draw it
a little way down within the lower segment, and so move it. The short
flexor muscle (_c_), which is attached at its lower end within the
upper segment of the pedicel, and the longer extensor (_d_), also,
attached within this same lower segment, serve, I believe, to move the
lower, partially confluent segments of each ramus as a whole. In the
case of these muscles, and of those last mentioned, I am surprised that
the extensors (_b_) and (_d_) are not attached nearer to the exterior
and dorsal surface. Other muscles (_e_, _f_) attached at their lower
ends within the upper segment of the pedicel, run up each of the two
rami to their tips, with some of the fasciæ terminating within each
segment: of these muscles, the outer one (_f_, _f_) appears to be
the extensor, and the inner one (_e_, _e_) the flexor. But besides
these, there are other short flexor muscles (_g_, _g_) which run on
the anterior face,[31] from segment to segment, serving to pull the
front edge of one segment within the edge of the next lower segment.
These muscles differ much in plainness in the several genera: they are
very distinct in Coronula. In some specimens of this genus, a few of
the articulations between the basal segments of the rami having been
obliterated, the short muscles (_g_, _g_) running from articulation to
articulation were absent, and their presence and nature in the upper
segments thus rendered the plainer. The muscular system in the several
pairs of cirri seems to be the same, with the exception of the first
pair, in which the muscle answering, as I suppose, to (_a_), namely,
the flexor of the upper segment of the pedicel, is much spread out at
its lower end, and is there attached to the exterior surface of the
lower segment.

    [31] For a considerable time I thought that there were muscles
    going to the spines, especially to those which arise from the upper
    dorsal edge of each segment; but I have since ascertained that
    these are the cases within which new spines, with their lower ends
    doubled like the fingers of a glove hastily pulled off, are in
    process of formation.

The backward and forward movements of the segments, both in the rami
and in the pedicels of the cirri, are apparently effected, as already
noticed, by the outer or inner (as the movement may be) basal edge of
one segment being drawn a little way down within the next succeeding
lower segment. If, at the same time, both the inner and outer margins
of all the segments were drawn one within the other, the whole limb
would necessarily be shortened; and I distinctly saw a shortening
action, with very slight movement in any other direction, in the
first and second pairs of cirri; and I think it almost certain that
this movement might be performed by the other cirri. If I correctly
understand a statement of Milne Edwards,[32] this is an important fact,
as he asserts that only the higher Crustaceans possess the power of
shortening their limbs.

    [32] 'Annales des Sciences Naturelles,' tom. xviii, 1852, p. 121.

When a Cirripede is alive, the action of the cirri is really beautiful:
from the position of the thoracic segments, the posterior cirri (three
pairs in the Balaninæ and four pairs in the Chthamalinæ) form a sort of
semicircle facing the mouth: the anterior cirri stand further apart,
and are opposed in pairs to each other, with the first pair pointing
beyond the mouth. Together the cirri form a hollow cone, not circular
but elongated, with the mouth situated at the lower anterior end. The
posterior cirri are protruded, by the movement of the whole thorax,
curled up, close along the carinal end of the orifice; as they are
protruded, they diverge, both by the movement of their pedicels, and,
as I believe, by the separation of the thoracic segments. As the two
rami of each separate cirrus are uncurled, they also diverge a little;
as do the double rows of spines on the segments in each ramus, by their
elasticity. By the movement of the thorax, the cirri are then swept
towards the rostrum; and, lastly, they are brought perpendicularly down
towards the mouth with a rapid movement, which would be beautifully
adapted to catch any object floating or swimming in the water; hence
I have called the action of the cirri, captorial. When the shell of
a Balanus is broken open, the second and third pairs of cirri are
repeatedly clasped over the mouth with a convulsive movement, in a
manner indicating, I think, that their chief function is to seize
and carry to the mouth any object entangled by the sweeping movement
of the three posterior pairs. The first pair is also well adapted
to aid in this seizing action; but I suspect that the long anterior
ramus likewise acts as an organ of touch, warning the animal of
danger. The mouth being itself moveable as a whole,--the outer maxillæ
being capable of a backward and forward sweeping action, and being
furnished with orifices apparently olfactory,--the inner maxillæ having
more diversified movements,--the toothed mandibles overhanging the
œsophagus,--and the œsophagus itself possessing a powerful swallowing
movement, are all admirably adapted to secure any prey, when once
entangled by the cirri.


_Mouth._

The mouth, in the sub-family Chthamalinæ, cannot be distinguished
from that of the Lepadidæ, which has been pretty fully described
in my former volume. In the Balaninæ, however, the labrum differs
considerably in not being swollen; that is, in its outer and inner
fold of membrane being close together, and in having a central notch:
the palpi are also larger, and the lower teeth on the mandibles, are
laterally (Pl. 26, fig. 5) double, as will be more particularly stated
under these two sub-families. I have given a drawing (Pl. 26, fig.
1) of the mouth, seen from above, of _Balanus perforatus_, with the
right-hand palpus (_d′_) and outer maxilla (_a′_) cut off, in order
that the labrum (_e_), mandibles (_c_), and inner maxilla (_b_) might
be better shown; the cut-off bases (_x_, _x_) of the first cirrus on
each side are also shown. In fig. 2 we have the deep supra-œsophageal
cavity in _Bal. improvisus_ torn open and laid flat, with the inner
surfaces of the labrum (_c_) and outer maxillæ (_a_) exhibited, the
palpi, mandibles, and inner maxillæ having been removed. Figs. 3 and
4 will presently be referred to; they are parts of the mouth, with
the muscles, &c. removed, of Coronula. The mouth differs extremely
little in the different genera and species of the Balanidæ, much less
than amongst the Lepadidæ. In the Balaninæ, the crest of the labrum
is sometimes hairy, instead of having, as is usual, from two to six
teeth on each side of the central notch: in _Balanus improvisus_ (Pl.
26, fig. 2) and _eburneus_, and in _Chelonobia_, the crest on each
side of the central notch (_e′_) is furnished with a row of finely
graduated teeth. A sub-triangular portion of the inner fold of membrane
of the labrum, which overhangs the œsophagus, is always thickened and
yellowish; it is also often punctured in patterns (Pl. 26, fig. 2,
_f_), which, I believe, give attachment to little muscles that serve
to open the upper end of the œsophagus. Opposite to this thickened,
sub-triangular portion of membrane, the thin membrane forming the
supra-œsophageal cavity (or the cavity surrounded by the gnathites)
is strengthened by a pair of curved ribs (_h_, fig. 2) of thickened
yellowish membrane, running down from the inner bases (_a′′_) of the
bilobed outer maxillæ to the opening of the œsophagus (_g_): a broad
branch from each of these ribs supports the sides of the orifice of
the œsophagus; and this branch almost joins on to a slightly thickened
rim or bar (_f′_), which branches off from the upper part of the
sub-triangular (_f_) inner fold of the labrum. This structure, in _Bal.
improvisus_, is represented in Pl. 26, fig. 2, as well as it could be,
considering that the deep supra-œsophageal cavity has to be torn open;
and then laid flat.

The _Palpi_ (Pl. 26, fig. 6) differ little, except in size, in the
different genera, being squarish, more or less elongated, or even
approaching to club-shaped: in most of the Balaninæ they are larger
even than the mandibles, of which they normally form a part. Their
upper margins, especially towards their free extremities, are always
thickly clothed with spines; and there is generally a single row,
either short (_r_) or long, of spines of greater length, which arise
from a little above, and stand almost in a parallel line to, the basal
margin. On the internal surface, there is sometimes a row (_t_) of
very short little spines, which overhang the crest of the labrum. The
_Mandibles_ (Pl. 26, fig. 5) have from three to five teeth; the lower
point or angle is generally pectinated. In Coronula and its close
allies, there are some small teeth intermediate between the four or
five main teeth; and in these genera, though members of the sub-family
Balaninæ, the lower teeth exhibit only rudiments of being laterally
double.[33] The _Maxillæ_ sometimes have a notch under the upper large
pair of spines, and in _Octomeris brunnea_ there is a double notch: in
many species of Balanus, the inferior corner stands up like a step (Pl.
26, fig. 7, _a_): in many other genera and species, the whole edge is
straight. In all, or almost all cases, the row of spines on the middle
portion is double. The _Outer Maxillæ_ are always bilobed on their
inner faces, and are clothed with bristles. On all the gnathites, the
bristles are often doubly serrated.

    [33] M. Martin St. Ange describes, in his 'Mémoire sur
    l'Organisation des Cirripèdes,' pp. 15 and 32, "_une petite
    langue_" in the mouth of Lepas; but I may venture to assert that
    such does not exist; it is merely the point of union between the
    outer maxillæ. M. St. Ange, in his comparison of the mouth of
    Lepas with that of Phyllosoma, compares the mandible of the latter
    with the palpus of Lepas; the first maxilla of Phyllosoma with the
    mandible of Lepas; and so on with the other gnathites.

_Muscles and functions of the Gnathites, and their confluence._--The
outer maxillæ appear at first like a deeply-lobed lower lip, for they
reach over almost to the labrum (Pl. 26, fig. 1), and thus partially
cover the other organs; they are separately capable of a strong and
rapid, to and fro movement, by which no doubt they sweep any prey,
entangled by the cirri, towards the other gnathites. Each outer maxilla
is furnished with a pair of muscles, apparently a flexor and extensor;
there is also a little muscle between the two maxillæ, I presume for
the purpose of bringing them together. The outer and inner maxillæ
generally stand close together, and in several genera a little way
apart, from the mandibles; but there is no trace of any labrum or true
lower lip, bounding the mandibles and orifice of the œsophagus. The
outer and inner maxillæ and mandibles are not opposed in pairs to each
other, but against the thickened inner fold of the labrum; almost in
the same manner as the posterior pairs of cirri are not opposed one to
the other, but to the mouth.

I have described pretty accurately the muscles of the mandibles in
my former volume, and there given a drawing (Pl. 10, fig. 1) of them.
There are four muscles: first, the depressor muscle, which is the
largest, and is attached, at its upper end, to ligamentous apodemes
under the free toothed portion of the jaw; and at its lower end,
spreading considerably out, is attached to a concavity close above the
basal margin of the labrum; to understand the action of these muscles,
it should be borne in mind that the mandible almost faces the labrum.
In some genera, as in Coronula,[34] the swelling near the basal margin
of the labrum (Pl. 26, fig. 3, _k_), caused by the internal concavity
for the above muscle, is conspicuous. The depressor muscle is opposed
by a small elevator, attached to the mandible close by the depressor;
thence it runs upwards, and is united at its upper end to the base of
the palpus, at the point where the latter adheres to the labrum: I have
ventured to call this muscle the elevator, from being apparently so
well fitted for this purpose; but I feel some little doubt, from having
observed an apparent slight movement in the palpi of living Balani;
and this is the only muscle entering those organs. The free part of
the mandible is articulated on a square, thickened piece of membrane,
forming part of the side of the mouth (Pl. 26, figs. 3, 4, _c^1_;
and Pl. 10, fig. 1, _a_, _b_, in my volume on the Lepadidæ); to this
square piece of membrane, two short muscles are attached, one above the
other, and which ought, in the Plate in my former volume, to have been
represented crossing the depressor muscle at nearly right angles; at
their further ends they are attached to about the middle of the labrum,
where, at least in Coronula (Pl. 26, fig. 3, _i_), a slight concavity
can be detected. The action of these two muscles must be to draw the
whole mandible against the labrum; and the depressor muscle might, at
the same time, draw the toothed edge downwards, and thus force any prey
into the œsophagus.

    [34] This is figured by Burmeister in his 'Beiträge zur
    Naturgeschichte der Rankenfüsser,' Tab. 2, fig. 6.

The inner maxillæ are likewise furnished with four muscles, very
nearly as figured in my former volume (Pl. 10, fig. 10); namely, two
muscles, one inside and the other outside the curious apodeme, which
in the Balanidæ (Pl. 26, fig. 7, _b′_) is as invariably present as in
the Lepadidæ: these two muscles are attached at their lower ends to
the outer membrane of the mouth, close to its basal articulation: the
outer one of these two muscles would, I presume, act as an elevator,
and the inner one as a depressor; the free part of the organ working on
the top of the apodeme, like an axe, on a hinge, on the top of a pole.
But there is also a larger depressor muscle, in an analogous position
with that (_i. e._ the first-mentioned muscle) of the mandibles; and a
fourth muscle, crossing the latter depressor at nearly right angles,
and attached (as far as I could make out) on the side of the orifice
of the œsophagus, close under the mandibles: the action of this latter
muscle would be to draw the whole organ towards the labrum.

I must not conclude my description of the mouth, without drawing
particular attention to its peculiar compounded nature. It is
prominent, and is capable, as a whole, of movement; it is separated
from the body by a fold or articulation, which can be traced all
round. It is, as we have seen, composed of a broad labrum and three
pairs of gnathites, which latter have only their terminal segments
free; and these surround a conical hollow, at the bottom of which
lies the opening of the œsophagus. The prominence of the whole mouth
appears to result from the lateral fusion of the two basal segments of
the three pairs of gnathites. I have examined the mouth of ordinary
Crustaceans, and can see no trace of a structure like this. That there
has been some union of the parts is indisputable; for the palpi, which
in ordinary Crustaceans are quite free, are here firmly united to the
upper and outer corners of the labrum; and indeed, at first appear to
be more intimately connected with the labrum than with the mandibles.
The palpus on its upper and exterior surface, is in direct continuity
with the square thickened piece of membrane, on which the mandible is
articulated, and likewise with that side of the upper or free portion
of the mandible which faces the labrum. This face of the mandible,
beneath the toothed edge, is hollowed out or arched (Pl. 26, fig. 5,
_p_), owing to the above-mentioned continuity of its membrane with
that of the palpus. On the lower surface, the palpus is firmly united
to the lateral corners of the labrum; or indeed the corners of the
labrum may be almost said to be formed by the soft, swollen bases of
the palpi: the point of union, when viewed from the outside, is seen
to form a knob on the shoulder of the labrum, beneath the level of
its crest, and at this knob (Pl. 26, fig. 3, close to _d′_) several
thickened bands in the surrounding membrane unite. The free portion of
the palpus stands out transversely behind (_i. e._ anteriorly to, in a
homological sense) the labrum. I suspect that the palpus possibly may
consist of two segments, of which the terminal one is free, and the
lower one confluent with the labrum.

Before proceeding any further, I should observe that figs. 3 and 4, in
Pl. 26, represent the membranes of the mouth of _Coronula diadema_,
perfectly cleaned. In fig. 3, all the front part of the mouth has been
removed, the mandible on one side, the labrum with the two palpi, and
the œsophagus being alone left, and these are viewed from the inner
side; the front part, however, of the supra-œsophageal cavity has been
cut away. In fig. 4, the labrum, with the œsophagus, has been removed,
whilst the two outer maxillæ, the right-hand inner maxilla and mandible
(with the exterior and basal portions, _d_, _d′′_, of one palpus) are
seen from the outside; but in order that these parts should all be
shown, the whole of the right-hand side of the mouth has been spread
out, for the teeth of the mandible should have stood in a vertical
line between the two outer maxillæ. In the mandibles, the free upper
part is separated, by a distinct articulation, from the square piece
of thickened membrane (fig. 3, _c^1_) on which it is supported; and
this latter is separated by a second articulation from a portion of
thickened membrane (_c^2_), the basal edge of which forms the third and
lowest articulation, separating the mouth from the body. This basal,
thickened portion of membrane curls round and inwards, towards the
outer maxillæ or front of the mouth, and its terminal points sometimes
even penetrate a little way within the muscles, like apodemes: it is
not distinctly separated by any line or suture from the membrane, which
forms the whole broad labrum; so that I at first concluded that the
labrum dipped under the mandibles, and thus afforded a support on which
they were articulated; but this appears so opposed to all analogy,
that it is more probable that the above basal thickened portion of
membrane is truly the basal segment of the mandibles, completely
confluent with the labrum; and it is, I think, not very improbable
that even a large portion of what in appearance belongs to the labrum,
namely, those concavities to which the muscles of the mandibles are
attached, may, also, be part of the basal segment of the mandibles.
Whether or no there really are two segments beneath the upper free
portion of the mandibles, which have become laterally confluent with
other parts, I must think that the square thickened piece of membrane
(_c^1_) represents at least one segment. I may here observe, that Prof.
Milne Edwards seems to consider the mandible of the higher Crustaceans
as answering homologically to the haunch of the leg; but, according to
M. Brullé,[35] there ought to be two basal segments (sous-maxillaire
and maxillaire) bearing the proper mandible, and giving rise, on the
outer side, to the palpus,--a structure which perfectly corresponds
with my view of the mandible and palpus in Cirripedes.

    [35] 'Annales des Scienc. Nat.,' 3d series, Zoolog., tom. ii, p.
    271.

_Maxillæ_: the point whence the long apodeme (_b′_, Pl. 26, fig.
4 and fig. 7) arises, according to Audouin's views, must mark an
articulation, and this would separate the upper free segment from the
lower segments, which I believe to be laterally confluent with the
organs on each side. The thickened membrane, of which the upper free
part is formed, extends a little distance beyond the insertion of the
apodeme; and this small portion beneath the point of insertion may
possibly answer to the square, thickened piece of membrane, or second
segment, supporting the mandibles. Beneath it, a rather wide expanse of
thin, flexible membrane reaches down to the basal fold surrounding the
mouth, and may thus form the third segment.

_Outer Maxillæ_: the upper free segment has a spinose lobe (_a′′_,
Pl. 26, figs. 2 and 4), on its inner face, which may indicate a lower
and second, almost free segment. Passing over this, we have, on the
outside of the mouth, beneath the free, upper segments, an expanse of
membrane, which, on the side, close to the inner maxillæ, is perforated
(Pl. 26, fig. 4, _n_) by orifices which I believe are olfactory. In
some species, as in _Bal. eburneus_ and _improvisus_, there is a
longitudinal medial suture in this expanse of membrane, which I suppose
indicates the lateral confluence of the middle segments of the two
outer maxillæ. A short, transverse articulation or fold separates this
middle segment (fig. 4, _a^1_) of each maxilla from the third or basal
segment; and this latter (_a^2_) is separated from the body by a very
distinct fold, which (at least amongst the Lepadidæ) sends inwards a
short, medial, tongue-formed apodeme. Here, then, we apparently have,
as in the mandibles, two segments under the upper free segment of
each outer maxilla, laterally confluent with the adjoining organs.
But I must state that, in old specimens, and only in old specimens of
_Coronula diadema_, I have found under the outer maxilla an additional
transverse ridge and fold, which plainly shows how easily a mere
thickening of the membrane might be mistaken for an articulation. I
can, however, hardly persuade myself that the articulated membrane,
under the free part of the mandibles, which has now been figured and
described, has no homological signification; and the fusion of the
palpus and labrum seems too plain to be mistaken. Hence I must conclude
that the mouth, in the Cirripedia, does truly exhibit a compounded
structure of a very peculiar nature.


_Cirri._

There are always six pairs; each biramous and multiarticulated,
supported on a pedicel formed of two segments. A shield-like swelling
at the exterior bases of these pedicels often appears like another
segment; but such, I believe, is not its nature. The five posterior
pairs answer to the five pairs of ambulatory legs in the higher
Crustaceans; and as in the case of the latter, the three, or the four
hindermost pairs almost invariably resemble each other. The first
pair, which is homologous with the outer maxillipeds of ordinary
Crustaceans, is separated by an interval from the second pair;--though
this is not the case with the legs of the pupa, from which the cirri
are metamorphosed. These anterior cirri are attached to the lateral
edges of the mouth, namely to the thickened rim of membrane, forming
the supposed basal segment of the mandibles. They are capable of more
diversified movements than the other cirri: the anterior ramus is
always elongated, with the terminal segments more or less tapering,
and is directed beyond (or anteriorly to) the mouth: the shorter
ramus closely resembles in structure the rami of the second pair.
In the Chthamalinæ the second pair, and in the Balaninæ the second
and third pairs (as will be more particularly described under these
sub-families) differ in structure from the posterior pairs, from which
they are separated by a slight interval. The number of segments on
the posterior cirri is often great, amounting in Chelonobia even to
fifty. Each segment normally is furnished on its inner face, which
is usually somewhat protuberant, with from two to rarely eight or
ten pairs of long spines or bristles, placed in a double row; the
two spines in the lower pairs stand nearer to each other, and are
shorter than the spines in the upper pairs. Between each pair of
spines there is either a single, very thin bristle, or often a tuft of
such. The pairs are directed somewhat upwards, and they diverge when
the cirri are uncurled; their function is obviously to entangle the
prey. On the dorsal or exterior surface of each segment, close to its
upper margin, there is a tuft of spines, often composed of thicker
and thinner spines; these, I believe, serve to prevent any creature
intruding within the sack. On both sides of the upper margin of each
segment, there is generally a row of short, blunt, excessively minute
spines, which only deserve notice, inasmuch as it is by their increase
in number and size, and by the spreading out of the dorsal tufts,
and, lastly, by the increase of the little tuft intermediate between
the pairs of spines situated in front, that the segments on the two
or three anterior pairs of cirri become covered, like brushes, with
bristles. The bristles or spines on the second and third cirri are
often, especially in Tetraclita, doubly and coarsely pectinated. The
bristles on the pedicels follow the same arrangement as on the rami;
namely, being in regular pairs on the posterior cirri, and crowded
thickly, like a brush, on the anterior cirri. The segments in the
shorter ramus of the first cirrus, and in both rami of the second,
and often of the third cirrus, are broader than the segments of the
posterior cirri; they are, also, especially in the genus Balanus,
frequently produced in their upper, ante-lateral corners, into
remarkable prolongations (see Pl. 29, fig. 4, of the third cirrus of
_Bal. perforatus_), clothed on their inner surfaces, and at their
extremities, by numerous bristles. The number of the segments in each
cirrus is in some degree variable, and increases with age; this is
likewise the case, to a certain extent, with the number of the spines
borne on each segment.

As compared with ordinary Crustaceans, I presume the two rami answer
to the "_tige_" and "_palpe_" of Milne Edwards; and the pedicel (as I
have called it) to the two basal segments of the leg.[36] The "_fouet_"
or flabellum does not appear to be developed in any Cirripede; for
though the filamentary appendages in certain genera of Lepadidæ, might
at first be thought to be of this nature, yet their usual position
_beneath_ the basal articulation of the first pair of cirri, and the
occasional presence of more than one, proves, I think, that such is not
the case.

    [36] According to this author's new nomenclature, the pedicel would
    consist of the coxopodite and basipodite; the tige would be the
    ischiopodite and following segments; and the palpe would be the
    exopodite; the epipodite or flabellum being absent. ('Annales des
    Sciences Naturelles,' tom. xviii, 1852.)

Though the structure of the cirri is very uniform, yet we meet with
some peculiarities. In Chelonobia, the segments of the posterior cirri
bear only two pairs of main spines; whereas in some varieties of
_Balanus balanoides_, they carry as many as ten pairs in a longitudinal
row; but in this latter species, the number of these spines varies,
in a singular manner, from six to ten pairs. In Tubicinella, the
pairs of spines on the segments of the posterior cirri are arranged
so closely one under the other, that they appear almost like a single
transverse row. Considering the whole family, the third pair of cirri
differs most in structure in the different genera. Thus, in _Chthamalus
antennatus_, the anterior (or outer) ramus (Pl. 29, fig. 3) is thicker
and much longer than the posterior (or inner) ramus; the number of the
segments in one instance being, in the two rami, 53 and 18; in the
longer ramus, the spines are arranged abnormally, tending to form a
little circle round each segment; and the whole ramus may be said to be
antenniformed, and I believe acts as an organ of touch: the relative
number of the segments, I may add, in the two rami and the arrangement
of their spines varies greatly in this species. In two other species of
the same genus Chthamalus, we have _occasionally_ the anterior ramus in
some degree antenniformed, so that this whole structure is variable.
In the allied _Chamæsipho columna_, it is the posterior or inner
ramus which is antenniformed, but this peculiar development is more
plainly marked in the case of the second pair of cirri than in that
of the third pair. In _Tetraclita porosa_ it is, also, the posterior
ramus of the third pair which is antenniformed; in this third pair,
and indeed in the other cirri, the relative numbers of the segments
vary extremely. A similar structure in the third pair, but in a lesser
and variable degree, may be observed in some of the other species of
Tetraclita. In _Balanus vestitus_, also, we have, in the third pair,
an analogous structure. It is scarcely possible to believe that the
circumstance of the second pair of legs, which answer to the third pair
of cirri, being antenniformed in certain decapod Crustaceans, is an
accidental coincidence; it must be owing to some special affinity in
the two groups.

In Chelonobia, the third pair of cirri is of unusual length compared
with the second pair, but does not otherwise differ from the type
of its sub-family: in Coronula and its allies, on the other hand,
the third pair is very short and broad, as may be seen (Pl. 29, fig.
5) in Xenobalanus: in this latter genus, the front surfaces of the
segments of the pedicels (fig. 6) of the posterior cirri, are extremely
protuberant, almost as in _Scalpellum vulgare_.

The last peculiarity in the cirri at all worth mentioning, is in
the sub-genus Acasta, in which, differently from in all other known
Cirripedes, the anterior ramus of the fourth pair does not absolutely
resemble the rami of the fifth and sixth pairs; in most of the species,
the spines on this anterior ramus are more crowded together, are
larger, and are mingled with some short thick points; and the spines
in the dorsal tufts are also longer than in the two posterior pairs of
cirri; but in _A. sulcata_ (Pl. 29, fig. 2), and in a lesser degree
in _A. cyathus_ and _A. purpurata_, the front margins of the lower
segments of this anterior ramus, and of the upper segment of the
pedicel, are developed into strong, downwardly curved teeth: it is very
remarkable that so beautiful a structure should be extremely variable,
as it certainly is in _Acasta sulcata_.


_Caudal Appendages._

With extremely few exceptions, these are present in all the Lepadidæ
and Verrucidæ; whereas amongst the Balanidæ they occur only in the
two species of Pachylasma, and in one species of Catophragmus;
these being the genera most closely allied to the Lepadidæ, and
where, consequently, their presence might have been anticipated.
These appendages are seated close together over the anus; they are
multiarticulate, each segment being sub-cylindrical, with a few small
bristles round its upper edge.


_Alimentary Canal._

I have not much on this head to add to what I have said under the
Lepadidæ. As in that family, the strong internal membrane of the
œsophagus terminates in a remarkable, bell-shaped expansion (Pl. 26,
fig. 3, _g′_), which, as observed by M. St. Ange, serves to keep
the upper broad end of the stomach expanded. The œsophagus is well
furnished with constrictor and radiating muscles for closing and
opening it; and it is thus capable of a strong swallowing movement.
The stomach runs down to the lower end of the prosoma, and then
doubling back on itself extends to the anus. As the prosoma is much
elongated in Tubicinella and Xenobalanus, so is the stomach of
unusual length in these genera. In several species of Balanus, the
upper edge of the stomach is surrounded by from six to eight cæca;
these cæca I ascertained, in _Balanus perforatus_, are branched, and
penetrate a considerable way into the body; and some of them at least
expand a little at their extremities. Each cæcum, from the manner in
which it retained fluid, must, I think, be furnished, at the point
where it enters the stomach, with a sphincter muscle. In Tetraclita,
Chthamalus, Tubicinella, Coronula, and Xenobalanus, there are no cæca;
but in Xenobalanus and _Coronula balænaris_, there are longitudinal,
approximate folds in the upper, broad end of the stomach, which would
serve to expose the food to a greater extent of digesting surface.[37]

    [37] The presence and absence of these cæca in genera so closely
    allied as Balanus and Tetraclita, shows, I think, that these
    cavities are not of high importance; and I must doubt whether Von
    Siebold's view ('Anatomie Comparée,' tom. i, p. 445), that these
    cæca form a passage to a true or isolated liver, such as exists in
    the higher Crustacea, can be admitted. Cæca are said by Von Siebold
    to occur in some of the Entomostraca, as Daphnia, Argulus, &c.

As in the case of the Lepadidæ, a transparent, structureless,
epithelial tube, composed of chitine, containing more or less digested
food, is found, in specimens preserved in spirits, occupying the whole
length of the stomach, and where there are cæca, sending branched
prolongations into them. It does not extend into the œsophagus or into
the rectum. This epithelial tube or model of the stomach, filled with
excrement, is expelled by the rectum, whole, that is in a single piece,
as I observed in some living specimens of _Balanus balanoides_: in
some specimens, however, of _Chthamalus stellatus_, the excrement was
ejected, perhaps from the animal being confined, in fragments, and the
sack thus became befouled. Beneath the epithelial layer, the stomach is
lined by a delicate, pulpy and cellular mucus layer, which easily peels
off in flakes: this is surrounded by a muscular layer with the fibres
closely approximated and transverse; and this by a layer of stronger,
longitudinal muscles, but more distant from each other. Lastly,
outside this double muscular layer, there is a rather thick, somewhat
laminated, pulpy layer, abounding with cells, often nucleated, and
frequently containing much oily matter. This structure agrees closely
with Dr. C. H. Jones's[38] account of the external covering of the
stomach in Daphnia, and which he believes to be hepatic: as in Daphnia,
there does not seem to be any ducts. I may here observe, that within
the upper part of the prosoma, but not immediately connected with the
stomach, I have often observed much white pulpy substance, permeated by
lacunal passages, and exhibiting no structure except some excessively
minute cells.

    [38] 'Philosophical Transactions,' 1849, p. 116. Karsten ('Nov.
    Actorum Acad. Nat. Cur.,' 1845, tab. xx) has excellently figured
    the testes, as the hepatic glands; and has indicated the ovaria
    as salivary glands; it is singular that this anatomist overlooked
    the ducts which lead from his supposed hepatic glands, into the
    vesiculæ seminales, within which he observed spermatozoa.

The rectum, lined by membrane continuous with that investing the
thorax (and seen through it, in Pl. 26, fig. 8, _c_), extends inwards
to about opposite the bases of the third or fourth pairs of cirri.
It is longitudinally plaited; the ends of the folds forming a sort
of valve where joined on to the stomach. It is coated by circular,
transverse muscular fibres: judging from the movements, the anus
itself is surrounded by a strong sphincter muscle. The anus opens on
the dorsal surface of the thorax (fig. 8, _b_); but as in the genera,
in which caudal appendages occur, it opens under them, the orifice, I
believe, is homologically terminal, and owes its dorsal aspect to the
aborted state of the whole abdomen, and to the great development of the
probosciformed penis; for the anus may be said to be situated on the
dorsal base of this organ.

Altogether we see that the alimentary canal is of a very simple
structure. The food, judging from the contents of the stomach, seems
generally to be composed of infusoria and minute animals: but in the
case of Tetraclita, I have been surprised at the size and number of the
included amphipod, isopod, and entomostracan Crustaceans, in one case,
together with an annelid. I have, also, sometimes seen some confervoid
matter within the stomach.


_Circulatory System._

On this subject I can add nothing, except to express my conviction that
there is no heart, or true vessels; the circulation being strictly
lacunal. A passage has often been quoted from Poli, in which he states
that he saw a pulsating organ, close above the anus; but I have seen
this movement, which appeared to me to be a convulsive twitching of the
sphincter muscle of this orifice. The largest lacunal channel extends
down the middle of the rostral compartment of the shell: and this
answers to the rostral channel down the peduncle in the Lepadidæ. Large
nerves and the main pair of unbranched ovarian tubes (Pl. 25, fig. 1,
leading into _g_) extend along this channel. At the basis (at least in
Coronula) this channel joins on to a large circular one, running all
round the sack, and sending off branches into the mass of ovarian tubes
and cæca.


_Nervous System._

It has been shown in my former volume, that in Lepas and in some
other genera of the Lepadidæ, there are six main ganglions; one
supra-œsophageal, and five infra-œsophageal, or thoracic. In
Pollicipes, however, there are only four thoracic ganglions, the
last ganglion supplying the three posterior pairs of cirri with
nerves, whereas in the other genera, the last ganglion supplies only
the fifth and sixth pairs of cirri. In this genus, moreover, the
lateral fusion of the ganglions has been so complete, that there is
no evidence of their having been formed by the union of two. Amongst
sessile cirripedes, we discover evidence of much higher concentration
even than in Pollicipes. My chief examination has been confined to
_Coronula diadema_, and to _Balanus tintinnabulum_: and in these
genera we find (and the fact appears to me highly remarkable) as high
a degree of concentration in the infra-œsophageal ganglion as in any
decapod Crustacean, for instance, as in Maia, judging from the figure
given by Milne Edwards; for all the nerves, with the exception of
those connected with the supra-œsophageal ganglions, radiate from a
single great ganglion.[39] The nervous system is, moreover, otherwise
complicated.

    [39] It must, however, be observed that, according to Mr. Dana,
    there is in certain suctorial Entomostracans, as in Caligus,
    only one infra-œsophageal ganglion. Mr. Dana speaks of this as
    resulting from reduction. In Cirripedes, from the gradation which
    may be observed from Lepas through Pollicipes into Balanus, the
    ganglions are certainly not reduced but concentrated. In Van de
    Hoeven's figure of the nervous system in Limulus, there is seen to
    be no chain of thoracic ganglions; all the nerves rising from the
    circa-œsophageal collar; but this, on the other hand, seems hardly
    developed into a ganglion.

To begin with _Coronula diadema_ the great infra-œsophageal ganglion
(Pl. 27, fig. 1, A) is seated nearly opposite to the anterior margin of
the second pair of cirri, which are homologous with the first pair of
legs in the decapod Crustaceans. This ganglion shows no trace of any
longitudinal medial suture; its shape is hardly discoverable, for it
is formed by the union of eleven principal pairs of nerves, besides
several arising from its under surface; in outline, however, it may be
said to be divided into a posterior and anterior half; the latter being
somewhat heart-shaped, and the posterior half elongated. The nerves
going to the five posterior pairs of cirri rise from the posterior
margin of the ganglion, and run for some distance in a sheet, parallel
and close together; the pair, however, going to the second pair of
cirri soon branches off from the others. Each of these nerves enters at
the inner and posterior margin of the cirrus to which it belongs, and,
at least in the case of the first pair, divides into two branches as it
enters. The nerves (Pl. 27, fig. 1, _r^5_, _r^6_) going to the fifth
and sixth pairs of cirri are more closely united together than are the
others, and appear, till they branch off, like a single large nerve.
That which belongs to the sixth cirrus gives off, opposite to the
fifth cirrus, a branch (_s_) nearly as large as itself, which enters
the probosciformed penis. I may remark, that homologically this is the
only abdominal nerve in any cirripede of the Order. From the under side
of the nerves which run to the five posterior pairs of cirri, small
branches are given off, extending dorsally into the thorax.

The anterior end of the great infra-œsophageal ganglion is formed by
the union of a set of nerves, extending parallel in a bundle in a
directly opposite direction to those running to the five posterior
pairs of cirri. These nerves consist of an outer larger pair (_r^1_)
entering the first pair of cirri; and within these, and rather dorsally
to their roots, we have the circa-œsophageal chord (_c_, _c_), or
collar nerve; between the roots of the latter, and on the ventral
surface (or near side of the figure), there are three closely united,
small pairs, running to the gnathites, and, as I believe, to the
olfactory sacks. From the under (or dorsal) surface of the anterior
end of the ganglion, two nerves, larger even than the circa-œsophageal
chord, and which I shall call the splanchnic pair (_d_, _d_) arise;
and the singular course of these nerves will presently be described;
between this great pair, there is a single (_b_) medial nerve, which
runs down and branches into that large diverging muscle, which is
attached to the upper ventral surface of the stomach. Posteriorly to
these three nerves, we have two pairs of much smaller nerves (not
figured), running dorsally into the body, so that we have seven nerves
rising from the under surface of the infra-œsophageal ganglion. I
need only further add, that on each side of this ganglion, between
the nerves going to the first and second pairs of cirri, there is a
moderately sized nerve (_k_), which appeared to run into the muscles of
the thorax: a nerve in a similar position is figured by Milne Edwards
in Maia.

The circa-œsophageal chord (_c_, _c_) nearly equals in length the whole
distance from the centre of the main ganglion to the posterior end
of the thorax. This collar bows out on each side, where passing the
œsophagus (_œ_), which is seated at its anterior end. From the collar a
branch is given off on each side, which I traced as far as between the
mandibles and maxillæ; from analogy with other Crustaceans, it perhaps
runs to the mandibles. The collar has not a transverse commissure, such
as described by Milne Edwards in the Podophthalmia, and as figured by
Van de Hoeven in Limulus.

The supra-œsophageal ganglions (B) present a singular contrast with
the infra-œsophageal ganglion in their little development, size, or
degree of confluence. They lie directly under the basal edge of the
labrum. They are laterally quite distinct, and consist merely of a
slight enlargement of the circa-œsophageal chord. From the anterior
edge of each ganglion, a broad nerve (_f_) extends for some distance in
a straight line, and, on close examination, can be seen to be formed of
two nerves closely united, of which the inner and smaller one, after
a space, appears to cross over the larger nerve: both become at this
point tortuous, and, giving off branches (_m_, _m_), form a plexus. The
two nerves (_f_) then bend inwards, and almost touching each other, run
down, together with the two ovarian simple ducts, along the rostral
compartment of the shell. No doubt, if the smaller branches from these
nerves could be traced, they would be seen to form a network over the
whole sack; and would therefore enclose, as in a cage, the rest of the
nervous system. These nerves correspond, I believe, to the two pair
of antennular nerves of ordinary Crustaceans, and hence I will call
them by this name. Just in front, at the outside corners of the two
supra-œsophageal ganglions (B), a branch (_e′_) arises, which I traced
to the ends of the adductor scutorum muscle, and to those several
muscles which serve to retract the interspace of membrane between the
mouth and the adductor.

The pair of great splanchnic nerves above alluded to, which arise from
the anterior and dorsal surface of the infra-œsophageal ganglion, are
in Pl. 27, fig. 1, _d_ _d_, (and in fig. 2), laid flat; but in nature
they first bow outwards, and then, penetrating deeper into the body,
approach each other, and running nearly parallel, pass round the lower
end of the œsophagus: their course consequently is nearly similar
to that of the circa-œsophageal chord, with this difference, that
the outwardly bowed portion is situated near the infra-œsophageal,
instead of near the supra-œsophageal ganglion. The splanchnic nerve,
a little beyond the supra-œsophageal ganglion, joins a plexus (_d′_);
and into this plexus another large nerve (_e_) which I will call the
supra-splanchnic nerve, sends branches; this nerve takes an almost
semicircular bend over the ovarian glands. The supra-splanchnic nerves
(_e_, _e_), though appearing to spring from the supra-œsophageal
ganglions, do really arise, as may be seen by tracing the constituent
fibres, from the circa-œsophageal chord. The plexus (_d′_) lies close
to the coats of the upper end of the stomach: several branches,
proceeding from it, run further on, but I was able to trace only a few
of them: one went (at least in the case of _Balanus perforatus_), to
the adductor scutorum muscle: another branch spread out on the flanks
of the prosoma: I strongly suspect that one branch goes to the acoustic
sack: it appeared, also, as if some of the small branches entered the
second plexus (_m_), where the inner antennular nerve and ophthalmic
nerve cross over the outer antennular nerve.

I have called the nerves (_dd_, _ee_) splanchnic and supra-splanchnic,
from their course and apparent function in supplying the viscera. In
the descriptions of the nervous system of other Crustaceans I can find
nothing analogous to my great splanchnic nerve (_dd_); the so-called
supra-splanchnic nerves (_ee_), which arise from the circa-œsophageal
chord, seem to be the analogues of the ordinary splanchnic nerves,
though these latter are always described as uniting into a single
medial branch. The plexus (_d′_) is the cervical ganglion of M.
Martin St. Ange,[40] who has likewise indicated the course of my
splanchnic and supra-splanchnic nerves; but the plexus, when viewed
as a transparent object, hardly appears to me to be ganglionic in its
nature. In my former volume on the Lepadidæ, I quite misunderstood the
course of these splanchnic nerves.

    [40] 'Mémoire sur l'Organisation des Cirripèdes,' p. 19.

From the commissure between the two supra-œsophageal ganglions, a
straight chord (Pl. 27, fig. 1, _g_) arises, which terminates in
a small ganglion (C), barely exhibiting traces of being formed of
two laterally confluent ganglions. This is the ophthalmic ganglion.
The chord connecting it with the two supra-œsophageal ganglions is
accompanied by a small nerve (_h_) which runs on to the muscles round
the adductor scutorum muscle; the chord is encased by much fibrous
tissue, and its dissection is thereby rendered difficult. From the
ophthalmic ganglion, on each side, a nerve (_i_) goes forth and crosses
the antennular nerve; these, if I could have traced them, would have
been found to run, as may be safely inferred from what is known in
_Balanus tintinnabulum_, to a pair (D, D) of eyes.

In _Balanus tintinnabulum_, the structure of the great infra-œsophageal
ganglion (Pl. 27, fig. 2, A) is essentially the same as described under
Coronula. The great pair of splanchnic nerves springing from its under
side, are here actually twice as large as the circa-œsophageal chord.
The plexus (_d′_) formed by the splanchnic nerve (_d_), on each side,
with the supra-splanchnic nerve (_e_), which arises close posteriorly
to the supra-œsophageal ganglion, is here much less complicated, but
is perfectly distinct; and there was no appearance of the cervical
ganglion of M. Martin St. Ange. The chord (_g_) running from between
the two supra-œsophageal ganglions to the ophthalmic ganglion, is
nearly as large as the double antennular nerve (_f_) on each side of
it. The ophthalmic chord (which is accompanied in its whole course by a
small branch running to the adductor scutorum muscle) terminates in a
small ophthalmic ganglion (C), which seems to be formed by the almost
complete fusion of two ganglions. This ganglion is hardly larger than
the chord which it terminates: it appeared to me to give rise to more
than one pair of nerves, and a single nerve (to my surprise) joined the
branch just mentioned, which goes to the adductor scutorum muscle.

From each supra-œsophageal ganglion, two closely united antennular
nerves (_f_) extend, of which the inner one crosses over the main
or exterior nerve, nearly opposite to the ophthalmic ganglion, and
here forms (_m_) a plexus. The structure of this plexus I was not
able, any more than in Coronula, to make out thoroughly, but I traced
quite distinctly a long nerve (_i_) running from it into what must be
considered as the eye. As in the case of Coronula, I traced a nerve
on each side from the ophthalmic ganglion into the plexus, where I
lost it; and as here in Balanus, I saw on each side of the ophthalmic
ganglion a cut off nerve, of about the size of that which runs from
the plexus on each side into the eye, I think we may safely conclude
that the latter or optic nerve does really arise from the ganglion here
called ophthalmic. I may add that the analogy of the nervous system in
the Lepadidæ most strongly confirms the view of this latter being the
ophthalmic ganglion.


_Eyes and Vision._

The optic nerve (_i_), running from the plexus to the eye, is of
considerable size; it runs nearly parallel to the main antennular
nerve, diverging from it a little. It retains nearly the same diameter
throughout; and gives off only one single, small, inner branch. It can
be traced beyond the basal edges of the scuta, to just under the upper
edge of the transparent opercular membrane, which unites the scuta
to the sheath of the rostrum. The nerve itself, at a little distance
from its further end, was, in a full-sized specimen, 5/1000ths of an
inch in diameter; widening a little, it expands slightly, and abruptly
terminates in a circular disc, about 8/1000ths of an inch in diameter,
(see Pl. 27, fig. 5). The nerve just beneath this slight expansion,
is coated all round by pellets of dark purple pigment-cells, but not
actually united into a continuous layer. These pigment-cells are
the more conspicuous from the surrounding parts being colourless. I
could not make out distinctly any cornea; and I suppose the external
transparent membrane, to which the above slight circular expansion is
attached, acts as such. This description very closely agrees with that
given of these organs in _Bal. rugosus_ of Gould, (_B. crenatus_?) by
Dr. Leidy,[41] who first discovered the eye in the adult cirripede,
but he did not observe the ophthalmic ganglion. These eyes differ from
those in some of the genera of the Lepadidæ, only in the greater length
of the optic nerve, and by standing laterally further apart from each
other.

    [41] 'Proceedings of the Acad. Nat. Sciences of Philadelphia,'
    vol. iv, 1848, p. 1. I may add that I have, also, observed the
    supra-œsophageal and ophthalmic ganglions in _Bal. perforatus_.

I may here mention that I tried a few simple experiments on the senses
of _Balanus balanoides_, _B. crenatus_, and _Chthamalus stellatus_.
I found these three species very sensitive to shadows, that is, to
an object like my hand passing even quickly, and at the distance of
about a foot, between them and the source of the light.[42] They were
indifferent to a gradual change from bright to obscure light; but
instantly perceived and drew in their cirri, when my hand was passed
between the basin in which they were kept and the window, even when
this was tried rather late on a dusky evening; and likewise when my
hand was passed between them and a single candle. I took, of course,
the precaution of passing my hand in other directions, but this
never produced any effect. These species are moderately sensible to
any vibration in the vessel in which they were kept, but they were
indifferent to noises made in the air, or in the water. I found it
impossible to touch, under water, an individual shell ever so lightly
with a needle, without all the immediately surrounding individuals,
when several adhered together, perceiving it, and retracting their
cirri: it made no difference whether the one touched had already
withdrawn its cirri and was motionless: from this fact, and from seeing
that a similar but slighter effect was produced by touching the rock
on which the specimens adhered, I infer that the perception by the
others of the one being touched, is communicated by vibration. When an
individual was touched under water, not by a needle, but by a pointed
camel-hair brush, it generally withdrew its cirri, but the neighbouring
specimens took no notice: when touched by a single hair of the brush,
no notice was taken, unless the skin of the orifice leading into
the sack was so touched. In these trials, it is of course necessary
carefully to avoid intercepting the light. I could not make out that
cirripedes perceived odours diffused in the water.

    [42] I find that this fact was long ago observed by Von Siebold,
    'Anatomie Comparée,' tom. i, p. 434.


_Acoustic Organs._

These are situated in the same position as in the Lepadidæ, namely,
in a slight swelling on the sides of the thorax (Pl. 25, fig. 1,
_d′_) just beneath the basal articulation of the first pair of cirri.
The orifice in Tubicinella and Xenobalanus is slightly produced, or
is tubular; the free part in the former genus projecting 5/100ths
of an inch. The structure of all the parts is essentially the same
as in the Lepadidæ, but I think all are proportionally larger. The
external membrane of the body is turned inwards at the orifice,
as a short flattened tube, which widens considerably (being, in a
middle-sized specimen of Coronula, 4/100ths of an inch in width) before
it abruptly terminates. The meatus, as I have called the sack-like
cavity which encloses the true acoustic sack or vesicle, is formed
of pulpy membrane, and is apparently continuous with the corium of
the whole body, but by dissection it can be separated entire. The
acoustic vesicle is of various shapes, as we shall immediately see; but
in all essential respects it is identical with the same part in the
Lepadidæ; it is formed of the same peculiar, soft, elastic, brownish,
transparent tissue, which seems to be composed of fine, transverse
pillars, becoming towards the outside fibrous, and at their inner ends
appearing when viewed vertically from above, like hyaline points. In
_Coronula diadema_, I observed on the outside of the acoustic vesicle,
some excessively minute bristles, only 1/3000ths of an inch in length,
seated on little eminences. I examined carefully the contents of the
vesicle in this species, in specimens well preserved in spirits, and
there was nothing within but a very little, thin, pulpy fluid, and a
few yellowish nucleated cells, here and there aggregated into small
groups. In Coronula, the flattened acoustic vesicle is elongated, with
a somewhat sinuous, but not very irregular margin (Pl. 27, fig. 4),
and is without any ridges on the surface; its neck or orifice projects
at right angles to the elongated portion, which stands obliquely to
the tubular orifice of the meatus. In a moderately-sized specimen of
_Coronula diadema_, the elongated portion of the acoustic vesicle was,
6/100ths of an inch in length. In Tubicinella, the acoustic vesicle
is heart-shaped, with the neck attached to its broader end; and the
surface is covered by zig-zag ridges. In _Balanus tintinnabulum_ (fig.
3), the acoustic vesicle is almost square at the lower end, with the
neck placed at one of the upper corners; on the external surface,
there is an oblique prominent ridge or fold, which sends off downwards
another ridge; its length, in a large individual, was 5/100ths of an
inch.

In all these cases, the acoustic vesicle is mainly attached by its
neck, to the upper end of the sack-like meatus; but there is likewise
a layer of soft, pulpy, cellular matter, slightly connecting that side
of the vesicle which is opposite to the neck, with the walls of the
meatus or outer sack. The mouth or orifice of the vesicle is closed
by a delicate lid or diaphragm, which can easily be separated; and
this diaphragm is formed by the expansion of a large nerve, which here
abruptly terminates. In a very large specimen of _Coronula diadema_ I
clearly made out the existence of this nerve, and traced its course
for some distance from the point where the summit of the meatus and
the neck of the vesicle are joined together; the nerve first runs
posteriorly, and then turns inwards and doubles back or anteriorly;
and I clearly followed it to the antero-lateral sides of the uppermost
end of the stomach, where it seemed to enter a ganglion, so that I
unfortunately cut it off, but found only a slight plexus, with the cut
off nerve apparently running onwards with nearly the same diameter.
The diameter is great, fully equalling, in its widest part, that of
the circa-œsophageal chord; but it is very much flattened, and so
has not nearly so much bulk as that nerve. Before it reached the
stomach, it gave off one branch, which ran towards the mouth. The only
nerves which, from their size, could, I think, be continuous with
this from the acoustic sack, are the main branches proceeding from
that plexus (_d′_) formed by the interbranching of the splanchnic and
supra-splanchnic nerves.[43]

    [43] I have always feared that anatomists would reject my view of
    these organs being acoustic, owing to the absence of otolithes;
    but I observe that so high an authority as Von Siebold ('Anatomie
    Comparée,' tom. i, p. 433) does not believe that otolithes occur
    in the acoustic organs even of the highest Crustacea. He considers
    an "ampoule volumineuse, a parois mince, remplie d'un liquide
    transparent," and a "membrane tympanique," though having a fissure
    in the centre, as sufficient. I may here remark, that the nerve
    proceeding from the acoustic vesicle in Cirripedes, and apparently
    running to the splanchnic nerve, may easily be placed in connexion
    with the antennular nerves, by the second plexus (_m_) in figs. 1
    and 2, pl. 27. I should infer from Von Siebold's remarks on his
    ampoule volumineuse in the higher Crustacea, that my acoustic
    vesicle answered to the labyrinth in higher animals.


_Olfactory Sacks._

I can add nothing to the account given of these organs under the
Lepadidæ: I saw them in all the genera which I examined for this
object. In _Coronula diadema_ the orifices are large; they are seated
in the usual position (Pl. 26, fig. 4, _n_), in the confluent segments,
beneath the free part of the outer maxillæ, and somewhat exteriorly,
or as near as possible to the inner maxillæ. In no sessile cirripede
are the orifices produced or tubular, as is the case with several
genera amongst the Lepadidæ. I failed, as heretofore, in tracing with
certainty the nerve, which appears to enter the base of the sack, to
its ganglion.


_Male Organs of Generation._

All the Cirripedes of the family we are now describing, are bisexual
or hermaphrodite; and no instance has been observed of the presence
of males or complemental males. I have very little to add to the
observations made by M. Martin St. Ange and R. Wagner,[44] and to
those given in my former volume. The testes seem always to be confined
to within the thorax, including the prosoma. With their ducts, they
resemble club-moss or stag's horns, with the extremities a little
enlarged: a figure[45] of a small portion from _Balanus perforatus_ is
given in Pl. 25, fig. 2. It is quite surprising how like in structure
and appearance the branching ovarian tubes often are to the testes with
their ducts; but the latter are of smaller diameter. Two main ducts
generally unite just before entering the broad, often reflexed, end of
the vesicula seminalis: in _Coronula balænaris_, however, I observed
four ducts entering this receptacle. The two vesiculæ seminales, lying
within the thorax and prosoma, are usually very long and tortuous: they
are formed of a thin inner tunic, which is strengthened by thicker
reticulated lines, and of an outer layer of transverse fibres, which
are either elastic, or probably muscular, as they serve to expel
the contents with force when the end is cut off. The inner tunic is
prolonged up the probosciformed penis, at the base of which the two
vesiculæ unite.[46] The contents of the vesiculæ are commonly pulpy and
cellular; and from the cells the spermatozoa are developed; soon after
their development, they are, as it appears, expelled.

    [44] The 'Report' on M. Martin St. Ange's memoir was laid before
    the Academy of Sciences, July 14, 1834, so that I suppose it was
    read previously to this date. R. Wagner's paper was published
    in 'Müller's Archiv,' 1834, p. 467. Burmeister's 'Beiträge
    zur Naturgeschichte der Rankenfüsser,' was published this
    same year, 1834; so that these three authors published almost
    contemporaneously.

    [45] A far better figure is given by Karsten ('Nov. Act. Acad. Cæs.
    Nat. Cur.,' 1845, Pl. 20, figs. 2, 3, 4), but under the erroneous
    supposition that these organs were hepatic.

    [46] In _Conchoderma aurita_, the ducts, as shown by Burmeister
    ('Beiträge,' &c. tab. ii, fig. 17), unite half way up the
    probosciformed penis.

I have seen the spermatozoa in _Balanus crenatus_, _perforatus_, and
_balanoides_, and in _Chthamalus stellatus_. The cells, from which the
spermatozoa are developed, and which are often found in vast numbers
within the vesiculæ, are on an average about 1/5000th of an inch in
diameter. The spermatozoa differ remarkably within the vesicula of
the same individual, according to their state of development. I have
observed in _B. perforatus_ and in the Chthamalus, that the shortest,
and therefore, I presume, the youngest (Pl. 29, fig. 7, _a_), had a
globular head with no projection in front: as they increased in size,
this head became less in diameter, and a short tapering filament, (_a_,
_b_,) like the tail, projected out of it. This anterior filament does
not lie in exactly the same line with the posterior filament, which is
straight as an arrow. In _Bal. crenatus_, the anterior filament was
1/2000th of an inch in length, and the posterior filament 4/2000th,
giving a total length of 5/2000th: in the longest and best developed
specimens of _Chthamalus stellatus_, the nodular enlargement was
much elongated and spindle-shaped, and not above half the diameter
it had in the earliest stage; the posterior filament (measured from
the front of the enlargement, this consequently being included) was
5/2000th in length, and the front part only 1/4000th, giving a total
length of 11/4000ths of an inch. These observations agree pretty well
with Kölliker's;[47] but this author states, that perfectly developed
spermatozoa are absolutely without any nodular enlargement: if this be
the case, I have never chanced to see the spermatozoa in their perfect
condition. Mr. Bate, also, figures some (Pl. 29, fig. 7, _c_) in this
state, without any enlargement.

    [47] 'Annales des Sciences Naturelles,' (2d series), tom. xix, p.
    348. Kölliker refers to Wagner's paper on the same subject, in
    Wiegmann's 'Archiv,' 1835, part ii, pl. iii, fig. 9. He also refers
    to Von Siebold's observations. Mr. C. Spence Bate has figured, in
    the 'Annals and Magazine of Natural History' (vol. viii, 2d series,
    1851), the spermatozoa of _Balanus balanoides_, _perforatus_, and
    of _Verruca (Clitia) Strömia_, and of these I have given copies,
    Pl. 29, fig. 7.

The probosciformed penis lies adpressed on the under side of the
thorax, with its apex generally projecting between the first and second
pairs of cirri. It presents the same ringed or articulated structure as
in the Lepadidæ: it arises from an unarticulated projection or support,
which also forms the posterior border to the anus. This support
often terminates, as first observed by Poli, in a very sharp point;
but this point cannot be of much functional importance, for though
present in _Balanus balanoides_, it is absent in the closely allied
_B. crenatus_; in Tubicinella there is only a rudiment of this point;
I have not observed it in any member of the Chthamalinæ. The strong,
transverse and longitudinal muscles with which the penis is furnished,
are attached to this support. The apex or orifice of the penis is, I
believe, invariably surrounded by some bristles. Its length varies
much, according to its state of contraction or relaxation; and this
again, I believe, is dependent on the condition of the male secreting
organs. In a small specimen of _Elminius modestus_, the penis was
actually thrice as long as the whole thorax, including the prosoma:
in Pachylasma and in _Octomeris angulosa_, the penis is very short,
being equal only to once and a half the length of the pedicel of the
sixth cirrus: in _Octomeris brunnea_, the unarticulated support is
much elongated, being as long as the pedicel of the sixth cirrus, in
which respect this organ resembles that of _Ibla quadrivalvis_, and of
no other Cirripede. From the attachment of the penis at the posterior
end and on the under side of the anus--from the position of the caudal
appendages (where such occur) over the anus--from the position of
these same appendages in the pupa--and lastly, from the position of
the papilla-like penis in the abnormal _Proteolepas_, I infer that,
homologically, the penis is situated at the apex of the abdomen, on
its ventral surface; and that, consequently, this organ cannot be
considered as the abdomen itself in a modified condition.


_Female Organs of Generation._

I have scarcely anything to add to the statements in my former volume.
These organs consist of the true ovaria, or glandular bodies seated
on each side, not far from the basal edge of the labrum; of the main
or unbranched ovarian ducts; and of the (Pl. 25, fig. 1, _g_) ovarian
branching tubes and cæca. I traced distinctly in Balanus, Tetraclita,
and Coronula, the two main ovarian ducts, running from within the
prosoma to the layer of inosculating, branching, ovarian cæca[48] which
overlie the basis. In _Coronula diadema_ one of these main ducts was
1/100th of an inch in diameter. Though I traced these ducts near to the
grape-like, glandular masses,[49] which I cannot doubt are the true
ovaria, I did not succeed in tracing them into actual connection. As
in the Lepadidæ, these ovarian glands lie on the sides, near the basal
margin of the labrum, and almost under, but rather to the outside of
the antennular nerves. The branching and inosculating ovarian cæca
form a layer, which corresponds with the mass filling up the peduncle
in the Lepadidæ. In Tetraclita they do not cover the whole basis, but
are confined to the circumference; they, however, likewise extend
up between the two layers of corium round the walls of the shell,
and chiefly in the interspaces between the depressor muscles of the
opercular valves. In Chelonobia, they enter between the radiating septa
in the thickness of the walls: in _Coronula diadema_, they extend from
over the basal membrane into the six large square chambers (Pl. 16,
fig. 7, _v_) separating the radii and alæ: in Tubicinella they are
confined to the basis: in Xenobalanus, they form a layer over the basis
and likewise round the upper part of the peduncle-like body, which
answers to the shell of other sessile cirripedes.

    [48] These are well described in Lepas, by R. Wagner, in 'Müller's
    Archiv,' 1834, p. 467. Von Siebold, I observe, refers to Burmeister
    as the first author who discovered the ovarian cæca within the
    peduncle; I had thought that M. Martin St. Ange had a prior claim.

    [49] These are obscurely figured by Karsten ('Nov. Act. Acad. Cæs.
    Nat. Cur.,' 1845, Pl. 20, fig. 1_d_) as salivary glands; they were
    so considered by Cuvier and M. Martin St. Ange: I may observe
    that salivary glands have not been positively recognised in any
    Crustacean.

As after the most careful and repeated examinations of various
Lepadidæ, I was convinced that there were no oviducts, so I have
come to a similar conclusion in regard to the Balanidæ; the ova
being brought to the surface, by the formation of a new membrane
round the sack underneath them, and by the subsequent exuviation of
the old membrane. The ova are united together by a most delicate
tunic investing each egg; the ovigerous lamellæ being thus formed,
as in the Lepadidæ. In the cases of _Chthamalus stellatus_, _Balanus
balanoides_, and _Platylepas decorata_, I saw a pair of very distinct
but fragile lamellæ. In Xenobalanus, the two ovigerous lamellæ form
two sub-cylindrical packets, pointed at their lower ends and often
cohering. There are no ovigerous fræna, for the attachment of the
lamellæ; the ova being sufficiently well retained, as it would appear,
by the well-closed shell. I have elsewhere stated my full belief that
it is the ovigerous fræna which have been metamorphosed into the
branchiæ of the Balanidæ. Most sessile cirripedes breed when very
young; and I have every reason to believe that they breed several times
in the year. The ova are ovate, and vary in length from 14/2000th of
an inch in Chthamalus, to 19/2000th in some species of Balanus, in
which this greater length was owing to a more elongated shape,--up to
25/2000th in some other species of Balanus. The ova are wonderfully
numerous, especially in the genus Coronula.

I may here mention the singular case of some elongated specimens
of _Balanus balanoides_, from Tenby, in South Wales: some of these
presented nothing abnormal; but in no less than seven specimens, the
two, three, or four posterior pairs of cirri, either on one or both
sides, were in an almost rudimentary condition, being of small size
and having a shrunk and wasted appearance. In six out of these seven
specimens, the probosciformed penis was quite short and abruptly
truncated, as if from abortion. By cutting off the truncated apex, and
cleaning the external tissue, I ascertained that it was imperforate,
apparently in all the cases, and I am certain of this fact in several
of the cases. In three of the specimens, I examined the vesiculæ
seminales; in one, I found some spermatozoa, but cohering together
in a peculiar manner; in the second, there were no spermatozoa; and
in the third, the vesiculæ were shrunk, empty, and quite rudimentary
in size. So that these three individuals certainly could not have
impregnated their own eggs; nevertheless, within the shell of these
very three, there were perfectly developed larvæ: I am led to conclude
from this fact, that adjoining specimens in a perfect condition had,
by means of their long probosciformed penis, effected the fecundation
of their imperfect neighbours. I need only further add, that some
out of the above six specimens, with more or less aborted cirri and
imperforate male organs, were infested by a peculiar parasite, allied
to Bopyrus,[50] and that these specimens did not contain ova.

    [50] I have given a short notice on this parasite, in my former
    volume on the Lepadidæ, in a foot-note to p. 55.


_Metamorphoses and Homologies, throughout the Order of Thoracica._

In my former volume, the metamorphoses were described under three
principal stages or heads; but whether these three included all
the main changes, I was then hardly able to conjecture. But now
I have reason to believe that such is the case, for in the genus
Cryptophialus, belonging to the Abdominalia, the whole course of the
metamorphosis, from the egg to the pupa, takes place within the sack of
the parent; and I found, when having, on the coast of South America,
numerous specimens to examine, that the egg-like larvæ (Pl. 24, fig.
15-18) could be naturally grouped into two main stages, but with many
transitional intermediate grades (answering to the successive moults in
the first stage of ordinary larvæ), before they passed into the third
or pupal stage. And the first two stages in these egg-like larvæ of
Cryptophialus, clearly seem to correspond with the first two stages in
ordinary larvæ; for in both the chief changes are, the shortening of
the terminal projection--the increase in size and approximation on the
ventral surface of the anterior horns or cases for the antennæ--and the
compression of the whole body. In all members of the Thoracica, the
metamorphosis seems to run a remarkably uniform course. The larva in
the first stage undergoes several moults and slighter changes--how many
is not known--before arriving at its second main stage, which has been
observed only in one single instance; and judging from Cryptophialus,
this second stage passes abruptly by one moult into the pupal stage;
and this, certainly, passes abruptly into the Cirripedial or mature
stage.


_Larva, First Stage._

The larvæ in this stage are known, amongst the Balanidæ, in Balanus,
Pyrgoma, Coronula, Platylepas, and Chthamalus; and these genera include
all the principal forms. Amongst the Verrucidæ they are known in its
one genus, Verruca. Amongst the Lepadidæ, in Scalpellum, Ibla, Alcippe,
Lepas, Conchoderma, &c.; and in all these genera the larvæ present no
important difference--hardly any difference which could be viewed as
generic, were these larvæ independent animals,--as may be inferred,
chiefly, from Mr. C. S. Bate's descriptions.[51] The abstract given in
my former volume was not accompanied by any illustrations, and I have
consequently here given (Pl. 29, fig. 8), a view of the larva, in the
first stage before moulting, of _Scalpellum vulgare_: the natatory legs
are not drawn with accuracy, only the relative position of the several
organs having been carefully attended to. I have also had copied from
Mr. Bate's memoir, a figure of the larva (Pl. 29, fig. 9) of _Balanus
balanoides_, in its first stage, _before_ moulting, with its ventral
surface exhibited; and another figure (with a few trifling alterations
made after examining specimens most kindly sent me by Mr. Bate) of
the larva of _Chthamalus stellatus_ (fig. 10), in its first stage, but
_after moulting once_. It should be observed that Mr. Bate has given
a drawing of the larva of this latter cirripede, in the first stage,
_before moulting_; and it does not differ essentially from that just
referred to (fig. 9), of _B. balanoides_, but is rather more fully
developed. These drawings suffice to show the character of the larvæ
in the first stage, both before and after the first moult, and even
after the second moult, throughout the Order of Thoracica. The larvæ
sometimes undergo their first moult within the sack of their parent, as
I have been informed by Mr. Bate, and as I have observed in Coronula.

    [51] 'Annals and Magazine of Natural History,' vol. viii (2d
    series), 1851, Plates 6, 7, and 8.

I will now make a few remarks on these larvæ in the first stage, before
and after the first moult, supplemental to those in my former volume.
Their shape is oval, and the whole dorsal surface is evidently covered
by a carapace. It is remarkable that the body exhibits no distinct
articulations; those given by Goodsir[52] being certainly erroneous.
Commencing at the anterior extremity, the eye varies considerably in
the state of its development; in _Platylepas decorata_ it is nearly
circular, and in most of the specimens very distinct; whereas in
the allied _Coronula balænaris_, before the first moult, it is very
imperfect, but afterwards square and of considerable size. In _Balanus
galeatus_, in the immature larvæ dissected out of the egg, the cellular
matter which was in process of conversion into the eye, formed a
transverse band, obscurely divided into two portions, and this seems
to indicate that the single eye is in fact formed by the confluence of
two eyes. In _Scalpellum vulgare_, this heart-shaped eye lies between
a V-shaped muscle, the nature of which I cannot understand, and which
has not been represented in (Pl. 29, fig. 8, _a_). I need only further
add, that in _Chthamalus stellatus_, after the first moult, the eye
exhibits, in specimens sent me by Mr. Bate, some appearance of tending
to become double.

    [52] 'Edinburgh New Philosophical Journal,' July, 1843, Pl. 3, 4.

Arising posteriorly to the eye, we see, in _Scalpellum vulgare_, a
pair of minute curved horns (_b′_), directed backwards; and within
these horns I distinctly saw an articulated organ. These horns are
difficult to be distinguished, and probably could not be made out
previously to the first moult, in any larva of less size than that of
_Scalpellum vulgare_. But after the first moult, Mr. Bate has seen, in
two species of Balanus, in Verruca and in Chthamalus (fig. 10, _b_),
a pair of articulated organs, in this same position, evidently now
forming antennæ, and directed anteriorly, and free from any envelope.
It is somewhat important, as we shall presently see, to bear in mind
that these antennæ first appear within an envelope or horn; and that I
detected that they included an articulated organ, before I had heard
of Mr. Bate's observations. These antennæ, from their small size, from
being seated internally with respect to the horns containing the other
pair of antennæ, and from the position which the latter assume in the
later stages of the larva, I believe to be the first or anterior pair.
Their position in appearance posteriorly to the large lateral horns,
containing the second pair of antennæ, is probably due to the anterior
cephalic segments having been driven inwards, the truncated outline
of the front of the head, and likewise, probably, the position of the
mouth between the bases of the natatory legs being thus caused.

In this same larva of _Scalpellum vulgare_, within the great lateral
horns just alluded to (fig. 8, _c_), filiform organs, supporting rows
of spines, could be distinguished; and these appeared to me to be
antennæ. These horns or cases resemble in structure the smaller pair
just described; they arise from the ventral surface, and can hardly,
therefore, be considered as prolongations of the carapace. After
the first moult (fig. 10, _c_) they are seen to have increased much
in length: in some cases they are of considerable length before the
first moult, as in Lepas: in the Balanidæ they seem to be generally
shorter than in the Lepadidæ; but in _Balanus galeatus_ I found them
one third of the entire length of the animal. Whilst within the
egg, these horns are adpressed laterally to the body, and so point
posteriorly; afterwards they project rectangularly from the sides,
or, as in _Scalpellum vulgare_, are directed somewhat anteriorly. As
in the larvæ of all ordinary Crustaceans, as yet known, the antennæ
are amongst the earliest developed organs; and as the first pair of
natatory legs (Pl. 29, figs. 8-10, _e_) in these Cirripedial larvæ,
might so very naturally be thought to be antennæ (as has been remarked
to me by Mr. Dana), both from their structure and from their position a
little anteriorly to the mouth, I am well aware that to prove my view
correct, namely, that these horns are the second pair of antennæ in
process of formation, it is not sufficient merely to have seen organs
resembling antennæ within them; nor is it sufficient to advance the
strictly analogical fact of the first-mentioned pair of antennæ, which
in Scalpellum indisputably appear in their earliest condition within an
envelope or horn. Further evidence is required, and this is presented
in Cryptophialus, in which the lateral horns of the egg-like larva, in
its first stage (Pl. 24, fig. 16), can be actually followed step by
step until, in the second stage (fig. 17), just before passing into
the pupa, the horns are seen to have become larger and more nearly
approximated to each other on the ventral surface; and whilst in this
condition, I several times dissected out the prehensile antennæ of the
future pupa with _every character perfectly recognisable_. Hence I
cannot doubt that in the larvæ of Cirripedes the law of development is,
that in their very earliest condition, the small first pair of antennæ
are enclosed in cases; and that the large second pair remains thus
enveloped until the pupal stage. This conclusion, we shall immediately
see, is in harmony with the late development of the succeeding
appendages or organs of the mouth, which certainly do not appear in the
first larval stage, and are not known to appear even till after the
final metamorphosis.[53]

    [53] According to M. Joly, ('Annales des Sciences Naturelles,' 2d
    series, tom. xix, p. 59) in the larva of the macrourous Caridina,
    the natatory legs appear before the gnathites or parts of the
    mouth; so that in ordinary Crustaceans there is no invariable order
    of development from the anterior towards the posterior end of the
    body, as has sometimes been supposed.

The mouth is more or less probosciformed (Pl. 29, figs. 8-10, _d_),
differing considerably in this respect in different species of the
Lepadidæ; and this, probably, is due to the larva being born in a more
or less mature condition. Its exact position likewise varies, for it
arises either between the first or second pairs of natatory legs. It is
known, from Mr. Bate's observations, to have the power of movement. It
is directed posteriorly, the œsophagus extending anteriorly; both these
directions being the same as in the mature cirripede. Certainly during
these early stages there are no jaws or gnathites; but the margin,
answering to the labrum, is furnished with some short, thick, sharp
spines, and with hairs. In _Scalpellum vulgare_ the orifice of the
œsophagus seems to lie rather beneath the upper prominent spinose edge,
which, as just remarked, probably answers to the labrum; but this is
one of the species in which the probosciformed mouth, at least before
the first moult, is not much developed.

We come, now, to the three pairs of natatory legs: the first (Pl. 29,
figs. 8-10, _e_) has throughout the order only one ramus, whereas the
two succeeding pairs (_f_, _g_) are biramous. I must here remark that
the straight and strong, and the curved plumose spines, with which
these limbs, after the first moult, become furnished, now appear to me
as more probably prehensile, rather than masticatory as I imagined in
my former volume. That these spines are important organs to the larvæ
I do not doubt. With regard to the homologies of these three pairs of
limbs, my first impression was that they were the mandibles and the
two pairs of maxillæ in their earliest condition; but I consider this
view as quite untenable, for several reasons; viz., the wide interval
between their bases and the mouth itself,--the somewhat variable
position of the mouth with respect to the legs,--and the position
which the latter occupy in the _second_ larval stage.[54] A far more
tenable view is that these three pairs of legs are the three pairs
of maxillipeds, in their earliest condition, in accordance with the
view of M. Joly[55] on the nature of the three very similar pairs
of natatory legs in the larva of Caridina, a macrourous Crustacean.
But, in Cirripedes, the three pairs of natatory legs, in the larva
in the first stage, are apparently the very same as the first three
pairs, in the larva in the second stage, and in the pupa. And in the
pupa the first three pairs, which certainly correspond with the first
three pairs of cirri in the mature animal, seem to me, for reasons
presently to be assigned, to be the second, third, and fourth thoracic
limbs. Hence I am led to the conclusion that the first pair of legs in
the larva in the first stage, are homologically the second thoracic
(answering to the third pair of maxillipeds in the higher Crustaceans),
and that the two succeeding pairs are the third and fourth thoracic
limbs; to be succeeded, in the _pupal_ stage, by the fifth, sixth, and
seventh thoracic appendages.

    [54] Mr. Dana, moreover, has remarked, ('Crustacea: United States
    Exploring Expedition,' p. 1386), "that he knows of no instance of
    a mandible becoming so completely a leg, as to lose wholly the
    mandibular function even of its basal portion."

    [55] 'Annales des Sciences Naturelles,' 2d series, tom. xix, 1843,
    p. 34. M. Joly's observations were made on the Caridina. I owe to
    the great kindness of Mr. C. Spence Bate, an examination of some
    larvæ of the allied genus _Hippolyte varians_, and I found, on
    dissection, the view of M. Joly, that the three pairs of natatory
    legs are the maxillipeds, so far strongly confirmed, that they
    followed closely, with equal intervals, the mandibles and two pairs
    of maxillæ. The first pair of natatory legs in Caradina, in its
    earliest condition within the egg, is uniramous, like the first
    pair in the larvæ of Cirripedes. There is one fact which seems
    rather strongly opposed to the view of these three pairs of legs
    in the larvæ of the macrourous Crustaceans being the maxillipeds,
    which is that Capt. Du Cane ('Annals of Nat. Hist.,' 1838, vol.
    ii, pl. 6, and 7) observed only three pairs of limbs in process
    of formation posteriorly to the first three pairs, whereas there
    should be found, in accordance with M. Joly's view, five pairs, _i.
    e._ all five pairs of ambulatory legs. This one fact countenances
    the view, which I hold on the nature of the legs in the larvæ of
    Cirripedes during their early stages, namely, that they are the
    second, third, and fourth thoracic limbs, to be succeeded by only
    three additional pairs.

Lastly, behind the natatory legs, on the ventral surface, (Pl. 29,
figs. 8, 9, _i_), the body is much produced, and terminates in a
horny fork, which, after the first moult (fig. 10, _i_), becomes much
elongated. Anteriorly to this fork, on the ventral surface, there
is another fork (_l_), and again above this I could distinguish, in
_Chthamalus stellatus_, after the first moult, another fork (_m_),
or at least a pair of short thick spines. From the structure of the
forked abdomen in the known larvæ of the Podophthalmia, I presume
that this portion of the body is the abdomen of the young Cirripede,
but it is not at all plainly articulated. After the first moult, the
posterior end of the carapace (_h_), which is always pointed, becomes
much elongated and serrated on both sides;[56] reminding one of the
structure of the carapace of the so-called Zoea, or larva of certain
Podophthalmia. Situated under this posterior prolongation of the
carapace, there is a swelling (_n_, with long hairs on both sides),
which apparently lies on the dorsal surface of the spinose and forked
abdomen; here, when the larva is compressed, the cellular and oily
contents of the body burst forth; and I suspect that this swelling
is the anus, for it is known from the researches of Rathke,[57] that
the anus in the higher Crustaceans opens during the earliest periods
dorsally.

    [56] I suspect that the account given by Goodsir ('Edinburgh New
    Phil. Journal,' 1848) of the posterior points of the carapace and
    abdomen in the larva of a Balanus, is not quite accurate.

    [57] 'Annales des Scienc. Nat.,' tom. xx, p. 451.


_Larva, Second Stage._

I have given, from Burmeister,[58] a lateral view (Pl. 30, fig. 1)
of the one single specimen, ever observed of a larva in this stage,
belonging, as is supposed, to the genus Lepas. The carapace has now
greatly altered its character. The two fleshy projections, as so
called by Burmeister, by which the larva adhered to the sea-weed,
were supposed by this author to include the great prehensile antennæ
of the pupa; from my observations, already alluded to, on the two
projections (Pl. 24, fig. 17) in the closely analogous egg-like larva,
in the second stage, of Cryptophialus, by which it also adheres, I
have not the least doubt that this is the case. The small, internal,
and anterior pairs of antennæ, are, as it would appear, now aborted.
The eye, according to Burmeister, has commenced becoming double;
but the two approximate eyes are not as yet compound. The mouth is
probosciformed (_m_), and does not differ much from its condition in
the first stage; no gnathites were observed by Burmeister, and they
could not be expected to be present, for they are not found even in
the pupa. The mouth, which in the larva in the first stage differs in
different genera, in being more or less advanced forward, here stands
some way anteriorly to the natatory legs, as in the pupal condition.
The first pair of legs is uniramous, and the two other pairs biramous;
this fact, together with the number of the legs in this second stage
being still three, and their structure being not very different,
leaves little doubt on my mind that we here have the same three pairs
as during the first stage. The abdomen has become much shortened, but
still space is left for the development, in the pupa, of the three
posterior pairs of legs. I may here remark that in the pupa the
anterior natatory legs have become, like the others, biramous; but yet,
as it were for the purpose of showing their metamorphosis from the
uniramous legs of the earlier stages, they have their bristles arranged
rather differently from those on the succeeding five pairs of legs.

    [58] 'Beiträge zur Naturgeschichte der Rankenfüsser,' tab. 1, figs.
    3, 4.


_Larva in the Last or Pupal Stage._

I have given a lateral view of the pupa of _Lepas australis_ (Pl. 30,
fig. 2), illustrative of the description in my former volume: the
specimen is drawn as if transparent, and it was to a certain extent
thus rendered by boiling in caustic potash. A sketch of the position
of the young Cirripede within the pupa, was made by the camera. At
first the drawing will perhaps hardly be comprehended: the darker
shaded portion to the left of the letter (_b_) shows the extent of the
sack, with the included thorax and natatory legs of the pupa: to the
right of the same letter, if we do not consider the young included
Cirripede, the only organs distinguishable in the mass of cellular and
oily matter, are the alimentary canal, the cement-glands (_t_), _i.
e._ the incipient ovaria, and the cement-ducts (_t′_) which enter the
antennæ. A view is also given (fig. 4) of the ventral surface of the
pupa; and a transverse section (fig. 7) of the carapace, taken close
to the eye-apodemes. On comparison with the larva in the second stage,
the changes in external appearance and structure are not very great;
the prehensile antennæ are freed from their cases; the two eyes stand
further apart; the three posterior pairs of legs have been developed,
and a small abdomen has become distinctly separated from the thorax.
Before proceeding to make a few additional remarks and corrections to
my former description of the pupa, it will be advisable, on account of
the importance of the subject, to discuss the homologies of the limbs.

From the presence of eyes and of two pairs of antennæ in the larva,
during its earlier stages, the front of the head consists, in
accordance with all analogy, of three segments; the mouth, likewise,
from being formed of three gnathites (which can be detected by
dissection in the pupal state), consists, also in accordance with all
analogy, of three segments, making altogether six segments--on the
nature of which I apprehend no objection will be raised. In two out
of the three orders into which Cirripedes may be divided, the mouth
is succeeded, in the adult animal, by eleven most distinct segments;
of which the first (_i. e._ the seventh cephalic) differs from the
succeeding seven thoracic segments; and these seven again differ
from the three abdominal and terminal segments. Hence it must be
admitted that, as far as the cephalo-thorax of the archetype Cirripede
is concerned, it consists, like that of the archetype Crustacean,
of fourteen segments, of which eight succeed the first-named six
that form the mouth and front of the head; and that, with the three
abdominal segments, there are altogether seventeen segments. In the
order Thoracica, however, which includes all common Cirripedes, both
in the pupa and in the mature animal, only six thoracic segments
with their appendages, succeed the mouth, two having been lost; and
the question arises which are these two, whether the seventh and
eighth, or the thirteenth and fourteenth (_i. e._ the two terminal
thoracic) segments; for there is no reason to suspect any other
segments of having disappeared. In my former volume, I inferred,
without sufficiently entering into my reasons, that it was the seventh
and eighth, _i. e._ the last cephalic and first thoracic segments,
which had disappeared; but I now find that Mr. Dana[59] believes
that, in ordinary Crustaceans, the abortion of the segments with
their appendages almost always takes place at the posterior end of
the cephalo-thorax. Nevertheless, after due deliberation and fresh
examination of the pupa, I must retain my former opinion, that it
is the last cephalic and first thoracic segments which have either
coalesced with the others, or wholly disappeared. In the pupa, the
mouth, although functionless, has its place most plainly marked by
being slightly prominent, and by the presence of a sort of labrum and
of a shrivelled œsophagus, round which latter the gnathites and the new
œsophagus of the future young cirripede are in process of formation.
Now between the mouth of the pupa and the first pair of natatory legs,
there is a space of membrane, equalling, when stretched out, the three
succeeding thoracic segments in length and breadth: this interspace, I
conceive, must have some homological signification; here then we have
at least an appearance of the abortion of appendages; whereas, at the
posterior end of the cephalo-thorax, no such appearance is presented.
Moreover this interspace of membrane is divided nearly in the middle by
a most conspicuous fold, which, on the view here adopted, would mark
the separation of the seventh (cephalic) from the eighth (thoracic)
segment; and the interspace and fold are thus simply explained. Lastly,
I have shown, in the Introduction (p. 18), that the first and five
succeeding pairs of cirri of the mature Cirripede present certain
small, but significant, resemblances in structure and in the origin
of their nerves, with the outer pair of maxillipeds and with the five
pairs of ambulatory legs in the Podophthalmia; which resemblances are
all futile, if the cirri belong to the 7th, 8th, 9th, 10th, 11th, and
12th segments of the cephalo-thorax, or those immediately succeeding
the mouth; but are full of meaning, if the six pairs of cirri belong,
as I believe, to the 9th, 10th, 11th, 12th, 13th, and 14th segments, or
the six posterior segments of the cephalo-thorax.

    [59] 'Crustacea: United States Exploring Expedition,' p. 22.

Before commencing on details, I may premise that I have examined
the pupa of _Lepas australis_, _pectinata_, _fascicularis_, and
_anatifera_, of _Conchoderma virgata_, partially of _Dichelaspis
Warwickii_, of _Ibla quadrivalvis_, and of _Alcippe lampas_; and in
the Balanidæ, of _Balanus balanoides_ and _Hameri_. In the pupæ of all
these genera there is a most close general agreement in structure,
excepting in minute details: I was surprised to find exactly the same
slight differences in the spines on the first pair of natatory legs,
as compared with the succeeding pairs, in _Balanus Hameri_, as in
Lepas. The abdomen and caudal appendages of the pupa in the abnormal
Alcippe, as we shall presently see, offer the only marked exception
to this uniformity of character throughout the Thoracica. The outline
of the carapace or shell is usually not so blunt at the anterior end,
as in the pupa of _Lepas australis_ (Pl. 30, fig. 2); more commonly
the shape is that of the pupa of Alcippe (Pl. 23, fig. 16). In _Lepas
pectinata_ the two posterior points of the carapace are produced into
two short spines. The surface of the carapace in _L. australis_ is
lined, as represented in fig. 4: the colour of this species when alive
was blue:[60] in _L. fascicularis_ the surface is punctured: in _L.
pectinata_ it is marked with curious points of various shapes, often
star-shaped, in parts reticulated, and confluent along the dorsal
margin, and in parts lined: in _B. balanoides_ it is very obscurely
punctured, and in _B. Hameri_ the punctures pass into lines. The whole
of what is externally visible consists of the carapace, for this is
produced not only backwards, so as to enclose the thorax and abdomen
with their appendages, but also forwards, so as to overhang the whole
front of the animal; and the prehensile antennæ, in Lepas, Ibla,
Balanus, and probably in all the genera, can be retracted within its
lower edge. The protection afforded by the carapace to the antennæ
is aided by two crests (Pl. 30, fig. 7, _c_) parallel to this lower
edge. The whole sternal surface is very narrow (fig. 4), and is
likewise protected by the carapace; that is, when the two sides are
drawn together by the adductor muscle. The shell, however, when thus
drawn together, gapes a little at the two ends, at least in the case
of _Lepas australis_. The adductor muscle, if introduced in fig. 4,
would have crossed close anteriorly to the basal margin of the mouth;
and in fig. 2, its end on the near side would have been attached under
the dark cæca, which enter the upper end of the stomach. The adductor
is shaped almost like an hour-glass, and so differs from this muscle
in the mature Lepas, in which it is of the same thickness throughout.
I may here add that the pupa of _Lepas australis_ could swim very
rapidly, and often on one side in a circle; it could walk by the aid
of its antennæ, but often fell over; being thus locomotive, and, as
we shall immediately see, well provided with senses, it cannot be
considered as very lowly organised.

    [60] I took this species alive in the Southern Atlantic Ocean; and,
    mistaking it for an independent Crustacean, was much perplexed
    where to class it. I had overlooked these specimens when publishing
    my former volume.

_Acoustic Organs._--Commencing at the anterior end, two small elongated
orifices, 10/6000th of an inch in diameter, (_e_, fig. 4, Pl. 30),
may be seen; these lead, as described in my former volume, into a
sack, with a bag suspended in it, which is provided with a large
nerve, and which I believe to be the acoustic vesicle. These orifices
occur in the carapace, either in the same position, or a little more
posteriorly, in the pupæ of all Cirripedes. In _Balanus balanoides_
they are minute, being only 2/6000th in diameter, but are surrounded
with a border: in _Conchoderma virgata_ they are also surrounded
by a border: in _Lepas pectinata_, the orifices are 3/6000th of an
inch in diameter, and are very singular from being seated on rounded
prominences, causing the carapace to have two short, blunt horns in
front. In _Lepas australis_, and I believe in the other species, the
corium round the acoustic orifices is darkly coloured; and these
coloured marks can be distinguished for some little time on the
peduncle of the young Cirripede, after the metamorphosis, and after
the entire organ, together with the whole pupal carapace and eyes, has
been moulted. Knowing the connection in the higher Crustacea, of the
acoustic organs and the antennæ, and seeing the very backward position
(figs. 2 and 4) of the one great pair of antennæ, I have always
imagined that these orifices probably marked the normal position of the
anterior pair of antennæ, which, since the earlier larval stages, have
disappeared. And I now find[61] that Schödler affirms, that in most,
if not in all Daphnidæ, there is a black spot in front of the eye,
which is connected with an opening in the basal portion of the anterior
antennæ, and he concludes that it is an organ of hearing.

    [61] Quoted by Dana, 'Crustacea of United States Exploring
    Expedition,' p. 1264.

_Antennæ._--These, from their present position, and from standing, in
their earlier stages whilst within their envelopes or horns, exteriorly
to the small medial pair (since aborted), I believe to be the second
pair; and this is Mr. Dana's opinion. In my former description of these
very singular and important organs (Pl. 30, figs. 4 and 8), I have
fallen into some considerable mistakes: the two plates or segments
(fig. 4, N), of which the posterior margins are inflected as apodemes
(_n_), carrying the eyes, are certainly, as may be clearly seen in the
pupa of Alcippe, Pl. 23, fig. 16, and as affirmed by Burmeister,[62]
the basal segments of the antennæ. The second or main segment
(formerly called by me the basal segment) has in some species an upper
portion of the membrane of which it is composed, next to the body,
excessively thin, and separated from the rest of the membrane composing
the segment, by an oblique line (fig. 8, _o_), which I mistook for
its articulation with the body.[63] We then come to the disc or third
segment; and lastly to the fourth, or ultimate segment. This ultimate
segment, generally, has its external corner projecting up, as a
step; and this sometimes, as in _Dichelaspis Warwickii_, gives the
appearance of its consisting of two segments; but a careful examination
of this part in Ibla, in which the step-like structure is carried to
an extreme, makes me believe that there is only one segment.[64] The
prehensile antennæ, therefore, like the natatory legs, are formed of
four consecutive segments, of which the basal segments give rise to the
singular apodemes, presently to be noticed (fig. 7), that carry the
great compound eyes. This basal segment, in all Cirripedes, is moulted
with the eyes, the three other segments invariably remaining cemented
to the surface of attachment.

    [62] 'Beiträge zur Naturgeschichte der Rankenfüsser,' p. 19.
    In tab. 1 of this work there are good drawings of the general
    structure of the pupa of a species of Lepas, probably _L.
    australis_. I believe this author was the first who made out the
    structure of the abdomen of the pupa.

    [63] In the table of measurements of the antennæ of the several
    genera and species of the Lepadidæ (p. 286) of my former volume,
    the articulation, called by me _basal_, I now know to be really
    the articulation between the basal and second segment. In the
    fourth column, headed "Length from end of the disc to the inner
    margin of the basal articulation," the term inner margin really
    applies to the oblique curved line separating the thin and scarcely
    visible membrane from the thicker membrane of that segment. These
    corrections do not in the least affect the object for which the
    table was given.

    [64] In a sketch, sent me by Mr. Dana, of this organ in the pupa
    of a Lepas from the Antarctic Ocean, I observe that he divides my
    ultimate segment into two segments.

In the Southern Atlantic I took some specimens of the pupa of
_Lepas australis_, not yet attached, and therefore with the muscles
of the antennæ, not having suffered any of that absorption, which
they undergo, as soon as the pupa is permanently cemented to some
floating object. In my former volume I noticed a pair of strong
muscles, attached to the tips of the middle forks (Pl. 30, fig. 7)
of the apodemes, and I now find two pairs attached to the bases of
the two outer forks, and directed dorso-anteriorly; and two other
pairs, also attached to their bases, but directed dorso-posteriorly,
so that altogether there are five pairs of muscles attached to the
apodemes; their chief function, I should think, was to draw the antennæ
posteriorly and upwards within the carapace; but as the apodemes cannot
be moved without the great compound eyes being likewise moved, the
muscles probably serve a double purpose. When the pupæ were alive, I
noticed that their eyes were constantly kept in a state of vibratory
movement. Flexor and extensor muscles are attached at one end to the
posterior margin of the basal segment, and at the other end to the
second or main segment; other powerful muscles attached to this latter
segment, are prolonged by ligaments into the disc. In Cryptophialus I
observed that the disc-segment had a movement almost like that of the
wrist. Whether any muscles enter the small terminal segment, I know not.

The drawing in Pl. 30, fig. 8, of part of the second segment, of the
third or disc segment, and of the fourth or ultimate segment, in
_Lepas australis_, is, I think, very accurate. The second segment
articulates on the upper or dorsal surface of the disc, and has the
articulation on one side constricted and formed of thin and flexible
membrane; the little terminal segment, which is turned outwards at
right angles, also, articulates on the disc. That the disc forms a
true segment is shown clearly in Cryptophialus (Pl. 24, fig. 18),
where the articulation with the second segment is not in such close
contact. The disc is either circular, as in Lepas, or hoof-shaped, as
in Ibla: in _B. balanoides_ the disc is rather hollowed out on the
inner side. It has the power of adhering even to so smooth a substance
as glass, placed vertically. It is surrounded by a rim of transparent
membrane. On the hinder margin some spines arise from the central and
more opaque part: in _Lepas australis_, there are no less than seven of
these spines (fig. 8): in _Conchoderma virgata_ there are only four,
in _Scalpellum_ and _Ibla_ only one. When the disc is placed on the
surface of attachment, these spines lie parallel to it. The middle
part of the disc is, almost always, nearly opaque; and in tracing
the cement-ducts from within the body of the pupa, or of the young
Cirripede, I in many cases traced them as far as this point, but here
lost them. From this same obscure central part of the disc, in most,
if not in all species of the Lepadidæ, spokes radiate, which sometimes
are branched, and are not regular, not always even resembling each
other on the opposite sides of the same individual. Round the proper
membranous border of the disc, a second one may be observed (fig. 8,
_p_), which differs in shape and extent in different specimens: under
favorable circumstances, and very high powers, it may be seen to
have a reticulated structure, and to be of a very pale brown colour;
towards the exterior margin, the reticulations become finer, and are
blended together and lost; on the inner margin, the substance forming
this membrane may be seen to come out of the spokes. This substance
is the cement, which has the power of adhering to whatever substance
it grows against; and thus the disc of each antenna becomes cemented
down, and soon both the antennæ are surrounded by a common border of
cement, which gradually increases, after the metamorphosis, in extent.
Occasionally the cement forms little projections, like short spines, on
the edges opposite to the orifices of the spokes.

The small terminal segment usually bears on its truncated extremity
six spines, some of which are occasionally hooked; in Scalpellum, two
spines, rather longer than the others, are borne on a step some way
down on the inner side of this segment; but in Lepas, two spines (fig.
8), very much longer than the others, arise from the outer corner
of the extremity. These two are very different from the other four
borne by this segment, or indeed any other spines on the body; for
they are quite flexible, and are furnished with a double row of very
long, straight, excessively fine hairs, which seem to be articulated
on them--the whole presenting a very beautiful appearance. These
spines are of considerable length, and in _Conchoderma virgata_ they
even equal in length the whole antenna. I can hardly doubt that these
beautiful, plumose, flexible spines, into the thick bases of which the
coloured corium could sometimes be seen to enter, serve as feelers.
Owing to the facts immediately to be mentioned, I erroneously stated,
in my former volume, that there were three long spines.

In three species of Lepas, in _Dichelaspis Warwickii_, and in
_Scalpellum Peronii_, after having torn the lately-cemented antennæ
from the surface of attachment, I observed proceeding from the end of
the terminal segment, from between the above two groups of spines, what
appeared to be a long narrow ribbon with its end torn off; and which,
in the case of _Lepas australis_, I fancied was one of the plumose,
long, flexible spines ripped open. But now that I have examined some
of the pupæ of this species before their attachment, I find (as
represented in Pl. 30, fig. 8, _v′_) a flattened tube, ending in a
blunt point, and having a peculiar ringed structure. I have noticed
similar appendages to the antennæ of specimens just attached of _Lepas
anatifera_. In the Dichelaspis and the Scalpellum, the tube was very
long, and seemed, from its torn appearance, to have been firmly
attached to the supporting surface. In both these cases, the tube came
out from within another slightly larger tube, which had been broken
off close to the extremity of the terminal segment of the antenna. In
the case of the _Lepas anatifera_, the tube expanded a little after
leaving the antenna. In the Dichelaspis, it had exactly the same
diameter as the cement-duct, which could be clearly distinguished
within the two lower segments. From these several facts, and from the
peculiar appearance of the tube itself, I believe it to be a tube of
cement-tissue which thus, sometimes even before the pupa is attached,
independently grows outwards. That the cement-tissue can grow outwards
and assume definite forms, we know from the singular case of _Lepas
fascicularis_, in which the cement proceeding from several apertures,
forms a vesicular float round the peduncle of not only a single
individual, but often of a group of specimens: we shall presently find
a somewhat analogous fact in the case of Coronula. It is possible
that this tube, proceeding from the extremity of the antenna, may be
the channel through which cement continues to be poured forth during
the continued growth of the above Cirripedes; and the manner in which
this is effected, considering how firmly the end of the peduncle is
cemented down, has always appeared to me a difficulty. In those pupæ
of _Lepas australis_, which I caught swimming about unattached, it is
surprising that the disc should have been edged with cement, and that a
tube, similarly formed, should have grown out of the ultimate segment:
it shows, I presume, that the cement-tissue will grow out, whether or
no the pupa has succeeded in finding a proper object for attachment.
Lastly, I have felt some surprise, in two or three instances in
observing some dark purple pigment-cells, like those in the corium,
within the terminal tube of cement; and likewise within the spokes of
cement in the disc: this is the only fact which causes me the least
doubt, whether I have rightly determined the nature of the terminal
tube, as wholly formed of cement tissue; or whether it may not be
covered by an outer integument, itself lined by true corium, coloured
purple.

Finally, I may add, that, excepting in small details, the prehensile
antennæ present no difference throughout the Order: I have minutely
examined them in several genera of the Lepadidæ; and in the Balanidæ,
I have seen them in Coronula, and in several species of Balanus. In
_B. balanoides_ I have examined them carefully; they are smaller and
thicker than in Lepas, with the second or main segment bowed outwards,
carrying its usual single spine; with the disc excised on its inner
margin and apparently without the spoke-like vessels for the cement;
and with the ultimate segment proportionably longer, and carrying,
I believe, six spines, of which two appeared to be longer and more
flexible than the other four shorter and somewhat hooked spines. In
_Coronula balænaris_, also, the terminal segment is, proportionably
to the others, of large size. Not only throughout the order, but
throughout the whole Class, the antennæ are singularly uniform in
structure, as will be seen, when the last two orders are described.


_Eyes._--These present no difference, except in size, throughout the
class; and have been sufficiently described in my former volume. The
true basal segments of the antennæ (incorrectly designated formerly as
sternal plates or segments) are separated from each other by a deep
fold; and are separated from the edges of the carapace on each side
by a crest and slight fold (Pl. 30, fig. 7, _c_; and 4); these folds
and crests die out posteriorly, and disappear. The hinder, rounded
margins of the basal segments are inflected inwards, and their corners
are produced far up into the body, thus forming the curious UU-like
apodemes. These apodemes exist throughout the whole class; and the
outer arms always carry the great compound eyes. I noticed, in _Lepas
pectinata_, that the two middle arms are proportionably longer than in
_L. australis_. Owing to the presence of these apodemes, and to certain
coloured marks on the adjoining corium, the eyes, though enclosed
fairly within the carapace, yet deceptively appear pedunculated,
so that even J. Vaughan Thompson was thus deceived. I have already
described the several muscles attached to these apodemes, and the
constant vibratory movement of the eyes. Whilst the pupa remains a
freely swimming animal, the eyes are included, not only within the
shell or carapace, but (as would naturally happen) within the corium or
true skin lining the carapace; but after the pupa has become attached,
preparatory to its final metamorphosis (in the state represented at Pl.
30, fig. 2.), not only are the muscles, as before remarked, which are
attached to the apodemes, absorbed, but so is the corium investing the
apodemes and the immediately adjoining parts of the carapace. Hence it
comes that the new corium of the young Cirripede within, is formed in
a deep transverse fold across the whole lower half of the animal, and
the apodemes with the eyes are thus, as it were, rejected from within
the corium, though still remaining within the carapace. Consequently in
this final stage, the eyes and apodemes are very conspicuous from the
outside, being seen only through the transparent carapace. I presume
that the eyes at this period have become functionless, with the optic
nerve divided and absorbed. The eyes, apodemes, and carapace soon
afterwards are all moulted together.

The eyes of Cirripedes certainly undergo a remarkable series of
changes: in the larvæ in the first stage, there is a single eye,
perhaps formed by the confluence of two eyes, occupying the normal
position in the front of the head: in the second stage, according to
Burmeister, the eye has become double, but the two are as yet simple;
they are now situated posteriorly to the second pair of antennæ: in
the third or pupal stage, they remain in the same situation, but have
become compound, of great size, and are attached to the apodemes of
the antennæ: in the mature and fourth stage, they have moved someway
posteriorly, and again have become simple, of minute size, and are
either confluent, as in the Lepadidæ, or tolerably far apart, as in the
Balanidæ. It must not be supposed that the eye of the mature Cirripede
is metamorphosed from the eye of the pupa, for such is not the case;
the new eyes and old eyes being formed someway apart, and frequently
both can be seen within the pupa (as in Alcippe, Pl. 23, fig. 16) at
the same time. It is scarcely possible that the eye of the larva in the
first stage, can be changed into the double eyes of the second stage;
though these latter may possibly be multiplied into the eyes of the
pupa, as both continue to occupy nearly the same position.[65]

    [65] Zenker, in his 'Physiological Remarks on the Daphnidæ,'
    ('Journal of the Microscopical Society,' 1853, p. 274), speaks of
    a "tripartite azygous eye" as common amongst Crustacea, and as
    occurring "in conjunction with the aggregated eyes in Artemia,
    Argulus, &c.; but as appearing regularly in all the Branchiopoda
    and Siphonostomata as the _earliest_ visual organ." Hence I
    conclude that this azygous eye is the homologue of that single eye
    which appears in the earliest larval stage of Cirripedes; and that
    the compound eyes of the cirripedial pupa, answer to the aggregated
    eyes of Artemia and Argulus, &c., with the difference, that in
    these latter genera the single eye is retained. See, also, Von
    Siebold, 'Anatomie Comparée,' tom. i, p. 435.

_Mouth_, _thorax_, _limbs_, _abdomen_.--I have nothing to add regarding
the mouth, except to confirm my former account; viz., that it is
functionless, consisting merely of crests, which project inwardly
between the gnathites of the young Cirripede, and of a shrivelled
closed tube representing the œsophagus. In fact the mouth is a model
of the outside of the mouth of the young Cirripede. I may remark that
some little way beneath the membrane answering to the labrum, a pair
of ligamentous apodemes, the use of which I do not know, slightly
penetrate the body. The degree of prominence of the mouth varies, but
it is far less than in the mature animal. On the limbs I have nothing
particular to add: the drawing of the first pair of legs (Pl. 30, fig.
5) is, I think, very accurate: I observed all the spines here figured,
on the corresponding leg of the pupa of _Balanus Hameri_. The five
posterior pairs of legs differ only in the outer ramus having five
plumose spines, instead of four, and one short simple spine at the
exterior angle, making six altogether. The legs, in their natural
position (fig. 2), have only the terminal segments of their two rami
directed posteriorly; and as a consequence the spine (close to _i_
in fig. 5), borne on the penultimate segment of the outer ramus, is
directed in the same line with that segment and with the pedicel,
namely, anteriorly, and at right angles to the natatory plumose spines.
This short spine acts, I imagine, as a defensive weapon; it has been
omitted in fig. 2. Of the thorax I need not give, from my notes, any
more details. The abdomen (fig. 6) is similarly constructed, as far as
I have seen, throughout the order, with the exception of Alcippe (Pl.
23, fig. 17), in which it is composed of only one segment instead of
three. In this genus the caudal appendages likewise consist of only one
segment, with very short spines. In the pupa of _Balanus balanoides_,
the three spines borne on each caudal appendage are very much more
unequal in size than in the pupa of _Lepas australis_, although in the
latter (fig. 6) the inner spine is considerably thicker than the two
outer. Whether the three segments of which the abdomen is composed, are
the three anterior or three posterior, of the normal seven segments,
I know not: on the view that they are the three posterior segments, I
presume, according to analogy, that the caudal appendages are borne on
the penultimate segment, and that the ultimate segment is here quite
aborted.

On the _internal viscera_ I have nothing to add. The cement-duct is
represented in Pl. 30, fig. 2, _t′_, on the near side, running into
the antennæ; and I repeatedly traced it, for the duct is very strong,
as far as the disc segment; at the other end it joins the cement-gland
(_t_) on the same side of the body. This cement-gland is proved, by
the clearest series of facts, to be converted into the incipient
ovaria and ovarian cæca. The cement-glands in the older pupæ could be
traced as far as the cæca of the stomach, exactly where the ovaria lie
in the mature animal; but in some young pupæ, they extended further
posteriorly, past the mouth, between the outer and inner membranes
of the overlapping carapace. I have faintly shown the course of the
stomach, with its two cæca at the upper end; the anus lies between the
caudal appendages, at the extremity (above _b′_) of the abdomen. At
this age there is no trace of the vesiculæ seminales, so conspicuous in
the mature Cirripede.


_Young Cirripede, whilst within the pupa._--I repeatedly succeeded in
dissecting the young _Lepas australis_ out of the pupa; and by the
previous action of boiling potash, and by a strong light, I was enabled
to make a camera sketch (Pl. 30, fig. 2) of the relative positions of
their several parts. The young Cirripede is drawn very faintly, and is
best seen by holding the plate in the same position with the mature
animal, of which a section is given in my volume on the Lepadidæ, Pl.
9, fig. 4. I may just notice how complicated are the membranes in a
longitudinal section taken at this period: we have, 1st, beginning at
the back, the horny tissue of the carapace or bivalve shell of the
pupa; 2d, the primordial valve (_z_, in fig. 3) of the young Cirripede;
3d and 4th, two folds of corium; 5th, the membrane of the sack of the
Cirripede; 6th, the membrane of the sack of the pupa; 7th, the outer
tunic of the thorax of the pupa; 8th, the outer tunic of the thorax
of the young Cirripede; 9th, the corium lining the latter membrane;
and these nine membranes would be repeated on the opposite side of the
section, if it were taken through either side of the shell or carapace,
bordering the orifice.

After the exuviation of the outer membranes of the pupa, certain
pre-existing coloured marks in the corium, such as those round the eyes
and round the acoustic orifices, along the ridge of the back and on the
borders of the orifice, &c., are still retained by the young Cirripede,
either temporarily or permanently; so that the correspondence of part
with part of the external surface admits of no doubt. Moreover, the
three terminal segments of the antennæ are invariably retained by
the young Cirripede, though in a functionless condition, and into
them the outer membrane of the body, and an important organ, viz.,
the cement-ducts are still prolonged; hence these prolongations must
be considered as aborted antennæ. Again, we have seen that the mouth
of the young Cirripede is formed under the rudimentary mouth of the
pupa, with the new œsophagus, round the old œsophagus, leading into
the same alimentary canal. The twenty-four extreme tips, likewise,
of the six pairs of biramous cirri of the Cirripede are formed
within the twenty-four extremities of the six pairs of biramous,
natatory legs of the pupa. Consequently, in the Cirripede and pupa,
thus far, part corresponds with part, notwithstanding that new eyes
are formed posteriorly to the old eyes, and new acoustic organs in a
quite different position from the old ones; but now we come to a most
important diversity in the metamorphosis, or rather to follow Professor
Owen,[66] in the metagenesis, of the young Cirripede. Although, as
just stated, the extremities of the cirri are formed within the legs
of the pupa, yet, from the great length of the cirri, they occupy more
than the whole of the thorax of the pupa; so that the thorax of the
young Cirripede is not formed within the pre-existing thorax of the
pupa, but within that part of the pupa, (homologically a portion of the
first three cephalic segments), which lies anteriorly to the thorax.
As a consequence of this, the pedicels and lower portions of the
cirri, the segments of the thorax and its dorsal surface, all come to
occupy a position at nearly right angles to that of the corresponding
parts in the pupa: this is shown in Pl. 30, fig. 2. And as a further
consequence, (and this is the more important point), the sack, which
both in the young Cirripede and pupa is formed by the overhanging and
produced portion of the carapace, and which is _internally lined by a
reduplication of the membrane of the thorax_, is necessarily, owing to
the changed position of the thorax, altered in extent and carried much
further; namely, from extending merely parallel to the longitudinal
axis of the pupa (from _b_ to _b′_), it is now in the young Cirripede,
in addition, carried (to _s′_) almost quite across the inside of the
animal. Hence it comes that the young Cirripede is, as I have said in
my former volume, internally almost intersected; and its body remains
attached only by a small space, (see the broken line, round _a_ and _b_
in Pl. 25, fig. 1, of a Balanus with the shell, &c., removed from one
side), to the sternal or ventral, inner surface of the carapace,--the
carapace being modified either into the capitulum and peduncle, or into
the shell with its operculum and basis. As a still further consequence
of this change of position of the body of the young Cirripede within
the body of the pupa, the alimentary canal becomes shortened to fully
half its former length. At the same time, the interspace between the
mouth and first pair of legs of the pupa, (consisting of the seventh
and eighth segments of the archetype), is quite lost in the Cirripede
by coalescence. The final cause of the thorax of the young Cirripede
not being developed within the thorax of the pupa, probably is, that
the cirri may be formed of considerable length, so as to be immediately
enabled to seize prey; and that the thorax, which supports the cirri
(and this probably is even more important) should be as free as
possible within the sack, so as to aid the captorial action of the
cirri.

    [66] 'Parthenogenesis,' pp. 13 and 26.

After these remarks, more especially with regard to the formation
of the sack, if any one will look at the sectional drawing of a
pedunculated Cirripede in my former volume, or of a sessile Cirripede
(Pl. 25, fig. 1) in this present volume, in which latter the shell adds
to the complexity, he will perceive the cause of the extreme difficulty
in understanding the relative position of the parts throughout the
whole class. Even after I had discovered that the prehensile antennæ
of the pupa might always be found in the centre of the basis or
surface of attachment, and which fact, it might have been thought,
should have convinced me that this was the anterior end of the whole
animal, yet still I fancied that the prominent mouth represented the
entire head, and that the shell was something quite distinct. It is
clear that others have been equally perplexed; for that which is the
anterior end in the eyes of one naturalist, is the posterior end in
the eyes of another; so with the dorsal and ventral surfaces: one
naturalist considers the peduncle of the Lepas as the abdomen; another
considers it as a pair of metamorphosed, thoracic limbs, &c.! The
probable position of the segments of the body of a mature Cirripede,
in relation to the three anterior cephalic segments, or carapace, is
shown in the diagram (Pl. 25, fig. 6) of the supposed position of the
mature Proteolepas within its pupal envelopes. Here, in the diagram,
the two segments immediately succeeding the mouth (_c_), which are the
seventh and eighth of the archetype, (for the mouth consists of three
segments, and all in front of the mouth of three other segments),
have come to adhere by their dorsal surfaces to the internal surface
of the carapace,--that is, of the first three segments, which ought
of course to have stood quite in advance of these two segments, and
these two segments again ought to have stood in advance of the mouth.
The mouth is directed posteriorly, instead of from the body; and the
three segments of which it is formed (closed at their anterior end by
the labrum), and are very small compared to the relatively monstrously
great, three anterior cephalic segments, composing the carapace. To
place the segments of the body of Proteolepas in proper sequence, in
respect to those of the carapace, and in accordance with the sequence
of the archetype Crustacean, it would be necessary, by seizing the
extremity of the abdomen (_a_), to tear the two segments succeeding
the mouth from their dorsal attachment, as far back as the basal
margin of the labrum; and then pull them till they stood posteriorly
to (or in the diagram, above) the mouth; which latter part would,
by the same movement, be made to project out at right angles to the
ventral surface, and would then be preceded only by the first three,
great, confluent segments of the head, which being produced backwards,
form the carapace. All that has just been said on the position, in
Proteolepas, of the segments of the body in relation to those forming
the carapace, I believe to be applicable to all ordinary Cirripedes,
with this difference, that in the latter, after the metamorphosis,
the two segments succeeding the mouth quite disappear on the ventral
surface, and dorsally are either aborted or have coalesced with the
adjoining segments.

_Act of Metamorphosis._

When the due time for the act of metamorphosis has arrived, the pupal
carapace splits along the dorsal ridge, and is cast off, together with
the acoustic sacks, the basal segments of the two antennæ, and the
great, black, compound eyes, hanging to the UU-like apodemes. The three
terminal segments of the antennæ invariably remain cemented to the
surface of attachment. The exuviæ usually continue for a time united to
the cemented antennæ, but are finally washed away. Besides the split
along the dorsal ridge, the carapace separates, all round the orifice,
from the delicate tunic lining the sack and investing the thorax and
natatory legs of the pupa; for these membranes are not moulted for some
considerable time afterwards. Hence all these inner parts retain for a
period the appearance and structure of the natatory pupa, whilst the
exterior resembles, in every respect, a fixed and perfect Cirripede.

In my former volume, I have insisted on the important and curious
results which ensue from the eye-apodemes penetrating so deeply into
the body (see Pl. 30, fig. 7, in which the proportions are more correct
than in fig. 2), with the eyes attached exteriorly to their outer
arms; for as these apodemes have to be ejected, the external membrane
of the young Cirripede (Pl. 30, fig. 2) has to be formed in a deep
fold or arch over them, and consequently the membrane on the sternal
surface is formed considerably longer than on the dorsal surface. From
this it follows, when all the membranes are free and are stretched
fully out after the moult, that the whole animal, posteriorly to the
cemented-down surface, turns vertically up, and assumes its normal
position at right angles to the surface of attachment, and to that
which it held in its pupal condition; for the pupa always adheres with
its sternal surface parallel to the surface of attachment. A young
Lepas, which has just moulted its pupal carapace, and has assumed its
proper vertical position, with the cemented antennæ and the surface of
attachment remaining as before, is shown at fig. 3, but is drawn on a
smaller scale than the pupa fig. 2, out of which it may be supposed
to have been excluded. In this fig. 3, it may be observed that the
natatory legs and caudal appendages of the pupa have not as yet been
moulted. The fact of the stretching out, in the young Cirripede, of the
fold of membrane, which in pupa, just before the metamorphosis passes
over the apodemes and eyes, is well shown by three darkly-coloured
bands in the corium, which in the pupa are curled, but after the moult,
are stretched straight out on the peduncle of the young Lepas.

The pupa, and consequently the young Cirripede, from being attached
at first by the antennæ, does not adhere by the actual anterior
extremity, but by the sternal surface near it; the anterior extremity,
however, soon becomes cemented down, and afterwards, in ordinary
cases, ceases to grow. In Cryptophialus, however, and in certain
genera of the Lepadidæ, as Alcippe, Lithotrya, and Anelasma, the
anterior or basal extremity does continue to grow, and is not cemented
down, and therefore comes to be prolonged beyond the original point
of attachment; in order to allow of this, the surface to which the
Cirripede is attached has to yield, apparently simply to the pressure
exerted in the case of Anelasma, but in the three other genera, to
the rasping action of the roughened surface of the extremity of the
peduncle.

When after a period the pupal membranes of the sack, thorax, and
natatory legs are moulted, the cirri of the young Cirripede are
curled up, and its thorax is raised towards the orifice, and we have
the animal in its ordinary position, and perfect with the exception
of a few parts to be further developed or modified. For, instead of
calcareous valves, we have at this period only the so-called primordial
valves, composed of chitine; and in the case of _Lepas australis_,
some minute spines and some coloured marks on the peduncle, which soon
disappear. The muscles, which enter the three terminal segments of the
antennæ in the pupa, have to be absorbed and converted into ligamentous
threads. In Lepas, the labrum has to become bullate; and the cæca have
to increase in number round the upper end of the stomach, and their
dark colour and that of the whole alimentary canal has to disappear or
be much weakened. The filamentary appendages at the bases of the cirri,
which generally contain some of the testes, have to be developed.
The probosciformed penis, which at first equals only the pedicels of
the posterior cirri in length, and is apparently imperforate, has to
increase greatly in length. The testes and vesiculæ seminales have to
be formed. And lastly, and this is a more important point, the two
gut-formed cement-glands (or incipient ovaria, _t_, fig. 2, Pl. 30)
which, at the period of the moulting of the carapace and eye-apodemes,
and when the whole animal was upturned, came to occupy, together
with the cement-ducts (_t′_), their normal position, _i. e._ nearly
parallel to the sternal surface, now undergo further changes. Their
upper and posterior ends lying near the cæca of the stomach, increase
in size, but retain nearly the same character, and thus form the two
true ovaria; their middle parts become emptied of their cellular
contents, and are converted into the two simple ovarian tubes; and
their lower ends branch out, inosculate, and form the inextricable mass
of ovarian tubes and cæca. The points of junction on each side between
the two cement-ducts and the newly branched ovarian tubes, become now
developed into the two cement-glands. The cement-ducts, which continue
throughout life growing, either still enter the old antennæ and there
pour out the cement-tissue, or they pour it out through special
orifices formed for this purpose in the lower part of the peduncle. The
changes, supervening during the metamorphosis, in the ovaria and in
the cementing apparatus, here described, I have no doubt are general
throughout the Order.

I have above alluded to the _primordial valves_; these are beautiful
objects when viewed under a high power: they are composed of chitine
without a trace of calcareous matter, but prefigure in shape, size,
and direction of growth, the shelly valves soon to be formed under
and round them. They are composed of an outer membrane, with its
margins separated by yellow thickened rims from the membrane uniting
the several primordial valves together; and this outer membrane is
underlaid by a single layer of generally hexagonal, thickish cells (Pl.
30, fig. 3 _a_), varying from 1 to 2/6000th of an inch in diameter.
These cells seem to contain a nucleus; and they are at first separated
from each other by clear interspaces. If a specimen be taken, only
a little before the formation of the calcareous valves, one or more
layers of membrane, marked by an hexagonal reticulation, can be
separated from the lower surface of the main hexagonal network. It
is a singular fact, that in those genera in which there are several
valves, the primordial valves occur only on five, namely, on the two
scuta, two terga, and the carina; and these are the most persistent
valves in the several genera. The other valves are prefigured only by
brownish membrane, without the hexagonal tissue. In the mature _Lepas_,
the membrane connecting the several shelly valves is not moulted, but
disintegrates; in the primordial valves, however, which stand far
separate from each other, this membrane is moulted; and immediately
after the first moult, the first layer of shell appears under and
a little way beyond each primordial valve; shelly matter likewise
appears, at least in some cases, between the cells of the hexagonal
tissue. The young shelly valves are connected together, at each
successive moult, by narrower strips of membrane, till, in the case of
Lepas, the valves when mature come to touch each other (Lepadidæ, Pl.
1, fig. 5). The primordial valves are often preserved for a long time
on the umbones, or centres of growth of the five valves, on which they
occur, in the same manner as the larva-shell is sometimes preserved on
the apex of certain spiral molluscs. Had not Cirripedes gone through
so many and such complicated metamorphoses, this last state, when
furnished only with primordial valves and with several internal organs
only partially or not at all developed, would have deserved to have
ranked as a special stage, and not as merely subordinate to the last or
pupal condition.

In the Balanidæ, or sessile Cirripedes, the young animal, immediately
after the metamorphosis, or still better if dissected out of the
pupal carapace, as I succeeded in doing with _Balanus balanoides_,
may be said to be pedunculated; for it is attached by a little disc
of cement closely surrounding the antennæ, the rest of the membranous
basis forming an almost semi-globular, flexible peduncle. The valves,
at this the earliest period, are all membranous, and do not overlap
each other. In the Balaninæ they do not present the peculiar structure
of the primordial valves of the Lepadidæ; but in the Chthamalinæ, in
Chthamalus, I saw traces of this structure. Calcareous valves are
soon formed under the membranous valves. The opercular valves, at
this early period, are much larger than the valves or compartments
of the shell, which are only four in number, for the carino-lateral
compartments are not yet formed. The compartments from the first are
surprisingly strong, and have their alæ already formed and overlapped
by the adjoining compartments; but of the radii there is as yet no
trace. The four compartments form a narrow but nearly circular hoop,
which, from its relatively large diameter, tends to draw down the
upper or posterior end of the animal, now forming the opercular valves;
and as the basis soon becomes throughout cemented to the surface of
attachment, the young Cirripede is much depressed. Soon the opercular
valves are drawn a little way down within the shell, becoming attached
to the sheath, instead of, as at first, to the very summits of the
compartments. In regard to the changes which take place in the shell,
in the number of the segments in the cirri, and in the number of spines
borne on these segments, &c., during the continued growth of the
animal, as they are chiefly important for the identification of the
species, I will here refer to a discussion on this subject under the
head of the Genus Balanus.


_On the Homologies of the Carapace and Shelly Valves._

In the pupa, the carapace is produced, not only posteriorly, but
anteriorly, so as to cover the entire animal, with the exception of a
narrow sternal surface (Pl. 30, fig. 4): in front it is notched, where
the sternal surface terminates, and from this notch a faint line runs
along the dorsal surface, separating its tergal elements. In the young
Cirripede, after the metamorphosis, there is no trace of this medial
dorsal suture, or of the wider sternal surface. Looking at the several
genera of the Lepadidæ, the external covering of the whole peduncle and
capitulum is so continuous and of so uniform a nature, that I think we
must consider the whole as a carapace, of which the sternal borders
have become completely confluent; formerly I was inclined to look at
the capitulum alone as formed by the carapace, and at the peduncle as
being composed of the two or three anterior cephalic segments, cased
only by their own integuments. As far as can be discerned, the carapace
in the pupa, and consequently in the Cirripede, consists only of the
tergal elements of the segments; and this seems likewise to be the
case with the carapace of the Podophthalmia. Until lately,[67] Prof.
Milne Edwards doubted whether the carapace in the higher Crustaceans
(to which I believe the carapace of Cirripedes must be compared) was
formed by the backward production of the third segment, which bears
the second pair of antennæ, or of the fourth, _i. e._ the mandibular
segment; but from the distribution of the nerves, he now argues that
it must mainly belong to the third segment. In Cirripedes, the course
of the nerves leads to the same conclusion; for the whole shell, sack,
and peduncle are supplied with nerves proceeding from the compounded
ganglion, which belongs to the second and third cephalic segments.[68]

    [67] Compare 'Histoire Naturelle des Crustacés,' tom. i, p. 27,
    with 'Annales des Sciences Nat.,' 3d series, tom. xvi, 1851, p. 233.

    [68] This conclusion is supported by the structure of Proteolepas:
    in this Cirripede there is not a vestige of a carapace, and as
    the whole of the animal in front of the mouth is almost utterly
    aborted, being reduced to a mere covering to the two cement-ducts,
    and as, on the other hand, the mouth with the mandibles, though
    peculiarly modified, is not at all aborted, there is a strong
    probability, that the abortion of the carapace is connected with
    the aborted state of the three anterior cephalic segments; and that
    the carapace in its origin is not any way related to the fourth or
    mandibular segment.

The whole of the head in front of the mouth, together with the
carapace, is, as we know, formed of three segments; and each of these
segments, homologically, ought to consist of eight elements; I recall
to mind these facts, inasmuch as the transverse separation between the
peduncle and capitulum in the Lepadidæ, and between the basis, the
shell, and the opercular valves in the Balanidæ, might be thought to be
connected with the separation of the three cephalic segments. So again,
as in the Balanidæ the shell normally consists of eight compartments,
these might be thought to be related to the eight elements of one or
other of the three segments. But I see no reason for admitting this
view; and in the case of the carina and rostrum, two of the most
persistent and important of the compartments, they exactly cover the
sutures which ought to separate the two tergal and two sternal elements
of the segment. The valves, moreover, often form many more whorls
than three, or the number of the true cephalic segments in front of
the mouth; and in each whorl the valves tend to stand in tile-like or
alternate order, with respect to those in the whorls both above and
below, which would not be the case, if they were the eight elements of
the segments.

For the true homologies of the sclerodermic plates, or Shelly valves,
with which the external covering of Cirripedes is so generally
strengthened, we must, I believe, look to the carapace of the
Podophthalmia. In these latter, we find the carapace composed of
sclerodermic plates, which, though closely joined and only occasionally
separated by sutures, yet in their origin are distinct;[69] they
tend, also, to be arranged in alternate or tile-like order. As the
animal grows, the old sclerodermic plates, all joined together, are
moulted, and new ones, also all joined together, of a larger size,
are formed beneath. Now let us imagine the growth to be more gradual
but yet periodical, and the new and larger sclerodermic plates, when
formed under the old ones, to adhere firmly to them; the older plates
would thus be prevented from becoming confluent, and instead of
being all moulted together, as is now the case, they would be almost
continually separated from each other, owing to the almost continuous
increase in size of the new underlying plates. Consequently lines of
splitting would run between the several plates, however numerous they
might be, instead of there being, as now, a single line of splitting
extending down the back. In fact, we should have the identical manner
of growth of the shell or carapace, which occurs in Cirripedes. It is
on this ground, and from the several points of homological resemblance
incidentally mentioned in the last few paragraphs, that, in the early
part of this Introduction (p. 13), when discussing the whole class, I
stated that I believed that the carapace of Cirripedes presented more
real resemblance with the carapaces of the Podophthalmia, or higher
Crustacea, than with those of the lower Crustacea, though in mere shape
they more nearly resembled the latter.

    [69] 'Annales des Sciences Naturelles,' 3d series, tom. xvi, pp.
    233, 236, 237.


_Cementing Apparatus._ (Plate 28.)

I have already (p. 128) given an account of the manner in which, in the
pupa of Lepas, the cement-tissue escapes from the prehensile antennæ,
and of the structure of the cement-ducts, and of the cement-glands
or incipient ovaria; and likewise of the changes by which these
organs assume their ultimate form in the mature Cirripede. In my
former volume, on the Lepadidæ, I described the cement-glands and the
cement-tissue in several genera, and I have there shown (singular as
the fact is) that the two cement-glands, with their contents, actually
consist of ovarian tubes with their contents (for there seemed to be
a relation in the state of fulness in both) in a modified condition.
In the Balanidæ, I am not able, from the difficulty of the dissection,
to confirm these conclusions, excepting in so far that the tubes on
which the cement-glands are formed, run into the mass of ovarian cæca;
but, I may add, that in the abnormal Proteolepas, belonging to another
Order (see the section, Pl. 24, fig. 1), nothing could be plainer than
that the membrane of the ovarian sack (_b_) formed the cement-ducts,
and that their cellular contents, which within the sack (_a_) were in
process of conversion into ova, within the ducts were converted into
the cement-tissue. This cement, by some unknown power, travels down the
ducts, and debouches at the antennæ.

In the Lepadidæ, there are only two cement-glands, which are situated
high up in the midst of the ovarian cæca, with one cement-duct
proceeding from each: both the glands and ducts increase in size with
the age of the animal:[70] the cement issues either permanently from
the prehensile antenna, or, after a short period, through apertures
in the peduncle, arranged irregularly or in straight lines,--the last
formed apertures being furthest from the central and basal point of
the peduncle. In the Balaninæ, on the other hand, at each period of
growth, a pair of new cement-glands is developed, larger than those
last formed, and making, with the older glands, a chain, connected
together by what I have called the cement-trunk. The cement-trunk
consists of a tube, which generally becomes enlarged just before
entering each gland. The glands, the cement-trunk, and cement-ducts,
all adhere to the basal membrane or basal shelly plate. Each gland
gives rise to two cement-ducts, these often bifurcate, and sometimes
repeatedly bifurcate and inosculate before pouring out their contents
round the circumference of the basis; and sometimes they all first
enter a circumferential cement-duct. The probable cause of the greater
complexity of the cementing apparatus and of the greater number of the
excretory orifices in the Balanidæ, compared with the Lepadidæ, is
that the entire surface of the broad basis, which answers to the whole
peduncle in the Lepadidæ, is firmly cemented down to the supporting
object, instead of merely the basal end of the peduncle. The cement
issues either in a cellular condition, or more commonly as a fine
network, which, at a short distance from the orifices (Pl. 28, fig.
4 _a_, _z_), becomes so fine as to form a sheet or layer: I may here
recall the fact, that in the cement proceeding from the disc of the
antennæ, in some species of Lepas, a similar structure was observed.
The cement itself presents the same transparent, brown, laminated,
structureless appearance, and the same chemical reaction, as described
in my former volume. The cement has the capacity of occupying and
filling up all inequalities in the supporting surface; I have seen it,
when spread over an encrusting Flustra, present an exact model of every
cell; in the case of Coronula, it seems, as we shall immediately see,
to have the power of penetrating into, and even almost blending with
the epidermis of the supporting Cetacean. The last-formed cement-glands
and cement-ducts present a delicate and transparent appearance, and
contain cellular matter; whereas the old cement-glands, and sometimes
the old cement-ducts, are filled with brownish cement, not acted
on by boiling potash. The foregoing remarks are confined to the
sub-family Balaninæ, for I have not been able to examine thoroughly the
Chthamalinæ, and can only affirm, that in Chthamalus and Pachylasma the
cement-ducts repeatedly bifurcate and inosculate, in the same manner as
in the Balaninæ. I will now proceed to describe, in some detail, the
cementing apparatus in the several following genera.

    [70] I had some slight reason to suspect in Pollicipes that new
    cement-glands were successively formed: this is more probable in
    this genus than in the others, inasmuch as it is the most nearly
    related to the Balanidæ.

_Coronula._--The cementing apparatus is here more simple than in any
other genus of the Balaninæ, and I have studied it more carefully.
The basal membrane of _Coronula balænaris_ is figured in Pl. 28, fig.
1 _a_, and must first be described; its relation to the shell will
hardly be understood without looking at the outline of the folded walls
of this species, in Pl. 16, fig. 5. The basal membrane closes the
central circular hollow, and is continuous with rays (not represented
in Pl. 28) extending under the doubled walls and terminal transverse
loops. It has eighteen concave sides, corresponding with the inner
ends of the folded walls, for each of the six compartments is trebly
folded. The membrane consists of successive, conformable slips (_c′_,
_c′_), bordered exteriorly by thickened yellowish rims, and internally
overlapping (when viewed from the inner side) the few last-formed
slips, and then thinning out. The membrane forming each slip is itself
laminated. The middle portion, about 1/50th of an inch in diameter,
is rather opaque, owing to the slips being so close together. Beyond
this central part, the slips suddenly increase in size, but yet have
a different shape from the 18-sided outline, which they ultimately
assume: this difference is owing to the great changes in shape, as
explained under the genus Coronula, which the shell undergoes, when the
walls at first assume their folded structure. The walls are invested
by longitudinally striated membrane (_p_, _p_, _p_, fig. 1 _a_), which
turns in under their basal edges; and this membrane is united with the
basal membrane, by what I shall call the circumferential slip (_b_),
and which is shaded in fig. 1 _a_, simply for the sake of catching
the eye. It is the circumferential slip of membrane which sends
rays under the spoke-like folded walls: thin as it is, this slip is
yet laminated, but is not bordered by thickened edges. The membrane
investing the walls is, like the basal membrane, formed of successive
slips with thickened edges, which overlapping (viewed from the inside)
the last-formed slips, project beyond them, and so face the edges of
the slips in the basal membrane; they are only obscurely indicated in
fig. 1 _a_. The circumferential slip (_b_) lies over (as viewed from
within) both the basal and wall membrane. This whole structure will,
perhaps, be best understood by the sectional diagram (fig. 1 _b_),
in which the letters (_c′_, _c′_) show the slips of basal membrane;
(_p_) the parietal membrane, coating the outside surface of the walls
of the shell, not here represented; (_b_) the circumferential slip
overlying both; and (_z_, _z_) the layers of cement, which may for
the present be disregarded. In order to allow, of the growth of the
shell, the circumferential slip (_b_) periodically splits in the
middle, all round, in a line exactly conformable to the edge of the
last-formed slip of basal membrane; and likewise in straight, medial
lines under the spoke-like (cut off in fig. 1 _a_) doubled walls. I
have seen, under a high power, the line of splitting, very shortly
after its formation, with the two edges ragged and near together, with
an extremely narrow, new circumferential slip just formed, between
and over the edges of the previously formed slip. What causes the
circumferential slip to split so symmetrically, I cannot say: the
opposed edges, after a time, become thickened, apparently from adhering
to the underlying layer of cement, as will presently be described. The
circumferential slip continues increasing in breadth till the period
of its splitting arrives, by which time it has become much broader
than the last-formed slip of basal membrane; and after the splitting
takes place, the interior half towards the basal membrane, forms a
new basal slip all round the basis, and the exterior half adds a new
slip to the membrane investing the walls. This latter membrane being
inflected under the basal edges of the walls, is, during the growth of
their edges, drawn straight down, the newly-formed portion taking the
inflected position.

In the sectional diagram, (1 _b_) the circumferential slip is not yet
broad enough to split; when it has become so, it will split under
the letter (_b_). The slips of basal membrane are, as may be seen in
fig. 1 _a_, narrower towards the circumference; but the two or three
last-formed slips, are out of proportion narrower than the others; and
it is certain, from the comparison of the basal membranes of specimens
of different ages, that these will afterwards increase in width.[71]
I have seen no other instance, in Cirripedes, of growth in membranes,
except at their extreme margins: I suspect that these last-formed slips
are pulled, during the downward and outward growth of the shell, a
little from over the last-formed slips, new and larger laminæ being
all the time thrown down, so as to prevent any fissure being formed. I
also suspect that the gradual increase in width of the circumferential
slip itself, is due to the opposed edges of the underlying and
last-formed circumferential slip being dragged further apart from each
other, new and wider laminæ of membrane being continually thrown down;
the new circumferential slip being thus, also, all the time thickened,
as well as rendered broader.

    [71] In the case of one _young_ shell, I found that the
    previously-formed circumferential slip must have split, long before
    it had assumed its proper and ordinary width; for the last-formed
    slip of basal membrane was of extreme narrowness, and would have
    to be considerably added to in width, whilst the new and narrow
    circumferential slip was likewise being added to in width. This
    slip of basal membrane, though so extremely narrow, had its own
    cement-ducts and glands.

The central slip or rather disc of membrane, is 3/400ths of an inch in
diameter; and this shows the basal diameter of the shell immediately
after the metamorphosis. In the middle of this little disc I saw,
in several specimens, the prehensile, pupal antennæ; I made out
distinctly the ultimate segment with its bristles, and, as I believe,
the disc-segment, which was 7/2000ths of an inch in diameter; but this
portion was much obscured by the quantity of cement. When the corium
is removed from the inner side of the basal membrane, the two chains
of glands, extending from exactly over the antennæ in the centre about
half way towards the circumference, are conspicuous. The cement-trunk,
connecting the glands, is thin, and at the further end is always broken
off, by the removal of the corium and overlying layer of ovarian cæca,
into which the two cement-trunks enter; and without which removal,
nothing could be seen. The two chains of glands form a very large
angle, open towards the rostral end of the shell, as represented at
fig. 1 _c_, much enlarged; by a mistake in fig. 1 _a_, the two have
been drawn in a straight line. The cement-trunk increases in diameter
in proceeding from the centre to the circumference, and the glands
likewise increase in size, at the same time altering somewhat in shape.
From near the lower side (the basal membrane being viewed from within)
of each gland, two cement-ducts proceed, which pour out their contents
beneath the basal membrane. The orifices of the ducts always exactly
face the middle folds of the two lateral, and two carino-lateral
compartments. In a full-sized specimen, there are from thirty-five to
forty cement-glands on each side, always corresponding exactly with
the number of slips of basal membrane, including the circumferential
slip, to which the last-formed pair of glands and cement-ducts belong.
In correspondence with the great number and narrowness of the central
slips of membrane, so are the cement-glands towards the centre numerous
and very small. All the glands, in the more central parts, consist of
a mere transverse enlargement of the cement-trunk; but the exterior
and larger glands, which are more closely packed together, are more
globular or pear-shaped; and the two ducts (of which the one on the
rostral side is considerably enlarged at its base) do not come out of
the gland exactly at the same level. The trunk, connecting the glands,
runs straight from one to the other. The ducts proceeding from the
outer and older glands, on the carinal side, are much curved (Pl. 28,
fig. 1 _c_). To give an idea of the dimensions of the several parts, I
may state that the largest ducts were 3/1000ths of an inch in diameter,
and the glands belonging to them nearly thrice as much, measured in the
direction of the cement-trunk; on the other hand, some of the ducts
from the small central glands had a diameter eighteen times less than
that of the largest ducts. Towards the circumference, the ducts that
proceed from the older and larger glands are piled one exactly over the
other--the last formed being the topmost, and all are imbedded in the
corium which overlies the basal membrane: this position of the ducts,
one over the other (which could not be well represented in figs. 1 _a_
and 1 _c_), is owing to their all debouching at the same exact point.
But the ducts form the smaller and younger glands, when the shell
had a different shape, are spread out, and are all attached to the
basal membrane. Altogether, the basal membrane of Coronula, when well
cleaned, and examined under a moderately high power, presents the most
singular and elegant appearance.

We now come to the cement-tissue: this lies on the under or outer
side of the basal membrane; it is not represented in figs. 1 _a_ or
1 _c_, but only in the sectional diagram, 1 _b_, by the letters _z_,
_z_: it presents its usual character and appearance, like solid glue
or brown gum, but is obliquely laminated and sub-laminated: it forms
a layer, much thicker than the basal membrane itself, being as much
as .004 or .005 of an inch in thickness. It is, however, difficult
to ascertain its thickness, from the singular manner in which it
penetrates into and almost blends with the epidermis of the whale's
skin; so much so, that for a considerable time I thought (not then
knowing anything about the cement of Cirripedes) that this transparent
horny substance probably answered to a corn on the human foot produced
by pressure. But I soon observed that this horny substance certainly
extended into and up the cement-ducts; and this fact first led me to
the examination of the whole subject in the several genera of Lepadidæ
and Balanidæ. It was not difficult to remove the cement-ducts, leaving
small portions of the contained cement projecting freely up as points
from the general surface of cement. The cement adheres slightly to the
whole basal membrane, but chiefly to the yellowish rims or edges of
the successive slips; and it is indeed this adhesion which, I believe,
produces the rims; for the circumferential slip, when first split, had
very thin ragged edges. The cement also extends under the spoke-like
prolongations of the circumferential slip, and likewise some way up the
sides of the walls.

The cement-glands, the trunk, and the ducts, except the two, three,
or even four last-formed ones, are all filled with an apparently
solid thread of transparent, brownish cement, differing in no respect
from the cement under the central parts of the basal membrane. In
one instance, in which the last-formed pair of glands and ducts had
apparently been only just developed, they were so perfectly transparent
that I could distinguish them only under certain lights, and I could
not perceive any contents. The last-formed glands and ducts always
appear very delicate, and include a tube of very delicate tissue,
containing more or less of granular matter. The next succeeding pair of
glands and ducts are always more opaque, and contain much more granular
matter; which, in the next, or next but one, may be seen passing into
the state of pale brown, transparent chitine. I have seen some most
distinct instances, in which, in the same duct, the part towards the
centre of the basis was filled with homogeneous cement, and the part
towards the circumference with still plainly granular matter. In the
successive circular slips of cement-tissue, lying attached under the
circumferential slip and under the two or three succeeding slips of
basal membrane, an exactly analogous series of changes is presented; as
indeed might have been expected, as the slips of cement are absolutely
continuous with the contents of the ducts. Moreover, if a vertical
section be made across one of the last-formed slips of cement, it
may sometimes be seen to be apparently in the act of separating into
layers, with the lower layers more pulpy, elastic, and white than the
upper layers, which are less coherent, and show as yet even still less
the character of cement. The cement under the circumferential and
adjoining slips, often presents a peculiar wrinkled appearance, in
lines conformable with the outline of the basal membrane; but I do not
believe that these are real wrinkles, though so like them; they seem
to consist of sinuous threads, longer or shorter, sometimes slightly
branched, crossing and inter-joined, and composed apparently of faintly
coloured cement. I suspect that these threads are formed by the union
and subsequent drawing out of aggregations of that matter, which within
the ducts is first granular, and then changes into cement; for at the
orifices of the ducts these wrinkled threads sweep outwards in curved
lines on both sides. The cement in these early stages adheres, with
very little force, to the basal membrane; and with only a little more
force to the underlying layers of cement; in fact, till it assumes the
brown translucent appearance, like solid glue, it hardly seems to act
as cement.

How the cement reaches the skin of the whale, will be best understood
by referring to the sectional diagram (Pl. 28, fig. 1 _b_). When the
circumferential slip of membrane (_b_) splits, a new circumferential
slip will be formed over it, together with new cement-glands and
ducts, and cement (_z_, _z_) will issue from four new orifices, and
will extend on both sides of these orifices, till the ends meet and
become united, thus forming a narrow, 18-sided, continuous, new slip
of cement, with 18 spokes proceeding from it. I have not noticed lines
of union in the cement of any one slip; but the matter forming each
slip, certainly has proceeded from four distinct orifices. Seeing how
perfectly successive layers of cement often become blended together,
lines of union or junction, could hardly be expected to be preserved
in the same individual layer. When the circumferential slip of basal
membrane splits, the underlying slip of cement, which we will call A,
does not split, but becomes stretched, so that the newly formed slip of
cement, which we will call B, does not reach the skin of the whale. As
the new circumferential slip of basal membranes goes on increasing in
width, A continues to be stretched, but does not split, till at least
another circumferential slip of basal membrane has been formed and
has been split, and till B has been also stretched. By this time, the
cement-tissue A has assumed its normal structure, and has the power of
adhering to the whale's skin, which it has now reached, owing to the
splitting of the under and older slips of cement. At the next period
of growth, A itself will split, and B will touch the whale's skin and
adhere to it; and this, also, will ultimately split. It results from
this action, that the cement has a stretched, and sometimes even a
fibrous appearance, with the lower edges of the layers, of which each
slip of cement is formed, thinning out. I have before stated, that the
two or three last-formed slips of basal membrane are formed at first
too narrow, and apparently have to be dragged outwards, over each
other; and it is perhaps owing to this circumstance, and to globules of
cement having first adhered to the under surface of the slips of basal
membrane, that these slips are studded beneath with parallel little
vermiform bodies, sometimes of considerable length, and furnished
with heads, all directed outwards. These tapering, vermiform bodies
have a considerable resemblance to the threads before mentioned,
which give the wrinkled, concentric appearance to the newly-formed
layers of cement, and have probably a closely analogous origin: in
one case, indeed, it appeared as if some of these concentric threads
were in process of being drawn out at right angles to their original
course. Lastly, it should be observed, that as the exterior half of the
membrane of the circumferential slip, after each splitting, is dragged
down, and thus comes to invest the outer surface of the wall of the
shell (the wall not being represented in the diagram, but standing
where the letter (_p_) stands), so it must be with the cement, which
thus likewise comes, in an unusual manner, to invest the outer surface
of the folded walls of the shell, and attaches them to the skin of the
whale,--which latter is always growing upwards, and tending to bury the
shell.

_Platylepas decorata._--This genus is closely allied to Coronula,
and the cementing apparatus is essentially similar. In one specimen,
I counted no less than forty-nine slips of basal membrane, each of
which, of course, had its pair of cement-glands, and each of the
latter its two ducts. The glands consists of a transverse enlargement
of the trunk, as in the early stages of Coronula. Neither the glands
nor the duct, when old, become filled up with cement, but only the
main-trunk. The ducts are very delicate and thin; the larger ones being
only 3/10,000 of an inch in diameter. The glands stand some way apart
on the two cement trunks; and the latter, instead of being straight
as in Coronula, are deeply serpentine; the glands are formed on each
bend, so that, though all on one side are connected on the same trunk,
they form a double row on each side of the basal membrane. The basal
membrane (in the centre of which I distinctly saw the antennæ of the
pupa) has six deep bays or excisions, corresponding with the midribs
(see Pl. 17, 1 _a_, 1 _d_) of the six compartments; and the two ducts
from each gland, on the right and left sides, debouch at the heads of
the four lateral excisions, exactly opposite the midribs of the lateral
and carino-lateral compartments. The later-formed glands, owing to
all of them being situated some way apart from each other on the two
cement-trunks, lie further from the centre of the basis than do the
orifices of their ducts; hence the later-formed ducts are directed a
little backwards, or from near the circumference towards the heads of
the deep excisions.

_Tubicinella._--The cementing apparatus is here less symmetrical; but
this, I believe, is chiefly owing to the basal membrane being formed
of successively larger discs (not slips) of membrane, thrown down not
quite concentrically one over the other; each new disc of membrane
seems to cover the last-formed cement-glands and ducts; and there
are as many ducts and glands as there are discs of membrane, all
adhering together. In one specimen, it appeared that normally there
were four sets of cement-ducts, as in the allied genera of Coronula and
Platylepas; but in other specimens, the ducts were distributed very
irregularly. In one case the two cement-trunks extended parallel and
close together, one of them terminating long before the other. I have
given a figure (Pl. 28, fig. 3) of three of the cement-glands, removed
from the basal membrane, together with their ducts. The cement-trunk
(_f f_) seemed to be a little enlarged, and to be crossed by septa, as
it entered the glands (_h_), but I could not make out this structure
clearly enough to be represented. Whilst young, the cement-glands stand
some little way apart from each other; and in the figure given of some
of the last-formed glands, they are hardly separate enough. Each gland
gives out obliquely, on one side, a cement-duct (_c_) which I traced
in several cases to the margin of one of the discs of basal membrane,
where cement issued from it; and on the opposite side, a tapering spur
(_b_), varying in length, which may be called, and I believe really
is, a rudimentary duct. Of these spurs we shall meet many instances
in other genera. The duct (_c_) and the spur (_b_), close to where
they entered the gland, in some specimens gave off, at about right
angles, short blunt points, or rudimentary branches. This duct and spur
correspond, I believe, with the two ducts in Coronula; but besides
these, a duct (_a_) is given off from one end of the gland, from the
surface opposite to that at which the cement-trunk enters. This duct
(_a_) is very singular, from always forming a loop (_a′_), with two
spurs projecting from it: these two spurs occasionally spring from a
common point: I have seen nothing like this structure in any other
Cirripede. This duct (_a_) runs, like the duct (_c_), to the margin
of its own disc of basal membrane, where it debouches. Besides these
ducts, in the best specimen which I examined, there were two other
sets of ducts, which were slightly zig-zag, and at each angular bend,
a mere knob or point, or at most a short branch, was given off; but
this branch seemed never to run to the margin of the basal membrane
or to give out cement; whereas the main branch did give out cement. I
was not able to trace these ducts to their glands. In these zig-zag
ducts, and in the rudimentary points sometimes observed at the base
of the duct (_c_), and likewise at the base of the spur (_b_), we see
the first indication of that tendency to bifurcation, so strongly
characteristic of the cement-ducts in all the genera, excepting those
already described, which are allied to Coronula.

_Chelonobia patula._--The cementing apparatus is here chiefly
remarkable for the thinness and straightness of the main trunk, (_f f_,
Pl. 28, fig. 2), and from the great distance at which the glands stand
apart; had another gland been drawn, it would, on the scale here used,
have stood exactly under the two upper, (_c′ c′_) in fig. 1 _c_. We
here see that the trunk (_f_), before entering the gland (_h_), has an
enlarged portion (_g_); this, I suspect, is a very general structure.
Each gland gives out, on opposite sides, two ducts (_a a_, _b b_),
larger even than the main trunk; and these ducts bifurcate repeatedly,
and inosculate. By this inosculation it is not improbable that all four
ducts, proceeding from the two glands of the same age, may be connected
together; certainly the bifurcating branches from the same duct thus
become repeatedly connected. For the first two or three bifurcations
the ducts decrease very little or not at all in diameter; but nearer
the circumference they become smaller. The ducts, also, proceeding
from the younger and smaller glands, are, of course, proportionably
smaller. In one case I was able to count four bifurcations in the duct
between the gland and the edge of the basal membrane. It follows from
this structure, that the basal membrane, at each period of growth,
is cemented down by cement issuing from several orifices; but we
shall presently find that in other genera the cement proceeds from
many more orifices. In fig. 2 there is represented, by the aid of the
camera, a small portion (from the outer (_a_) to the outer (_b_) being
12/100ths of an inch in length) of the basal membrane, with all the
several cement-ducts adhering to it, which I could distinguish, and
drawn of their proper relative sizes; this figure also shows some of
the bifurcations, but no inosculation happened to be included in the
space here given; the basal membrane itself has not been represented.
In taking a view of a considerable portion of the basal membrane,
especially towards the circumference, some parallelism in the branches
could be perceived; one set of branches tending to run in the direction
of the ray of the circle, and the other set in the line of the
circumference.

_Elminius Kingii._--The cement-glands here resemble those of
Chelonobia, but the trunk does not seem to be enlarged before
entering the gland. The glands are situated rather far apart; and
the chief peculiarity is, that the trunk connecting the glands is
as tortuous as the track of a worm. Each gland gives out two ducts,
which bifurcate repeatedly, and often inosculate, making, in parts,
an hexagonal mesh-work: some of the branches do not debouch on the
basal membrane, but terminate in blunt points. So numerous are the
ducts, that the basal membrane may be compared to pieces of paper
with the fine fibrous branching roots of some plant dried and heaped
on it. Some of these ducts are very regularly jointed, and resemble
a conferva,--an appearance which I believe is owing to divisions in
the contained cement; other ducts are partially marked by little
wrinkles, as presently to be described under Balanus. The cement,
instead of, as heretofore, invariably forming a slip round and beneath
the circumference of the basal membrane, here often forms little,
independent, circular, and irregularly-shaped discs, each of which has
issued from a single orifice. I may here add that in two species of
_Tetraclita_ I saw the cement-ducts repeatedly bifurcating, with some
of the branches inosculating, as in Elminius and Chelonobia.

In _Balanus balanoides_, which, like all the Cirripedes hitherto
mentioned, has a membranous basis, I could only make out an amazing
number of bifurcating and inosculating cement-ducts, of very various
diameters. The cement-tissue, on the under side of the basal membrane,
generally consisted of little circular discs, on an average from 2 to
5/1000ths of an inch in diameter; but there were also globules and
short tortuous threads of cement. In some very much elongated and
crowded specimens--in which, during the downward growth of the walls,
the basal membrane had ceased to reach the surface of attachment, and
being thus suspended had become, as viewed from the outside, deeply
concave--the cement had apparently continued to try to reach the rock,
and now hung down in the form of two thickish roots, some tenths of
an inch in length. These roots were round, and tapered either to a
fine or blunt point; one was doubled on itself, and so had become
united; in the other, I could perceive five layers or sheaths of the
cement-tissue, one within the other; the innermost of these layers,
which once, no doubt, formed the outside surface of the root, was only
a quarter of its present length.

In _Balanus tintinnabulum_, the basis is calcareous: when its upper
surface is cleaned, dried, and examined under a good light, the
numerous larger cement-ducts can be seen, even by the naked eye, or
under a weak lens, and present an elegant appearance. These larger
ducts run in parallel lines from the two chains of glands towards the
circumference. They are all encrusted with calcareous matter, and in
the more central parts are hidden under it; at each period of growth,
when the basis is added to round the circumference, it would appear
that a layer of excessive tenuity of shell is thrown down over the
whole surface, just in the same way as in Tubicinella, at each period,
a new and larger disc of membrane was thrown down over the pre-existing
membranes with their cementing apparatus. The cement-glands, in the
middle of the basal plate, seem often to give rise to small abnormal
depositions of calcareous matter. When the basis (it is best to take
a young specimen) is slowly dissolved in acid, all the cementing
apparatus is left uninjured, adhering to the delicate tissue which
before existed in a calcified condition. Near the middle I saw the two
antennæ of the pupa; and from them the two cement-trunks extended about
half-way towards the circumference. These two chains of glands are
often placed very irregularly, but they tend to form, as in Coronula,
a large angle, open towards the rostral end of the shell. The glands,
close to the old antennæ, commence abruptly, of rather large size: the
later-formed glands, with their ducts, are in regular order larger than
the younger ones, and stand much closer together. After immersion in
acid all the glands and ducts appeared empty, instead of the older
ones being, as in Coronula, filled with cement. In one case I counted
on each trunk twenty-five glands, besides some smaller obscure ones
close to the centre.

In Pl. 28, fig. 4 _b_, I have given a drawing of two of the cement
glands: the cement-trunk (_f f_) is smooth and apparently cylindrical:
it becomes enlarged (at _g_) before entering the gland: it seems even
to be prolonged across the gland under the form of a narrow bar (not
represented), which apparently serves to keep the two ends of the
trunk, on the two sides of the gland, in their proper relative places
and distances. The gland itself is an elongated bag (_h_), which
properly lies exactly over the enlarged portion (_g_) of the trunk,
but in the drawing has been purposely displaced: it gives rise, in the
later-formed glands, to a sort of neck (see the upper gland), which
is either so long as to deserve rather to be called a duct and which
soon bifurcates, or is quite short (see the lower gland) and gives rise
to two separate ducts. On the opposite side of these glands, there
is a spur (_m_), of greater or shorter length, which is evidently a
rudimentary duct, for in the younger glands it existed as a perfect
duct. Moreover, the first-mentioned duct often gives off branches
(_t′_), having an exactly similar appearance with the spur (_m_). The
membrane of which the cement-trunk (_f_), with the enlargements (_g_),
is composed, is smooth, but that of the glands and of all the ducts,
presents a very peculiar appearance, which at first would be called
scaled, but more properly perhaps notched,--each notch being apparently
formed by a line of thickened membrane, extending obliquely round only
a short portion of the tube, and indenting it. The ducts, which I
measured, were between 1/3000th and 4/3000ths of an inch in diameter.

In fig. 4 _a_, I have given a drawing of the two chains of glands, but
with only those ducts figured which proceeded from the last-formed
pair of glands. The specimen here drawn was old; and it is rare to
find the structure of the ducts so simple. From both glands[72] a neck
or thick duct arises, which soon bifurcates; one branch runs direct
into the circumferential duct, and the other (_t_) bifurcates again;
of the latter, one branch unites with its fellow from the opposite
gland, and then forming a single duct (_t′_) enters, as do the two
other branches, the circumferential duct. Thus, into the latter, five
main ducts enter: the position of their points of entrance, with
respect to the shell, varies considerably; but I think the five points
tend to face the middle of the rostrum, and middle of the two lateral
compartments on each side. In some other specimens, in which the
ducts were nearly as simple, I observed that the neck or main duct at
once divided into three branches, instead of into two, with one soon
bifurcating; and on one side a rudimentary branch or spur was given off
(above _t_), indicating a tendency to an additional bifurcation. In
the later-formed glands, the ducts proceed only from the outer sides
and form the ends of the glands furthest from the centre; but in the
earlier-formed and smaller glands of the same individual, other ducts
proceed from the inner sides, where in the older glands the spurs (_m_)
are situated: moreover, in the younger glands, all the ducts bifurcate
much oftener (how often I was not able to ascertain), before entering
the circumferential duct; many of the branches, however, terminating in
spur-like points. Now if we imagine twenty or thirty repetitions of the
ducts given in fig. 4 _a_, (independently of the greater complication
of the ducts of the younger glands), each a very little smaller than
the other, and placed, with the main branches parallel, one over and
within the other, we shall gain some insight into the wonderfully
complicated structure of the cementing apparatus in this and many other
species of Balanus.

    [72] It should be observed that fig. 4 _b_ ought to have been drawn
    with its present upper end downwards, to make it correspond in
    position with fig. 4 _a_.

I have as yet only alluded to the _circumferential duct_ (_i_, _i_, Pl.
28, fig. 4 _a_): we have not hitherto met with this duct, but I suspect
that the branches which in Chelonobia inosculate, and which seem to
run nearly parallel to the circumference of the basal membrane, answer
the same purpose of connecting the ducts together, and are, perhaps,
strictly homologous. In this, and some other species of Balanus, the
last-formed circumferential duct runs round the margin of the upper
lamina of the basal plate, close to the basal edges of the walls; and
as these latter have projecting longitudinal ribs, the duct curves
a little round each rib; so that the whole duct is formed by as many
short inwardly curved portions as the walls have ribs, or longitudinal
septa. Between the basal extremities of these parietal, longitudinal
septa, the extremities of the radiating septa of the basis project
and enter; and along the crests of the latter, little branch-ducts
(_i′_), proceeding from the circumferential duct, extend. In the basis,
beneath the tubes formed by the just-mentioned radiating septa, there
is a cancellated shelly mass (which, in fig. 4 _a_, was of unusual
thickness), and along the crests of the branching ridges forming this
cancellated mass, the sub-branches of the above branch-ducts (_i′_)
run; these soon become so minute as not to be distinguished by the
highest powers, and thus form a sheet of cement, which attaches the
last-formed zone of the shelly basis to the supporting surface. At
what point the membrane forming any one duct ceases, the cement-tissue
being alone left, I was not able to ascertain; but the lower parts
of the reticulated slip (_z_, _z_, fig. 4 _a_) closely resembled the
cement-tissue which surrounds the disc-segment of the pupal antennæ
in _Lepas australis_. The circumferential duct, here and there, forms
little loops, as may be seen in fig. 4 _a_: and often two branches,
running along the crests of two adjoining basal septa, proceed from
a common point of the circumferential duct. The cement itself, under
different parts of the basis, appears as little separate discs, as
threads, globules, and as a fine network, but most commonly as simple
layers. As each thick zone of shelly matter is added round the basis,
the exterior branches of the ducts, between the circumferential duct
and the new layer of cement beneath, are fairly imbedded in shell, and
are for ever hidden, without, indeed, acid be used for the dissolution
of the calcareous matter: so, also, the pre-existing ducts and glands,
and the main trunk, would all have been hidden, if the layer of
calcareous matter, which, I believe, is thrown down at each period of
growth over the entire surface, had not been of excessive tenuity.

I cursorily examined the cementing apparatus in _Balanus galeatus_,
_improvisus_ and _crenatus_, which have all calcareous bases, and
belong to different sections of the genus; and the structure seemed to
be essentially the same. In _Bal. galeatus_, I found the cement-ducts
varying in diameter from 1/4000th to 1/10,000th of an inch in diameter.
In _B. improvisus_, the cement-glands do not differ much from those
of _B. tintinnabulum_; but the cement-ducts bifurcate often before
entering the circumferential duct; and the little branches, which
proceed from the latter, are very short, and almost immediately, owing
to the thinness of the basis, blend into a slip of cement.

       *       *       *       *       *

I hope to be excused for describing at such length, the apparatus by
which sessile cirripedes are permanently attached to a supporting
surface; for this is the great leading character of the sub-class,
not hitherto observed in any other Crustacean.[73] It is not easy to
overstate the singularity and complexity of the appearance of the
basal membrane of a Balanus or Coronula: and when we consider the
homological nature of the apparatus, the subject becomes still more
curious: I feel an entire conviction, from what I have repeatedly
seen in several genera of the Lepadidæ, both in their mature and
pupal condition, and from what I have seen in Proteolepas, that the
cement-glands and ducts are continuous with and actually a part of an
ovarian tube, in a modified condition; and that the cellular matter
which, in one part, goes to the formation of ova or new beings, in the
other and modified part, goes to the formation of the cementing tissue.
To conclude with an hypothesis,--those naturalists who believe that
all gaps in the chain of nature would be filled up, if the structure
of every extinct and existing creature were known, will readily admit,
that Cirripedes were once separated by scarcely sensible intervals
from some other, now unknown, Crustaceans. Should these intervening
forms ever be discovered, I imagine they would prove to be Crustaceans,
of not very low rank, with their oviducts opening at or near their
second pair of antennæ, and that their ova escaped, at a period of
exuviation, invested with an adhesive substance or tissue, which served
to cement them, together, probably, with the exuviæ of the parent,
to a supporting surface. In Cirripedes, we may suppose the cementing
apparatus to have been retained; the parent herself, instead of the
exuviæ, being cemented down, whereas the ova have come to escape by a
new and anomalous course.

    [73] Rathke has described ('Acta Nova,' 1839, p. 147), in some
    siphonostomatous crustaceans, a pair of curious organs, which serve
    to secrete a substance that holds the eggs attached together in a
    mass to the parent's body: these organs Rathke has designated by
    a similar name to that which I have used, namely, the cementing
    organs or receptacles; they are distinct from the oviducts, but
    enter them near their external orifices. As in Cirripedes, the
    cement-glands and ducts are certainly continuous with an ovarian
    tube; and as they occupy a quite different position in the animal's
    body, these organs of Rathke, though in some degree analogous in
    function, must be homologically distinct.


_Affinities, Classification, Variation._

Under the Order it has been stated that the Balanidæ, are, on the
cirripedial type, the highest in the class; that is, they are the
most complicated, but not (to use Professor Owen's term) by mere
vegetative repetition. Amongst the Balanidæ, the first section of the
genus Balanus may be taken as typical; here we have the structure of
the shell extremely complicated, yet beautifully adapted for strength,
and for the protection of the included body. The cementing apparatus
is here, also, most complicated. I have divided the Balanidæ into two
natural sub-families, the Balaninæ and Chthamalinæ, in accordance
with certain differences in the structure of the shell and of the
animal's body: that this division is natural, might almost be inferred
from the one fact, that all the characters by which the Chthamalinæ
differ from the Balaninæ, are those by which the former approaches
the family of the Lepadidæ; moreover, certain anomalous characters
in the Chthamalinæ, as the supplemental whorls of compartments in
Catophragmus, and the presence of caudal appendages in this same genus
and in Pachylasma, reveal this same affinity. The only objection which
I can see to the separation of the sixteen genera into the above
two sub-families, may be drawn from the degree to which they blend
together; thus, as far as the shell is concerned, Chelonobia, in one
important internal point of structure, tends to assume the character
of the Chthamalinæ; and on the other hand, Pachylasma, a member of the
Chthamalinæ, has a shell, which if not examined during its earliest
growth, would be placed without doubt amongst the Balaninæ. But it
fortunately so happens, that in no one character of the body does
Pachylasma approach the Balaninæ more nearly, than do the other members
of its sub-family; or Chelonobia approach, in the same respects, the
Chthamalinæ. It is only in Chthamalus, of which the shell clearly
places it in the sub-family bearing its name, that in some of the
species, the less bullate labrum,--the larger palpi,--the lower teeth
of the mandibles being laterally double,--and the lower segments of
the third pair of cirri being thickly clothed, like the lower segments
of the second pair, with bristles--all show that these species make an
approach in structure to the Balaninæ.

It will be seen that I have divided the Balaninæ into two little
groups, according as whether the branchiæ consist of one or of two
plicated folds of membrane, and as whether or not the scutum and
tergum are articulated together. I have been greatly tempted to
follow Drs. Leach and Gray, who have separated the second of these
groups, containing the genera Coronula, Tubicinella, Xenobalanus,
and Platylepas, into the sub-family of the Coronulinæ. Certainly
these genera have a peculiar aspect in common, and agree in being
parasitic and imbedded in the skin of Cetaceans, as is the case
with the first three genera, or in that of turtles, manatee, and
sea-snakes, as in Platylepas. Though these genera possess several
peculiar characters, yet I can find none common to all four, excepting
their imbedment in the skin of Vertebrata, their double branchiæ, and
their non-articulated opercular valves; and these I do not think of
sufficient importance to serve for the separation of a sub-family; for
in Chthamalus, one species has double branchiæ, one species has no
branchiæ at all, and the other species have single branchiæ; so again
in Chelonobia, the scutum having only a horny articular ridge, makes
an approach to Coronula and its allies. I may further specify that the
folded walls, a singular character common to Coronula, Platylepas and
Xenobalanus, fails in Tubicinella; the open tubes and the imperfect
outer lamina of the parietes towards their bases, are characters which
fail in one species of Platylepas; the muscles running to the opercular
valves being thinly spread out, and partially without transverse striæ,
is also a character which fails in Platylepas; the simplicity of the
cement-ducts partially fails in Tubicinella; and lastly, the existence
of small intermediate teeth on the mandibles, fails in Xenobalanus:
hence, I repeat, I have not thought it prudent to admit the sub-family
of the Coronulinæ though in many respects a very natural group.

The genera in the Balaninæ and Chthamalinæ are founded chiefly on
the number of the compartments (the number being apparently due, as
previously explained, to the fusion or abortion of certain of the
eight typical compartments); and secondarily, on the nature and even
form of the basis, and on the porosity of the walls. In Coronula and
its allies, the non-articulated opercular valves and deeply folded
walls come into play. As a justification for using these characters in
distinguishing the genera, and even to a certain extent in separating
the two sub-families, I must call to mind that the shell, with the
basis, is not merely a dermal envelope, as amongst Molluscs, but
actually consists of the first three segments of the head. The parts of
the mouth and the cirri are of very little service in distinguishing
the genera,--a singular fact, considering that most of the genera
amongst the Lepadidæ could be distinguished by these organs,--though
trifling details in their structure sometimes come in useful as
specific characters. Balanus, with the sub-genus Acasta; Pyrgoma, with
the sub-genus Creusia; Tetraclita, and Elminius, are genera of about
equal value; though perhaps the two latter are rather more nearly
related together than to the others. Chelonobia is more distinct; it
shows some little affinity to the Chthamalinæ, and likewise to the four
following genera. Coronula, Platylepas, Tubicinella, and Xenobalanus,
are genera quite distinct from the foregoing, and from each other;
yet, as we have just seen, palpably allied together. Amongst the
Chthamalinæ, Pachylasma, Octomeris, and Catophragmus, are more closely
related to each other than to the other two genera of the sub-family;
yet Pachylasma, as far as the shell is concerned, leads into the
Balaninæ, and Catophragmus into the Lepadidæ; Octomeris leads towards
Chthamalus, and Chthamalus towards Chamæsipho.

_Variation._--The discrimination of the species in most of the
genera, offers very great difficulties. I cannot too strongly impress
on any one intending to study this class, not to trust to external
characters: he must separate and clean, and carefully examine the
internal structure and form of the compartments, and more especially
of the opercular valves. After considerable experience, when numerous
varieties of a species have been carefully examined, the eye acquires a
sort of instinctive knowledge, by which it can recognise the species,
though the character cannot be defined by language; but I have found
that no amount of experience with some of the commonest species,
will save frequent and grave errors, as long as external characters
alone are trusted to. Not only does every external character vary
greatly in most of the species, but the internal parts very often
vary to a surprising degree; and to add to the difficulty, groups of
specimens not rarely vary in the same manner. After having given up
several years to the study of this class, I must express my deliberate
conviction that it is hopeless to find in any species, _which has a
wide range, and of which numerous specimens from different districts_
are presented for examination, any one part or organ,--which from
differing in the different species is fitted for offering specific
characters,--absolutely invariable in form or structure. I may in one
respect even go further, and affirm, that, if in a species, any part or
organ differs remarkably from the same part in its congeners, then if
many specimens are examined, especially when collected from different
districts, such part or organ will be found _eminently_ variable. I
may instance the antenniformed third pair of cirri in _Chthamalus
antennatus_, the teeth on the posterior cirri in _Acasta sulcata_,
the terga in _Pyrgoma dentatum_, the adductor ridge of the scuta in
_Pyrgoma cancellatum_ and in Creusia, and other such cases: hence it
will not do to found a species on a slight, or sometimes even on a
considerable difference, in _any single point or organ_. On the other
hand, I am far from asserting, that if only half-a-dozen specimens of
some rare species of Cirripede be brought from some one quarter of the
world, characters beautifully defined may not be readily discovered. In
determining what forms to call varieties, I have followed one common
rule; namely, the discovery of such closely-allied, intermediate forms,
that the application of a specific name to any one step in the series,
was obviously impossible; or, when such intermediate forms have not
actually been found, the knowledge that the differences of structure in
question were such as, in _several allied forms_, certainly arose from
variation,--for instance, in the case of two shells otherwise identical,
one being longitudinally ribbed and the other smooth, a character which
we know to vary,--but I have always used this argument from analogy with
great caution. Finally, as in the large genus Balanus, there is an
especial amount of variation, I have there entered in detail on this
subject; and I hope that those interested in it, will refer to that
discussion, which is almost verbally applicable to some other genera of
the family, as Tetraclita and Chthamalus.


_Rate of Growth, Exuviation, Powers of Repairing Injuries._

In my former volume I have shown that the pedunculated cirripedes
grow rapidly; this is likewise the case with the Balanidæ. Mr.
Stutchbury informs me that he has seen numerous specimens of _Balanus
tintinnabulum_ from 2 to 3 inches in height and from 5 to 6 inches
in circumference (and this is nearly the full size which the species
attains), on a vessel which had been to sea only during one year. At
Coquimbo, in Chile, I have seen a specimen of _B. psittacus_ 1.3 of
an inch in basal diameter, and .8 in height, adhering to a chain that
had been only six months under water. Poli, also, gives the case of
some Balani (probably _B. perforatus_), which, in about four months,
had attained a basal diameter of 1 inch, and a height of 1-1/6th of an
inch. _Balanus balanoides_ is a smaller species, and of slower growth;
for the late Mr. W. Thompson, of Belfast, found that in three months
from July 3d, certain marked specimens had increased from 2-1/2-3
lines to 4-1/2 lines, which is the usual maximum size attained in that
locality. From other observations, Mr. Thompson believes that the
extreme duration of life of this species is about two years: whether
the other and apparently quicker-growing species, are shorter-lived, I
have no means of judging.

In accordance with this rapid growth is the frequency of the periods
of exuviation. Mr. Thompson kept twenty specimens of _B. balanoides_
alive, and on the twelfth day he found the twenty-first cast-off
integument, showing that all had moulted once, and one individual
twice, within this period.[74] This frequency of exuviation, together
with the durability of the cast-off integuments, explains the
astonishing masses of exuviæ, which Mr. Peach assures me he annually
has observed off the coast of Cornwall; they are most abundant in
April and May, but he has seen quantities also in September; he could
easily, as he tells me, have filled several quart-measures with them.
The specimens sent to me consisted chiefly of _Balanus balanoides_,
_perforatus_, and _Chthamalus stellatus_. The opercular membrane,
with a narrow strip from between the two scuta, and another narrow
strip from between the two terga, are moulted together, in connection
with the more delicate membrane lining the sack, and investing the
plicated branchiæ. This membrane, likewise, is continuously connected
with that covering the whole body and its appendages. As I have stated
under the Lepadidæ, the inner tunic of the œsophagus, of the rectum,
of the olfactory pouches, and that which enters a little way into the
acoustic meatus, and the apodemes of the maxillæ, are all moulted.
On the cirri and jaws, new spines are formed with their upper ends
enclosed within the old spines, but with their lower ends projecting
inwards, beyond the bases of the old spines, and inverted like the
fingers of a glove hastily pulled off. The membranes of the body, in
the act of exuviation, split, I believe, only over the prosoma. How the
neat separation of the opercular membrane, from all round the sheath
and opercular valves, is effected, I do not fully understand; but it
is, probably, analogous to the splitting of the thick carapace of the
common crab. I suspect in Coronula, in which genus and its allies the
opercular membrane is not periodically moulted, that the membrane
lining the sack is not always thrown off at the same exact time with
that of the body. In _Chthamalus stellatus_, in the act of moulting,
the opercular membrane is the last part that separates from the new
underlying membranes: I find that this species can moult when kept in a
damp box out of water. The new membranes of the body, immediately after
the exuviation, are not lax in any extreme degree. The exuviæ of the
genus Chthamalus, and of some other genera amongst the Chthamalinæ, can
at once be recognised by the divergence of the posterior _four_ pairs
of cirri: in the case of _Chthamalus stellatus_ I have also noticed
that the animal generally dies with these cirri in the same divergent
position. Finally, I cannot doubt[75] that the Triton described by
Linnæus was only the exuviæ of some Balanus (probably _B. porcatus_);
Linnæus mistaking the probosciformed penis for the mouth of his
imagined distinct animal.

    [74] In Daphnia, Straus ('Mém. du Museum,' tom. vi, p. 151) found
    that the individuals moulted every five or six days.

    [75] Linnæus speaks of the included body (inhabitant as he calls
    it) of other Cirripedes, as a Triton; and this, I think, shows
    that Lesson's conjecture that the Triton was an Alepas cannot be
    correct; for Linnæus could hardly have supposed that a pedunculated
    cirripede inhabited another pedunculated or sessile cirripede.

I have seen a few specimens showing that when the shell has been broken
it can be repaired; and this I believe is effected by the growth
of a crest of corium between the broken edges, and the subsequent
calcification of this crest. Mr. Stutchbury possesses a monstrous
specimen of _Chelonobia testudinaria_, in which one of the lateral
compartments on one side has not been developed. The cirri not rarely
get cut off, but are, as it appears, soon repaired. I have observed
a singular number of examples of the act of reparation in a group of
the Australian _Balanus vestitus_. The manner in which the cirri are
repaired seems to me curious: the cut-off end is closed by a rounded
scale of yellowish chitine, and then the corium, in the four or five
subjacent segments, separates from the external articulated membrane,
which now serves only as a case or capsule. The tube of corium thus
enclosed, with its contained muscles, shrinks a little, and soon can be
perceived to be in process of dividing into new and smaller segments,
in one instance ten in number,--which at the next exuviation would, no
doubt, be invested with an external membrane, and be freely exposed. In
another instance, the pedicel of a posterior cirrus had been cut off
and subsequently closed; in this instance, a whole, immature, miniature
cirrus, with the two rami, each having fifteen minute segments, was
thus enclosed in what had been the single lower segment of the pedicel.
I have seen several specimens of _Balanus balanoides_, as described
under that species, with several of the cirri and the penis truncated;
but I believe this was owing to monstrosity, which seemed particularly
to affect the male organs of generation; for no reparation seemed to be
in progress. In a specimen of Coronula, however, the penis appeared to
have been really cut off by accident; it had been closed, by a scab,
with concentric lines, like the articular rings on the penis itself;
and within the case thus formed, the corium had healed, and had become
pointed, but _inverted_; I presume that the point would, after another
exuviation, have been everted, and its length thus increased.


_Geographical Range and Habits._

With respect to range, the results arrived at have no particular
interest, for the species are not sufficiently numerous; and, what is
still more adverse, the genera, with unimportant exceptions, range over
the world; so that there is no scale of differences, and it cannot
be said that these two regions differ in their genera, and these two
only in their species. In all the following remarks, I have trusted
exclusively to my own specific identification; and have rejected all
assigned localities which appeared from any cause to be doubtful.
Sessile cirripedes are found in every sea, from lat. 74° 18′ north to
Cape Horn. The area included between the north point of the Philippine
Archipelago and the south point of Australia, extending on the right
hand to New Zealand, and on the left to Sumatra,--an area, which,
though including two distinct Cirripedial regions, is small compared
with the surface of the globe, yet includes a greater number of
species, especially of peculiar species, than the whole rest of the
known world. This is, probably, in chief part due to the broken nature
of the land, affording diversified habitats, and to much of the coast
being rocky. Cirripedes, from requiring to be attached, cannot live
where the shores and adjoining bottom are sandy or muddy or formed
of moving shingle; hence, no doubt, it arises, that there is such a
remarkable contrast in the great number of the species inhabiting
the bold rocky western coast of South America, and the few species
living on the sloping, and often sandy or muddy or shingly, eastern
shores of this continent. Hence, also, I believe, it is that not many
species have been brought from India. Coral-reefs are not favorable to
Cirripedes, consequently but few are known to inhabit the islands of
the Pacific Ocean. Where Cirripedes can live, though the species in no
district are numerous, the individuals abound in infinite numbers: I
have walked over the coast-rocks of the Falkland and Chonos Islands,
of Chile and Van Diemen's Land, fairly encrusted over wide spaces with
a continuous layer of Cirripedes, consisting of only two or three
species; in the same manner as may be observed on many parts of the
shores of Great Britain, and, I believe, of North America.

With respect to the effects of temperature on the range of Cirripedes,
no genus (having more than one species) is confined to the torrid
zones. Pyrgoma, from being always attached to corals, is, of course,
ordinarily found in the hotter seas; but one species ranges from the
Cape de Verde Islands in the torrid zone to the southern shores of
England and Ireland. Tetraclita is not found in the colder seas, but
is numerous in species and in individuals, on the southern shores of
Australia and at the Cape of Good Hope. I may here add, that the two
genera with the most confined ranges, are Chamæsipho and Elminius;
the former has only two species, one inhabiting Australia, and the
other the East Indian Archipelago; Elminius has four species, confined
to the southern hemisphere, and inhabiting Australia, New Zealand,
and South America. To return to the effects of temperature; in Mr.
Dana's great work on Crustacea, an excellent chart is given, in which
the _isocrymal_ lines, or those exhibiting the mean temperature of
the waters along their course, for the coldest thirty consecutive
days in any season of the year, are given; and which lines Mr. Dana
has shown are the most influential in governing the distribution of
marine animals. At the isocryme of 68°, Mr. Dana divides the torrid
and sub-torrid zones from the several temperate zones; and at 44°, the
temperate from the sub-frigid and frigid zones; but as no Cirripedes
are exclusively confined to these frigid zones, we may here disregard
them. From Mr. Dana's[76] table of the areas of the torrid and
temperate ocean-zones, on both sides of the equator, it seems that
they are nearly as 337 to 278, in relative area; and consequently, he
remarks, that the marine species in any class, if distributed equally
over the two, would be one fifth more numerous in the torrid than in
the temperate zones. Now of Cirripedes, taking all the orders, there
are at present known 147 species; of these, 7 have doubtful habitats,
leaving 140 for comparison. Of these 140, nearly one quarter, or
37, inhabit both the torrid and temperate zones, as above defined;
46 are found exclusively in the torrid, and 57 exclusively in the
temperate zones; so that the temperate zones, though less in area,
and having, proportionally, even a considerably lesser length of
coastline, nevertheless have a preponderance in the number of species.
But it should be borne in mind, that there are _two_ great temperate
districts, separated from each other by _one_ great torrid district;
and, inasmuch as the number of species in any region seems to depend in
some degree on the isolation of the sub-regions, we might have expected
(the other conditions now being, and the _past_ conditions having been
alike), that the two great temperate areas would have contained more
species, perhaps doubly more, than the single great torrid area.

    [76] 'Crustacea: United States Exploring Expedition,' p. 1476
    (corrected).

The proportional numbers, above given, are not very widely different,
whether we take separately the Balanidæ, the Lepadidæ, or all
together. Mr. Dana has shown[77] at length, that generally amongst
the Crustacea, the species which he considers of highest rank, belong
to the extra-torrid zones: there seems to me in all such cases to
be some degree of vagueness in the attempt to determine which are
highest or lowest, but I have already elsewhere stated that Balanus
is, probably, the most eminently Cirripedial form, and exhibits in the
strongest manner all the characters by which Cirripedes differ from
other Crustacea; as this genus is the largest, containing 36 species,
of which the habitats are known, I may state that of these, exactly one
third, or 12, inhabit both zones; 9 exclusively inhabiting the torrid,
and 15 exclusively the temperate zones. According to the proportions of
the whole class, the numbers should have been 9 torrid, to only 11.11
temperate; so that evidently the genus Balanus (in one sense typical)
inclines towards the temperate regions more strongly than does either
the family or the sub-class to which it belongs.

    [77] Ibid., p. 1528.

With respect to the relation between the size acquired by the different
species of sessile cirripedes, and the temperature of the localities
inhabited by them, the genera Chthamalus, Tetraclita, and Balanus,
alone can serve for comparison: in Chthamalus much the largest species
is found in the temperate zone: on the other hand, the two largest
species of Tetraclita are from the torrid zone, though one of them
also sometimes ranges into the temperate seas: in Balanus, the largest
species, and six other species having a basal diameter sometimes over
two inches, inhabit the temperate regions; and two out of these seven
species, flourish even in the Arctic seas; whereas, within the torrid
zone, there are only three species with a diameter sometimes exceeding
two inches, but two of these frequently become very large and massive;
so that Balanus, judging from the size of the species, as well as
from their range, does not require for its highest development the
temperature of the torrid zones.

The greater number of the species of the Balanidæ have wide ranges, as
might be inferred from the fact of between one third and one fourth
of the total number inhabiting both the torrid and temperate zones;
but it should not be overlooked, that those species, as _Balanus
tintinnabulum_, _amphitrite_, _improvisus_, and, in a lesser degree,
_B. trigonus_ and _Tetraclita radiata_, which seem to range over
nearly the whole world (excepting the colder seas), are species which
are habitually attached to ships, and which could hardly fail to be
widely transported. Indeed, it appears to me surprising, that such
species as _Balanus psittacus_ and _eburneus_, which often become
attached to vessels, should still be confined, the one to Southern,
and the other to Northern America. But some other Cirripedes, which
I have never seen attached to vessels, have likewise immense ranges:
thus _Tetraclita porosa_ is found in every tropical and warmer sea,
and _Chthamalus stellatus_ ranges round the world in the northern
hemisphere, and, along the eastern side of America, far south of the
equator: _Balanus spongicola_, and _Acasta spongites_, extend from the
shores of Britain to the Cape of Good Hope: _Balanus lævis_ ranges from
Tierra del Fuego to California. I may further remark, that the only two
other species of Balanus, and the one Chthamalus, inhabiting Tierra
del Fuego, are, also, found on the shores of Peru. But to show the
powers of endurance in some species, I may specify the case of _Balanus
improvisus_, which flourishes on the coast of Nova Scotia, amongst
the West Indian Islands, in Southern Patagonia, and near Guayaquil.
Even more striking is the case of _B. crenatus_, of which I have seen
specimens from latitude 74° 48′ north, from the West Indies, and the
Cape of Good Hope! In these two latter localities, however, it seems to
be rare, and may have been first transported to them from the shores of
Europe, on the bottoms of vessels, to which it sometimes adheres.

The several species of Balanidæ live attached either to coast-rocks,
or to objects at various depths, down to, as in the case of _Balanus
crenatus_, 50 fathoms. _Balanus balanoides_ sometimes adheres to rocks
or wood so high up, that it is not covered by water during the neap
tides. Mr. Thompson has informed me, that he once accidentally kept
some specimens of this species out of water for seven days in a warm
room, and that they were then quite lively. This species, is very
easily killed by brackish water, as are some other species, whilst _B.
improvisus_ and _eburneus_ can flourish in it; and at the Falkland
Islands, I saw _Elminius Kingii_ attached to rocks at the mouth of
a fresh-water brook, so as to be covered by pure water during the
ebb of each tide. Sessile cirripedes adhere to all sorts of objects,
floating and fixed, animal and vegetable, living and dead, organic and
inorganic. Chthamalus is, perhaps, more commonly attached to rocks than
are the other genera. Living Mollusca are, I think, the most frequent
objects of attachment: Mr. Cuming has remarked to me, that shells
covered by an epidermis, as Patella, Haliotis, and Mytilus, are the
greatest favorites. Acasta is always imbedded in sponges, or in the
sponge-like bark of Isis; Pyrgoma and Creusia in corals; Chelonobia is
attached to turtles, and one species to crabs or very smooth shells;
Coronula, Tubicinella, and Xenobalanus, are imbedded in the skin of
Cetaceans; and Platylepas in that of manatee, turtles, or sea-snakes.

If we attempt, with our present not very imperfect materials, to
divide the globe into provinces, according to the amount of difference
in their Cirripedial inhabitants, including all orders and families,
and disregarding entirely, as I think we ought, all probabilities or
conclusions deduced from the distribution of other tribes of animals,
we find that the globe may be divided into the four following great
provinces and one sub-province. I should premise, that in the following
remarks and tables,[78] the species of Lepas, Conchoderma, Chelonobia,
Coronula, Platylepas, and Tubicinella, are excluded, owing to their
being attached to floating or swimming objects, and being consequently
widely and irregularly distributed.

    [78] As the number of Cirripedes in the whole class is not very
    great, I have given lists of the species in the four main provinces
    and in the one sub-province.

The first, or North Atlantic province, is that of Europe and the
eastern shores of North America, from the arctic regions to lat. 30°:
the island of Madeira, part of the north-west coast of Africa, and the
whole Mediterranean being included. In this province (the above-named
genera being excluded) we have 31 species, of which 22 are not found in
any other distant quarter of the globe. As some few of these species
range into the West Indies, I have not, on this account, excluded them
from the 22 peculiar forms. Had I included the West Indies[79] in my
first province, the total number of species would have been 42, of
which 28 would have been peculiar. The coast of Brazil, even as far
south as the Rio Plata, might, also, have been included, for I have not
seen from it a single species not included in the above 42 West Indian
species. So also, by adding a single species, might the west coast of
equatorial Africa. The two coasts of South America and Africa, which
face each other within the torrid zone, seem to be remarkably barren
in Cirripedes. Europe has several more species than the United States,
which is inhabited by only ten species, including even the probably
imported _Balanus tintinnabulum_ and _amphitrite_. Of these ten United
States species only two are not found in Europe; and both these two
range into the West Indies, and as far as the northern shores of South
America, and therefore cannot be considered as peculiar to the United
States.

    [79] The total number of species which I have seen from the West
    Indies, is 19 or 20; of these, only 6 are peculiar to it, or 8, if
    the United States be likewise included, the other 12 or 14 species
    being found in other quarters of the world. Six peculiar species
    out of 19 or 20, has not appeared to me sufficient to institute
    even a sub-province.

I have formed my single sub-province for the southern extremity of
Africa; for although I know of only 11 species from this comparatively
short and uniform line of coast, yet I was not able to group these
eleven in any of the main provinces: 5 of the species are peculiar,
1 Australian, 3 European and West Indian, and 2 almost universally
distributed.

The second province includes the west coast of North and South America,
from Tierra del Fuego to Behring's Straits: on this enormously long
line of coast, only 22 species are known to exist, but of these no
less than 15 are peculiar. Of these 15, four are not found south of
the torrid equatorial region, and eight are not known to occur north
of this same region; so that this long line of coast might have been
divided into two sub-provinces, of which the southern would have been
the most peculiar; but as eight species are found both north and south
of the equatorial region, I have not made this sub-division. Two of
the species occurring on the western coast of North America, seem to
represent species found on its eastern coast, and in Europe; thus,
_Balanus glandula_ takes the places of _B. crenatus_, and _B. cariosus_
that of _B. balanoides_. Not a single species, excepting a few which
are also widely distributed over other parts of the world, is known to
be common to the east and west coasts of the two Americas.

The third province is that of the East Indian Archipelago, and includes
the Philippines, Borneo, New Guinea, Sumatra, Java, Malacca, and the
eastern coast of India. Here we find 37 species, of which 24 are
peculiar. I may remark, that I have received no species from Madagascar
or the eastern coast of Africa; few from India, or from the coast of
China; and I suspect, that on most of these coasts, only few exist.
Probably our third province will hereafter be found to include the
whole Indian Ocean.

The fourth province is that of Australia, including New Zealand: it
has 30 species, of which 21 are peculiar. Had the temperate Australian
coasts (_i. e._, those south of the isocryme of 68°) been alone
considered, the number of the species would have been probably 25, of
which 20 would have been peculiar,--that is, if we admit within the
20, several species which range from the temperate into the torrid
zone, but do not extend beyond the Australian shores. Owing to the
widely-extended ranges of most Cirripedes, no Arctic or Antarctic
provinces can be said to exist.

To recapitulate the above results, bearing in mind that, although the
total number of known existing Cirripedes is 147, yet that the habitats
of seven are unknown, and that eighteen are excluded owing to their
being attached to floating or swimming objects, so that there are only
122 species referred to in the following table:

  +--------------------------------------------------+--------+---------+
  |                                                  | Total  | Species |
  |                                                  | number |confined |
  |                                                  |   of   | to the  |
  |                                                  |species.|province.|
  +--------------------------------------------------+--------+---------+
  |(1.) First, or North Atlantic Province, to lat.  }|        |         |
  |     30° N. (If the West Indies had been included}|   31   |   22    |
  |     the numbers would have been 42 and 28)      }|        |         |
  |(2.) Sub-province of South Africa                 |   11   |    5    |
  |(3.) Second province, or West Coast of North and }|        |         |
  |     South America                               }|   22   |   15    |
  |(4.) Third province, or East Indian Archipelago   |   37   |   24    |
  |(5.) Fourth, or Australian province               |   30   |   21    |
  +--------------------------------------------------+--------+---------+

The least prolific of these provinces contains 22 species, or between
1/5th and 1/6th of the total number of species, and the most prolific
between 1/3rd and 1/4th of this same number. In each of these
provinces, it is remarkable that the peculiar species are very nearly
two thirds of the whole of its inhabitants. These facts, I think, show
that the above provinces are natural divisions of the world, as far as
their Cirripedial inhabitants are concerned.

As Cirripedes belong to the great class of Crustacea, and as the
distribution of the latter has lately been fully discussed by Mr. Dana,
it may be worth while briefly to compare my results with his; more
especially as they are so very different. I should premise, as perhaps
accounting to a certain extent for this difference, that, owing to the
wide range of many species, and the almost universal extension of the
same genera, my provinces are founded merely on a certain proportion
of the species, namely, two thirds, being peculiar or confined to a
region of considerable dimensions: whereas, in the case of ordinary
Crustaceans, the greater number of the species are distinct even in
the sub-provinces, and the provinces are founded mainly on generic
differences. Mr. Dana divides the surface of the globe into three great
sections, or provinces, the _Africo-Europæan_, the extent of which is
shown by its title; the _Occidental_, which includes both the east and
west coast of both Americas; and the _Oriental_, including the Indian
and Pacific Oceans, with the East Indian Archipelago, and Australia.
Thus Mr. Dana entirely separates the Eastern shores of North America
from Europe; whereas, as far as their Cirripedial inhabitants are
concerned, they are most intimately allied, and form my first or North
Atlantic province; and to this, as I have shown, even the West Indies,
the coast of Brazil, and equatorial West Africa might have been added.
It follows, from this similarity in the Cirripedes on the two sides of
the Atlantic, and from their dissimilarity with those on the shores of
the Pacific, that the east and west coasts of the two Americas form two
quite distinct Cirripedial provinces; though, in the northern half,
some connection is shown by a few representative species: on the other
hand, Mr. Dana unites both sides of the whole American continent,
into his single Occidental province. The South-African province is not
brought out by Mr. Dana so prominently, as I have found necessary. Mr.
Dana joins the East Indian Archipelago and Australia into his single
Oriental province, and makes New Zealand, as a sub-province, apparently
as distinct from Australia, as Australia is from the East Indian
Archipelago: whereas I find that the Cirripedes of New Zealand clearly
belong to Australia; and that the Australian Cirripedes, especially if
the temperate shores be alone considered, are as distinct from those of
the East Indian Archipelago, as from those of any other quarter of the
whole world. I believe that the provinces deduced from the distribution
of Cirripedes, accord better with the Molluscan provinces, than with
those given by Mr. Dana for the rest of the great class of Crustaceans.

In the following tables, an asterisk means that the species is not
found in any other distinct region of the globe. When found in one
of the five provinces, a corresponding number, within brackets, is
appended, to show in which province or sub-province it has been found.


(1.) FIRST OR NORTH ATLANTIC PROVINCE: _Europe and the Eastern United
    States, from the Arctic Regions to 30° north latitude._

  Balanus tintinnabulum        (1 to 5).
          tulipiformis*        confined to Europe.
          calceolus            Europe and India.
                             { confined to North America and West
          galeatus*          {   Indies.
          spongicola           (2) and West Indies (?).
                             { confined to Europe, but possibly in the
          perforatus*        {   West Indies.
          amphitrite           (1 to 5).
          eburneus*          { confined to North America and West
                             {   Indies.
          improvisus           Europe and North America and (3).
          porcatus*            Europe and North America.
          crenatus                 "             "  W. Indies and (2).
          balanoides*              "             "
          Hameri*                  "             "
  Acasta spongites             Europe and (2).
         cyathus*              Madeira and West Indies.
                             { confined to Europe, but ranges as far at
  Pyrgoma anglicum*          {   least as the Cape de Verde Islands.
  Xenobalanus globicipitis*    confined to Europe.
  Chthamalus stellatus         Europe and North America and (3 and 4).
  Pachylasma giganteum*        confined to Europe.
  Verruca Strömia              Europe and Red Sea.
          Spengleri*           Madeira.
  Pœcilasma aurantia*             "
            crassa*               "
  Dichelaspis Lowei*              "
  Oxynaspis celata*               "
  Alepas minuta*               Europe.
         parasita*             Europe and Atlantic Ocean.
  Anelasma squalicola*         Europe.
  Alcippe lampas*                 "
  Scalpellum vulgare*             "
  Pollicipes cornucopia*          "

Here we have 31 species, of which 22 are not found in any other great
region of the world.


(2.) SUB-PROVINCE: _Africa, South of lat. 30°._

  Balanus tintinnabulum        (1 to 5).
          Capensis*
          spongicola           (1) and West Indies (?).
          amphitrite           (1 to 5).
          crenatus             (1) and West Indies.
  Acasta spongites             (1).
  Tetraclita serrata*
             rosea             (4).
  Chthamalus dentatus*         also on West Coast of Africa.
  Octomeris angulosa*
  Scalpellum ornatum*

In this small region we have only 11 species, of which five are
peculiar: _Balanus Capensis_ and _Tetraclita serrata_, seem to be
representatives of _B. psittacus_ of S. America and of _T. porosa_ of
that and several other regions.


(3.) SECOND PROVINCE: _West Coast of South and North America, from
    Tierra del Fuego to Behring's Straits._

  Balanus tintinnabulum        north and south of the equator (1 to 5).
          psittacus*           south.
          vinaceus*              "
          trigonus             north and south (4 and 5).
          lævis*                 "
          concavus               "             (4 and 5).
          pœcilus*             south.
          improvisus           south (and north?) (1).
          nubilus*             north.
          glandula             north, and Southern Pacific Ocean.
          cariosus*            north.
  Tetraclita porosa            north and south (4 and 5 and W. Indies).
  Elminius Kingii*             south.
  Chthamalus stellatus         north.
             cirratus*         south.
             scabrosus*          "
             fissus*           north and south.
             Hembeli*          north.
  Verruca lævigata*            south.
  Pollicipes elegans*          south and north.
             polymerus*          "
  Cryptophialus minutus*       south.

Here we have on this long line of coast, 22 species, of which 15 are
peculiar.


(4.) THIRD PROVINCE: _Indian Archipelago (including the Philippines,
    Malacca, Borneo, Sumatra, Java, and New Guinea, and eastern coast
    of India)._

  Balanus tintinnabulum        (1 to 5).
          Ajax*
          navicula*
          stultus              and West Indies.
          trigonus             (3 and 5).
          concavus             (3 and 5).
          amphitrite           (1 to 5).
          patellaris*
          amaryllis            (5).
          quadrivittatus*
  Acasta lævigata              and Red Sea.
         fenestrata*
         purpurata*
         sporillus*
  Tetraclita porosa            (3 and 5) and West Indies.
             costata*
             vitiata           (5).
             cœrulescens       Pacific Ocean.
             radiata           (5) and West Indies.
  Pyrgoma cancellatum*
             grande*
             milleporæ*
             crenatum*
             monticulariæ*
  Creusia spinulosa            and West Indies.
  Chthamalus stellatus         (1 and 3).
             intertextus*
  Chamæsipho scutelliformis*
  Octomeris brunnea
  Pœcilasma fissa*
            eburnea*
  Dichelaspis Warwickii*
  Ibla Cumingii*
  Scalpellum rostratum*
  Pollicipes mitella*
  Lithotrya Nicobarica*
            truncata           and Pacific Ocean.

Here we have 37 species, of which 24 are peculiar to this province.


(5.) FOURTH PROVINCE: _Australia (including New Zealand)._

  Balanus tintinnabulum        (1 to 5).
          nigrescens*
          decorus*
          trigonus             (3 and 4).
          concavus             (3 and 4).
          amphitrite           (1 to 5).
          amaryllis            (4).
          allium*
          vestitus*
          imperator*
  Acasta sulcata*
          glans*
  Tetraclita porosa            (3 and 4).
             rosea             (2).
             purpurascens*
             vitiata           (4).
             radiata           (4) and West Indies.
  Elminius plicatus*
             simplex*
             modestus*
  Chthamalus antennatus*
  Chamæsipho columna*          and Pacific Ocean (?).
  Pachylasma aurantiacum*
  Catophragmus polymerus*
  Alepas tubulosa*
  Ibla quadrivalvis*
  Scalpellum Peronii*
  Pollicipes spinosus*
             sertus*
  Lithotrya cauta*

Here we have 30 species, of which 21 are peculiar.


_Geological History._

The ancient history of the Balanidæ is a brief one. No secondary
species has hitherto been discovered; in my monograph on the fossil
Lepadidæ[80] I have shown that the negative evidence in this case is of
considerable value, and consequently that there is much reason to doubt
whether any member of the family did exist before the Eocene period.
The existence of a Cretaceous Verruca is an apparent exception to the
rule, as this genus has hitherto always been ranked amongst sessile
cirripedes; but Verruca, as we now know, must be placed in a family by
itself, quite distinct from the Balanidæ. Balanus is the oldest genus
as yet known; it first appeared in Europe and North America, during
the deposition of the eocene beds; and was at that time, as far as our
information at present serves, represented by very few species. In
South America, one species of Balanus abounds in individuals in the
ancient Patagonian tertiary formation. I have seen, in the British
Museum, specimens, said to have come from the eocene nummulitic beds
near the mouth of the Indus, belonging to the second section of the
genus. Generally, the extinct forms belong to the last section of
this genus, which has the parietes not permeated by pores. During the
miocene and pliocene ages, sessile cirripedes abounded. No extinct
genus in this family has hitherto been discovered. It is singular,
that though the Chthamalinæ approach much more closely than do the
Balaninæ to the ancient Lepadidæ, of which so many species have been
found fossil even in the older Secondary formations, yet that only one
species of one genus of this sub-family has been hitherto found in any
deposit; and that species is the still existing _Pachylasma giganteum_,
in the modern beds of Sicily. During the epoch of the Glacial deposits
in Scandinavia, Scotland, and Canada, the still existing species seem
to have abounded; and they attained larger average dimensions than
the same species now do on the shores of Great Britain, or even on the
shores of the northern United States, where the average size seems
larger than on this side of the Atlantic.

    [80] Since the note to page 5 of that work was written, I have been
    informed that the so-called cretaceous _Tubicinella maxima_ is not
    a Cirripede.

Under the genus Balanus, I have given my reasons for never naming
species in this large and difficult genus, without examining the
opercular valves: it has been owing to this, as it appears to me,
proper want of caution, that there are so many nominal species. Thus
it is made to appear in catalogues, that the tertiary seas abounded
with species of Balanus to an extent now quite unparalleled in any
quarter of the world. Bronn,[81] for instance, in his invaluable
'Index Palæontologicus,' gives the names of 35 Balani, found fossil in
Europe, and I have not counted those found only in alluvial deposits,
as they would certainly be the same as the still living species. Now
I know only 11 recent Balani on the shores of all Europe, from the
North Pole to lat. 30°; and of these I doubt whether _B. balanoides_
and _improvisus_ have been found fossil. In the Red Crag there is one
extinct Balanus: in the Coralline Crag, which seems to have been very
favorable to the existence of Cirripedes, there are six species of
Balani, of which two are absolutely extinct, and one does not occur
in any neighbouring sea: in the Eocene formations the species seem to
have been rare, and I have seen only one, and that is an extinct form.
Taking these several facts into consideration, and bearing in mind
that Cirripedes usually range widely, I do not believe, if all the
specimens of Balani hitherto found in the several tertiary formations,
from the eocene to the glacial deposits, throughout Europe, were
collected together, they would amount to 20 species. I have myself
seen, in a recognisable state, only 12 fossil species, of which five
are extinct, or not found in any neighbouring sea: I think it probable
that three other recent species, viz. _B. tulipiformis_, _perforatus_,
and _amphitrite_, may occur in the Mediterranean formations; and this
would make 15 species. Therefore in the above estimate of 20 species,
five are allowed for species existing in European collections,
but not hitherto seen by me; and this, I believe, is a very full
allowance. Consequently, even on the supposition that the five
species just admitted as possibly existing in cabinets, and that the
other five extinct species, which I have seen and examined, have all
been previously named by other authors, a supposition excessively
improbable, even then there would be 15 superfluous names in Bronn.

    [81] To save any other person, interested in fossil Cirripedia,
    going through the several works quoted by Bronn, I have given some
    remarks on his list of species, in an appendix at the end of the
    Balanidæ.

The following short table shows how Cirripedes, including all three
Families, were represented in Great Britain, throughout the several
TERTIARY STAGES.

  A = Living species but found fossil in some tertiary deposit.
  B = Mammilliferous crag, and glacial deposits.
  C = Red crag.
  D = Coralline crag.
  E = Eocene.

  +----------------------------+---+---+---+---+---+
  |            Name.           | A | B | C | D | E |
  +----------------------------+---+---+---+---+---+
  | Balanus tintinnabulum      | * |   | * |   |   |
  |         calceolus          | * |   |   | * |   |
  |         spongicola         | * |   |   | * |   |
  |         concavus           | * |   | * | * |   |
  |         porcatus           | * | * | * |   |   |
  |         crenatus           | * | * | * | * |   |
  |         Hameri             | * | * | * |   |   |
  |         bisulcatus         |   |   | * | * |   |
  |         dolosus            |   | * |   |   |   |
  |         inclusus           |   |   |   | * |   |
  |         unguiformis        |   |   |   |   | * |
  | Acasta undulata            |   |   |   | * |   |
  | Pyrgoma anglicum           | * |   |   | * |   |
  | Coronula barbara           |   |   | * |   |   |
  | Verruca Strömia            | * | * | * | * |   |
  | Scalpellum magnum          |   |   |   | * |   |
  |            quadratum       |   |   |   |   | * |
  | Pollicipes reflexus        |   |   |   |   | * |
  +----------------------------+---+---+---+---+---+
  | Total, 18, recent and    } |   |   |   |   |   |
  | extinct, found fossil in } | 9 | 5 | 8 |10 | 3 |
  | Great Britain, in some   } |   |   |   |   |   |
  | tertiary deposit         } |   |   |   |   |   |
  +----------------------------+---+---+---+---+---+

As affording some standard of comparison by which to compare the number
of fossil species, at any period, in relation to the number of species
probably existing in the neighbouring seas during the same epoch, I
may state that there are now living and propagating on the shores of
Great Britain, 18 species belonging to the three Families included
in the above table. I have not counted three species, in the genera
Alcippe and Conchoderma, which, from the minuteness of their valves,
it is hardly possible would be found fossil. On the other hand, I have
included in the 18, five species of Lepas, which from floating and
being oceanic, are more likely to be cast up on beaches, than to be
imbedded in sedimentary deposits; so that 13 would, perhaps, be a safer
number, as a standard of comparison. Now in the coralline crag, which
seems to have been eminently favorable for the existence and subsequent
preservation of Cirripedes, and which has been so well worked, only
nine fossil species, as may be seen in the table, have been as yet
discovered.



Sub-Family--BALANINÆ.

_Shell with the rostrum having radii, but without alæ; lateral
compartments all having alæ on one side and radii on the other side;
parietes generally either porose, or longitudinally ribbed on their
inner surfaces._

_Mouth with the labrum notched in the middle, not swollen; palpi large,
almost touching each other; mandibles generally with the lower teeth
laterally double; third pair of cirri with their segments resembling
those of the second pair._

_First Section._†

_Scutum and tergum articulated together, or overlapping each other;
each branchia composed of a single plicated fold._

_Genera_--Balanus; Acasta; Elminius; Tetraclita; Pyrgoma; Creusia;
Chelonobia.

_Second Section._††

_Scutum and tergum (when both are present) not overlapping each other;
basis membranous; parietes often deeply folded, with the outer lamina,
towards the basis, generally imperfect; each branchia composed of two
plicated folds; shell attached to living vertebrata._

_Genera_--Coronula; Platylepas; Tubicinella; Xenobalanus.[82]

    [82] At the end of the volume a Synopsis is given, which will serve
    as a systematic index for the discovery of generic and specific
    names.


The Balanidæ may be divided into two sub-families; namely, the Balaninæ
and Chthamalinæ; and, in the former, the genera, as we see, may be
very naturally grouped into two sections. The Balaninæ differ from the
Chthamalinæ, as far as the shell is concerned, in the rostrum having
radii but no alæ, all the lateral compartments having both radii and
alæ; on the other hand, in the Chthamalinæ, the rostrum has alæ, and
the rostro-lateral compartments radii on both sides, and therefore
no alæ. These differences probably arise, as already explained, from
the perfect confluence, in the Balaninæ, of the true rostrum with the
rostro-lateral compartments. In Chelonobia, belonging to the Balaninæ,
we see an intermediate state, with the fusion not quite effected:
on the other hand, in one genus amongst the Chthamalinæ, namely,
Pachylasma, we must look to the shell at a very early age, to find the
rostrum with its alæ, distinct from the rostro-lateral compartments.
In Tetraclita, Elminius, and Creusia, the carino-lateral compartments
are aborted, or possibly confluent with the lateral compartments,
making altogether only four: in Pyrgoma all the compartments are
fused together and form a solid ring. The sub-genus Acasta is, in
one sense, very natural, as it includes species most closely allied:
in another sense it is far from natural, as some of the species can
hardly be distinguished from those species of Balanus, which live
attached to Gorgoniæ: I almost regret I did not merge the species of
Acasta into Balanus. In the Balaninæ generally the parietes are either
porose, or are furnished on their internal surfaces with regular ribs,
representing the longitudinal parietal septa, which in other species
form the tubes or pores; there are, however, many exceptions to this
rule in several species of Balanus, in Acasta and Elminius, all of
which have the parietes of their shells internally quite smooth, or
only irregularly roughened with points.

Looking to the animal's body, in the Balaninæ, the labrum is always
notched in the middle, and is never swollen or bullate, for the outer
and inner folds of membrane of which it is composed lie close together.
The palpi are large, so that their tips almost touch each other. The
mandibles, generally, have their lower main teeth laterally double.
Of the cirri, the third pair invariably much more closely resembles,
in its whole structure, and in its action, the second than the fourth
pairs; and it is also generally separated by a small interval from the
fourth pair.

I have already under the Family sufficiently entered on the relations
of the Balaninæ to the Chthamalinæ, and of the genera, one to the
other, so that I need not here add anything.

I can point out no difference in habits or geographical distribution
between the Balaninæ and Chthamalinæ.



1. _Genus_--BALANUS, Auct.[83]

    [83] The name Balanus was used, almost as at present, by Lister and
    Hill, before the introduction of the binomial system. Since that
    period the first two authors, as far as I know, who used this name,
    were Da Costa, in his 'Hist. Nat. Test. Brit.,' in 1778; and Bock,
    in the 'Naturforscher,' for the same year; Bock, however, applied
    it to a Chelonobia.

  CONOPEA (pars generis). _Say._ Journal Nat. Sc. Philadelphia, vol.
        ii, part ii, 1822.

  MESSULA (do.). _Leach._ Zoological Journal, vol. ii, 1825.

  CHIRONA (do.). _J. E. Gray._ Philosoph. Transacts., 1835, p. 37.

_Compartments six; basis calcareous or membranous; opercular valves
sub-triangular._

  _Distribution._--Mundane: in the warmer seas.


_General appearance._--The shape of the shell in the different
species varies from depressed conical to cylindrical; the latter
form being generally assumed when specimens are crowded together; but
some species, as _B. balanoides_, _crenatus_, and _lævis_, seem more
subject than others to be thus affected. The colour is either white,
generally tinted by the yellowish or brownish epidermis, or any colour
intermediate between bright pink and rich blue, purple being the
prevailing tint. The persistence of the so-called epidermis is very
different in different species, being even sometimes highly variable in
the same species. The surface is either smooth or more commonly folded
longitudinally, or sharply ribbed. The orifice differs in form from
diamond-shape to trigonal; the carinal end, owing to the shape of the
carina, being always sharper or narrower than the rostral end. The size
of the orifice, in proportion to the shell, varies accordingly as the
latter is more or less conical or cylindrical. The orifice is either
entire or more or less deeply toothed, in proportion to the degree of
obliquity of the summits of the radii and alæ. The radii almost always
have smoother surfaces than the parietes. In some few species the
radii are not developed, the sutures being marked only by fissure-like
lines; in others they are very narrow, and in this case their upper
margins are generally rounded and smooth, instead of being straight
and jagged. The carino-lateral compartments are usually much narrower
than the lateral compartments, occasionally in an extreme degree, as in
_B. allium_. The shell is generally strong, sometimes to a wonderful
degree; but the strength and thickness vary in the individuals of some
of the species. By the action of hot caustic potash, the compartments
in several species, such as _B. Hameri_ and _crenatus_, separate on a
touch; in others, they adhere so strongly as to prove that the sutures
must be calcified together. In this genus we have the largest known
sessile cirripede, viz., the _B. psittacus_, and on the other hand many
small species; but it is very difficult, except in well-known species,
to ascertain the average or even the maximum dimensions.

_Scutum._--This valve is almost triangular, with the basi-tergal corner
more or less rounded off. The prominent lines of growth are sometimes
crossed by longitudinal striæ. Internally, the articular ridge projects
to a very different degree in the different species; its lower end is
sometimes (as in _B. lævis_, Pl. 4, fig. 2 _c_) produced downwards as a
small, sharp, free style; there is always an articular furrow receiving
the inflected margin of the tergum. There is always an impression left
by the attachment of the adductor scutorum muscle; and often its lower
side is bounded more or less closely by a sharp adductor ridge, running
some way down the valve; this ridge is occasionally almost confluent,
in its upper part, with the articular ridge, and in this case sometimes
it forms, together with the inflected tergal margin, a large tubular
cavity, running up, as in _B. psittacus_ (Pl. 2, fig. 3 _c_), almost
to the apex of the valve. Almost invariably there is a slight pit or
depression for the lateral depressor muscle; sometimes within the
depression there is a little ridge, as in _B. perforatus_ and _nubilus_
(Pl. 4, fig. 3 _a_, and Pl. 6, fig. 2 _a_); and in the case of _B.
vestitus_, _flosculus_, and _imperator_ (Pl. 8, figs. 3 _a_, 4 _a_),
there are regular crests for this same purpose. The rostral depressor
muscle is usually attached in a small pit formed by the folding over of
the lower part of the occludent margin: in _B. imperator_ (Pl. 8, fig.
4 _a_) there are regular crests for its attachment, and traces of them
may be discovered in _B. vestitus_.

_Tergum._--This valve is more nearly triangular than any other shape,
with the spur more or less prominent. The apex generally projects a
little above the level of the scutum; in some species it consists of
a triangular and solid, in others (Pl. 2, fig. 3 _b_) of an almost
cylindrical, extremely sharp, inwardly curved, and very prominent beak.
This beak is generally purple; it is sometimes hollow, and occupied by
a thread of corium. Its formation, and the apparent sliding up of the
whole tergum, so as to project above the scutum, has been described
under the family. From an account given to me by a person who kept
_B. porcatus_ alive, the beaks appear to be used, when the operculum
is touched, as an organ of defence,--the animal striking with them.
The tergal margin is more or less inflected; and the carinal margin
is convex in different degrees, and, in some species, is added to
by upturned zones of growth. The basal margin either forms a nearly
straight line on opposite sides of the spur, or more commonly slopes
towards it in various manners. The spur, or basal projection, is
rarely placed in the middle of the basal margin, generally near,
sometimes close to the basi-scutal angle; it varies much in length
and breadth, and is sometimes even half the width of the valve. The
surface of the valve is almost always more or less depressed, sometimes
so much as to form a deepish furrow, the "longitudinal furrow," which
extends from the apex to the extremity of the spur. When the furrow is
deep, its sides, as the specimen grows old, almost always become folded
inwards, so as to touch, and then the furrow becomes converted into a
closed fissure: in this latter case the folded sides generally form a
central crest on the spur. Internally, in the middle of the upper part
of the valve, the articular ridge is more or less prominent, forming
the carinal margin of the articular furrow, in which the articular
ridge of the scutum is lodged; occasionally, however, this articular
ridge can hardly be said to exist. In most species the tergal depressor
muscle is attached to sharp crests on the basi-carinal corner of the
valve, but these are almost obliterated in other species.

_Compartments._--The external appearance of the shell has already been
described. In the most typical species, the parietes consist of an
outer and inner lamina, separated by strong longitudinal septa; these
septa are denticulated on both sides at their bases, but only close
to the inner lamina; in fact the inner lamina is apparently formed by
the union, thickening, and production, of some of the denticuli. As
it is not the innermost of the denticuli on the basal edges of the
longitudinal septa, which thus become united into a solid layer, the
longitudinal septa form slightly projecting, longitudinal ribs on the
inner lamina. These internal ribs are longitudinally striated; in
old specimens they often become obliterated, especially in the upper
part of the shell. The parietal tubes or pores (occupied by threads
of corium) are generally square and large; but in _B. Ajax_ they
are very small, and in _B. glandula_ often extremely minute. In the
upper part of the shell, and sometimes low down, they are generally
crossed by thin, transverse, calcareous septa: in some species, as
in _B. perforatus_, and in some varieties of _B. amphitrite_, the
upper ends of the tubes are filled up solidly with shell. In some
varieties of _B. crenatus_ and of _amphitrite_, the longitudinal septa,
near the outer lamina, divide, thus giving rise to a very imperfect
row of outer short tubes. In _B. vinaceus_ (Pl. 2, fig. 7 _d_) the
inner lamina is cancellated instead of being solid, which is caused
by the basal denticuli of the longitudinal septa being simply united
together by their ends and crossed by transverse septa, instead of
being consolidated into a mass. In several species, as in _B. Hameri_,
the walls consist only of the outer lamina with longitudinal ribs, no
inner lamina having been formed; the ribs here evidently answer to the
longitudinal septa in the foregoing species. In _B. flosculus_ and
_imperator_ the walls are solid, their basal margins being formed of
irregular, elongated points, and little ridges (Pl. 8, fig. 4 _c_),
which apparently prefigure the more regular longitudinal ribs or septa.
In _B. balanoides_ the walls are generally either nearly smooth and
solid, or irregularly cancellated; in _B. cariosus_ (Pl. 7, fig. 3 _b_)
two or three rows of short irregular tubes are formed by unequally
branching septa, almost as in the genus _Tetraclita_.

The _Radii_, in all the species, are constructed essentially on the
same plan as the parietes; thus, in the typical forms, there is an
outer and inner lamina, with septa, which, near the inner lamina, are
furnished with denticuli on both sides; hence the radii are permeated
by pores or tubes, like the parietes; but this holds good only in the
first section of the genus, for, in the other species, the tubes are
filled up quite solidly. The denticuli on the septa often occur only on
one side, or disappear altogether; and, lastly, the septa themselves
often appear merely like little teeth, or disappear altogether as in
_B. Hameri_, or occur only near the bases of the radii, as in _B.
amaryllis_. A slight furrow in the compartment, against which each
radius abuts, is generally marked by the septa and their denticuli. In
regard to the _alæ_, their lateral or sutural edges are either thin
and smooth, or, more commonly, finely crenated or ribbed. The little
transverse crenations are homologous with the septa in the radii and
parietes. The edges of the alæ are usually received in a furrow. The
diametric growth of the shell is effected by the growth of the radii
and alæ, and chiefly by that of the former. The sutural and lateral
edges of both radii and alæ are added to, either quite up to their
summits, or only low down, and during the continued growth of the
shell, lower and lower down; in accordance with this difference in
growth, the summits of the radii and alæ become either very oblique,
or they extend parallel to the basis, that is, from tip to tip of the
adjoining compartments. When the radii and alæ are added to, as is most
usual, above the level of the opercular membrane, and therefore above
the sack, ribbons of corium run up the sutures from the sack, higher
or lower, according to the height to which, in the different species,
the edges of the radii and alæ continue to be added to. The obliquity
of the summits of the radii and alæ varies, in some cases, in the same
species. It often happens that when the summits of the radii are very
oblique, the summits of the alæ are but little so; and the converse;
both, however, are often either equally oblique, or both have square
summits. The _sheath_ extends either one third or more than half down
the shell; its basal margin often (Pl. 25, fig. 1, K′) freely depends
or overhangs the inner lamina of the walls.

_Basis._--In typical species the basis is calcareous, and consists of
an upper and lower lamina, separated by radiating septa, forming pores.
In the same manner as the septa of the parietes sometimes, though
rarely, become irregularly divided near the outer lamina, forming
outer pores, so it is, but in a much more marked degree, with the
basis. The basis in such cases becomes extremely thick, and consists
of an upper, thin lamina, with the regular radiating septa and pores,
and of an underlying, thick, cancellated mass, which seems wholly to
result from the dividing and sub-dividing of the septa. The basal
radiating pores, like the parietal pores, are closed at intervals by
calcareous transverse septa. The basal points of the parietal septa
enter the orifices of the basal pores, and the threads of corium pass
into the latter, between the denticuli of the parietal septa. In some
species, as in _B. crenatus_ and _Hameri_, the basis is perfectly
solid, the upper lamina being absent, just as in some species, the
internal lamina of the parietes is absent. In _B. flosculus_ the basis
is calcareous, but consists of so excessively thin a film as hardly
to be distinguished: it presents, moreover, as also is the case with
_B. imperator_, a beaded structure. Again, in some few species, as
in _B. balanoides_, the basis is simply membranous. When the basis
is thin, it is always flat, and is closely moulded to the irregular
surface of attachment; and in this case, when specimens are crowded
together, their elongation is effected exclusively by the growth of the
walls; but, when the basis is thick, it sometimes becomes, in crowded
groups, deeply, but irregularly, cup-formed, or cylindrical, as in _B.
psittacus_ and _perforatus_. In _B. allium_, however, which inhabits
massive corals, the basis is as regularly concave or cup-formed as
in the genus Pyrgoma: in _B. calceolus_ and its allies, and in some
varieties of the fossil _B. inclusus_, the basis is boat-formed, with
its lower surface deeply grooved longitudinally from clasping the
stem of the Gorgonia or other zoophyte, to which it was attached.
In certain varieties of _B. lævis_ it is very remarkable that the
deeply cup-formed basis becomes, owing apparently to the whole shell
having grown too deep for the animal, half-filled up with irregular,
calcareous, transverse plates (Pl. 4, fig. 2 _a_), resting one upon the
other by irregular points or pillars. The cementing apparatus has been
sufficiently described under the Family.

_Mouth._--The _labrum_ is always notched; sometimes it has no teeth,
but generally there are three on each side; in _B. balanoides_ there
are five or six on each side; and in _B. improvisus_ and _eburneus_
there is a whole row of teeth (Pl. 26, fig. 2, _e′_), graduated in
size, on each side of the notch. The _palpi_ are large, with their
apices nearly touching, and furnished with long spines. The _mandibles_
have, as it appears, normally, five teeth, but the two lower teeth
are always small and often rudimentary, and almost confluent with
the inferior, sometimes spinose angle. The _maxillæ_ have either a
simple edge, or a notch under the pair of large upper spines, or the
lower part forms (Pl. 26, fig. 7) a step-formed projection: there are
generally two lower spines, placed singly or not in pairs, larger than
the others, with the exception of the uppermost pair. The _outer
maxillæ_ are, on their inner faces, obscurely divided into two lobes.

_Cirri._--The rami of the first pair are unequal, the shorter one
sometimes not being more than half the length of the other ramus:
the segments of the shorter ramus are broad, and are, together with
the lower segments of the longer ramus, thickly clothed with spines;
in some species, as in _B. perforatus_, the anterior surfaces of the
segments, more especially of the shorter ramus, and of both rami of the
second pair are produced (Pl. 29, fig. 4), so as sometimes to form very
remarkable projections. The segments of the second and third pairs are
always thickly clothed with spines, as also are their pedicels. The
third pair is rather longer than the second; but in _B. vestitus_ and
_imperator_ it is much longer, and is otherwise somewhat different.
The dorsal and basal margin of the pedicel of the third pair, in some
of the species, as in _B. tintinnabulum_, is produced backwards on the
thorax, and forms a membranous plate fringed with fine spines. The
three posterior and longer pairs of cirri have from three to rarely
eight or ten pairs of long spines on each segment, with generally one
or two minute spines in the middle between each pair: their pedicels
have a regular double row of spines.

The _penis_ is long and hairy: in most of the species there is, at its
dorsal basis, a small, sharp, flattened, imperforate projection; first
observed by Poli: but this is sometimes absent, as in _B. crenatus_,
though present in the closely allied _B. balanoides_; and its presence
is variable in _B. tintinnabulum_. All the species have large plicated
_branchiæ_. The base of the _sack_ in several species is furnished
with inwardly projecting filamentary appendages. In _B. perforatus_,
_crenatus_, and _improvisus_, and I believe in other species, the upper
part of the _stomach_ is furnished with a circle of branching cæca.


_On the variation of the species_; _their arrangement and affinities_;
_value of the characters used_; _changes during growth_.--Owing to the
great variation in external characters, to which almost all the species
are subject, and likewise to the genus being a very natural one, that
is, to the species following each other in close and natural order, it
is not easy to exaggerate the difficulty of identifying the species,
except by a deliberate examination of the internal and external
structure of each individual specimen. Every one who has collected
sessile cirripedes must have perceived to what an extent their shape
depends on their position and grouping. The surface of attachment has
a great effect on that of the shell; for as the walls are added to at
their bases, every portion has at one time been in close contact with
the supporting surface; thus I have seen a strongly-ribbed species
(_B. porcatus_) and a nearly smooth species (_B. crenatus_) closely
resembling each other, and both having a peculiar appearance, owing
to their having been attached to a pecten. Dr. Gray has pointed out
to me specimens of _B. patellaris_, curiously pitted like the wood
to which they had adhered; and numberless other instances might be
added. Quite independently of the effect produced by the surface of
attachment, the degree to which the longitudinal folds and ribs are
developed on the parietes, is variable in most of the species, as in
_B. tintinnabulum_, _vestitus_, and even in _B. porcatus_; the presence
or entire absence of these ribs often surprisingly alters the whole
aspect of the shell. The persistence of the so-called epidermis is in
some degree variable; and in _B. lævis_ we have groups of specimens
absolutely naked, and others uniformly clothed with a brown membrane.
Again, some species in certain localities are all subject to the
disintegration of the entire outer lamina of the walls; and in such
cases (as with _B. perforatus_) there is not the smallest resemblance
between the corroded and perfect specimens. The size of the orifice,
and consequently of the operculum, compared with the shell itself,
varies accordingly as the shell is more or less conical or cylindrical;
in the latter case, the summits of the radii are generally more oblique
and the aperture consequently more deeply toothed than in the more
conical varieties. Size is a serviceable character in some cases, but
very many specimens are required to ascertain the average or maximum
size of each species, for there is no method of distinguishing a
half-grown from a full-grown specimen; and I believe, as long as the
individual lives, so long does it go on moulting and growing. _Colour_
is of very considerable service; though the precise tint varies greatly
in almost every species; and what is a far more serious evil, the
majority of the species have their white or nearly white varieties, the
latter sometimes as numerous as the coloured ones: in _B. perforatus_,
_lævis_, _flosculus_, _amphitrite_, and in several other species, the
common white varieties are eminently deceptive.

Besides the slight variation in the obliquity of the summits of the
radii and alæ, dependent on the more or less cylindrical form of
the shell, in some species, as in _B. tintinnabulum_, _amphitrite_,
_improvisus_, _trigonus_, and _porcatus_, their obliquity also varies
occasionally from unknown causes, and thus greatly affects the general
appearance of the shell. In some few species, as in _B. perforatus_,
the radii are often either not at all developed, or are of very
variable width; in others, when the shell has become cylindrical, or
when very old, the radii cease to grow, and from the disintegration of
the whole upper part of the shell, with the continued growth of the
lower part, the radii at last come to exist as mere fissures: I have
seen instances of this in _B. psittacus_, _nigrescens_, and _porcatus_.
Nevertheless, the obliquity of the upper margin, and the breadth of the
radii are useful characters; and still more useful is the fact whether
the upper margins are smooth and arched, or straight and jagged. The
fact of the terga being more or less beaked is useful: as is, likewise,
the presence of striæ, or furrows, or rows of pits, radiating from the
apices of the scuta; but to ascertain the presence of these marks,
it is almost invariably necessary to take out the scuta, clean, and
examine them with a lens; these ridges and furrows, moreover, in some
species, as is strikingly the case with _B. tintinnabulum_, and in
less degree with _B. trigonus_, _lævis_, and _concavus_, appear and
disappear, and vary without any apparent cause.

Now if we reflect that form, size, state and nature of the surface,
presence of epidermis, relative size of the orifice, presence of
longitudinal ribs, tint, and often the existence of any colour, are
all highly variable in most of the species; and that the obliquity
of the summits of the radii, and the presence of longitudinal striæ
on the scuta, are variable in several species; we shall perceive how
difficult it must ever be to distinguish the species from external
characters. As some evidence of this, I may mention that, after having
described nearly 40 species, and when my eye was naturally able to
appreciate small differences, I began carefully to examine varieties of
_B. tintinnabulum_, _amphitrite_, _improvisus_, _porcatus_, _vestitus_,
&c., without even a _suspicion_ that they belonged to these species, at
that time thoroughly well known to me; yet in the cases here referred
to, there could be no doubt, when a perfect series was examined,
that the specimens were only varieties. From this cause the labour
of naming a collection is great. Let no one attempt to identify the
species of this genus, without being prepared to disarticulate, clean,
and carefully examine with a microscope the basis and parietes, and
both the under and upper surfaces of the opercular valves; for I feel
convinced, that he will otherwise throw away much labour. Moreover, in
many cases, it is almost necessary, on account of the variability of
the characters, to possess several specimens. From these facts, I have
not hesitated to form my sections on characters which require close
examination, though I would gladly have seized on external characters,
could I have found such even moderately constant.

The least varying, and therefore most important characters, must
be taken from the internal structure of the parietes, radii, and
basis: not that these characters are absolutely invariable; thus the
porosity of the parietes is slightly variable in _B. glandula_, and
highly variable in the fossil _B. unguiformis_; it is also highly
variable in _B. balanoides_, but under a systematic point of view
this is unimportant, as the section including this latter species is
well defined by the membranous basis. The porosity of the basis is in
some degree variable in _B. nubilus_, _improvisus_, and _patellaris_;
and in _B. flosculus_ we see signs of a passage from a calcareous to
a membranous basis. Characters derived from the general shape, and
from the ridges and depressions on the under side of the scuta and
terga, especially of the scuta, are highly serviceable. The terga,
indeed, in many species, as in _B. amphitrite_, vary considerably,
and are affected by the general shape of the shell. Unfortunately the
differences are not very great between the scuta of the different
species. The cause of the opercular valves offering more useful
characters, as far as outline is concerned, than do the walls of
the shell, is no doubt due to their being almost independent of any
influence from the nature of the surface of attachment. Even the ridges
and depressions on the under side of the scuta, which are in direct
connection with the muscles and soft parts of the animal, vary to a
certain extent: thus the length and prominence of the adductor ridge
is decidedly variable in _B. concavus_ and _tintinnabulum_, and in a
less degree in _B. lævis_; the size and form of the little cavity for
the lateral depressor muscle varies in many species; so does the exact
shape and degree of prominence of the articular ridge. There is one
character in the terga, which at first would be thought very useful,
namely, whether an open longitudinal furrow, or a closed fissure
runs down the valve from the apex to the spur; but it is found that
the furrow almost always gradually closes up during growth; and as a
consequence of this, the width of the spur compared to that of the
whole valve, as well as its distance from the basi-scutal angle, and
the form of its basal extremity, all vary in some degree. The length
of the spur sometimes varies considerably, as in _B. concavus_ and
_amphitrite_. The parts of the mouth are only occasionally serviceable;
for the teeth on the labrum, and the state of the lower teeth on the
mandibles, and the presence of a step-formed projection at the lower
angle of the maxillæ, are all often variable. The relative lengths of
the two rami of the first pair of cirri, the degree of protuberance
of the segments, and the number of pairs of spines on the segments of
the posterior pairs of cirri, are sometimes useful; but the relative
lengths of the cirri, and more especially the numbers of pairs of
spines on the posterior cirri, are apt to vary. Finally, I must
express my deliberate opinion, that every part and organ, internal and
external, in Cirripedes, is liable to some amount of variation in some
of the species.

I must now point out the principal changes which supervene during
growth, and which cannot properly be called variations. In the first
place, I think, it is scarcely possible to recognise a species when
under the 1/10th of an inch in diameter. In some cases, as in _var.
d'Orbignii_ of _B. tintinnabulum_, the shell is invariably coloured
when old, but quite white when very young. Generally the tints become
very much darker with age. Some species, which usually or invariably
have, when mature, longitudinally folded walls, as _B. flosculus_
and _balanoides_, are perfectly smooth in youth. The walls in _B.
eburneus_, when young, have white, hyaline, longitudinal lines, and
are naked; whereas, with advancing age, these lines disappear, and the
subsequently formed shell becomes covered with membrane. The summits
of the radii are apt to be oblique in the young of _B. Capensis_,
_psittacus_, and _tintinnabulum_, whereas they are generally quite
square in old specimens. In _B. eburneus_, _cariosus_, and in a lesser
degree in _B. psittacus_, the scuta become longitudinally striated
only with age. On the other hand, in very young specimens of _B.
tintinnabulum_, the scuta sometimes are deeply impressed by little
pits placed in rows. I have already alluded to the longitudinal
furrow on the tergum so entirely changing its character, owing to the
edges becoming, during growth, folded inwards; this likewise causes a
decrease in the proportional breadth of the spur. In old specimens of
_B. flosculus, var. sordidus_, the whole tergum is much more elongated
than in young specimens. The basal margin of the sheath is hollow
beneath in the young of _B. cariosus_ and of some other species, but
in the old it is continuous with the inner surface of the walls. The
inner lamina of the parietes generally loses, to a certain extent, its
longitudinally ribbed character in old age. The basis is solid, instead
of being porose, in very young specimens of _B. improvisus_. In all the
species, the carino-lateral compartments, in early age, are very narrow
in proportion to the width of the lateral compartments; and in all, at
this early period, the operculum is large in proportion to the whole
shell. The number of spines on the edge of the maxillæ, the number of
segments in all the cirri, and the number of spines on each segment,
are few in early youth, and go on increasing with each successive
exuviation: the pedicels of the cirri are long in proportion to the
rami at this same early age.[84]

    [84] In some specimens of _Balanus perforatus_ I made the following
    enumeration of the number of segments in the cirri:--

    +--------------+--------------+--------------+------------------+
    |              |Basal diameter|Basal diameter|   Medium sized   |
    |              |   of shell   |   of shell   |  specimen about  |
    |              |  1/20th of   |   1/5th of   |3/4ths of an inch |
    |              |   an inch.   |   an inch.   |in basal diameter.|
    +--------------+--------------+--------------+------------------+
    |First cirrus }|      ?       |      11      |        17        |
    |shorter ramus}|              |              |                  |
    |              |              |              |                  |
    |Second cirrus |    4 or 5    |       9      |        13        |
    |              |              |              |                  |
    |Sixth cirrus  |   9 or 10    |      19      |31, in another 36 |
    +--------------+--------------+--------------+------------------+

    In the specimen 1/5th of an inch in basal diameter, each segment
    of the posterior cirri carried five pairs of spines; whereas,
    in full-grown specimens, there are six or seven pairs. In the
    1/20th of an inch specimen, on the inner maxillæ, there were
    no spines between the upper large and the lower large pair of
    spines; whereas, in the 1/5th of an inch specimen, there were five
    intermediate spines, and in larger specimens nine or ten spines.

Notwithstanding the difficulties now enumerated, I hope that, owing to
having examined a vast number of specimens of the most varying species,
I have not fallen into very many errors. I have endeavoured to err
on the side of making too few instead of too many species. In those
cases, however, in which I have seen only a few specimens, I have been
sometimes compelled to decide without sufficient evidence.

I would gladly have divided this genus, already including 45 species,
into smaller genera; but so far from being enabled to do so, I
have been compelled to form my Sections (immediately to be given)
on characters not absolutely invariable, and far from obvious. I
was particularly anxious to separate the elongated species with a
boat-formed basis, which are attached to Gorgoniæ, and which form the
genus Conopea of Say, but I was unable to effect their separation even
as a sub-genus; for _B. navicula_ and _cymbiformis_ graduate in the
most insensible manner through _B. galeatus_ (the type of Say's genus)
and _B. calceolus_ into _B. stultus_, and this into _B. Ajax_; yet
this latter species has even been described as a mere variety of the
typical _B. tintinnabulum_! Indeed, so insensible is this graduation,
that the first and second sections of the genus are hardly distinct. I
fully admit, that if _B. stultus_ and _Ajax_ had never existed, _B.
calceolus_ and its three allies might have formed as natural a little
group, though difficult to be characterised, as does the sub-genus
Acasta; or perhaps this group and Acasta might have been combined
together. These same species, viz., _B. calceolus_ and its allies,
are intimately allied to _B. terebratus_ and _inclusus_, which are
contained in the last section (F) of the genus; and this shows that Dr.
Gray's proposed genus Chirona, including the species with non-porose
parietes in sections (E) and (F), could hardly have been instituted,
even if the porosity of the parietes had not been variable in _B.
unguiformis_, _balanoides_, and _glandula_. My fourth section (D),
founded on the basis not being porose, is perhaps the weakest of the
divisions, as may be seen in the list of exceptions appended to the
sectional headings.

The arrangement of the species is, I think, as natural as a linear
one could be made: every one knows how irregularly and in how many
directions the lines of affinity in every natural genus branch out.
Some few species stand rather isolated, as _B. declivis_; and _B.
allium_, _cepa_, and _quadrivittatus_ in a little group by themselves.
I have already shown how the species in the first and second sections
(A and B) blend into each other; and that the blending species are
likewise allied to some in the last section (F); furthermore, I shall
have occasion to show that these same species can hardly be separated
naturally from the sub-genus Acasta. The first section, moreover,
passes into the third (C) by _B. tulipiformis_; and the third into
the fourth (D) by _B. improvisus_, _nubilus_, _corrugatus_, and
_patellaris_: the fifth and sixth (E and F) sections are closely
connected together by _B. cariosus_ and _flosculus_; and these two
sections blend into the fourth (D) through _B. unguiformis_ and
_balanoides_, and lastly, into the third (C) section by _B. dolosus_
and _improvisus_.

The genus, as we have just seen, is hardly separated from the sub-genus
Acasta; by _B. allium_ it tends to pass through the sub-genus Creusia
into Pyrgoma; and by _B. imperator_ and _flosculus_ it graduates into
Elminius and Tetraclita.


_Geographical Distribution._--This, which is much the largest genus of
sessile cirripedes, has its species scattered over the whole world,
from the arctic regions, in lat. 74° 48′, where we have _B. crenatus_
and _porcatus_, throughout the tropical seas, to Cape Horn, where
_B. flosculus_ adheres to the coast-rocks. Many of the species have
individually very wide ranges; thus _B. tintinnabulum_ and _amphitrite_
are found throughout the warmer seas; but the wide distribution of
these species may be partly due to their frequent adhesion to ships'
bottoms: _B. crenatus_ ranges from the frozen seas, in lat. 74° 48′
north, to the West Indies and Cape of Good Hope--a wonderful endurance
of the most opposite climates. _Balanus improvisus_, again, extends
from Europe to Nova Scotia, thence southward to Patagonia, and up
the western coast of S. America, someway north of the Equator. Most
of the species have considerable ranges; thus of the six species
found on the eastern shores of northern America, five of them occur
in Great Britain. Of the thirty-six species of which the habitats
are known, exactly one third, or twelve, inhabit both the torrid and
temperate zones, these being divided by the isocryme of 68°; nine
are found exclusively in the torrid, and fifteen exclusively in the
temperate zones. Within the warmer latitudes, and especially in the
southern hemisphere, Tetraclita and Elminius to a certain extent
supplant Balanus. In depth, the species range from the upper limits
of the tidal zone to even fifty fathoms. _Balanus improvisus_ and
_eburneus_ are able to survive in brackish water. The different species
are attached to various surfaces--rocks, shells, timber, floating
objects, sea-weed, lamelliform corals, Milleporæ, Gorgoniæ, and even to
sponges. Mr. G. B. Sowerby has remarked[85] that in the species from
the southern hemisphere it is the basis, and in the species from the
northern hemisphere it is the parietes, which are elongated, when the
individuals, from being crowded together, become cylindrical; but this
is erroneous; _B. perforatus_, in the northern hemisphere, sometimes
has an elongated basis; but no doubt the basis of our commonest
species, as _B. balanoides_, _crenatus_, and _porcatus_, from being
either membranous or thin, does not become cup-shaped; whereas this
structure is conspicuous in _B. psittacus_ and _lævis_, the two
commonest species in southern South America.

    [85] Darwin's 'Geology of S. America,' p, 264.

_Fossil Species._--Having already given, under the Family, some account
of the geological history of sessile cirripedes, short as it is, I here
only allude to the subject in order to state my conviction that species
cannot be satisfactorily distinguished in a fossil state, and rarely
in a recent state, without an examination of the opercular valves.
Nothing, indeed, could have been easier than to have affixed names
to many groups of specimens, having different aspects, but to feel
sure that these were really distinct species requires better evidence
than can be afforded by the shell, without the operculum. No doubt,
in some of the smaller sections of the genus--for instance, in that
characterised by a membranous basis--it would have been possible to
have distinguished some or several fossil species; but such have not as
yet been found. When the specimens are much fossilised, it is, indeed,
difficult to make out the primary points of structure--namely, whether
the parietes, radii, and basis are porose: to do this it is sometimes
necessary to rub down, polish, and carefully examine, a transverse
section of a piece of the shell.


_Sections of the Genus._

[A]

_Parietes, and basis, and radii permeated by pores._

[B]

_Parietes and basis sometimes permeated by pores, sometimes not; radii
not permeated by pores; shell elongated in its rostro-carinal axis;
basis boat-shaped, attached to Gorgoniæ and Milleporæ._

[C]

_Parietes and basis permeated by pores; radii not permeated by pores._

[D]

_Parietes permeated by pores. Basis and radii not permeated by pores._

[E]

_Basis membranous._

[F]

_Parietes and radii not permeated by pores. Basis sometimes permeated
by pores, sometimes not, sometimes excessively thin and hardly
distinguishable._[86]

    [86] The following species might, owing to variation, or to the
    obscurity of their structure, without care, be classed in the wrong
    sections:--in A, _Balanus Ajax_, from living attached to Milleporæ,
    is sometimes elongated in its rostro-carinal axis, and from having
    its radii only finely porose, it might be classed in B: on the
    other hand, _Bal. stultus_ is sometimes but little elongated, and
    the basis hardly boat-formed, and hence might be classed in A. The
    distinction between all the species in (B) and the sub-genus Acasta
    is artificial.

    In sections C and D, I have seen one specimen of _B. spongicola_
    with a solid basis, and very young specimens of _B. improvisus_
    are thus characterised, and therefore these species are liable to
    be placed in the wrong section, D: _Bal. nubilus_, also, has part
    of its base non-porose, and therefore likewise might be placed
    in D: on the other hand, the circumference of the basis in _B.
    patellaris_ is often porose, and hence this species, which belongs
    to D, might be placed in C.

    In _Bal. glandula_, in D, the parietes of the compartments, without
    several were examined, might be thought to be solid, and therefore
    this species might be wrongly placed in F; on the other hand, the
    fossil _B. unguiformis_, in F, often has porose parietes, and such
    specimens would naturally be placed in D. Lastly, without care, _B.
    flosculus_ might be thought to have a membranous basis, and so get
    placed in E.

    The genus Pachylasma, without an examination of the animal's body,
    might easily get misplaced in the wrong genus, amongst the species
    in the last section (F) of Balanus, yet there can be no doubt that
    Pachylasma belongs to the Chthamalinæ.



_Section_ A.



1. BALANUS TINTINNABULUM. Pl. 1, fig. _a_-_l_; Pl. 2, fig. 1 _a_-1 _o_.

  LEPAS TINTINNABULUM. _Linn._ Syst. Naturæ, 1767.

  ---- ---- _Ellis._ Phil. Transact., vol. 50, 1758, Tab. 34, figs. 8
        and 9.

  ---- ---- _Chemnitz._ Neues. Syst. Conch., 8 B. (1785),
        Tab. 97, figs. 828-831.

  BALANUS TULIPA. _Bruguière._ Encyclop. Meth., 1789; sed non _B.
        tulipa alba_, in _Chemnitz_; neenon _B. tulipa_, _O. F.
        Müller_, Zoolog. Dan.; neenon _B. tulipa_, Poli, Test. ut.
        Siciliæ.

  ---- ---- _G. B. Sowerby._ Genera of Recent and Fossil Shells, Tab.
        Genus Balanus.

  LEPAS CRISPATA (_var._) _Schröter._ Einleitung Conch. vol. iii, Tab.
        9, fig. 21.

  ---- SPINOSA (_var._) _Gmelin._ Linn. Syst. Nat.

  ---- TINTINNABULUM, SPINOSA, CRISPATA ET PORCATA. _W. Wood._ General
        Conchology, 1815, Pl. 6, figs. 1, 2. Pl. 7, figs. 4, 5. Pl. 8,
        figs. 1-5.

  BALANUS TINTINNABULUM. _Chenu._ Illust. Conch.[87]

  ---- D'ORBIGNII (_var._) _Chenu._ Illust. Conch., Tab. 6, fig. 10,
        sed non Tab. 4, fig. 13.

  ---- CRASSUS (Foss.) _Sowerby_ (!). Min. Conch., 1818. Tab. 84.

    [87] Chenu gives several admirable figures of this species; but
    he confounds some forms certainly distinct under this name, for
    instance the _B. tulipiformis_ of this work.

_Shell varying from pink to blackish purple, often striped and ribbed
longitudinally: orifice generally entire, sometimes toothed. Scutum
with the articular ridge broad and reflexed. Tergum with the basal
margin generally forming a straight line on opposite sides of the spur._

  _Var._ (1) communis (Pl. 1, figs. _a_, _b_, _f supra_; Pl. 2, figs. 1
  _a_, 1 _c_, 1 _d_, 1 _e_, 1 _i_, 1 _k_): _conical or tubulo-conical;
  smooth or moderately ribbed longitudinally; colours varying from
  purplish-pink to blackish-purple; often in obscure longitudinal
  stripes; orifice of shell rounded-trigonal_.

  _Var._ (2) vesiculosus (_young_) (Pl. 2, fig. 1 _h_): _exterior
  surface of the scuta impressed with small square holes, arranged in
  two or more rows, radiating from the apex of the valve_.

  _Var._ (3) validus (Pl. 1, figs. _c_, _f infra_): _globulo-convex;
  coarsely ribbed, ribs flexuous; either smooth or rugged; pale
  chocolate purple or pink; shell extremely strong; orifice almost
  circular_.

  _Var._ (4) zebra (Pl. 1, fig. _g_): _conical; rich chocolate purple
  with broad snow-white ribs; sheath bright chesnut colour; summits of
  alæ oblique; orifice almost circular_.

  _Var._ (5) crispatus (_Schröter_) (Pl. 1, fig. _h_): _pale blueish
  or pinkish-purple, with irregular rough projections, or with short,
  sharp, needle-like points; scuta with their exterior surface either
  plain, or with radiating lines formed of hood-like projecting points_.

  _Var._ (6) spinosus (_Gmelin_) (Pl. 1, fig. _i_): _globulo-conical
  or cylindrical; shell rather thin, with long, upcurved, nearly
  cylindrical, very sharp points; colours very pale; attached to other
  specimens, and to Lepas anatifera_.

  _Var._ (7) coccopoma (Pl. 1, fig. _d_; Pl. 2, fig. 1 _f_, 1 _l_,
  1 _o_): _globulo-conical; orifice small, rounded; walls generally
  smooth, thick; intense rose-colour, sometimes most faintly striped
  longitudinally with varying shades of pink; radii tinged with
  purple; scutum sometimes as in_ var. communis, _sometimes with its
  basi-tergal corner much cut off, with the adductor ridge prominent,
  the pit for the depressor muscle deep, and the articular ridge broad
  and hooked; tergum sometimes as in_ var. communis, _sometimes with a
  broader spur, placed nearer to the basi-scutal corner of the valve_.

  _Var._ (8) concinnus (Pl. 1, fig. _e_; Pl. 2, fig. 1 _g_):
  _globulo-conical; walls finely ribbed; dull purple, tinged and
  freckled with white; scutum, with a broad, hooked, articular ridge,
  with an extremely sharp plate-like adductor ridge, and with a cavity,
  bordered by a plate, for the rostral depressor muscle; tergum as in_
  var. 1.

  _Var._ (9) intermedius: _radii with their summits slightly oblique;
  parietes pale blueish purple, with narrow dark purplish-blue
  longitudinal lines; sheath with the internal surface of the rostrum
  and lateral compartments much darker colored than the internal
  surface of the carina and carino-lateral compartments; scuta and
  terga as in_ var. communis.

  _Var._ (10) occator (Pl. 1, fig. _k_; Pl. 2, 1 _b_): _radii with
  their summits slightly oblique; parietes smooth, or ribbed, or
  spinose; very pale blueish-purple, with narrow darker longitudinal
  lines; sheath with the internal surface of the rostrum and lateral
  compartments dull blue, whilst the corresponding parts of the carina
  and carino-lateral compartments are white; scuta with small, sharp,
  hood-formed points, arranged in straight radiating lines; terga with
  the spur placed at either its own width, or less than its own width,
  from the basi-scutal angle_.

  _Var._ (11) d'Orbignii (_Chenu_) (Pl. 1, fig. _l_; Pl. 2, 1 _m_, 1
  _n_): _radii with their summits oblique, and the orifice of the shell
  rather deeply toothed; shell conical or tubulo-conical, smooth, or
  rugged; colour dull purplish-lilac, with the tips of the parietes and
  a band along one side of the radii quite white; sheath rather darker
  at the rostral than at the carinal end; scuta as in_ var. 1; _terga
  as in_ var. occator.

  _Habitat._--West coast of Africa, on mytili; Madeira, on rocks; West
  Indies; Cape of Good Hope, on a patella and on kelp; mouth of the
  Indus; East Indian Archipelago; Sydney, Australia, attached to Lepas
  anserifera, adhering to a floating cane; Peru; Galapagos Islands;
  West Mexico; California. Extremely common on ships' bottoms arriving
  from West Africa, India, and China, often associated with _B.
  amphitrite_.

  _Fossil_ Red Crag, England; Mus. S. Wood and J. de C.
  Sowerby.--Touraine (?); Mus. Lyell.


  _General Remarks._--This, the first species of Balanus, is, perhaps
  with the exception of _B. amphitrite_, the most difficult and
  variable in the genus. There are some other species which vary quite
  as much in external appearance; for instance, _B. perforatus_; but
  _B. tintinnabulum_ also varies in far more important points, as in
  the proportions and structure of the opercular valves. The difficulty
  in determining whether or not the differences are specific, is
  wonderfully increased by whole groups of individuals varying in
  exactly the same manner. I have seen three most distinct varieties
  taken from the bottom of the same vessel, so that I did not at first
  entertain the least doubt that they were three distinct species. I
  may mention, as showing the vacillations which I have experienced
  on this subject, that beginning with the impression, that the above
  three varieties were really distinct species, after going over the
  several immense collections of specimens placed at my disposal,
  I came to the conclusion that the above three, and several other
  forms presently to be described, were only varieties; yet after an
  interval of some months, having to look at some of these specimens
  again, I could not but think that I had come to a false conclusion,
  and so went into all the details again, and satisfied myself that I
  had followed a right course; after another interval, I had to repeat
  the same process, and even now I can never look at a group of the
  beautifully coloured shells with their small rounded orifices of
  _var. coccopoma_ attached to the _Avicula margaritifera_, or again
  at _var. d'Orbignii_, with its toothed orifice and white tips to
  the compartments, without thinking that they must be specifically
  distinct from the dull-coloured specimens with large entire orifices
  so common on ships' bottoms; yet I can produce a full series of
  intermediate forms, and I can further show, in each variety, that
  the several points of difference by which each is characterised, are
  variable. I may be permitted to add, in order to show that it has
  not been from indolence that I have combined so many forms, that I
  had named and already written out full descriptions of most of the
  varieties, before determining to sacrifice them.

  Seeing that _B. tintinnabulum_ and _amphitrite_ are the two most
  variable species in the genus, more especially in the important
  characters derived from the opercular valves, and knowing that these
  species are attached so very frequently to ships' bottoms, one is led
  to suspect that their extreme tendency to vary may be due to their
  being exposed to varying and peculiar conditions, whilst transported
  to new and distant localities. It is even just possible, as may be
  inferred from the facts given in the Introduction (p. 102) in regard
  to certain monstrous specimens of _Bal. balanoides_ having been
  apparently impregnated by adjoining individuals, that the varieties
  may interbreed, and so produce numerous intermediate forms. Whether
  or not this could take place, I am inclined to look at these two
  species, as in an almost analogous condition with our domestic
  animals, which give rise to such infinitely numerous varieties. It
  appears to me probable, that several of the varieties keep true
  to their peculiarities, as long as they continue to breed in the
  same locality; but that when their larvæ become attached to ships'
  bottoms, and are thus transported and exposed to new conditions, they
  give rise to new and ever-varying varieties. I will first give a full
  description of the more common forms of _B. tintinnabulum_, which
  undoubtedly belong to the same species, only alluding to the less
  frequent points of difference, and then separately describe the more
  marked varieties.

  _General Appearance._--Shape of shell generally tubulo-conical, or
  conical, or globulo-conical, rarely depressed. Orifice either large
  and rounded-trigonal, or small and oval, either entire or less
  frequently toothed. Surface quite smooth, or longitudinally ribbed;
  ribs of variable strength, not unfrequently flexuous or branching,
  sometimes roughened with blunt or sharp projecting, irregular points,
  or more rarely with almost cylindrical, upturned, long spines; the
  simple longitudinal ribs are generally most strongly marked in
  young specimens. _Colour_, generally varying from pink, to pink
  tinged with purple, to dark, inky purple, more or less striped,
  longitudinally, with white or pale tints; rarely the shell is of the
  brightest rose-colour, either uniform or longitudinally striped;
  sometimes it is pale purplish, or dark blue; and sometimes dark
  chocolate-purple: the ribs, when present, are generally more or less
  white, sometimes snow-white. That there is much variation in colour,
  and in the prominence of the longitudinal ribs, is quite certain,
  as the two sides of the same individual sometimes differ greatly in
  these respects. The radii are generally rather darker coloured than
  the parietes, but sometimes they are lighter, even in the darkest
  tinged specimens. The surfaces of the radii are occasionally finely
  plaited in lines parallel to the basis. In some infrequent varieties
  the radii have oblique summits, making the orifice of the shell to
  be toothed. The sheath is generally feebly coloured, but sometimes
  bright chesnut-brown, and sometimes blueish. The _strength_ of the
  shell varies considerably; some of the globulo-conical varieties are
  extremely massive. _Size_; basal diameter of largest specimen very
  nearly three inches; height of the highest specimen three inches.

  Young specimens are apt to have a peculiar aspect; for their shell
  is often strongly ribbed longitudinally, and the summits of their
  radii are sometimes oblique. Their scuta are sometimes deeply
  pitted in radiating lines. Their colours are generally pale. I
  have seen specimens attached to kelp from the Cape of Good Hope,
  with their parietes white and ribbed, and their radii mottled with
  pinkish-purple; I have seen other young specimens from the Galapagos
  Archipelago, of a uniform grayish-blue.

  The _Scuta_ generally have their lines of growth moderately
  prominent; occasionally they are longitudinally striated, with the
  lines of growth flexuous and upturned at intervals into small,
  sharp, hood-liked projections, which are symmetrically arranged in
  straight lines radiating from the apex of the valve; I have seen
  this structure in some specimens of _var. crispatus_ and in _var.
  occator_ (Pl. 2, fig. 1 _b_); and I have noticed an intermediate
  state in _var. communis_. The degree to which the basi-tergal corner
  of the valve is rounded off varies much even in _var. communis_ (Pl.
  2, fig. 1 _a_, 1 _d_, 1 _e_). The articular ridge (1 _c_, 1 _e_)
  is broad and much reflexed; and often, but not always, distinctly
  hooked (1 _f_). The adductor ridge is confluent with the articular
  ridge, and runs straight down the valve, bounding the cavity for
  the depressor muscle; generally the adductor ridge is blunt, and so
  little prominent as barely to deserve notice; but I have seen it
  sharp and prominent in one specimen of _var. communis_, and it is
  generally prominent in _var. coccopoma_ (1 _f_), and most remarkably
  so in _var. concinnus_ (1 _g_). The cavity for the lateral depressor
  muscle is generally very slight; but in the two vars. just mentioned,
  and sometimes in _var. communis_, it is deep. In _var. concinnus_ (1
  _g_) there is a remarkable plate developed for the attachment of the
  rostral depressor muscle. The scuta are coloured either dull-purple
  or reddish, or striped longitudinally white and blue. The surface is
  sometimes externally depressed in the line of the adductor ridge; and
  in young specimens there is sometimes, along this line, a chain of
  pits (1 _h_), as in full-grown specimens of _B. trigonus_ and _lævis_.

  The _Tergum_ (Pl. 2, figs. 1 _i_ to 1 _o_) is broad, with a generally
  closed longitudinal furrow; this furrow is open in young specimens,
  and it is often, but not always, open in rather large specimens of
  _var. occator_; it is always open in _var. d'Orbignii_ (Pl. 2, fig. 1
  _m_), and sometimes in _var. concinnus_. Apex barely beaked, except
  in _var. spinosus_, in which it is sensibly produced. Spur placed
  either very nearly in the middle of the basal margin, or when least
  medial, it stands at above its own width from the basi-scutal angle;
  yet in some specimens of _var. occator_ the spur is less than its own
  width from this angle. The basal margin (1 _i_), on opposite sides of
  the spur, either forms a nearly straight line, or the scutal portion
  descends lower than the carinal portion, and curves very regularly
  towards the spur; this is the case in _var. d'Orbignii_ (1 _n_), and
  in some specimens of _var. occator_. The carinal half of the basal
  margin generally forms an angle with the spur of only a little above
  a rectangle. The spur varies a little in length and breadth, but
  never exceeds one fourth of the greatest breadth of the valve. The
  scutal margin is broadly inflected, the inflected portion forming
  either a right angle, or somewhat less than a right angle, with the
  exterior surface of the valve. Internally the articular ridge is
  prominent, and is either considerably or slightly curved; it extends
  down either about half, or three fourths, of the length of the valve.
  The spur is produced for a considerable distance up the internal
  surface of the valve as a prominence. The crests for the tergal
  depressor muscle are very feeble. In one specimen, in which both the
  shell and operculum had undergone much disintegration, the scuta and
  terga were calcified together.

  _Compartments_: their exterior appearance has been already described.
  The parietal tubes are not large; they are generally crossed by
  transverse septa in their uppermost part; but they are sometimes
  almost solidly filled up by dark shelly layers. The internal surface
  of the parietes is more or less plainly ribbed; in old specimens,
  however, it generally becomes smooth. The _radii_ have their septa
  denticulated on both sides; and they are porose, that is, the
  interspaces between the septa are not filled up solidly. The radii
  generally extend from tip to tip of the adjoining compartments, that
  is, their summits are parallel to the basis; but in three not common
  varieties, viz., _vars. intermedius_, _occator_, and _d'Orbignii_
  (the former of which at least must, without the smallest doubt,
  be ranked as a mere variety), their summits are oblique. I have
  occasionally met with specimens of _var. communis_ with oblique
  radii; and this is not very infrequent in young shells. Exteriorly
  the radii are generally smooth, but sometimes finely ribbed
  horizontally, owing to the projection of the septa. The _alæ_ have
  their sutural edges smooth; their summits are usually parallel to
  the basis, but they are often much broken; in _var. zebra_, however,
  in every specimen which I examined, the summits were oblique. The
  sheath varies much in colour: in _var. occator_, and in a less
  degree in _var. intermedius_ and _var. d'Orbignii_, the portion
  lining the rostrum and lateral compartments is much darker than
  the other parts of the sheath. The _Basis_ generally has a thick,
  underlying, cancellated layer. Sometimes the basis (Pl. 1, fig. _b_)
  is irregularly cup-formed.

  _Mouth_: labrum with four or six minute teeth: mandibles with five
  graduated teeth; inferior point more or less spinose. Maxillæ, either
  with or without a small notch, beneath the upper pair of spines; in
  the lower part there are two spines longer than those immediately
  above them. _Cirri_, the first pair has the rami unequal, in the
  proportion of about 19 segments in the longer ramus, to 16 in the
  shorter. The segments in the latter have their anterior surfaces very
  protuberant. The second pair is short, with the anterior surfaces of
  the segments protuberant. On the thorax (Pl. 25, fig. 1), on each
  side, at the bases of third pair of cirri, there is a projecting
  membranous plate fringed with fine bristles. The three posterior
  pairs have their segments shield-shaped in front, generally bearing
  four pairs of spines, of which the lower pair is minute; between
  these pairs there are some minute spines. In some young specimens
  from the Cape of Good Hope, and in _var. concinnus_, I found six
  pairs of spines on the segments of the posterior cirri.

  _Geographical Distribution._--This species is extremely common over
  the whole of the warmer seas. It ranges from the Island of Madeira
  to the Cape of Good Hope, and on the west coast of America, from
  Monterey, in lat. 37° N., in California, to Peru. It is attached to
  rocks and sub-littoral shells, to floating timber, to kelp, and to
  _Lepas anatifera_. It is attached in wonderful numbers to ships'
  bottoms arriving at our ports, from West Africa, the West Indies,
  the East Indian Archipelago, and China. It is generally associated
  with _B. amphitrite_ and _amaryllis_. I have already stated that,
  on the bottoms of vessels, the different varieties are generally
  grouped together; and this makes me believe that they are local. In
  Mr. Stutchbury's collection there are numerous specimens taken from
  a ship which first went to the west coast of Africa for guano, and
  then to Patagonia for the same object, and it was interesting to see
  the manner in which numbers of _B. psittacus_, a Patagonian species,
  had become attached on the African _B. tintinnabulum_. The varieties
  from the west coast of America seem eminently peculiar; we there find
  _var. coccopoma_ and _concinnus_; and a blueish, rugged variety, with
  peculiar opercular valves.

  _Geological History._--I have seen specimens in Mr. S. Wood's
  collection from the Red Crag of England, which, though not
  accompanied by opercular valves, I cannot doubt belong to this
  species. The specimens named by Mr. Sowerby, in the 'Mineral
  Conchology,' as _B. crassus_, and which I have seen through the
  kindness of Mr. J. de C. Sowerby, also belong to this species. I
  further believe that a specimen in Sir C. Lyell's collection, given
  to him by M. Dujardin under the name of _B. fasciatus_ from Touraine,
  is likewise _B. tintinnabulum_.


_Varieties._

  With respect to _var. communis_, I have nothing to remark. The
  second, _var. vesiculosus_ (Pl. 2, fig. 1 _h_), is confined to young
  specimens, and may, perhaps, be due to a want of calcareous matter.
  With respect to _var. validus_ (Pl. 1, figs. 1 _c_, 1 _f infra_),
  I may observe that some of the coarsest and strongest specimens
  which I have seen were said to have been attached to a surface of
  iron. I have seen two large lots of _var. zebra_ (1 _g_), taken by
  Mr. Stutchbury, from the bottoms of ships, arriving from Bengal and
  China, and in both cases associated with _var. communis_, and in one
  case with _var. coccopoma_. I at first thought that this variety,
  _zebra_, was specifically distinct, but now I feel no doubt, that it
  is a mere variety; its body was in every respect identical with that
  of _var. communis_.

  Nor have I any doubt that _var. crispatus_, of Schröter (Pl. 1, fig.
  _h_), is only a variety, although the scuta in some specimens have a
  peculiar appearance, externally like these valves in _var. occator_
  (Pl. 2, fig. 1 _b_): the scutum is here broader and flatter than in
  _var. communis_, and the adductor ridge is very feebly developed,
  but we shall see how variable this ridge is in all the varieties:
  externally, the sharp, hood-like points formed by the upturned lines
  of growth, have a very remarkable appearance, from being arranged in
  quite straight radiating lines. This structure is evidently caused by
  the same tendency which produces on the walls the sharp, upturned,
  irregular points; but it is singular that the scuta are smooth in
  some specimens with very rough parietes; and, on the other hand,
  bristling with the symmetrically arranged, hood-like projections, in
  other specimens on which I could with difficulty detect only a few
  exceedingly minute points on the walls. In _var. communis_ I have
  seen a few specimens with a slight tendency in the scuta to become
  striated longitudinally. The tergum in _var. crispatus_ presents no
  difference from that in _var. communis_. Some of the roughest and
  best characterised specimens of _var. crispatus_ appear to have come
  off copper-sheathed vessels.

  I believe _var. spinosus_ of Gmelin (Pl. 1, fig. _i_), has been
  correctly considered by me as a variety, but I have unfortunately
  seen only one set of specimens with their opercular valves preserved:
  these were attached to _Lepas anatifera_. The colour of the shell
  varies from reddish-purple to nearly white; the radii are sometimes
  quite white; the walls are slightly ribbed. The scuta in the above
  specimens, externally were smooth; the adductor ridge was rather more
  distinct from the articular ridge than in any other variety; and the
  terga rather more plainly beaked. The tubular, up-curved, calcareous
  spines sometimes occur only on one side of the shell, and often only
  in the lower part. These spines are often coloured brighter than
  the walls. Their presence cannot be accounted for (any more than
  the state of the scuta in the foregoing _var. crispatus_) by the
  nature of the surface to which they are attached; for I have seen one
  set attached to a large rugged specimen of _B. tintinnabulum_, and
  another to the very smooth valves of _Lepas anatifera_. I believe
  that this form is almost always associated with _var. communis_,
  which is an argument that it is only a variety.

  To _var. coccopoma_ (Pl. 1, fig. _d_) I alluded in my introductory
  remarks as having so strongly the aspect of a distinct species. I
  possess a beautiful group, with a globulo-conical, smooth shell, of
  the finest rose colour, with a rather small, rounded orifice. These
  specimens were attached (mingled with _B. trigonus_) to _Avicula
  margaritifera_, from, as Mr. Cuming believes, Panama. I can never
  look at this set of specimens without doubting the correctness of the
  determination at which I have arrived. In the British Museum there
  are two sets of specimens taken off a vessel, on the west coast of
  South America, almost identical in external appearance with those in
  my possession, but rather more rugged. Mr. Stutchbury has sent me
  some specimens from a ship, direct from China, which are rather paler
  pink, and more striped, and come near to some ordinary varieties of
  _B. tintinnabulum_. The scuta (Pl. 2, fig. 1 _f_) in the above three
  sets of specimens agree in having the adductor ridge more developed,
  and the pit for the lateral depressor muscle deeper than is usual.
  The tergum (1 _l_, 1 _o_) in most, but not in all these specimens,
  has a rather broader spur; and some of the specimens have the
  carinal portion of the basal margin considerably hollowed out; the
  spur, also, is placed nearer the basi-scutal angle than in ordinary
  cases. On the other hand, in Mr. Cuming's collection, there are two
  specimens taken off a vessel, identical in external appearance with
  the foregoing, but which have scuta and terga in every character
  exactly as in _var. communis_; hence I am compelled to consider all
  these specimens as mere varieties.

  _Var. concinnus_ (Pl. 1, fig. _e_) is, perhaps, the most remarkable
  of all the varieties; I have seen three sets of specimens from the
  west coast of South America,--all identical in appearance, having
  longitudinally-ribbed walls, either rosy or of a dull purple, striped
  and freckled in a peculiar manner with white. I have, however,
  seen an approach to this colouring in some few specimens of _var.
  communis_; and the shell itself offers no other peculiarities. The
  scutum (Pl. 2, fig. 1 _g_) resembles, in general shape, that of _var.
  coccopoma_; but the adductor ridge is here much sharper and more
  prominent; and the rostral depressor muscle, instead of being lodged
  in a little cavity formed by the folding over the occludent margin,
  has, in addition, a small plate on the under side, which tends to
  convert the pit into a tube. The tergum exactly resembles that of
  _var. communis_. The segments in the sixth cirrus bear six, instead
  of four, pairs of spines,--a circumstance which I have noticed
  only in some young specimens of _var. communis_, from the Cape of
  Good Hope. From these several peculiarities, until quite lately, I
  resolved to keep this form specifically distinct; but I have finally
  concluded that they are not sufficient. For firstly, I have seen a
  scutum in _var. communis_ (Pl. 2, fig. 1 _d_), with the adductor
  ridge nearly as sharp; and this ridge is always strongly pronounced
  in _var. coccopoma_; secondly, with respect to the plate for the
  rostral depressor muscle, although I have not seen this in any other
  variety, yet in _Bal. concavus_ a closely analogous plate, situated
  in the lateral depressor cavity, is highly variable, and I am not
  willing to found a new species on one minute point of structure,--a
  structure which is variable in another species of the same genus.

  I have seen some cylindrical and conical specimens of _B.
  tintinnabulum_, from the coast of Mexico and California, only
  noticeable, as far as the shell was concerned, from being rugged,
  and of a dull blueish-purple; but which had opercular valves
  exactly like those of _var. coccopoma_, and therefore, as far as
  the scutum is concerned, approaching closely in structure to _var.
  concinnus_,--all three from the west coast of America. Hence I was
  at one time led to believe that there existed a species on this line
  of coast, which represented _B. tintinnabulum_, and which varied in
  external shape and colour in an analogous manner to that species. But
  as the opercular valves in _var. coccopoma_ are sometimes identical
  with those of _var. communis_, and as this is always the case with
  the tergum of _var. concinnus_, and as the shell itself presents
  no differences, it is scarcely possible to admit the existence on
  the west coast of America of this supposed representative of _B.
  tintinnabulum_.

  With respect to _var. intermedius_ I have little to say in addition
  to the character given above: I have seen only two groups of
  specimens in Mr. Cuming's collection: the chief interest in this
  variety is that it shows that the next form must be ranked as a
  variety, and not as a distinct species.

  Of _var. occator_ (Pl. 1, fig. _k_) I have seen several specimens,
  mostly taken off the bottoms of vessels, and one specimen, marked
  in Mr. Cuming's collection "South Seas." After having carefully
  examined these specimens, I came to the conclusion, that the slightly
  oblique radii--the general colouring, and more especially that of the
  sheath--the scuta (Pl. 2, fig. 1 _b_), with their sharp hood-like
  points, in radiating lines--and the terga, with the spur so near to
  the basi-scutal angle, were amply sufficient to distinguish it as
  a good species. Subsequently, however, I found that the scuta in
  _var. crispatus_ presented, both externally and internally, exactly
  the same peculiar appearance. In _var. intermedius_, I found the
  summits of the radii equally oblique, and the general colouring
  nearly the same, and more especially a close approach to the singular
  circumstance of the sheath differing in colour towards the opposite
  ends of the shell. So that the position of the spur of the tergum was
  the chief remaining character; and this evidently varied considerably
  in the four or five specimens examined by me, being either its own
  width, or much less than its own width, from the basi-scutal angle:
  the outline, also, of the small portion of basal margin, between the
  spur and the basi-scutal angle, likewise varied much, being either
  angularly indented, or gradually curved down towards the spur: so
  also the tip of the spur varied in shape. The longitudinal furrow
  is unusually apt, in this variety, to remain open. We know that the
  position of the spur varies considerably in _var. communis_. Hence,
  although the spur, on an average, lies closer to the basi-scutal
  angle in this than in any other variety, even than in _var.
  d'Orbignii_, it would, I conceive, be preposterous to found a species
  on this one character. In the animal's body, every part agrees
  perfectly with that of _var. communis_.

  Lastly, we come to _var. d'Orbignii_ (Pl. 1, fig. _l_): until quite
  recently I did not even suspect that this form was only a variety
  of _B. tintinnabulum_: I have examined a great number of specimens
  in Mr. Stutchbury's collection, which had come attached on a vessel
  from Java, and likewise a few other specimens in other collections.
  They all closely resemble each other in shape, and even in size,
  and differ only in tint of colour, and in the surface being either
  very smooth, or longitudinally ribbed, sometimes with rugged, sharp
  points. From this circumstance--from the peculiarity of the tint,
  with the tips of the parietes and one side of the radii perfectly
  white--and from the obliquity of the summits of the radii, I was led
  to think this form specifically distinct. But the colour does not
  differ from that of some other varieties of _B. tintinnabulum_; the
  circumstance of the colour being uniform or not striped, is common
  to the sub-varieties of several varieties, and the white tips to
  the parietes, and the white borders to the radii, result simply
  from the shell, whilst young, having been wholly white, and this is
  not rarely the case with _var. communis_. Dismissing, therefore,
  colour, it will be found that hardly any other characters remain by
  which this form can be separated from _var. occator_; in both the
  summits of the radii are oblique, in both the sheath is coloured in
  nearly the same manner, in both the opercular valves, especially the
  terga (Pl. 2, figs. 1 _m_, 1 _n_), resemble each other; the scuta,
  however, are smooth in _var. d'Orbignii_ and rough in _var. occator_.
  This latter form, certainly, cannot be specifically separated from
  _var. intermedius_, and this assuredly is only a variety of _B.
  tintinnabulum_. Hence I am led to conclude that _Balanus d'Orbignii_
  of Chenu, peculiar as its whole aspect is, must be ranked only as a
  variety of _B. tintinnabulum_; its oblique radii resulting from the
  same cause, whatever that may be, which has given this structure
  to _var. intermedius_ and _occator_; and its peculiar colouring to
  having been exposed (owing probably to having been transported on
  vessels) to different conditions, whilst young and old.



2. BALANUS TULIPIFORMIS. Pl. 2, fig. 2 _a_-2 _d_.

  BALANUS TULIPIFORMIS EX CORALLIO RUBRO. _Ellis._[88] Philosoph.
        Transactions, vol. 50 (1758), tab. 30, fig. 10.

  LEPAS TULIPA. _Poli._ Test. utriusque Siciliæ, tab. 5, fig. 1. et 6
        (1791).

  BALANUS TINTINNABULUM (_var._) _Chenu._ Illust. Conch., tab. 3, f. 5.

    [88] According to the letter of the Rules of the British
    Association, Ellis's name ought to be retained, as it was published
    in 1758, the same year during which the 10th edition of the
    'Systema Naturæ' appeared, in which edition the binomial method
    was first used. But as Ellis himself did not then know of, or
    follow this method, it might be disputed whether, according to the
    _spirit_ of the law, his name ought to stand. The only other name
    given to this species is that of _tulipa_, affixed by Poli in 1791,
    but this name had been previously used by Müller in 1776, and by
    Chemnitz in 1785, for another species, the _B. Hameri_ of this
    work; and likewise, also previously to Poli, by Bruguière in 1789,
    for still another species, viz., _B. tintinnabulum_ of this work:
    under these complicated causes of confusion, I think it is highly
    advisable to adopt Ellis's name. I may add that the _B. tulipa_ of
    Mr. G. B. Sowerby is the _B. tintinnabulum_ of this work. It is
    possible that the _B. conoides_ of Brown, 'Illustrations Conch.'
    (1st edit. pl. 6, fig. 7), may be our present species; but without
    details of structure it is hardly possible to identify, in many
    cases, the species of Balanus.

_Shell dark rose-coloured, sometimes tinged with purple; orifice
toothed. Scutum externally very smooth, covered by membrane. Tergum
with distinct crests for the depressor muscles._

  _Hab._--Sicily, Malta, Malaga, (associated with _B. perforatus_),
  Madeira. Often growing in clusters and associated with _Pachylasma
  giganteum_. Attached to _Millepora aspera_, oysters, and other
  shells. According to Poli, an inhabitant of deep water; yet in mus.
  Cuming there are two fine specimens attached to the always floating
  _Lepas anatifera._ Mus. Lowe, Macandrew, Stutchbury.


  _General Appearance._--Shell tubulo-conical or conical: orifice
  large, toothed, approaching to pentagonal. Surface moderately smooth,
  naked. Colour rosy, or tile-red, with a slight tinge of purple; or
  beautiful rich purple. Radii nearly as dark as, or darker than, the
  parietes. The portion of the alæ seen externally is generally white.
  Internally the whole shell is nearly white. Generally the tints
  outside vary in transverse fasciæ; sometimes there are very fine,
  dark, longitudinal lines. Largest specimen (from Malta), 1.4 of an
  inch in basal diameter; usually full-sized specimens are about three
  quarters of an inch in basal diameter.

  _Scuta_ (Pl. 2, fig. 2 _a_, 2 _c_) very smooth, with the
  growth-ridges very little prominent, sometimes there are obscure
  traces of longitudinal striæ; surface covered by an unusually thick
  and persistent yellow membrane: valve narrow, with the upper part
  commonly reflexed: the basal margin forms, with the occludent margin,
  a smaller angle than is usual: the tergal margin of the valve is
  rectangularly inflected, instead of being, as is usual, merely
  bowed inwards. Internally, the articular ridge is rather prominent.
  The depth of the _slight_ pit for the lateral depressor muscle is
  variable; it sometimes includes a minute, central, longitudinal ridge.

  _Terga_ (2 _b_, 2 _d_): the longitudinal furrow is deep, with the
  sides folded in; the spur is placed at about its own width from
  the basi-scutal angle; it is moderately long, with its lower end
  obliquely rounded off; but the length, breadth, and precise outline
  of the lower end varies a little. The basal margin on the opposite
  sides of the spur, forms a nearly straight line, but with the
  portion on the carinal side very slightly hollowed out. Crests for
  the depressor muscle are well developed.

  _Compartments._--The radii and alæ always have their summits oblique:
  the sutural edges of the radii are deeply penetrated by pores between
  the strongly denticulated septa: the sutural edges of the alæ are
  quite smooth: the tubes in the parietes are crossed in the upper
  part of the shell by septa. _Basis_ tubular, with an underlying
  cancellated mass.

  _Mouth._--Labrum with the teeth either absent or very small:
  mandibles with the fourth and fifth teeth rudimentary: maxillæ with
  a small notch under the two upper spines; near the lower angle,
  two spines, one beneath the other, are larger even than the upper
  pair; beneath the lower pair, there is a tuft of fine spines.
  _Cirri_, segments protuberant in one ramus of the first cirrus and
  in both rami of the second cirrus; posterior cirri with the segments
  short and broad, each bearing three pairs of spines, with a small
  intermediate tuft.

  _Affinities._--This species in all essential respects comes very near
  to the three last varieties of _B. tintinnabulum_, which have the
  orifices of their shells toothed. The smoothness of the scutum, with
  its persistent epidermis,--its peculiar shape,--its small and not
  reflexed articular ridge,--together with the crests on the tergum for
  the depressor muscles, are sufficient diagnostic characters. Even in
  general habit and tint of colour, this species has a different aspect
  from _B. tintinnabulum_. In some respects _B. tulipiformis_ leads
  into the species included in the third section of the genus.



3. BALANUS PSITTACUS. Pl. 2, fig. 3 _a_-3 _d_.

  LEPAS PSITTACUS. _Molina._ Hist. Nat. Chile (1788), vol. i, p. 223.

  BALANUS PICOS. _Lesson._ Zoolog. Voyage de la Coquille (1829).

  ---- TINTINNABULUM (var. c). _Ranzani._ Mem. di Storia Nat. tab. 3,
        fig. 1-3 (1820).

  ---- CYLINDRACEUS. _Lamarck_, in _Chenu_. Illust. Conch. Tab. 4,
        fig. 17, Tab. 5, fig. 7, sed non var. (_c._) in Lamarck,
        Animaux sans Vert., (1818).

  ---- PSITTACUS. _King_ and _Broderip_. Zoolog. Journal, vol. v
        (1832-1834), p. 332.

_Shell, pale dirty pink; orifice hexagonal. Scutum with the articular
ridge very small, confluent with the very prominent adductor ridge,
forming a tubular cavity, which extends up to the apex of the valve.
Tergum with the apex produced, needle-like, purple: spur placed at less
than its own width from the basi-scutal angle._

  _Hab._--Peru, Chile, Chiloe, Patagonia. Fossil in an ancient tertiary
  deposit, Coquimbo; and in a superficial, recent bed at S. Josef, in
  Patagonia.


  _General appearance._--Shell either almost cylindrical or steeply
  conical, generally flesh-coloured, sometimes pale pink; surface
  either smooth (when not disintegrated) or sometimes with the parietes
  distinctly and rather strongly ribbed, with the ribs distant from
  each other: I have seen six or seven ribs on the rostrum alone. The
  orifice in the most perfect specimens is nearly equilateral and
  hexagonal. The radii generally are very broad, but occasionally
  quite narrow, and even linear. The basis is generally deeply and
  irregularly cup-formed.

  _Size._--This is the largest species in the family: I have seen a
  specimen six inches in length and three and a half in diameter; and
  another specimen no less than nine in length, though only two and a
  half inches in diameter.

  _Scuta._--In full-sized specimens the surface is finely striated
  longitudinally, caused by the lines of growth being minutely sinuous;
  but in young specimens, until they attain a basal diameter of above
  half an inch, the surface is smooth. The valve is transversely
  arched, a line of flexure running from the apex to the basal margin,
  at about one third of the width of the valve from the tergal margin.
  The basal margin is curved nearly continuously, and extends nearly
  half-way up the valve; hence the basi-tergal corner is largely
  rounded off. The articular ridge is but little prominent, and is not
  reflexed: the articular furrow is very narrow. The adductor ridge
  consists of a sharp, much-projecting plate, running down close to the
  basal margin, and is confluent with the lower part of the articular
  ridge. This plate and the inflected tergal margin of the valve,
  together form a large and deep cavity, which extends up almost to the
  apex of the valve. The depressor muscle is attached in the middle, at
  the lower, open end of this cavity.

  _Terga._--These are strongly beaked, the beak being from one third
  to one fourth of the total length of the valve, including the spur:
  the beak is very sharp, somewhat flattened, and bowed; when young,
  and when well preserved, it is coloured purple: it is penetrated
  by a fine tubular cavity, occupied by a thread of corium, which
  extends about half-way up it. The whole valve is narrow, being about
  thrice as long as wide. The spur is also long and narrow; it is
  seated at less than its own width from the basi-scutal angle. The
  scutal margin is not much inflected. The longitudinal furrow has its
  sides, in full-grown specimens, closely folded together. The basal
  margin slopes down on both sides to the spur. There are no crests,
  or only traces of them, for the attachment of the depressor muscle.
  Internally, the spur is prolonged, as a prominent ridge, upwards to
  the beak, and serves as an articular ridge. In the middle, in the
  upper part (Pl. 2, fig. 3 _d_), between this articular ridge and the
  carinal margin, there is a second narrow ridge, which extends from
  the lower part of the beak half-way down the valve, and then dies
  out. The space between these two ridges, and the ridges themselves,
  are coloured purple, and consist of harder shell than the rest of
  the valve; hence, when the outer surface and the adjoining scutal and
  carinal margins disintegrate, this part remains, and so forms the
  beaked, purple apex.

  _Compartments._--The parietal tubes are unusually large in proportion
  to the size of the shell, and run up to the summit without any
  transverse septa: the longitudinal septa are strongly denticulated.
  The radii are penetrated by large tubes; their septa are very
  strongly denticulated, and the denticuli themselves often subdivide
  and branch out at their extremities. The sutural edges of the alæ
  are smooth, or with a high power can just be seen to be crenated.
  The radii are generally very highly developed, so that their summits
  are even wider than the bases of the parietes; but, on the other
  hand, in some few large specimens, the radii are either very narrow
  or absolutely linear. In these latter cases, the diametric growth
  has nearly or altogether ceased, whilst the walls of the shell have
  continued to be added to at their bases, their summits at the same
  time suffering disintegration; and thus the orifice has increased in
  size.

  _Basis_ generally, and occasionally very deeply, cup-formed. An
  unusually thick cancellated layer in most cases forms the under side
  of the basis.

  _Mouth._--Labrum apparently without teeth, or with very minute ones:
  mandibles with three teeth, of which the third is thicker than the
  first or upper one: the fourth and fifth teeth are confluent with the
  inferior angle. The maxillæ have a small notch under the upper pair
  of spines; inferior part projecting and supporting two spines, placed
  one below the other, and equalling in size the upper pair. _Cirri_:
  the rami of the first cirrus are unequal by four or five segments;
  shorter ramus and both rami of the second cirrus with the segments
  extremely protuberant: posterior cirri not much elongated, with the
  segments rather broad, supporting six pairs of spines.

  _General Remarks._--This, which is much the largest known species
  of the genus, ranges from Peru (Arica being the most northern spot,
  whence I have seen specimens), along the coast of Chile, where it
  is very abundant at a few fathoms' depth, at least as far south as
  Southern Chiloe; it is said by Captain King to attain the largest
  size at Conception. On the coast of Eastern Patagonia, I dredged
  up this species from nineteen fathoms, in lat. 49°. In lat. 42°
  (S. Josef), on the same eastern coast, I found fossil specimens
  in beds of sand upraised between eighty and one hundred feet. In
  the tertiary formation at Coquimbo, in Chile, it occurs in the
  middle bed, associated with the recent _B. lævis_, and with various
  mollusca, all of which are apparently extinct, indicating that the
  formation is of considerable antiquity. In the living state, on
  the coast of Chile, it is often associated with _B. lævis_. As it
  frequently adheres to large specimens of the Concholepas, it must
  sometimes be an inhabitant of shallow water. I have seen one specimen
  attached to _Mytilus Magellanicus_. Mr. Cuming believes that about
  six fathoms is the usual depth at which it lives. Numerous specimens
  are often congregated together into great masses. Mr. Stutchbury has
  some interesting specimens which he procured from a ship that had
  first sailed to Ichaboe, on the coast of Africa, and afterwards
  to Patagonia; consequently numerous specimens of _B. psittacus_
  had become attached on _B. tintinnabulum_, and subsequently during
  the voyage home, some few of the latter again had adhered on _B.
  psittacus_: the contrast in the paler colour and hexagonal orifice
  of this species, with the darker tints and more trigonal orifice of
  _B. tintinnabulum_ was striking. At Coquimbo, in Chile, I procured
  a specimen of _B. psittacus_, attached to a chain cable which had
  been in the water only six months; this specimen measured 1.3 of an
  inch in basal diameter, and .8 in height: this shows a rapid rate of
  growth. Lastly, I may mention that it is asserted by Molina, and I am
  assured by Mr. Cuming that the statement is perfectly correct, that
  this Balanus, when cooked, is universally esteemed as a delicious
  article of food.



4. BALANUS CAPENSIS. Pl. 2, fig. 4 _a_, 4 _b_.

  BALANUS CAPENSIS ORE OBLIQUO. _Ellis._ Phil. Transact., vol. 50
        (1758), Tab. 34, fig. 14.

_Shell shaded, and often longitudinally striped with bright pink.
Scutum as in B. psittacus. Tergum with the apex produced and
needle-like, white: spur placed at its own width from the basi-scutal
angle._

  _Hab._--Cape of Good Hope. Attached to stems of Fuci, Algoa Bay. Mus.
  Brit. and Bowerbank. Attached to a Patella, Mus. Darwin, Mus. Cuming,
  and Stutchbury. Attached to floating kelp, Lagulhas Bank, Mus. James
  Ross, associated with _B. tintinnabulum_ and _spongicola_.


  This species comes extremely close to the South American _B.
  psittacus_, and I should hardly have attached a specific name to
  it, had I not examined many specimens, young and old, of the true
  _B. psittacus_, from Peru, Chile, and Eastern Patagonia, and found
  them all identical in the few, apparently trifling points, in
  which that species differs from _B. capensis_. The animal's body
  and the shell agree in every respect, excepting that the shell is
  decidedly pinker, being often most distinctly and prettily striped
  longitudinally with pale and bright pink. In some of the specimens
  the basis is cup-formed: in some, the broad radii are pale pink,
  in others they are quite white, and in this latter case a singular
  aspect is given to the pinkish varieties. In very large specimens
  (and I have seen one fully two inches in basal diameter) the pink
  colour is extremely feeble, and the whole shell has a very rugged,
  disintegrated, coarse, and sometimes dirty appearance: in most
  of these large specimens the walls are more massive than in _B.
  psittacus_, and the orifice of the shell rather smaller; in some,
  however, the walls certainly are of unusual thinness.

  The _Scuta_ differ from those of _B. psittacus_ only in the
  basi-tergal corner not being so much rounded off, and consequently
  in the articular ridge, which is rather more reflexed, descending in
  proportion lower down the valve: the cavity at the basi-tergal corner
  is in proportion broader. The valves in the two species differ, also,
  but only in young specimens, in the occludent half being tinted,
  both externally and internally, purple, whereas in _B. psittacus_
  the whole valve, at all ages, is white. In the _terga_ the spur is
  removed fully its own width from the basi-scutal angle, whereas it
  is not half this distance in _B. psittacus_. The scutal margin is
  here much more inflected. In _B. psittacus_ there is a remarkable
  patch of purple on the inside of the valve, between the articular
  ridge and a second special ridge; of this purple patch there is here
  no trace, consequently the beak or apex is white. The beak, also, is
  less prominent. The special ridge, just alluded to, here runs much
  nearer to the articular ridge, and is less prominent: indeed, in old
  specimens, it is often almost obliterated. Finally, the whole valve,
  in proportion to the Scutum, is rather broader.

  I have seen a young specimen, about a quarter of an inch in basal
  diameter, with the orifice of the shell toothed owing to the
  obliquity of the summits of the radii; and this gave the shell a very
  peculiar aspect. The largest well-coloured specimen which I have seen
  is 1.2 of an inch in basal diameter; but in Mr. Cuming's collection
  there are two rugged, disintegrated specimens, two inches in basal
  diameter, and two and a half in height. Some specimens, 1.3 in basal
  diameter, in Mr. Stutchbury's collection, are remarkable from the
  radii having been obliterated--the shell being merely divided by six
  sutures, as we have seen is likewise sometimes the case with large
  specimens of _B. psittacus_.

  This species is evidently a South African representative of the South
  American _B. psittacus_.



5. BALANUS NIGRESCENS. Pl. 2, fig. 5 _a_, 5 _b_.

  BALANUS NIGRESCENS. _Lamarck_, (1818) in _Chenu_. Illust. Conch.,
        Tab. 4, fig. 16.

  ---- GIGAS. _Ranzani._ Memoire di Storia Nat., 1820, Tab. 3, fig. 5,
        6, 7.

  ---- ---- _De Blainville._ Dict. des Sc. Nat., Tab. 116, fig. 2, 2
        _a_.

_Shell cinereous, tinted with pale or blackish blue, or wholly white.
Scutum with the articular ridge terminating downwards in a small,
sharp, free point: adductor ridge prominent. Tergum with the apex
produced and needle-like._

  _Hab._--Swan River, West Australia, Mus. Brit., attached to
  sandstone. Attached to sandstone and to each other in a group, Mus.
  Cuming. Twofold Bay, S. E. Australia, attached to tidal rocks and
  Patellæ, Mus. Darwin.


There can be no doubt that this species is the _B. nigrescens_ in
Chenu, who had access to Lamarck's original specimens; and there can be
equally little doubt that it is the _B. gigas_ of Ranzani, collected,
during Baudin's expedition, at King George's Sound: it is essentially
allied to _B. psittacus_, but in external appearance strikingly
resembles some of the varieties of _B. tintinnabulum_.


  _General Appearance._--Shape tubulo-conical: walls smooth, sometimes
  longitudinally ribbed: colour ashy-gray tinged with blue, but many
  specimens are dark purplish-blue, owing to the disintegration of the
  outer lamina, and consequent exposure of the almost solidly filled
  up, dark blueish parietal tubes; on the other hand, some specimens
  are quite white. Ranzani describes the colour as earthy-violet, which
  is very characteristic of some of the specimens. The orifice is apt
  to be rather small, compared to the size of the specimens, and tends
  to be hexagonal. The radii are often rather narrow. The opercular
  valves are tinted pale blue. The basal diameter of the largest
  specimen is two inches, and its height two and a quarter.

  The _Scuta_ have their basi-tergal corner much rounded off, as in
  _B. psittacus_, so that the tergal margin does not extend more than
  half down the valve. The surface is somewhat prominent, along a line
  running from the apex to the point of chief curvature in the basal
  margin. The surface is not striated. Internally, the articular ridge
  is little prominent, and not reflexed; the lower end depends as a
  free, sharp style or point. The adductor ridge is moderately sharp,
  and stands some little way distant from the articular ridge: it is
  produced downwards, and forms a moderately deep and large cavity for
  the depressor muscle; but this cavity is not closed, and does not
  extend up, as in the two last species, to the apex of the valve.

  _Terga_, narrow, with a sharp, prominent, needle-like beak. Spur,
  long, narrow, placed at less than its own width from the basi-scutal
  angle: the basal margin on both sides slopes down to the spur: the
  scutal margin is not inflected. Internally, the articular ridge is
  very feebly developed, but extends down close to the basi-scutal
  angle. On the under surface in the upper part of the valve, there is
  a short, very slight ridge, extending on the carinal side, near and
  parallel to the articular ridge; this slight ridge plays an important
  part, as in the two foregoing species, in the formation of the beak
  or apex. Crests for the depressor muscle are hardly distinguishable.

  The _Walls_ appear to vary in some degree in strength and thickness;
  as is likewise the case with the opercular valves. In some of the
  thinner specimens, the parietal tubes are large, and the longitudinal
  septa are furnished with small, sharp denticula. The tubes are often
  thickly lined or almost filled up solidly with blue shell; they are
  not crossed by transverse septa.

  The _Radii_ vary in width; externally they are often finely ribbed
  transversely, at other times they are smooth; their septa are fine
  and thin, with their delicate denticuli not extending to the outer
  lamina: they are very porose. The _alæ_ have their summits parallel
  to the basis; their sutural edges are most finely crenated. The
  sheath is blueish, excepting the wedge-like portions of the alæ which
  have been added during the diametric growth, and these are white.

  _Mouth_: labrum without teeth: mandibles with five sharp teeth:
  maxillæ with the edge straight. _Cirri_, first pair with the rami
  very slightly unequal; segments of the shorter ramus and of both rami
  of the second pair protuberant: posterior cirri with the segments
  shield-shaped in front, bearing four pairs of spines, of which the
  upper pair is much longer than the lower pairs; each pair has a small
  intermediate tuft of minute spines.



6. BALANUS DECORUS. Pl. 2, fig. 6 _a_, 6 _b_.

_Parietes pale pink; radii rather darker. Scutum with a small articular
ridge. Tergum with the longitudinal furrow very shallow and open; basal
margin on both sides sloping towards the spur._

  _Hab._--New Zealand. Mus. Brit., and Flower: attached to shells.


  _General Appearance._--Shell conical or tubular, with a large
  rhomboidal orifice; very pale pink, but tinted yellowish from the
  persistent epidermis, and sometimes faintly striped longitudinally;
  radii and sheath of rather a darker pink; scuta in themselves white,
  though lined by purple corium; the carinal half of the tergum pink.
  Walls extremely smooth. Largest specimen above one inch in basal
  diameter.

  _Scuta_, with the finest striæ radiating from the apex; growth-ridges
  moderately prominent; articular ridge small; there is a very slight
  and blunt adductor ridge: the hollow for the lateral depressor muscle
  is rather narrow and deep.

  _Terga_, with the apex slightly prominent or beaked; the longitudinal
  furrow is of very little depth; on its scutal margin there is a
  narrow, rounded, slightly prominent ridge, which, however, appears
  more like a furrow than a ridge. Spur moderately long and blunt;
  placed at half its own width from the basi-scutal angle; the
  basal margin on both sides of the spur, slopes gently towards it.
  Internally, the articular ridge is pretty well developed; the scutal
  margin is not much inflected; the carinal portion of the under
  surface of the valve is rough; the crests for the carinal depressor
  muscle are entirely absent.

  _Compartments._--Walls moderately strong; parietal tubes small, with
  transverse septa in their upper ends; inner surface of the walls much
  less strongly ribbed than is usual. _Radii_ broad, with their summits
  parallel to the basis; their septa are strongly denticulated. _Alæ_
  with their summits oblique; their sutural edges are barely crenated.
  _Basis_, thin, flat, or cup-formed. _Body_ unknown.

  _Affinities._--In general appearance this species comes near to _B.
  psittacus_; but in all essential characters it comes much closer
  to the following species, from which, however, it can easily be
  distinguished by colour, and by the inner lamina of the parietes not
  being cancellated.



7. BALANUS VINACEUS. Pl. 2, fig. 7 _a_-7 _d_.

_Shell purplish dark brown: inner lamina of the parietes cancellated.
Scutum finely striated longitudinally. Tergum with the longitudinal
furrow shallow and open; basal margin on both sides sloping towards the
spur._

  _Hab._--West Coast of South America. Mus. Cuming.


  _General Appearance._--Shell conical, with a large, rhomboidal
  orifice; walls rather thin, coloured, together with the radii and
  operculum, dark purplish-brown; sheath nearly colourless. Walls
  smooth, slightly irregular, very finely striated longitudinally.
  Basal diameter of largest specimen .8 of an inch.

  _Opercular Valves_, unusually smooth, that is without prominent
  growth-ridges. _Scuta_, finely striated longitudinally, with the
  sharp striæ closely approximate. The teeth on the occludent margin
  are sharp, and stand some way apart from each other. Internally, the
  whole surface is remarkably flat and smooth: the articular ridge is
  of moderate breadth, and slightly reflexed: there is no adductor
  ridge, and the oval depression for the lateral depressor muscle is
  extremely slight. _Terga_, with the longitudinal furrow very slight;
  the bottom of this furrow is feebly striated longitudinally, and
  there is a trace of a fine, rounded ridge on the scutal margin, as in
  _B. decorus_. The basal margin slopes on both sides towards the spur,
  which is of moderate length and breadth, with its lower end truncated
  and parallel to the carino-basal margin; the spur stands at about
  once and a half its own width from the basi-scutal angle. Internally,
  the valve is lined by very dark, purplish-brown corium; the articular
  ridge is prominent; in the upper part of the valve, parallel to the
  articular ridge, there are two or three feeble ridges; there are no
  crests for the tergal depressores.

  The _Parietes_, though moderately thick, yet are light and fragile;
  the denticuli at the bases of the longitudinal septa are prominent,
  and those on the adjoining septa are united together, making a
  network (Pl. 2, fig. 7 _d_), but the interspaces between them are not
  filled up by solid calcareous matter (as is the case with every other
  species of the genus), but are only crossed at successive levels by
  fine transverse calcareous septa; the internal lamina thus becoming
  cancellated, and, though thick, fragile. Hence, in a transverse
  section of the parietes, the ordinary parietal tubes or pores are
  seen to be lined on their inner sides by five or six rows of very
  minute pores. I have not seen any other instance of this structure.
  The internal lamina is ribbed, as usual, on its inner surface, by
  the projection of the longitudinal septa. The ordinary parietal
  tubes are open, to nearly the summit of the shell. The _radii_ are
  rather thin, and unusually fragile; their summits are parallel to
  the basis: their septa, as seen on the sutural edges, are extremely
  thin and denticulated on both their upper and lower surfaces, on the
  side towards the internal lamina: towards the external lamina, the
  septa are simple, and the small square pores thus formed, are open
  or not filled up. The _alæ_ have their summits extremely oblique,
  being added to very little during the diametric growth of the shell;
  the narrow margin, however, which is thus added, is coloured red,
  the rest of the sheath being nearly colourless: the sutural edges
  of the alæ are smooth. The _basis_ has a thick, underlying, finely
  cancellated layer of shell.

  _Animal's body_ unknown.

  A young specimen, .2 of an inch in basal diameter, differed from
  the above in being of a much paler purplish-brown. This species is
  distinct from all its congeners, in its peculiar colour, and likewise
  in the structure of the inner lamina of the parietes. As already
  stated, it comes nearer to _B. decorus_ than to any other species.



8. BALANUS AJAX. Pl. 3, fig. 1 _a_-1 _d_.

  BALANUS TINTINNABULUM (_var._) _Chenu._ Illust. Conch., Tab. 2, fig.
        8.

_Shell globulo-conical, often elongated in the rostro-carinal axis,
pale pink, smooth, extremely massive: parietal pores, close to the
basal margin, circular and very small. Scutum with the articular ridge
broad and reflexed._

  _Hab._--Philippine Archipelago, attached to _Millepora complanata_,
  Mus. Cuming. Mus. Brit. and Stutchbury.


  _General Appearance._--Shell globulo-convex, sometimes much elongated
  in its rostro-carinal axis; smooth; walls excessively strong,
  massive, and heavy. Orifice oval, rather small in proportion to the
  size of shell, this being chiefly due to the infolding of the upper
  part of the rostral compartment. Parietes pale pink, feebly tinted
  with purple: radii either paler, or tinted of a bright chesnut-brown:
  sheath rich purplish chesnut-brown. Basal diameter of the largest
  specimen nearly 3-1/2 of an inch; height 2-3/4: another specimen had
  a basal longitudinal diameter of 2.9 of an inch, and a transverse
  diameter of only 1.6; this great difference in the two diameters
  being caused by the prolongation of the basal portion of the rostrum
  in the line of the branch of the Millepora, to which the shell had
  adhered; the height of this same specimen was 1.5; and the diameter
  of the orifice, both transversely and longitudinally, .75 of an inch.

  _Scuta_, broad, feebly tinted with pink; exterior surface rough,
  with sharp hood-formed projections, arranged in straight lines
  radiating from the apex; an inflected portion of the valve along the
  tergal margin is not roughened. Internally (Pl. 3, fig. 1 _d_), the
  articular ridge is broad and reflexed. An adductor ridge can hardly
  be said to exist, but a slight prominence borders the gentle hollow
  in which the lateral depressor muscle is attached. The basal margin,
  on its inner face, is slightly toothed. _Tergum_ white, with the
  narrow part of the valve, on the scutal side of the spur, rough with
  the little projecting hoods, like those on the scutum; the other and
  larger half is smooth: spur rather long, narrow, placed at twice its
  own width from the basi-scutal angle; on the carinal side, about half
  of the basal margin slopes down towards the spur. The longitudinal
  furrow is either quite or nearly closed. Internally, the spur is
  produced upwards on the valve, as a prominence: the articular ridge
  is not very prominent. There are no crests for the tergal depressor
  muscle.

  Altogether the opercular valves strikingly resemble those of _B.
  tintinnabulum_, but all the characters above mentioned have not been
  observed in any one variety of this species; perhaps _var. coccopoma_
  comes nearest, both in the external appearance of the shell and in
  the structure of the opercular valves, to _B. Ajax_.

  The _Compartments_ are remarkably compact and solid; the parietal
  tubes are cylindrical and quite minute even close to the basis; they
  extend, however, nearly up to the top of the shell; the parietal
  septa at the basis are thick, and with blunt denticuli; the thickness
  of the walls in the upper part of the shell is excessive; in the
  lower part, it is also unusually great, owing to the thickness of the
  inner lamina, and hence the ribs, generally formed by the projection
  of the longitudinal septa on the inner lamina, are here visible only
  close to the basis. The _radii_ are rather wide; their summits are
  parallel to the basis; the septa on their sutural edges are thin,
  straight, and closely approximate, and most symmetrically furnished
  with little denticuli of equal sizes on both sides: the interspaces
  are nearly filled up solidly, but with some pores still left open.
  In the upper part of the shell, the radii, like the walls, are of
  extraordinary thickness: the septa are transverse and horizontal,
  as seen externally by slight variations in the colour of the radii;
  internally, as seen in a vertical section of the shell, the septa
  dip inwards at an angle of above 45°. The _alæ_ are thin, and have
  their summits oblique: their sutural edges are smooth. The pores in
  the basis are crossed by numerous transverse septa, and there is an
  underlying cancellated layer: the internal surface is very smooth.

  _Animal's body_ unknown.

  The strength of this Balanus is truly remarkable; and when, by
  repeated blows, a specimen which I was examining at last yielded,
  the radii broke sooner than separate at their sutures. In most of
  its characters, this species approaches _B. tintinnabulum_, and I
  believe has been included by Chenu as one of its varieties; but
  it comes almost equally near to _B. stultus_, to which it is much
  more closely allied in its habit of being attached to Milleporæ.
  By a close and unbroken chain of affinities, _B. Ajax_, through _B.
  stultus_, is connected with _B. calceolus_ and its allies in section
  (B), which live attached to Gorgoniæ. Some of the specimens of _B.
  Ajax_, are almost as much elongated in their rostro-carinal axis, as
  are the species in section (B); and there is an affinity in the same
  direction in the smallness of the pores in the radii of _B. Ajax_;
  indeed, had the basis in this species been generally more boat-or
  cup-formed, I should have placed it as the first species in section
  (B), instead of, as at present, the last species in section (A). The
  intermediateness of the characters of _B. Ajax_ has been one chief
  cause why I have rejected the genus Conopea, which was instituted by
  Say for the species living attached to Gorgoniæ.



_Section_ B.

_Parietes and basis sometimes permeated by pores, sometimes not: radii
not permeated by pores: shell elongated in its rostro-carinal axis:
basis boat-shaped: attached to Gorgoniæ and Milleporæ._



9. BALANUS STULTUS. Pl. 3, fig. 2 _a_-2 _d_.

_Parietes and base porose: shell white, or faintly tinged with purple.
Scutum with the basal margin protuberant in the middle. Tergum with
the longitudinal furrow closed in the upper part: spur not closely
adjoining the basi-scutal angle._

  _Hab._--Attached to Milleporæ, Singapore, Mus. Cuming. West
  Indies,[89] Mus. Brit.--Mus. Stutchbury.

    [89] This specimen in the British Museum was purchased at the sale
    of the Rev. L. Guilding's collection, and therefore it is not
    certain that this habitat is correct; but as it was sold in the
    same lot with a Cirripede certainly West Indian, and as the main
    collection was made in the West Indies, this habitat may, I think,
    be trusted.


I have considerable doubts whether it would not have been more correct
to have placed this species in the last section, instead of where
it now stands; it certainly is more closely allied to _B. Ajax_,
especially in its operculum, than to the following species; yet the
fact of the radii not being permeated by pores does not permit of its
admission into the last section; and both in habits and structure it
undoubtedly comes very near to the following species. Those varieties
which are not much elongated, and which have the basis nearly flat,
would certainly, if considered by themselves alone, not have gained
admission into our present section.


  _General Appearance._--Shell conical, somewhat globular, more or less
  elongated in the rostro-carinal axis, owing to the basal production
  of the rostrum. Orifice, rather small, entire, oval, pointed at the
  carinal end. Radii moderately broad, with their summits parallel to
  the basis. Colour dirty white, often faintly tinged with purple;
  sheath, pale purplish-blue. Surface extremely smooth; the parietes
  are generally covered (as viewed through a lens) by a very thin,
  yellowish epidermis, giving to the whole a glistening, granular
  aspect: the radii are generally destitute of this epidermis, and are
  therefore of a dead white. The basis is concave, and sometimes deeply
  cup-formed; it is, however, not symmetrical; sometimes it is flat.
  Basal diameter of largest specimen, including the basis itself, 1.5
  of an inch in the longitudinal axis; transverse diameter, 1 inch; the
  inequality in the length of the two diameters is rarely so great as
  in this unusually large specimen.

  _Scuta_, externally very convex, with the growth-ridges extremely
  prominent; basal margin sinuous, the middle portion being prominent;
  this is best seen in young specimens (Pl. 3, fig. 2 _d_). Internally,
  the articular ridge is broad and reflexed. The adductor ridge in the
  upper part is almost confluent with the articular ridge; it runs down
  to the most prominent point of the basal margin; in young specimens
  it is sharp and prominent; in old specimens it is very blunt and
  little prominent. There is a rather deep hollow for the lateral
  depressor muscle. In young specimens there is a small, depending,
  blunt tooth at the basi-tergal angle, which helps to make the basal
  margin more deeply sinuous.

  _Terga_, with the longitudinal furrow closed, except on the spur
  itself, where it is open. The spur is moderately long and broad, but
  varies in breadth; it is placed at rather less than its own width
  from the basi-scutal angle; its lower end is obliquely rounded; the
  basal margin on the opposite sides of the spur, together form a
  nearly straight line. The whole valve is rather broad. The crests for
  the tergal depressores are barely developed.

  The _Compartments_ have rather large parietal tubes; the septa are
  coarsely denticulated at their bases; the internal lamina is smooth,
  except close to the basis. The _radii_ have their summits parallel
  to the basis; their sutural edges are formed of rather thick septa,
  which stand at an unusual distance apart from each other, and have
  perfectly symmetrical, minute denticuli on each side. The interspaces
  between the septa are filled up solidly to within a short distance of
  the surface; but yet not so completely as in the following species,
  and as in those in the succeeding sections of the genus; this is what
  might have been expected from the close affinity of _B. stultus_ to
  _B. Ajax_, in which latter the radii are still permeated by pores,
  though smaller than is general in the species of our first section
  (A). The _alæ_ have their summits extremely oblique, and their
  sutural edges, I believe, smooth. _Basis_ porose, with an underlying,
  finely-cancellated layer.

  _Mouth_: labrum with six small teeth; mandibles with the 3d tooth
  blunt; the 4th minute, and the 5th almost confluent with the inferior
  angle. Maxillæ with the edge straight and simple. _Cirri_ partly
  destroyed; on each segment of the sixth pair there were five pairs of
  spines.



10. BALANUS CALCEOLUS. Pl. 3, fig. 3 _a_-3 _e_.

  BALANUS CALCEOLUS KERATOPHYTO INVOLUTUS (?) _Ellis._ Phil. Trans.,
        vol. 50 (1758), Tab. 34, fig. 19.

  LEPAS CALCEOLUS (?) _Pallas._ Elench. Zooph., p. 198, (sine
        descript.) (1766).

  CONOPEA OVATA (?) _J. E. Gray._ Annals of Philosophy, vol. x, 1825.

_Parietes and basis porose. Scutum with the pit for the lateral
depressor muscle small and deep._

  _Hab._--Attached to Gorgoniæ, West Coast of Africa. Tubicoreen, near
  Madras, (Dr. Johnston), associated with _B. navicula_. Mediterranean
  (?). Mus. Brit., Cuming, Stutchbury.

  _Fossil._ Coralline Crag; Mus. S. Wood.


I must premise, with respect to the nomenclature of this and the three
following quite distinct species, that in the published descriptions
no allusion is made to any one of the characters by which alone
they can be distinguished: hence I have been guided by geographical
probabilities in assigning the specific name of _calceolus_ to the
present species, as Ellis's specimens came from the Mediterranean; and
that of _galeatus_ to the North American and West Indian specimens, as
Linnæus' original specimens (according to a statement by Spengler) came
from the West Indies. I have assigned new names to the two remaining
East Indian species. I may here add that Spengler ('Skrifter af
Naturhist.' 1 B, tab. 6, fig. 3, 1790) has described, under the name of
_B. cassis_, an allied form attached to the _Gorgonia placomus_ from
the seas of Norway; but I do not believe that it is the same with our
present species.


  _General Appearance._--The degree of elongation of the shell in
  its rostro-carinal axis varies considerably (3 _a_, 3 _b_): the
  elongation is due to the production of the rostrum and of the
  corresponding end of the basal cup. These two portions of the
  shell always form together an angle, and sometimes an acute angle,
  whereas in all the many specimens which I have seen, the carina and
  the carinal end (or heel) of the basis together form a straight
  line; yet I should not be surprised if this end of the shell was
  sometimes produced. The surface of the shell is smooth, or sometimes
  marked with very minute projecting points: it is almost always
  covered by the horny bark of the Gorgonia. The colour is either dull
  purplish-red or dull purple, with obscure longitudinal stripes,
  and often more or less transversely banded with white. The rostrum
  is either white or very feebly tinted, being always paler than the
  rest of the shell: the radii are usually paler than the parietes,
  and are sometimes white: the carinal end of the basal cup is tinted
  of the same colour with, but rather paler than, the compartments.
  The orifice is rather small compared to the shell, and nearly
  heart-shaped. The carino-lateral compartments are about one-third of
  the width of the lateral compartments. The shell is very strong, and
  the sutures resist the action of boiling caustic potash. The largest
  specimen which I have seen was .7 of an inch in extreme length, and
  under .25 in extreme breadth.

  _Structure of the shell and basis._--The parietes are permeated by
  quite distinct pores,--a character sufficient by itself to separate
  this from the following species; the longitudinal septa forming the
  tubes are slightly denticulated at their bases. The radii have their
  summits quite square, extending from apex to apex of the adjoining
  compartments. The alæ have oblique summits. The sutural edges of
  the radii have approximate septa, which are obscurely denticulated:
  the interspaces are filled up solidly, so that the radii are not
  porose. The basis is distinctly porose, by which this species can be
  distinguished from _B. navicula_ and _cymbiformis_. The basis has a
  deep furrow on the under side, from clasping the thin horny axis of
  the Gorgonia: the basal point of the rostrum is also notched from the
  same cause, and, as a consequence, its upper surface becomes slightly
  furrowed along its whole length.

  The _Scuta_ have an articular ridge but moderately prominent, and
  only slightly reflexed; the basi-tergal corner is rounded off; there
  is no adductor ridge; there is a small, rather deep, distinct pit
  for the lateral depressor muscle. _Terga_; externally the surface is
  considerably depressed in the line of the spur. The spur is between
  half and one-third of the width of the valve: its lower end is square
  and truncated, or in some degree rounded; it is sometimes (3 _e_)
  dentated with a few, minute, sharp teeth. The articular ridge is but
  slightly developed; the crests for the depressor muscle are very
  feeble.

  _Animal's body_ unknown.



11. BALANUS GALEATUS. Pl. 3, fig. 4 _a_-4 _c_.

  LEPAS GALEATA (?) _Linnæus._ Mantissa altera Holmiæ, 1771.

  CONOPEA ELONGATA. _Say._[90] Journal of Acad. Nat. Sci. Philadelphia,
        vol. ii, part 2, p. 323, 1822.

    [90] If I have assigned the specific title of _galeatus_ to
    the wrong species, yet Say's name of _elongatus_ ought not
    strictly to be admitted; as the _Lepas elongata_ of Gmelin is a
    Balanus,--probably a variety of _Balanus crenatus_. I may add, that
    as the _Lepas galeata_ of Schröter ('Einleitung in die Conch.'
    &c.), was attached to a Gorgonia from the East Indies, it cannot be
    our present species, but probably is one of the three other allied
    species, which all occur in India.

_Parietes not porose; basis porose. Tergum, with the apex square,
caused by the great development of the articular ridge._

  _Hab._--Charlestown, South Carolina; Florida; West Indies; Central
  America; attached to Gorgoniæ; Mus. Brit., Agassiz, Cuming,
  Stutchbury.


  _General Appearance._--This and the two following species come so
  close in general appearance to the last, that it will be quite
  superfluous to do more than describe the few points of difference.
  The shell and basis are generally quite as much elongated as in the
  last species, and sometimes much more so, owing to the carinal end
  (fig. 4 _a_), with the corresponding portion of the basal cup, being
  produced like the rostral end, into a flattened, sharp point: I have
  seen a specimen in this state .9 of an inch in length, and only .25
  in breadth in the broadest part. In many specimens, however, the
  shape is exactly as in _B. calceolus_; but the rostrum seems less
  usually furrowed from clasping the stem of the Gorgonia. The colour
  is paler, pinker, and more distinctly striped longitudinally than
  in _B. calceolus_; I have, however, seen some not-striped, purple
  specimens (and one transversely freckled with white) from the West
  Indies. The parietes are strongly-ribbed internally, and are not
  permeated by pores. The radii have their sutural edges crenated. The
  basal cup is permeated by pores.

  The _Scutum_ differs from that in the last species, only in the pit
  for the lateral depressor muscle, being much shallower, and less
  defined, and in the apex being truncated. The _Tergum_ is remarkable
  from its broad, square, truncated summit, which underlies the whole
  broad apex of the scutum: the square summit of the tergum is formed
  by a great and peculiar development of the uppermost part of the
  articular ridge. The spur is a little narrower than in _B. calceolus_.

  _Mouth_: on the crest of the labrum there are two teeth on each side
  of the central notch. The mandibles have five teeth, of which the two
  lower are very small. The maxillæ show a trace of a notch under the
  upper large pair of spines; near the inferior angle there are two
  long spines. _Cirri_: in the first pair, one ramus is nearly twice as
  long as the other: the segments are not very protuberant. There is
  a sharp point at the dorsal basis of the penis. The branchiæ are of
  moderate size, and plicated on one side.



12. BALANUS CYMBIFORMIS. Pl. 3, fig. 5 _a_, 5 _b_.

_Parietes and basis not porose. Scutum and Tergum with very small
articular ridges. Tergum broad, almost equilateral._

  _Hab._--Attached to a Gorgonia, Tubicoreen, near Madras, (Dr.
  Johnston). Hab. unknown, Mus. Cuming.


  _General Appearance._--I have seen only two specimens, kindly sent me
  by Dr. Johnston, and a single specimen in Mr. Cuming's collection.
  In most points this species agrees with the two last species. The
  shell (excepting the rostrum), and even the opercular valves in Mr.
  Cuming's specimen were of a very fine purplish-red; in the other
  specimens they were feebly tinted purple. The parietes are strongly
  ribbed internally, and are not permeated by pores. The basal cup is
  not porose, but its inner surface is ribbed in lines radiating from
  the centre, and in both these respects this species differs from the
  two foregoing. The Radii are rather narrow; they are paler coloured
  than the parietes; they have their sutural edges plainly crenated.
  The alæ have extremely oblique summits; the narrow rim added during
  the diametric growth of the shell is white, the rest of the sheath
  being, in Mr. Cuming's specimen, finely coloured like the parietes.
  Basal diameter of the longer axis of the largest specimen, .4 of an
  inch.

  _Scutum_, rather narrow, with the basi-tergal corner much rounded
  off; externally the lines of growth are little prominent. Internally,
  the articular ridge is extremely little developed, and not at all
  reflexed; there is no adductor ridge; there is a minute pit for the
  lateral depressor muscle, placed almost on the edge of the valve.
  The _Tergum_ is broad, forming (the spur being excepted) an almost
  equilateral triangle. The articular ridge is remarkably little
  prominent, and placed close to the scutal margin. The spur is nearly
  half as broad as the valve, with its extremity or basal margin in one
  case obliquely truncated, and in another case nearly square.

  _Animal's body_ unknown.



13. BALANUS NAVICULA. Pl. 3, fig. 6 _a_-6 _d_.

_Parietes and basis not porose: carino-lateral compartments very
narrow, and of nearly the same width from top to bottom: radii with
their sutural edges smooth. Scutum externally striated longitudinally._

  _Hab._--Attached to Gorgoniæ, Tubicoreen, Madras (associated with _B.
  calceolus_), Dr. Johnston. Hab. unknown, Mus. Brit. and Darwin.


This is a very distinct form, though nearer to the foregoing than to
the other species. Its separation from the sub-genus Acasta is quite
artificial; its affinity to this sub-genus is shown by its weaker
shell, non-porose parietes and basis; by the radii having their sutural
edges smooth, and their summits not quite square; by the carino-lateral
compartments being very narrow; by the less elongated basis, not
furrowed, from not clasping the branches of the Gorgonia; and by the
longitudinally striated scuta; nevertheless, from the similar habits,
and from the graduated structure in the five foregoing species, it
cannot be removed out of the genus Balanus. I have seen three sets of
specimens of this species.


  _General Appearance._--Shell, sometimes with the rostrum, and
  sometimes with the carina, and corresponding portions of the basal
  cup, elongated; but not, apparently, to so great a degree as in
  the foregoing species; basis not furrowed, from not clasping the
  branches of the Gorgonia. Colour pale blueish-purple, with the
  radii whiter. The surface is studded with small calcareous points.
  The carino-lateral compartments are very narrow, not more than one
  tenth of the width of the lateral compartments; they are, moreover,
  scarcely wider at the base than at the summit. The summits of the
  radii are, apparently, a little oblique, or at least not so square as
  in the foregoing species. The shell is not nearly so strong as in the
  last three species; and the compartments separate by gentle force,
  and from the action of caustic potash. The largest specimen was .4 of
  an inch in basal diameter.

  Internally, the parietes are not very strongly ribbed, or they are
  almost smooth, and there are no pores. The basis is concave and
  smooth within, and is not porose. The sutural edges of the radii are
  quite smooth, or sometimes they exhibit, in the lower part, mere
  traces of septa,--a character by itself sufficient to separate this
  from the foregoing species. The alæ have oblique summits, and the
  rather narrow portion added during the diametric growth of the shell,
  is white.

  The _Scutum_, externally (6 _d_), has raised striæ, radiating from
  the apex; valve rather thick; internally, the articular ridge is but
  slightly prominent, and its lower end is rounded off: the depression
  for the lateral depressor muscle is slight; between this depression
  and that for the adductor muscle, the surface of the valve is
  prominent. _Tergum_, somewhat beaked; externally, the surface is
  depressed in the line of the spur: the carino-basal margin slopes
  towards the spur.

  _Animal's body_ unknown.



_Section_ C.

_Parietes and basis permeated by pores. Radii not permeated by pores._



14. BALANUS TRIGONUS. Pl. 3, fig. 7 _a_-7 _f_.

_Parietes ribbed, mottled purplish-red; orifice broad, trigonal, hardly
toothed. Scutum thick, with from one to six longitudinal rows of little
pits. Tergum without a longitudinal furrow; spur truncated, fully one
third of width of valve._

  _Hab._--Java; East-Indian Archipelago; Peru; West Columbia;
  California; Sydney; New Zealand. Mus. Brit., Cuming, Stutchbury,
  Dunker, &c.


  _General Appearance._--Shell conical, generally depressed; orifice
  broad, triangular, almost equilateral; walls coloured or only mottled
  with purplish-pink, having either irregularly branching, or regular,
  longitudinal ribs, which are generally white. The radii are pale
  pink, or nearly white: the opercular valves have either their upper
  parts, or nearly their whole surface, clouded with pinkish-purple:
  the epidermis is not persistent: the walls are moderately strong: the
  largest specimen was one inch, but generally full-grown specimens are
  about half an inch in basal diameter.

  The _Scuta_ have the lines of growth highly prominent. From one to
  five or six rows (7 _b_, 7 _c_) of nearly circular, or transversely
  oblong, deep pits, extend down the middle of the valve; rarely there
  is not even one row; in this latter case, the valve is not striated
  longitudinally. These little pits are caused by one or more deep
  longitudinal furrows, crossed by the lines, or rather ridges, of
  growth. In the same group of specimens, I have seen individuals with
  three, five, and six rows; and even a few specimens with only one
  row, or none at all. The outline of the valve is elongated, with the
  apex slightly reflexed: the inner surface is protuberant, sometimes
  to a remarkable, but variable degree. The articular ridge is not
  very prominent, but it extends fully half-way down the valve, and
  generally ends in a small free point. There is a short adductor
  ridge, and a deep narrow pit or cleft for the lateral-depressor
  muscle. _Terga_, externally smooth, flat, with scarcely a trace of
  a longitudinal furrow; spur broad (7 _e_, 7 _f_), varying from half
  to one third of the width of the valve, with the end truncated,
  situated either near or quite close to the basi-scutal angle. The
  crests for the depressor muscles are moderately well developed.

  _Compartments._--The parietal tubes are, in their upper parts, filled
  up solidly, without transverse septa. The radii generally have their
  summits slightly oblique, and this is almost always the case with the
  radii of the rostrum; the other radii sometimes extend from tip to
  tip of the parietes, and are parallel to the basis; rarely the radii
  are considerably oblique. The septa of the radii are very obscurely
  denticulated, and the interspaces between them are filled up solidly.
  The alæ have their sutural edges thin and smooth.

  _Mouth._--Labrum with three teeth close together on each side of the
  central notch: mandibles with four teeth, the fourth being small, the
  fifth either absent or scarcely distinguishable from the inferior
  angle: maxillæ without any notch, with the two lower spines rather
  longer than the others. _Cirri_: In the first pair, one ramus is only
  half the length of the other; in the second pair, both rami are short
  and about equal in length; in the posterior pairs, the segments,
  which are not protuberant, bear four pairs of spines, of which the
  three lower pairs are short.


This species is widely-distributed, and where found seems to be common.
It is generally attached to shells of mollusca, but I have seen it also
attached to wood. I have found it associated with _B. tintinnabulum,
var. concinnus_, and _coccopoma_, with _B. psittacus_, _improvisus_ and
_amphitrite_, and with _Elminius modestus_.

Young specimens bear a considerable resemblance to certain young
varieties of _B. tintinnabulum_, and can indeed be distinguished
from them only by a careful examination of the opercular valves; for
it should be borne in mind, that in certain cases the scuta in _B.
tintinnabulum_ are pitted with little cavities. This species in some
respects is, I think, allied to _B. porcatus_, but it is far more
closely related to _B. spongicola_, and can be discriminated with
difficulty from certain varieties of this latter species. In Mr.
Cuming's collection, there is a group of small specimens, crowded
between some older specimens, which are remarkable from the shell being
oval in a transverse section,--from the smoothness of the walls,--and
from the absence of pits on the scuta; yet there could be no doubt that
these specimens belonged to our present species.



15. BALANUS SPONGICOLA. Pl. 4, fig. 1 _a_-1 _d_.

  BALANUS SPONGICOLA. _Brown's_ Illustrations of the Conchology of
        Great Britain (1827), pl. 7, fig. 6: 2d edit. (1844), pl. 53,
        figs. 14-16.

_Parietes generally smooth, sometimes longitudinally folded; coloured
pink: orifice toothed. Scutum longitudinally striated. Tergum, with the
apex produced, without a longitudinal furrow; spur truncated, about one
third of width of valve._

  _Var. with the walls slightly folded longitudinally._

  _Hab._--South coast of England, and Tenby in South Wales, often
  imbedded in sponges; attached also to shells and rocks in deep
  water; Mus. Brit., Jeffreys. Algiers, on Mytili and Serpulæ, with
  _B. perforatus_, Mus. Mac Andrew. Madeira, with _B. tulipiformis_,
  Mus. Lowe. Lagulhas Bank, Cape of Good Hope, on detached kelp, with
  _B. Capensis_, Mus. Sir J. Ross. Imbedded in sponge with _Acasta
  spongites_, Mus. Bowerbank. _Var._ West Indies.

  _Fossil_ in Coralline Crag, Mus. S. Wood.


  _General Description._--Shell tubulo-conical; orifice of moderate
  size, rather deeply toothed; colour dull pink, or purplish, or dark
  flesh-colour; sometimes the radii are paler, sometimes of the same
  colour with the parietes. Surface smooth when well preserved, having
  transverse rows of minute spines. In the West Indian variety the
  walls are slightly or much folded, but I will describe this form
  separately. Size of largest specimen (Mus. Jeffreys), .6 of an inch
  in basal diameter.

  _Scutum_, with fine ridges radiating from the apex, and with the
  lines of growth, crenated: internally, the articular ridge is small,
  adductor ridge short and barely distinct: there is a rather deep
  and narrow pit for the lateral depressor muscle. The whole valve
  is much thinner than in _B. trigonus_, which in most respects it
  closely resembles. _Tergum_, with the apex pinkish purple, produced
  or beaked, but the beak is not needle-like, as in _B. psittacus_
  and its allies, for the carinal margin is perfectly preserved up to
  the tip. Externally the valve is nearly flat, for the longitudinal
  furrow is very shallow. The spur is about one third of the width of
  the valve; its lower end is abruptly truncated: in European specimens
  (1 _b_) the whole basal margin, on the carinal side, slopes down to
  the spur in a straight line, which, together with the sharpness and
  production of the basi-scutal angle of the spur itself, gives to the
  whole valve a peculiar appearance: in the specimen (1 _c_) from the
  Lagulhas Bank, the basal margin on the carinal side is a little more
  hollowed out, but it is quite impossible to doubt about the specific
  identity of these specimens: in the West Indian variety (1 _d_) the
  basal margin on the carinal side forms a distinct but obtuse angle
  with the spur. In all cases the crests for the depressor muscles are
  very feebly developed.

  The _Compartments_ have their radii developed to a rather varying
  degree, with their summits oblique; hence the orifice is toothed:
  the sutural edges of the radii have their septa barely denticulated;
  the sutural edges of the alæ are smooth. The basis, as with the
  other species of this section, is permeated by pores; yet I found
  one specimen, from the Cape of Good Hope, with the basis apparently
  solid, thus offering a very singular anomaly. In the specimen
  imbedded in sponge, the basis, as viewed externally, is concave;
  whereas in Acasta, which always inhabits sponges, the basis is highly
  convex or hemispherical.

  The _Mouth_ and _Cirri_ resemble those of _B. trigonus_, and I can
  point out no distinguishing character.

  With respect to the variety from the West Indies, I have seen two
  sets of specimens differing somewhat in external appearance, one set
  attached to a coral from St. Vincent's, and another set to an Avicula
  from an unknown locality; at first I described these specimens, with
  some hesitation, as a distinct species, and I am very far from sure
  whether this would not have been the more correct course, although
  I am unable to point out any sufficient diagnostic characters. This
  form differs from the ordinary _B. spongicola_, in the walls being
  more rugged, stronger, and slightly or deeply folded longitudinally;
  in this latter case the shell in external aspect differs much from
  ordinary specimens of _B. spongicola_; but this is a variation so
  common that I dare not place any reliance on it. The colour is more
  purple; the summits of the radii perhaps rather less oblique. In the
  scuta the only difference is that the articular ridge seems rather
  longer, and the adductor ridge perhaps more prominent: in the terga,
  as already remarked, the basal margin on the carinal side does not
  slope so straight into the spur. These differences I consider all too
  slight to be of specific value. The difficulty in determining the
  nature of this variety is added to by its approach to _B. trigonus_
  in all those points in which it departs from the ordinary _B.
  spongicola_, so that for a short time I was even tempted to consider
  both these species as varieties of one form. But until _B. trigonus_
  is found with its scutum longitudinally striated, and with its tergum
  beaked, it can hardly be confounded with _B. spongicola_; for it
  should be observed that when in _B. trigonus_ the rows of little pits
  disappear from the scuta, as sometimes happens, though rarely, yet
  these valves do not become longitudinally striated.

  _Balanus spongicola_ occurs, mingled with _B. tulipiformis_, in the
  Mediterranean, and by the external characters of the shell alone
  cannot be distinguished from that species; but the striated scuta and
  beaked terga suffice to separate them. Again, this species, at the
  Cape of Good Hope, occurs mingled with _B. Capensis_, and from the
  non-striped young varieties of that species, it can, externally, be
  distinguished only by the beak of the tergum not being sharp like a
  needle. I have seen a single, perfectly characterised specimen, with
  its opercular valves preserved, found by Mr. S. Wood in the Coralline
  Crag at Sutton, mingled with _B. inclusus_.



16. BALANUS LÆVIS. Pl. 4, fig. 2-2 _g_.

  BALANUS LÆVIS. _Bruguière._ Encyclop. Meth. (1789), Pl. 164, fig.
        1.[91]

  ---- DISCORS. _Ranzani._ Mem. di Storia Nat., 1820, Tab. 3, figs. 9
        to 13.

  ---- COQUIMBENSIS. _G. B. Sowerby_, in Darwin's Geology of South
        America (1846), Tab. 11, fig. 7.

    [91] M. Deshayes, in his descriptions of the plates, considers this
    figure, I have no doubt erroneously, as that of _B. perforatus_, of
    Bruguière. The _B. Coquimbensis_ of Sowerby, is a different species
    from the _B. Coquimbensis_, of Chenu, 'Illust. Conch.,' tab. 6,
    which latter is unknown to me.

_Shell covered by brown membrane, or naked and white or pale purple;
orifice small; radii very narrow. Scutum with one or two deep
longitudinal furrows._

  _Var._ nitidus (fig. 2): _shell not covered by membrane, white or
  pale purple: orifice but slightly toothed: scutum generally with two
  furrows_. Hab.--Chile, as far south as Concepcion; Peru; California.

  _Var._ Coquimbensis (fig. 2 _a_): _with the basal cup partly filled
  up with thin, irregular, calcareous layers, making a cancellated
  mass_. Fossil, and recent.

  _Hab._--Strait of Magellan, ten to twenty fathoms, attached to
  shells; often entirely surrounding pebbles, forming globular masses;
  associated with _Verruca lævigata_. Chile and Peru, (generally _var.
  nitidus_), often attached to _Balanus psittacus_. California. Very
  common.

  _Fossil_ in an ancient tertiary formation (middle bed) at Coquimbo,
  Chile. In a recent deposit (_var. nitidus_) at the height of 1000
  feet at Valparaiso; with Human remains at San Lorenzo, Callao, Peru.


I may premise that, having myself collected this species from the same
locality, the Strait of Magellan, where no allied species occurs,
attached to the same Mytilus and associated with the same Verruca, I
feel confident that it is the _B. lævis_ described by Bruguière; and
there can hardly be any doubt that it is the _B. discors_ of Ranzani.
With respect to the old tertiary specimens from Coquimbo, named _B.
Coquimbensis_ by Sowerby, they differ from the recent in no respect,
except in being considerably larger; and therefore I cannot consider
them specifically distinct. At first I was unwilling to believe that
the specimens with a single very broad longitudinal furrow, and those
with two rather broad, or with one narrow furrow, on their scuta,
could belong to the same species; but I soon found that all these
varieties occurred mingled together, and that they differed in no
other respect whatever. Generally, however, all the individuals in the
same cluster had the same variety of scutum,--thus adding one more to
the many instances amongst cirripedes of variations common to whole
groups of specimens. Still more unwilling was I to believe that _var.
nitidus_ and the common variety could belong to the same species. Their
general aspect is totally unlike: _var. nitidus_ has a smooth, clean,
naked shell, either white or pale purple, somewhat globulo-conical,
often with a nearly entire orifice; whereas the other common variety
generally has a more steeply conical shell, with a toothed orifice,
and is covered by a dirty brownish membrane. Moreover, though I have
seen hundreds of specimens from Tierra del Fuego, I have not seen one
specimen of _var. nitidus_, or even of an approach to it in appearance;
and, on the other hand, _var. nitidus_ is the common form in Chile and
Peru; though I have seen one or two specimens of the membrane-covered
variety from Valparaiso. Such facts strongly induced me to believe that
these forms were specifically distinct; but upon careful examination
I could find no other or more important differences than those just
specified. Some specimens from northern Chile are in an intermediate
condition; and from Concepcion, in the south of Chile, where the
climate approaches in character to that of the more southern parts of
the Continent, there are many specimens, in so intermediate a condition
that I know not whether or no to rank them under _var. nitidus_. Thus
I became convinced that these forms are only varieties. At Concepcion,
some few specimens are pale purple, and yet are wholly invested by
thick brown membrane, thus uniting the two extreme varieties. From
California I have seen both varieties, but I do not know which is
most common there. With respect to the great difference in aspect
between the specimens from northern Chile and Tierra del Fuego, we
shall hereafter see a strictly analogous case in _Balanus flosculus_.
Finally, I may add that _B. lævis_ seems to represent in the southern
hemisphere and on the west coast of North America, the _B. perforatus_
of Europe and Western Africa.


  _General Appearance._--Shell conical, sometimes slightly globular;
  surface smooth (that is, not folded), either naked, and in that case
  white or pale purple, or covered by dirty yellowish-brown membrane.
  Orifice small, more or less toothed, rarely exceeding one third of
  the basal diameter. Radii very narrow, often not developed, the six
  sutures forming in all cases deep and narrow clefts. The largest
  recent, but much depressed, specimen which I have seen (from the
  Strait of Magellan) was three fourths of an inch in basal diameter;
  specimens growing congregated are often much elongated. I have seen
  one with the basal cup between two and three times as deep as the
  height of the compartments. Of the ancient tertiary specimens, the
  largest had a diameter of three fourths of an inch, and a total
  length of actually two inches (fig. 2 _a_); another of these fossils
  had a basal cup in depth equalling four fifths of the entire length
  of the shell and basis.

  The _scutum_ has either one very broad and deep longitudinal furrow
  (2 _b_), or two moderately broad and deep (2 _e_), or two narrow and
  deep, or less frequently one narrow and inconspicuous longitudinal
  furrow (2 _f_); rarely there is not one furrow; sometimes there are
  none towards the apex, whilst furrows have been formed in the lower
  part of the valve. In young specimens the furrows extend down to
  the actual basal margin, but in old specimens they often fall short
  of this, and, as a consequence, the furrows become crossed by one,
  two, or three calcareous ridges, which ridges at successive periods
  formed the basal margin of the valve. The external surface is covered
  by yellow membrane; and fragments of several successive opercular
  membranes are often attached to the zones of growth.

  Internally the articular ridge is not very prominent, but is
  remarkable (2 _c_, 2 _d_) from its lower point being produced into a
  long, sharp, sub-cylindrical, free style (like the hinge of a common
  gate), which is generally broken off in disarticulating the valve
  from the tergum. The adductor ridge is either sharp and prominent
  or blunt: it extends up the middle of the valve nearly to the apex,
  and downwards it trends a little towards the occludent margin. The
  pit for the lateral depressor muscle is minute but deep: the basal
  margin is sometimes hollowed out under this pit. Sometimes there is
  a distinct, but blunt ridge, caused by one of the furrows outside,
  parallel to the adductor ridge, and placed between it and the little
  pit for the lateral depressor; in this case, the basal margin, as
  viewed internally, is rendered sinuous (2 _d_), as is best exhibited
  in the great fossil specimens from Coquimbo.

  _Tergum_ (2 _g_).--Spur of moderate length and breadth, with its
  lower end obliquely truncated and rounded. The longitudinal furrow
  has its edges somewhat folded in. The basal margin on the carinal
  side of the spur is sometimes a little hollowed out. The crests for
  the depressor muscles are well developed; but the corner of the valve
  supporting them is extremely thin, and is often imperfectly calcified.

  _Compartments._--The parietal tubes are not crossed by transverse
  septa, but in their upper parts are filled up solidly. The radii are
  always very narrow, with their summits oblique, though to a variable
  degree: their sutural edges have fine and closely approximate septa,
  with minute denticuli: the sutural edges are received in a furrow, on
  the opposed compartment, of unusual depth; hence the lines of suture
  run, in the lower part of the shell, almost exactly in the middle
  between each two compartments. The alæ are added to above the level
  of the opercular membrane.

  The _Basis_ is often thick, with an underlying layer, largely
  cancellated or honeycombed. When many specimens grow crowded
  together, the basis is generally deeply cup-formed, or even
  sub-cylindrical; and equals as much as four fifths of the length of
  the entire shell. In such cases, in some few recent specimens, and
  in all the large or even quarter-grown old tertiary specimens, but
  not in the quite young fossil specimens, a structure is presented,
  which I have not seen in any other Cirripede, namely, the basis (Pl.
  4, fig. 2 _a_) is filled up for one third, or even for more than
  half its depth, by successive, separate, calcareous, transverse
  layers or septa. It would appear as if the basal cup had grown too
  large for the animal's body, and so required filling up. The layers
  are thin and fragile; a single layer never stretches across the
  whole shell; each is irregularly mammillated or blistered, with the
  convex surfaces generally directed upwards; the layers are furnished
  on their under sides with little pillars and short ridges, resting
  on the layers beneath; it rarely happens that the supports of one
  layer lie directly over those of another, though this is sometimes
  the case. In a vertical section, the mass formed by these irregular
  layers has a coarsely cancellated structure. This structure, although
  confined to this one Cirripede, is not so anomalous as might at first
  be thought, for in most species of the genus, each time that the
  circumference of the basis is added to, an excessively thin calcified
  film is thrown down over its whole inner surface; and in any of these
  species, if the films had been formed thicker and had rested only
  on certain points, instead of over the whole underlying layer, the
  cancellated structure above described would have been produced.

  _Mouth_: the labrum is either destitute of teeth, or has two or three
  very minute teeth. The palpi have a tuft of very long spines at their
  ends. The third tooth of the mandibles is thicker and larger than
  the two upper ones. The maxillæ have either a nearly straight edge,
  or the inferior corner is obliquely truncated, and projects much
  beyond the rest of the edge. In the _Cirri_, none of the segments are
  very protuberant: in the first pair, one ramus is nearly twice as
  long as the other: in the posterior pairs, the segments are not much
  elongated, but each supports seven pairs of spines.

  _Var. nitidus_: with respect to this variety I have little to add
  to my preliminary remarks on its peculiar appearance, owing to its
  smooth, naked condition, and pure white or pale purple colour. This
  colour, when examined through a lens, is seen to consist of very
  fine longitudinal stripes; and is produced by the calcareous matter
  within the longitudinal parietal pores being thus coloured. Generally
  the scuta have two longitudinal furrows; but I have seen a scutum
  of one perfectly characterised specimen with only a single broad
  furrow, like that which frequently occurs in the membrane-covered
  variety. _Var. Coquimbensis_, as before stated, differs only in its
  greater size: the scutum, in the one specimen examined, had two
  broad longitudinal furrows; neither it, nor the tergum differed from
  certain varieties now found on the coast of Chile.



17. BALANUS PERFORATUS. Pl. 5, fig. 1 _a_-1 _d_; Pl. 4, fig. 3 _a_-3
_c_.

  BALANUS PERFORATUS. _Bruguière._ Encyclop. Meth., 1789, Tab. 164,
        fig. 12 _infra_.

  LEPAS ANGUSTA. _Gmelin._ Syst. Naturæ, 1789.

  ---- ORE ANGUSTIORE. _Chemnitz._ Vol. viii, Tab. 98, fig. 835.

  BALANUS CORNUBIENSIS CONICO ORE MINORE. _Ellis._ Phil. Trans. vol.
        50, 1758, Tab. 34, fig. 16.

  LEPAS BALANUS ET FISTULOSUS. _Poli._ Test. Siciliæ (1795), Tab. 4,
        fig. 5, Tab. 6, fig. 1.

  BALANUS COMMUNIS. _Pulteney._ Dorset Catalogue, 1799.

  ---- ---- _Montagu._ Test. Brit., 1803.

  LEPAS ANGUSTATA. _Wood._ General Conchology, 1815, Pl. 6, fig. 5.

  BALANUS CRANCHII. _Leach_ (!). (_B. Blainvillii_ in Tab.) Encyclop.
        Brit. Suppl., vol. iii, 1824.

  ---- ---- _Brown._ Illust. Conch., 1827, Pl. 7, fig. 9, 10, and 2d
        Edit., Pl. 53, fig. 9-12.

  ---- PERFORATUS. _Chenu._ Illust. Conch., Tab. 3, fig. 9, Tab. 6,
        fig. 15.[92]

    [92] I have very little doubt regarding any of these references:
    I have no means of ascertaining the priority, within the same
    year, of Gmelin and Bruguière, but have given it to the latter,
    as _perforatus_ is much the best known specific name. English
    conchologists seem generally to suppose that the _B. communis_ of
    Pulteney and Montagu is the _B. porcatus_ of this work; but I have
    not the smallest doubt that I have given it rightly as a synonym
    of the present species; the indistinctness of the compartments,
    the multitude of fine ridges, the smallness of the orifice, the
    longitudinal furrow on the terga, the colour, size, and habitat,
    all given by Pulteney or Montagu, will agree with no other British
    species. The _Lepas balanus_ of Poli, which is certainly a synonym
    of our present species, has been erroneously considered by several
    authors to be the same with the _L. balanoides_ of Poli, which
    latter undoubtedly is the _B. amphitrite_ of this work.

_Shell pale purple, or white, or dirty ash-colour; smooth, or, from
being corroded, finely ribbed longitudinally; sheath purple; orifice
generally small; radii generally narrow or absent. Scutum, internally,
with a short minute ridge, parallel and close under the prominent
adductor ridge. Tergum with the apex somewhat produced._

  _Var._ angustus (_Gmelin_) Pl. 5, fig. 1 _a_: _pale dull purple
  or white; orifice small or of moderate size; radii very narrow or
  moderately wide, white or pale purple, with oblique summits_.

  _Var._ Cranchii (_Leach_) Pl. 5, fig. 1 _b_: _corroded, covered with
  fine longitudinal ridges owing to the exposed, filled-up, parietal
  tubes; dark dirty ash-colour, with a tinge of purple: radii not
  developed, or very narrow with oblique summits; orifice small_.

  _Var._ fistulosus (_Poli_) Pl. 5, fig. 1 _d_: _shell cylindrical,
  white or dull purple; orifice of moderate size or small; basis deeply
  cup-formed_.

  _Var._ mirabilis, Pl. 5, fig. 1 _c_: _bright purple; radii white,
  very broad, with their summits parallel to the basis; orifice entire,
  large_.

  _Hab._--Southern shores of England; South Wales; Mediterranean;
  Western Africa, southward to Loanda, in 9° S.; West Indies (?).
  Generally adhering to rocks at a low tidal level; in one case
  attached to the floating _Lepas Hillii_, Mus. Jeffreys.


This is a well-marked species, and in its essential characters does
not vary much; but owing to the shell being almost as often white as
purple,--to its being remarkably subject to disintegration,--to its
often becoming cylindrical,--to the radii being either not at all, or
slightly, or moderately, or largely developed, and consequently to the
orifice of the shell varying in size, the general external appearance
of the different varieties is singularly diversified; but when a series
of specimens is examined, it is easy to see how one form passes into
another.


  _General Appearance._--Shell conical, with the orifice oval,
  unusually small, being generally only from one third to half of the
  basal diameter; sometimes moderately large; in one single instance as
  wide as the basis. Radii, often represented by mere lineal fissures,
  or they are narrow, or sometimes moderately wide. Colour pale, dull
  purple, sometimes lilac, often passing into a dead pure white: the
  same individual will occasionally have one part of its shell white,
  and another purple: the purple tint almost invariably is nearly
  uniform, or not in stripes. The radii are generally white, when
  the whole shell is purple, but sometimes they are pale purple: the
  sheath is apparently always coloured of a fine claret-purple, with
  the triangular portion of the alæ, added during diametric growth,
  generally white, but sometimes purple. The surface is quite smooth,
  but very often, especially on the shores of England, whole groups of
  specimens (excepting the very young ones,) have had the outer lamina
  of the parietes entirely corroded and removed; in this case the shell
  assumes a dirty, more or less dark, ash-colour, feebly tinted with
  purple, and the whole surface, owing to the exposure of the solidly
  filled-up parietal tubes, becomes finely striated, or covered with
  very narrow, longitudinal ridges. When specimens are crowded together
  they often become cylindrical, and much elongated, owing to the
  basis becoming deeply cup-formed: I have seen specimens, half an
  inch in diameter in the widest part, one inch and a half in height,
  the walls forming only a third of this. The largest specimen which I
  have seen (from the southern shores of England) had a basal diameter
  of 1.2 of an inch; some very steeply conical specimens were .9 of an
  inch in height, and .8 in basal diameter.

  _Scuta_, externally, slightly convex; growth-ridges approximate,
  moderately prominent. Internally (Pl. 4, fig. 3 _a_) the articular
  ridge is moderately developed, with the lower end produced downwards
  into a freely depending, flattened style, somewhat variable in
  size, but not so long as in _B. lævis_, and easily broken in
  disarticulating the valves. The adductor ridge is very prominent,
  running from almost the apex of the valve, close to the articular
  ridge, to near the basal margin. The basi-tergal portion of the
  valve is converted by the adductor ridge into a rather deep cavity,
  within which there is a short, sharp, and minute ridge, close and
  parallel to the adductor ridge, and bounding the impression left by
  the lateral depressor muscle: this insignificant ridge was present
  in every specimen; it occurs only in very few other species, as in
  _B. nubilus_ and _cariosus_. The thickness of the valve sometimes
  varies a little, and when thick the adductor ridge does not appear
  quite so prominent. _Tergum_, with the apex moderately beaked and
  produced; beak triangular in section, coloured dark purple, as is
  the upper internal surface of the valve; the longitudinal furrow is
  deep, and has its edges folded in, and even quite closed. The spur
  is moderately long and narrow; but its width varies a little (Pl. 4,
  fig. 3 _b_, 3 _c_), and consequently it stands at either rather above
  or at twice its own breadth from the basi-scutal angle: its lower
  end is either bluntly pointed or square, and generally is feebly
  toothed on the under-side. The basal margin of the valve generally
  slopes a little, on both sides, towards the spur. Internally, the
  scutal margin is but slightly inflected: the articular ridge is but
  slightly prominent, and but little curved; in the upper part of the
  valve there are generally several very minute ridges, parallel to the
  articular ridge, on the side towards the scutum. The internal surface
  of the spur itself is sometimes concave. The crests for the carinal
  depressor muscle are barely developed. It may here be mentioned that
  on the opercular membrane many long spines stand rudely arranged in
  rows.

  _Parietes_: the parietal tubes have not transverse septa; but
  are solidly filled up in their upper parts by dark-purple layers
  of shell. The _radii_, as already stated, are either not at all
  developed, or are extremely or only moderately narrow, with their
  summits more or less oblique: in Mr. Cuming's collection, however,
  there is an unique specimen, _var. mirabilis_ (Pl. 5, fig. 1
  _c_) with the aperture of the shell as wide as the basis, with
  bright purple parietes, and white, very broad radii, having their
  summits parallel to the basis. The septa of the radii are finely
  denticulated, and the interspaces are filled up solidly. The _alæ_
  have very oblique summits, and their edges are finely crenated.
  _Basis_, flat, or deeply cup-shaped; there is often an underlying,
  coarsely-cancellated layer.

  _Mouth_: labrum finely hairy, but without any teeth; mandibles, with
  the 4th tooth small; the 5th confluent, with the sometimes smooth,
  sometimes pectinated inferior angle. Maxillæ, rather broad, with a
  slight notch under the upper pair of spines. _Cirri_, first pair,
  with one ramus, having 29 segments, and above one third longer than
  the shorter ramus, having 17 segments; these latter segments are
  remarkable by the extent to which their upper front surfaces are
  laterally produced into projections, twice as long as the breadth of
  that portion of the segment which is articulated to the adjoining
  segment. These projections have a double row of serrated spines on
  their upper edge, and a beautiful radiating bundle at the end; the
  projections decrease in length, both in the upper and lower segments.
  The second cirrus (Pl. 29, fig. 4) has the segments (13 in number, in
  the same individual with the segments above enumerated) of both rami
  produced in the same singular manner as in the first pair. The third
  pair have only inverted conical segments, coloured darker purple than
  the other cirri. The sixth pair had in the same individual 31 or 32
  segments, and therefore one or two more than in the longer ramus of
  the first pair.[93] The segments in the posterior cirri have their
  anterior faces shield-shaped, and bear 6 or 7 pairs of spines, with
  some minute intermediate spines. There is the usual point at the
  dorsal basis of the penis.

    [93] Under the Genus (p. 190) I have given the numbers of the
    segments in the cirri of this species at successive ages.

  _Range._--This species is common on the southern shores of England
  and in the Channel Islands: the largest specimens which I have seen
  came from these quarters. The most northern point whence I have
  seen specimens, is Tenby, in South Wales. This species is common
  throughout the Mediterranean; I have seen specimens from Malaga,
  Sicily, Algiers, and Smyrna; thence it ranges down the western
  coast of Africa, as far south as the Gambia and Loanda, in 9° south
  latitude. I believe British specimens are more often corroded than
  those from further south. Amongst some old, ill-kept specimens in a
  box in the British Museum, marked "Kingston, Jamaica," there were
  some of this species: also I received some specimens, marked "S.
  America," from Mr. G. B. Sowerby: again, Ellis, in Phil. Trans., vol.
  50, part 11, gives a figure (Tab. 34, fig. 15) of some specimens from
  the West Indies, which I believe to be _B. perforatus_: hence, it is
  in some degree probable that this species, like _B. tintinnabulum_,
  and _amphitrite_, and _improvisus_, may be found on both sides of the
  equatorial Atlantic. _Balanus perforatus_ is attached, together with
  _B. tulipiformis_, _trigonus_, _amphitrite_, _Chthamalus stellatus_,
  and _Pollicipes cornucopia_, usually to rocks, near the lower limit
  of the tidal level; but I believe, from specimens kindly sent me
  by Mr. Mac Andrew, that it is frequently obtained by dredging;
  one specimen was even marked 30 fathoms. According to Poli, it is
  sometimes attached to the bottoms of vessels; and I have seen a
  specimen adhering to the floating _Lepas Hillii_.

  _Affinities._--This is a distinct species, closely allied to no other
  species, but comes nearest to _B. lævis_, which is its representative
  in Southern America, and on the whole west coast of that continent.
  It is allied to that species, and differs from most other species,
  in the general form of the shell, its small orifice, narrow radii,
  and often deeply cup-formed basis. It agrees to a certain extent in
  the colouring, though the purple here is much more prevalent, and
  is not confined to the shelly matter filling up the parietal tubes.
  It agrees with that species in the general structure of the scutum;
  but the two or three deep, longitudinal furrows are here absent; and
  the minute ridge, parallel to and almost under the adductor ridge,
  is a peculiarity confined to this and very few species in the genus.
  The terga differ from those of _B. lævis_, chiefly in the spur
  being narrower, and in the apex being beaked. Lastly, the highly
  protuberant segments of the one ramus in the first cirrus, and of
  both rami in the second pair, are here remarkable. With regard to the
  varieties, I have nothing to add to their short diagnostic characters
  above given.



18. BALANUS CONCAVUS. Pl. 4, fig. 4 _a_-4 _e_.

  BALANUS CONCAVUS. _Bronn._ Italiens Tertiär-Gebilde (1831) et Lethæa
        Geognostica, b. ii, s. 1155 (1838), Tab. 36, fig. 12.[94]

  ---- CYLINDRACEUS, _var._ c. _Lamarck._ Animaux sans Vertèbres
        (1818).

  LEPAS TINTINNABULUM. _Brocchi._ Conchologia Sub-Appen., t. ii, p. 597
        (1814).

    [94] I suspect that _B. pustularis_, _miser_, and _zonarius_, all
    figured by Münster, in his 'Beiträge,' b. iii, Tab. 6, may be this
    species.

_Shell longitudinally striped with white and pink; or dull purple;
sometimes wholly white. Scutum finely striated longitudinally;
internally, adductor ridge very or moderately prominent._

  _Hab._--Panama; Peru; S. Pedro in California; Philippine Archipelago;
  Australia. Mus. Brit., Cuming, Stutchbury, Aug. Gould.

  _Fossil_ in Coralline Crag, England; Mus. Brit., S. Wood, Bowerbank,
  Lyell, J. de C. Sowerby, Tennant. Sub-Appennine formations, near
  Turin, Asti, Colle in Tuscany, Mus. Greenough, &c. Tertiary beds,
  near Lisbon, Mus. D. Sharpe and Smith. Bordeaux (?) Mus. Lyell.
  Tertiary beds, Williamsburg; and Evergreen, Virginia, Mus. Lyell.
  Maryland, Mus. Krantz. Recent formations[95] near Callao, Peru, Mus.
  Darwin. Red Crag (Sutton) Mus. S. Wood.

    [95] I procured this specimen from the Island of S. Lorenzo, off
    Callao; it was imbedded, together with seventeen species of recent
    shells and with human remains, at the height of eighty-five feet.


This species has caused me much trouble. Looking first to the recent
specimens, I examined several from Panama and California, which,
though differing greatly in colour, resembled each other in their scuta
having the adductor ridge extremely prominent, and in having (Pl. 4,
fig. 4 _a_), an almost tubular cavity for the attachment of the lateral
depressor muscle,--characters which at first appeared of high specific
value; but I soon found other specimens from Panama in which these
peculiarities were barely developed. I then examined a single specimen
from the Philippine Archipelago, resembling in external appearance one
of the Panama varieties, but differing in the scuta being externally
strongly denticulated in lines instead of being merely striated,--in
the adductor ridge being far less prominent,--and in the spur of the
tergum being broader and more truncated; I therefore considered this
as a distinct species. I then examined a single white rugged specimen
from the coast of Peru, which differed from the Philippine specimen
in the shape of the well-defined denticulations on the scuta, and in
some other trifling respects, and in the segments of the posterior
cirri bearing a greater number of spines; with considerable doubt, I
also named this as distinct. But when I came to examine a large series
of fossil specimens from the Coralline Crag of England, from northern
Italy, from Portugal, and from the southern United States, I at once
discovered that the form of the denticuli on the scuta was a quite
worthless character,--that in young specimens the scuta were only
striated,--that the prominence of the adductor scutorum ridge and the
depth of the cavity for the lateral depressor muscle varied much (as
in the case of the recent specimens), owing apparently to the varying
thickness of the valve,--that in the terga the spur varied considerably
in length and breadth, the latter character being in part determined
by the varying extent to which the edges of the longitudinal furrow
are folded in,--and lastly, that in young specimens the basal end of
the spur is much more abruptly truncated than in the old. Hence I have
been compelled to throw all these forms, originally considered by me
as specifically distinct, into one species. I must repeat that this
considerable variation in the prominence of the adductor ridge, and in
the depth of the pit for the lateral depressor muscle--the pit in some
cases becoming even tubular--is a very unusual circumstance.

With respect to the fossil specimens[96] from the above-stated several
distant localities, I consider them as certainly belonging to one
species, though varying considerably in several points of structure.
When compared with the recent specimens, they differ from them in often
attaining a considerably larger size; in the parietes being often, but
not always, longitudinally ribbed; and in the radii often having more
oblique summits. On the other hand, considering the many points of
identity between the fossil and the recent specimen, I have concluded,
without much doubt, that they ought all to be classed together. I may
remark that, in the Coralline Crag specimens, the spur of the tergum
(Pl. 4, fig. 4 _d_), is unusually long and narrow; it is broader and
shorter in the Italian specimens (4 _e_), and variable in this respect,
in the United States specimens; the scuta of the Lisbon specimens
are remarkable for the greater prominence of the adductor ridge, and
for the depth of the lateral depressor cavity. Some of the specimens
from all the several localities are identical with the recent ones
from the coast of Peru. The walls of the shell in the Coralline Crag
specimens, are generally ribbed longitudinally. I have entered into
the above particulars, on account of, in the first place, its offering
an excellent example how hopeless it is in most cases to make out the
species of this difficult genus without a large series of specimens;
secondly, as showing how the characters alter with age; and thirdly, as
a good instance of the amount of variation which seems especially to
occur in most of the species which have very extensive ranges.

    [96] These will be fully illustrated in the monograph on the Fossil
    Balanidæ, to be published by the Palæontographical Society.

Some of the pink-striped Panama varieties, though having a somewhat
different aspect, can be distinguished from certain varieties of _B.
amphitrite_ only by their scuta being longitudinally striated,--a
character in this species variable in degree, and in most cases of
very little value. Some of the other recent varieties are sufficiently
distinct from _B. amphitrite_; and the great fossil Coralline Crag
specimens, which stand at the opposite end of the series of varieties,
with their ribbed walls, very oblique radii, and coarsely striated
scuta, are extremely unlike _B. amphitrite_. With respect to the
nomenclature of the present species, I have little doubt that I have
properly identified the Italian fossil specimens with _B. concavus_ of
Bronn, who has given a very good figure of this species in his 'Lethæa
Geognostica;' it must, however, be confessed that the longitudinal
striæ on the scuta are not there represented. Considering the large
size and frequency of this species in Europe and in the United States,
it has probably received several other names, besides the two incorrect
synonyms, quoted at the head of this description. I should add that
the true _B. cylindraceus_ (not _var._ C) of Lamarck, according to the
plate given by Chenu in his 'Illust. Conch.,' is the _B. psittacus_ of
South America. I have seen in collections specimens of _B. concavus_
labelled as _B. tulipa_ of Poli (_B. tulipiformis_ of this work),--a
very natural mistake, without the opercular valves be carefully
examined.


  _General Appearance._--Shell conical, often steeply conical; orifice
  rather small, with the radii narrow, and generally in the fossil
  specimens very oblique; surface generally smooth, sometimes rugged,
  and in the coralline crag specimens generally ribbed longitudinally,
  the ribs being narrow. Colour various, either dull reddish-purple
  with narrow nearly white, or wider dark longitudinal bands; or,
  again, pale rosy-pink with broad white bands; or lastly, wholly
  white. The radii are either darker or paler than the parietes. The
  opercular valves are either dark purple or nearly white. Pale pink
  and white stripes are visible on some of the Italian and Portuguese
  tertiary specimens; and in most of the fossils the sheath is tinged
  dull red.

  _Dimensions._--The largest actually recent specimen which I have
  seen, from the Philippine Archipelago, had a basal diameter of 1.2
  of an inch; the Peruvian pleistocene specimen is 1.7 in diameter;
  specimens from the crag and from the Italian deposits, however,
  sometimes slightly exceed two inches in basal diameter, and three in
  height.

  _Scuta_: these in young and moderately-sized specimens are striated,
  sometimes very faintly, but generally plainly, causing the lines of
  growth to be beaded; but often, in large and half-grown specimens,
  the lines of growth are extremely prominent, and being intersected by
  the radiating striæ, are converted into little teeth. As the striæ
  often run in pairs, the little teeth frequently stand in pairs,
  or broader teeth have a little notch on their summits, bearing
  a minute tuft of spines. In very old and large specimens, the
  prominent lines of growth are generally simply intersected by deep
  and narrow radiating striæ. In one case, a single zone of growth in
  one valve was quite smooth, whilst the zones above and below were
  denticulated. The valve varies in thickness, which I think influences
  the prominence of the lines of growth and the depth of the striæ.
  These striæ often affect the internal surface of the basal margin,
  making it bluntly toothed. The articular ridge is rather small,
  and moderately reflexed: the adductor ridge (as already stated,)
  varies remarkably; in most of the Panama specimens, it is extremely
  prominent, and extends down to near the basal margin; in other
  specimens it is but slightly prominent, especially in some of the
  fossil specimens from Virginia. The cavity for the lateral depressor,
  also, varies greatly; it is often bounded on the side towards the
  occludent margin by a very slight straight ridge, which occasionally
  folds a little over, making almost a tube; this, at first, I thought
  an excellent specific character, but far from this being the case,
  the cavity often becomes wide, quite open, and shallow.

  _Terga_, very slightly beaked; the surface towards the carinal end of
  the valve, in some of the fossil specimens, is very slightly striated
  longitudinally. There is either a slight depression, or more commonly
  a deep longitudinal furrow, with the edges folded in and touching
  each other, extending down the valve to the spur, and causing the
  latter to vary in width relatively to its length. When the furrow
  is closed in, the spur is about one fourth of the entire width of
  the valve, and has its lower end obliquely rounded, and stands at
  about its own width from the basi-scutal angle: when there is only
  a slight depression and no furrow (as is always the case with young
  specimens), the spur is broader, equalling one third of the width
  of the valve, with its lower end almost truncated, and standing at
  about half its own width from the basi-scutal angle. But the absolute
  length of the spur, also, varies considerably; it is often very long,
  compared to the whole valve. The basal margin on the carinal side
  is sometimes slightly hollowed out; when the furrow is closed, this
  latter side slopes towards the spur. Internally, the articular ridge
  and crests for the tergal depressor muscle are moderately prominent.

  _Parietes_, the longitudinal septa sometimes stand near each
  other, making the parietal pores small. The _radii_ have oblique
  summits, but to a variable degree; their septa are unusually fine,
  and are denticulated on their lower sides; the interspaces are
  filled up solidly. The _alæ_ have their summits very oblique,
  with their sutural edges nearly or quite smooth. In most of the
  fossil specimens, and slightly in some of the recent specimens, the
  surface of the sheath presents an unusual character, in a narrow,
  longitudinal, slightly raised border, running along the sutures, on
  the carinal side of each compartment.

  _Basis_ thin, porose; sometimes with an underlaying cancellated layer.

  _Mouth_: labrum with six teeth: mandibles with the fourth and fifth
  teeth small, either sharp, or blunt: maxillæ with a straight edge,
  or with the inferior part slightly prominent. _Cirri_ with the rami
  of the first pair unequal by four or five segments: the segments in
  the shorter ramus are extremely protuberant. The segments in the
  second cirrus only moderately protuberant: but all the specimens were
  in bad condition, and it appeared as if, in the Panama specimens,
  the segments of the second cirrus were more protuberant than in the
  Philippine Island specimens. In the posterior cirri there are from
  three to five pairs of spines on each segment: even amongst the
  Panama specimens some had three and some four pairs, and a white
  Panama specimen had five pairs of spines.


All the recent specimens which I have seen, were, with one exception,
attached to various shells and crabs, and to each other. The Peruvian
specimen was associated with _B. flosculus_. The tertiary specimens are
often congregated together into great masses. Including the recent and
fossil specimens, this species encircles the globe. During the miocene
period it seems to have been the commonest existing sessile cirripede;
now, it does not appear to be common, excepting, perhaps, at Panama:
Mr. Cuming procured only one specimen from the Philippine archipelago.



19. BALANUS AMPHITRITE. Pl. 5, fig. 2 _a_-2 _o_.

  LEPAS RADIATA. _Wood's_ General Conchology (1815), Pl. 7, fig. 7.

  ---- MINOR? _Wood's_ General Conchology (1815), Pl. 7, fig. 6.

  ---- BALANOIDES. _Poli._ Testacea utriusque Siciliæ (1795), Tab. 5.

  BALANUS BALANOIDES. _Risso._ Hist. Nat. de l'Europe Merid., tom. iv,
        1826.

_Shell longitudinally striped with purple or pink; sometimes with the
stripes confluent; sometimes wholly white. Scutum internally with a
prominent broad adductor ridge._

  _Var._ (1) communis: (2 _e_, 2 _h_, 2 _l_,) _nearly white, with
  pale or dark violet-coloured longitudinal stripes: epidermis rarely
  persistent: shell either thin or thick: radii white or freckled
  with reddish mahogany colour, with their summits either oblique,
  sometimes in a high degree, or nearly parallel to the basis: basal
  point of spur of the tergum either square or bluntly pointed_. Hab.
  Mediterranean, W. Indies, S. Africa, Philippine Archipelago, New
  South Wales.

  _Var._ (2) venustus: (2 _a_,) _white or pale pink, with narrow bright
  pink, or broad pinkish-purple stripes; orifice either much dentated
  or nearly entire. Tergum with the carinal half of the basal margin
  sometimes much hollowed out_. Hab. W. and S. Africa, Ceylon.

  _Var._ (3) pallidus: (2 _c_, 2 _k_,) _white, with or without a
  yellowish persistent epidermis; sometimes with the edges of the
  compartments tinted purple: radii moderately oblique: tergum
  generally narrow, with the spur sharp, and the basal margin on its
  carinal side much hollowed out_. Hab. W. Africa, Madagascar, Red Sea.

  _Var._ (4) niveus: (2 _f_,) _white, with longitudinal hyaline lines;
  epidermis not persistent_. Hab. W. Indies, Florida, S. Africa, &c.

  _Var._ (5) modestus: _upper part of shell white, lower part uniform
  blueish-gray, opercular valves as in_ Var. (1). Hab. unknown.

  _Var._ (6) Stutsburi: (2 _d_, 2 _i_, 2 _m_, 2 _n_, 2 _o_,) _white,
  with or without pinkish-purple stripes, which are often confluent,
  rendering the lower part of the shell of a uniform purplish tint;
  epidermis persistent: radii very narrow: tergum narrow, spur sharp,
  varying in form and in exact position; carinal margin sometimes
  highly protuberant; basal margin on the carinal side of the spur
  generally, but not invariably, much hollowed out_. Hab. West Africa.

  _Var._ (7) obscurus: (Pl. 5, fig. 2 _g_,) _with narrow, approximate,
  obscure and often almost confluent, slaty, or pale purplish-brown,
  or dark slate-coloured stripes_. Hab. West Indies, Australia, and
  unknown.

  _Var._ (8) variegatus: _with narrow, approximate, dusky,
  claret-coloured stripes, transversely freckled with white; shell
  conical; walls very thin: scutum with the adductor ridge small_. Hab.
  New Zealand.

  _Var._ (9) (_an. spec._?) cirratus: (fig. 2 _b_,) _shell very pale
  purplish-brown, with faint, more or less plain longitudinal stripes,
  transversely freckled with white; walls thin: scuta with the lines of
  growth beaded: basis, in specimens growing in groups, irregularly cup
  formed: maxillæ with the inferior corner extremely prominent_. Hab.
  Mouth of Indus, Australia, Philippine Archipelago.

  _Hab._--Warmer temperate and tropical seas; extremely common;
  Mediterranean, Smyrna, Sicily, Coast of Portugal; West Coast of
  Africa, River Gambia, West Indies, Demerara, Natal, Madagascar, Red
  Sea, Mouth of the Indus, Ceylon, Philippine Archipelago, East Indian
  Archipelago, Pacific Ocean, east coast of Australia, New Zealand;
  extremely common on ships' bottoms; often attached to floating
  timber, canes, &c.; often associated with _B. tintinnabulum_;
  attached to pebbles and various shells.


With respect to the nomenclature of this extremely common species,
which is widely distributed in all the warmer seas (excepting, as
far as I have seen, on the west coast of America), there is some
difficulty. I have no doubt that it is the _Lepas radiata_ of Wood
(1815), but Bruguière, in 1789, gave this same name to a Balanus which
he had not seen, but which is figured in Chemnitz, Tab. 59, fig. 842.
I should have thought that this also had been the present species,
but Spengler, in describing (Skrifter af Naturhist. Selskabet i, B.
1790) this individual specimen, which he calls _L. purpurea_, states
that it is 13 lines in basal diameter; now this is a size which is
never acquired by _B. amphitrite_; and the description, habits, and
size, would apply equally well to the species which I have called _B.
amaryllis_; but when no notice is taken of such points of importance,
as whether the walls are permeated by pores, whether the radii are
smooth-edged, whether the scuta are striated, it is impossible to
identify with any approach to certainty sessile Cirripedes; and
the names given ought, in my opinion, to carry little weight with
them. With respect to Lamarck's _Balanus radiatus_ (1818), the
synonyms quoted exhibit some great and inextricable confusion. The
_B. radiatus_, again, of Risso, is a fossil and apparently distinct
species. There can be no doubt that the present species is the _Lepas
balanoides_ of Poli, (and of several authors who have followed him),
and equally little doubt that the present species is not the true _L.
balanoides_ of Linnæus, which has a membranous basis, and which I have
not seen from the Mediterranean. Under these circumstances I have
concluded that less confusion would be caused by giving a new name to
this species than by taking that of Wood, which ought not to have been
used by him, considering Bruguière's previous adoption of it.

Under the head of _B. tintinnabulum_ I have alluded to the great
variation of _B. amphitrite_, which consists not only in a vast
diversity in the colouring and in the general aspect, but likewise
in the degree of obliquity of the summits of the radii, in the form
of the terga, and slightly in that of the scuta. In order to show
that it has not been from indolence that I have put so many forms
together, I may state that I had already named and fully described
in detail eight of the following forms as species, when I became
finally convinced that they were only varieties: it would require at
least thirty figures, which I have not the power to give, fully to
illustrate the transitional forms. As with _B. tintinnabulum_, the
deception is wonderfully enhanced by whole groups of specimens from the
same locality exactly resembling each other, and sometimes differing
from other groups attached to the very same object. If a person were
to get together only some fifty or sixty specimens from only half a
dozen different localities, he would almost certainly come to the same
conclusion, as I at first did, that several of the varieties are true
species; but when he gets several hundred specimens from all quarters
of the globe, he will find, to his trouble and vexation, that character
after character fails and blends away by insensible degrees, and he
will be led, as the more prudent course, to include, as I have done,
and I hope rightly, all under one specific name. I have experienced
more doubt regarding the last variety, _cirratus_, than on any other,
on account of its peculiar colouring, and from the basis being often
irregularly cup-formed. Under _B. concavus_ I have remarked how closely
some of its varieties approach to _B. amphitrite_, and it is to this
last variety that they approach; almost the only difference being that
the scuta in _B. concavus_ are longitudinally striated. Yet some of the
varieties of the two species are so distinct that it would be puerile
to class them together. I will only add, that after studying such
varying forms as _B. tintinnabulum_ and _amphitrite_ it is difficult to
avoid, in utter despair, doubting whether there be such a thing as a
distinct species, or at least more than half a dozen distinct species,
in the whole genus Balanus.

As with _B. tintinnabulum_, I will first give a full description of the
more common forms, alluding only to each less frequent variation, and
then separately describe briefly the more marked varieties.


  _General Appearance._--Shape conical, either steep or considerably
  depressed; sometimes tubular; orifice either nearly entire or deeply
  toothed, not large, varying from rhomboidal to rounded-trigonal.
  Surface of shell smooth, never ribbed, generally naked, but
  occasionally the yellowish epidermis is persistent; in the same
  individual, I have seen all the lower part of the shell thus covered
  and the upper part naked, the line of separation being defined.
  The _colour_ varies much, even sometimes considerably on the same
  individual; generally white or pale gray, with dull violet-coloured,
  longitudinal, moderately broad stripes; these stripes are sometimes
  equidistant, but more usually they are arranged so as to leave
  broad white spaces; the stripes fade away by endless variations,
  the edges of the compartments and the carinal end of the shell
  longest retaining any colour, until we have a uniformly white
  shell, generally covered with a yellowish epidermis; or the white
  is longitudinally marked with hyaline lines; this latter variety
  has a very peculiar aspect, and I did not doubt it was specifically
  distinct, until, in a number of specimens on a ship from the West
  Indies, I got the most perfect series, and another scarcely less
  perfect series from the Mediterranean, graduating into common
  coloured varieties. Rarely the dull violet or purple stripes become
  approximate and dark, so that the whole shell is tinted of a brownish
  slate-colour, occasionally freckled with white. Again, we have
  another set of very pretty varieties, with a white or very pale
  pink ground, with either narrow bright pink or broad pinkish-purple
  stripes. Again, I have seen numerous specimens of a variety, _var.
  Stutsburi_, from the west coast of Africa, in which the upper part
  of the shell is white, and the lower part shaded with pinkish or
  dark purple approximate stripes, which often become confluent; in
  one group, the whole shell being thus uniformly coloured, without
  any vestige of stripes. I have seen another group from an unknown
  locality, in which the lower part of the shell was uniformly
  blueish-gray. A variety from Australia has narrow approximate dark
  claret-coloured stripes, transversely freckled with white. Lastly, in
  the variety _cirratus_, the whole shell is very pale purplish-brown,
  with indistinct longitudinal brownish stripes, transversely freckled
  with white lines. I considered this as a distinct species, until
  quite lately finding forms which I could not possibly determine
  whether to class as _B. cirratus_ or _amphitrite_.

  The radii are generally snow-white, or freckled with a bright
  mahogany tint, or rarely clouded with purple, or in the pink
  varieties with pink. The scuta are dull purple or pink, generally
  with a white band along their tergal margin; often, however, they
  are white, with merely one or two purple fasciæ. The thickness or
  strength of the shells varies much; some specimens attached to a
  floating cane, from Natal and the Philippine Archipelago, were
  extremely strong; others, from the Mediterranean and Australia,
  and some tubular varieties from the West Indies, were very thin,
  translucent, and fragile. _Size_: large specimens generally attain a
  diameter of from half to three quarters of an inch in basal diameter;
  and I have seen one or two specimens an inch in diameter.

  _Scutum_; sometimes the surface is very smooth, but generally the
  growth-ridges are moderately prominent; the latter are occasionally
  very finely beaded, and this seems always the case with _var.
  cirratus_. Internally, the articular ridge is prominent and reflexed:
  the adductor ridge is sharp, very prominent, and straight; it runs
  parallel to the occludent margin; close to its lower side there is
  often a depression (Pl. 5, fig. 2 _i_), sometimes bounded by a slight
  ridge, as if for the attachment of a muscle, but there certainly is
  no muscle here: rarely the adductor ridge is only slightly prominent:
  there is a small and shallow little pit of variable depth for the
  lateral depressor muscle.

  _Tergum_ (2 _k_-2 _o_); this valve is here far more variable than in
  any other species: in the commonest purple-striped forms (2 _l_), the
  valve is rather broad, the basal margin lies in nearly a straight
  line on the opposite sides of the spur, which is placed at rather
  less than its own width from the basi-scutal angle; the spur is
  rather short, and in width about one fourth of the entire valve; its
  lower end is either bluntly pointed or more commonly nearly square
  (2 _k_) and parallel to the basal margin: in young specimens it is
  generally sharper than in older ones. Externally, in the line of
  the spur, there is either a slight longitudinal depression, or more
  rarely a deep furrow. The carinal margin is more or less convex, and
  is formed by upturned lines of growth: the scutal margin is broadly
  inflected. Internally, the articular ridge in the upper part is very
  prominent: the crests for the tergal depressor muscle are moderately
  prominent, but very variable. Sometimes the carinal portion of the
  basal margin is slightly hollowed out. In _var. Stutsburi_ (2 _m_, 2
  _n_, 2 _o_), and in some white varieties, which differ most in the
  shape of the tergum from the commoner varieties, the whole valve is
  narrower, the spur is much sharper and narrower, the carinal half of
  the basal margin is much hollowed out and slopes down towards the
  spur, with the crests for the depressor muscles depending beneath the
  basal margin, and with the carinal margin sometimes extremely convex
  or protuberant. But the shape and position of the spur, and the
  outline of the carinal half of the basal margin vary much in nearly
  all the varieties.

  _Compartments._--The upper parts of the parietal pores are either
  filled up solidly with, generally coloured, shell, or they are
  crossed by thin transverse calcerous septa: the longitudinal parietal
  septa occasionally bifurcate at their bases close to the outer
  lamina, making an irregular outer row of minute pores. The _Radii_
  have their septa rather fine, and finely denticulated on both sides,
  but sometimes only on the lower side; the thickness of the septa
  varies a little; the interspaces are filled up solidly; the summits
  of the radii are jagged and oblique, and usually form an angle of
  about 45° with the basis, not being added to above the level of the
  opercular membrane; but not rarely they reach up much higher, and
  are very nearly parallel to the basis, extending from tip to tip of
  the compartments. Again, in some ordinary varieties, and always in
  _var. Stutsburi_, the summits of the radii are extremely oblique, the
  radii themselves forming a mere border to the compartments to which
  they belong. In no other species have I seen so great an amount of
  variation in the form of the summits of the radii. The _alæ_, in like
  manner, have their summits either very oblique, not being added to
  above the opercular membrane, or they are only slightly oblique; it
  often happens that in those specimens in which the summits of the
  radii are nearly parallel to the basis, the alæ are very oblique, and
  the converse: in other individuals, both radii and alæ have equally
  oblique summits. The sutural edges of the alæ vary in thickness,
  being either very thin and obscurely crenated, or moderately
  thick and ribbed. The _basis_ is porose; but I have never seen an
  underlying cancellated layer of shell, as is so common in several
  species.

  _Mouth_: labrum, with from four to eight, generally with six, little
  teeth: mandibles with three teeth, and two minute lower teeth, or
  mere knobs: maxillæ with the edge straight, or with the inferior
  part forming a slightly step-formed projection. _Cirri_: the rami
  of the first pair are unequal by three or four segments, but in
  some specimens by five or six segments, with the front surfaces of
  the segments in the shorter ramus extremely protuberant. The second
  pair of cirri are short, with the front surfaces of the segments
  moderately protuberant: the third pair have a tuft of bristles at
  their bases on the thorax. The segments in the sixth pair have from
  four to six pairs of spines on the segments; equal-sized specimens
  seem to vary in this latter respect. There is a small sharp
  projection on the dorsal base of the penis.


_Varieties._

  With respect to _var._ 1, _communis_, I have nothing further to
  remark, except that I have seen specimens identically similar from
  the Mediterranean, Natal, the Philippine Archipelago, and Sydney;
  at the latter place it is said to be rare, but in most places it is
  the commonest variety, and is often attached to ships' bottoms. Of
  _var._ 2, _venustus_, I have seen specimens from the west coast of
  Africa, Natal, and Ceylon, in groups by themselves, and associated
  with _var. communis_; it is much less common than _var._ 1. The third
  variety, _pallidus_, is not uncommon; I have seen many specimens
  from the bottoms of ships, from the West Indies, and the west coast
  of Africa. Of the _var._ 4, _niveus_, I have seen the most perfect
  graduated series passing into _var._ 1, both from the West Indies,
  Florida, and the Mediterranean: I have seen other specimens from the
  Red Sea and Madagascar. Of the _var._ 5, _modestus_, I have seen
  only one group from an unknown locality; it is only remarkable from
  its uniform colouring. The _var._ 6, _Stutsburi_, is more remarkable
  than the foregoing; until quite lately I did not doubt that it was
  specifically distinct; but as I have seen every character graduate
  into other varieties, I am now convinced that it is not a true
  species: all the specimens which I have seen have come on shells,
  or on ships' bottoms, from West Africa. Of _var._ 7, _obscurus_, I
  have seen three or four groups of specimens from unknown quarters,
  both on pebbles, shells, and on cork (probably from the Atlantic
  ocean); and likewise some specimens taken from the bottom of Her
  Majesty's ship "Fly," on the east coast of Australia; these latter
  are intermediate in character with the next _var. variegatus_;
  from the Australian seas, which I at first ranked as an undoubted
  species, but I have subsequently failed in discovering any sufficient
  diagnostic character. Lastly, of _var. cirratus_, I have seen several
  groups of specimens from India and the Philippine Archipelago, and
  a group intermediate in character between this and the first and
  third varieties, from Australia; I retained this variety owing to
  its peculiar freckled, pale brown colouring and beaded scuta (of
  which, however, I have seen decided traces in the common variety),
  as a distinct species, after I had given up all the foregoing forms.
  I entertain some doubts whether I have now acted right; but when I
  found some specimens which, I found it impossible to decide, whether
  to rank as _amphitrite_ or _cirratus_, I determined to take the more
  prudent course, and sink the latter as a species. This variety, also,
  seems to connect _B. amphitrite_ and _concavus_ very closely.



20. BALANUS PŒCILUS. Pl. 5, fig. 3 _a_, 3 _b_.

_Shell dull red, freckled with white. Scutum internally without an
adductor ridge; tergum with the spur, sharply truncated, almost one
third of width of valve._

  _Hab._--West coast of South America, Mus. Cuming; attached to an
  Avicula.


The appearance of the fragile shell, in the one group of specimens
which I have seen, leads me to suspect that they may have grown under
unfavorable circumstances. This species differs considerably in general
aspect, but not much in essential characters, from _B. amphitrite_; the
absence, however, of an adductor ridge to the scutum, and the sharply
truncated spur of the tergum, are sufficient to distinguish them. In
the opercular valves this species comes near to _B. vinaceus_, also
from the west coast of South America; but the striated scuta of that
species, the cancellated inner lamina of the parietes, the general
colouring, and square porose radii, are amply diagnostic characters.


  _General Appearance._--Shell fragile, tubulo-conical, orifice large,
  passing from diamond-shaped into oval. Colour fine dark rose,
  freckled with transverse, sharply pointed, fine zig-zag white lines:
  the pink is also so arranged as to obscurely give to the walls a
  longitudinally striped appearance: radii generally rather whiter than
  the walls, and similarly freckled: terga similarly freckled: scuta
  dull red, with a white band along the scutal margin. Basal diameter
  of largest specimen half an inch.

  _Scutum_, externally smooth: internally, articular ridge moderately
  developed, slightly reflexed: there is no adductor ridge: there is
  a distinct pit for the lateral depressor muscle. _Tergum_, with the
  scutal margin unusually prominent, toothed: longitudinal furrow
  shallow, the edges apparently having no tendency to fold in: spur
  short, barely one third of width of valve, with the lower end sharply
  truncated, parallel to the basal margin: articular ridge and crests
  for the depressores moderately prominent.

  _Compartments._--Walls very fragile, with the outer lamina not
  thicker than the inner lamina. _Radii_ fragile, broad, with their
  summits moderately oblique; their sutural edges have the septa
  plainly denticulated on both sides, with the interspaces filled up
  solidly nearly to the tips of the septa. _Alæ_, with their summits
  more oblique than those of the radii; their sutural edges smooth.
  _Basis_ with an underlying cancellated layer. _Mouth_: labrum with
  three unusually large teeth on each side of the notch: mandibles with
  the fourth tooth tolerably well developed, the fifth being confluent
  with the inferior angle. _Maxillæ_ simple. _Cirri_,--first pair with
  one ramus longer by about four segments than the other ramus, which
  has considerably protuberant segments: second pair with segments only
  moderately protuberant: sixth pair with segments much elongated, but
  bearing only four pairs of spines.



21. BALANUS EBURNEUS. Pl. 5, fig. 4 _a_-4 _d_.

  BALANUS EBURNEUS. _Aug. Gould_ (!). Report on the Invertebrata of
        Massachussetts, 1841, fig. 6.

_Shell yellowish white. Scutum striated longitudinally: tergum with the
spur truncated, the basi-carinal margin generally much hollowed out,
and the carinal margin protuberant in the upper part._

  _Hab._--United States, from about lat. 42° to Charlestown; West
  Indies; Honduras; Venezuela; attached to shells and floating wood.
  Attached to ships' bottoms from Trinidad and Jamaica, associated with
  _B. tintinnabulum_, _amphitrite_, and _improvisus_. Brackish water,
  Salem, Massachussetts, according to Mr. Stimpson. Mus. Aug. Gould,
  Agassiz, Stutchbury, Cuming, W. Dunker, &c.; very common.


  _General Appearance._--Shell conical, or almost tubular; white, with
  the surface very smooth, covered by thin yellowish epidermis, but
  with the radii naked. Orifice large, passing from rhomboidal into
  pentagonal, moderately toothed. Average full size, about one inch in
  basal diameter; I have seen a specimen 1.3 in basal diameter, and the
  same in height.

  _Scutum_, plainly striated longitudinally: the teeth on the occludent
  margin small. Internally, the upper surface is roughened: the
  articular ridge is prominent, and either slightly or not at all
  reflexed: the pit for the adductor muscle is distinct; the adductor
  ridge is prominent in a variable degree, and is almost confluent
  with the articular ridge. In one specimen from Beverly Bay, U. S.,
  the scuta were extraordinarily disintegrated, and I could perceive
  no trace of the external radiating striæ. _Tergum_, with the basal
  margin on the carinal side of the spur sometimes deeply (Pl. 5, fig.
  4 _b_), and sometimes only slightly (fig. 4 _d_), and rarely hardly
  at all, hollowed out: when much hollowed out the valve may almost
  be said to be two-pronged, with the carinal prong narrower than
  the spur. There is no distinct longitudinal furrow, but the whole
  scutal margin projects above the general surface of the valve. In the
  carinal margin, in the upper part, there is a remarkable convexity
  or protuberance in the same plane with the valve, from which it
  is separated by a very slight and narrow ridge. The spur is about
  one fourth of the width of the valve, with its lower end abruptly
  truncated. Internally, the upper surface is much roughened with
  finely crenated ridges: the distinct crests for the depressores cover
  the whole of the so-called carinal prong.

  _Compartments_; the radii and alæ have their summits oblique,
  sometimes a little rounded, but not smooth. The septa on the sutural
  edges of the radii are remarkably fine, and closely approximate; the
  denticuli are excessively minute. The sutural edges of the alæ are
  most delicately crenated; the alæ are largely added to during the
  diametric growth of the shell, and above the level of the opercular
  membrane. The parietal pores are square and rather large: they are
  crossed by transverse septa almost close down to the basis: the
  longitudinal septa have tolerably large denticuli at their bases.
  The pores in the basis are crossed by numerous transverse septa.
  When specimens grow in a group, the basis is sometimes irregularly
  cup-formed.

  _Mouth_: _labrum_ serrated with small teeth, decreasing in size
  downwards, on each side of the central notch. _Mandibles_ with the
  third tooth rather thick and blunt, and with the fourth and fifth
  knob-like. _Maxillæ_, with the inferior part projecting much beyond
  the rest of the edge, and bearing two long single spines: between
  these two spines and the large upper pair, there are, in a full-sized
  specimen, about seven pairs of moderately long spines, feathered on
  their sides. _Outer maxillæ_ thickly clothed with very fine spines,
  and remarkably prominent.

  _Cirri_: first cirrus, with one ramus having twenty-six segments,
  and longer by ten segments than the shorter ramus, which has sixteen
  segments: the shorter ramus, and both ramii of the second pair, have
  their segments remarkably protuberant in front; the protuberance,
  in the upper segments, equalling in length the supporting part of
  each segment: rami of the second cirrus unequal in length by five
  segments. Third cirrus with the segments only slightly protuberant;
  rami considerably longer than those of the second cirrus: at the
  dorsal base of the pedicel of this third cirrus there is no tuft of
  fine hairs, as is common in many other species. _Sixth pair_, with
  the upper segments elongated, bearing from six to seven pairs of
  spines; dorsal spines short, thin, and few.

  _Affinities_: in external appearance of the shell, this species can
  hardly be distinguished from some of the white varieties of _B.
  amphitrite_; and there is a considerable resemblance, in some of the
  varieties, in the opercular valves; but the longitudinally striated
  scuta of _B. eburneus_ suffice to distinguish these certainly very
  distinct species. Equally, or even more like _externally_, is this
  species to the _B. Hameri_, so that I have received from an eminent
  naturalist in the United States both species mingled in the same lot,
  all bearing the same name of _B. eburneus_; but when the internal
  structure of the shell is examined, the species are at once seen to
  be far removed from each other. Still more close is the affinity
  of this species to _B. improvisus_, both in internal and external
  characters: it agrees with this species in the singular habit of
  being able to live in brackish water: these two species are the only
  ones which have the labrum serrated with teeth, graduated in size, on
  each side of the central notch. In the case of _young_ specimens of
  the _var. assimilis_ of _B. improvisus_, an inhabitant of the same
  seas with _B. eburneus_, the diagnosis is most difficult without long
  practice; for in the young of _eburneus_, the compartments are only
  partially covered by yellow epidermis, and have a striped appearance,
  the radii are sometimes very oblique, the scuta externally have not
  acquired their longitudinal striæ, and internally the adductor ridge
  lies not so close to the articular ridge as it does subsequently;
  hence I for some time mistook the _var. assimilis_ of _B. improvisus_
  for the young of _B. eburneus_. But I found in the latter, that the
  rami of the first pair of cirri, are always, even in the earliest
  youth, more unequal in length, and that each segment of the posterior
  cirri bears a greater number of pairs of spines, there being, even in
  very minute specimens, seven pairs. Moreover, after having examined
  scores of specimens, I found I could almost always distinguish the
  two species by the smoothness and curvature of the summits of the
  radii of _B. improvisus_; I entertain no doubt whatever about the
  distinctness of the two species; indeed, when both are mature,
  besides the greater size, striated scuta, &c. of _B. eburneus_, their
  general aspect is very different.



22. BALANUS IMPROVISUS. Pl. 6, fig. 1 _a_-1 _c_.

_Shell white: radii narrow, with their upper margins smooth, slightly
arched, very oblique. Tergum with a longitudinal furrow; spur with the
end rounded._

  _Var._ assimilis, _with longitudinal white hyaline lines_.

  _Hab._--England, Scotland, Belgium (?), Nova Scotia, United States,
  West Indies, Rio Plata, Southern Patagonia, Guayaquil, West Colombia;
  attached to wood, shells, rocks, ships' bottoms, from low tidal level
  to twenty fathoms depth.


  _General Appearance._--Shell conical, with a rather large
  diamond-shaped orifice, moderately or but little toothed; very
  smooth; walls never folded longitudinally; white, with an extremely
  thin pale-yellow persistent epidermis. The radii are very narrow,
  with their summits very oblique, rounded, and smooth; the epidermis
  is generally more persistent on the radii than on other parts, and
  this is exactly the reverse of what is common with _B. eburneus_. The
  specimens from nearly fresh-water in the R. Plata (hereafter to be
  mentioned), are brownish, and have undergone a remarkable degree of
  corrosion, the outer lamina of the walls having been entirely removed
  to near the base; hence the external aspect of these specimens is
  wholly different from ordinary individuals. The _var. assimilis_
  has also a very different appearance, owing to the dead white of
  the walls being relieved by narrow approximate longitudinal hyaline
  lines, corresponding with and caused by the longitudinal parietal
  septa being externally visible through the outer lamina of the
  parietes; the epidermis on the radii is also of a rather brighter
  yellow. The largest specimens which I have seen are those from the
  Plata, and those attached to a ship from the West Indies, and they
  had a basal diameter of .6 of an inch: from .4 to .5 of an inch is
  the more usual full average size.

  _Scuta_, with the lines of growth but little prominent: articular
  ridge prominent, but little reflexed: adductor ridge straight and
  very prominent, varying a little in its distance from the articular
  ridge; there is scarcely any depression for the lateral depressor
  muscle; the upper internal surface of the valve is roughened with
  ridges. _Terga_, with a moderately deep longitudinal furrow; spur
  short, rather narrow, with the end rounded, placed at less than its
  own width from the basi-scutal angle; in the Rio Plata specimens the
  spur is close to this angle: the basal margin is generally straight
  on opposite sides of the spur, but sometimes on the carinal side it
  is a little hollowed out. The lines of growth are upturned along the
  carinal margin, which consequently is a little protuberant, but to a
  varying degree. The crests for the depressores are extremely distinct
  and prominent. In the varieties having the basi-carinal margin
  hollowed out, and the carinal margin protuberant, there is a marked
  resemblance to the peculiar tergum of _B. eburneus_.

  _Walls_: the parietal pores are tolerably large, and are crossed by
  numerous transverse septa: the longitudinal septa are very finely
  denticulated at their bases, but occasionally almost smooth. The
  _radii_ are, as stated, extremely narrow, and very remarkable from
  their smooth rounded edges; their septa are barely denticulated.
  The _alæ_ are remarkably protuberant; they have their summits much
  less oblique than those of the radii, and sometimes they are almost
  parallel to the basis: their sutural edges are coarsely crenated.
  _Basis_, flat, thin, permeated by pores, but the pores do not
  generally run to the very centre; they are, as usual, crossed by
  transverse septa.

  _Mouth_: the _labrum_ is the most remarkable part; on each side of
  the central notch there are generally two teeth; and on the two
  sides of the notch itself nine or eleven smaller teeth, decreasing
  regularly in size downwards till they become so minute as to be
  hardly visible even under the compound microscope; thus, in the two
  specimens closely examined, there were altogether twenty-two and
  twenty-six teeth on the labrum. _Mandibles_ with the two inferior
  teeth reduced to mere knobs: _maxillæ_ with the lower part of the
  edge bearing two large spines, and generally, but not always,
  forming a step-formed projection. _Cirri_: the ramii of the first
  pair are but slightly unequal; in one specimen examined there were
  fifteen segments in one ramus and twelve in the other: segments very
  protuberant in front. Second cirrus with the segments only slightly
  protuberant; segments thirteen. Third cirrus longer than the second
  pair, with the rami rather unequal in length: there is a tuft of long
  spines on the basal segment of the pedicel of this cirrus. Fourth
  cirrus twenty-two segments. Sixth cirrus, in the same individual,
  thirty-four segments: on each of these segments there are five or six
  pairs of spines. I may specify that the longer ramus of the first
  cirrus of a large Rio Plata specimen had twenty-four segments.

  _Varieties, affinities._--When I first met with the _var. assimilis_,
  misguided by its general aspect, I did not doubt that it was
  specifically distinct; I was strengthened in this view by the
  absolute identity of several hundred specimens attached to two
  vessels from Jamaica and Trinidad, in the West Indies, with one
  specimen from Charlestown, in the United States, sent me by Prof.
  Agassiz, and with several in three lots from the western tropical
  shores of South America: yet on close examination I can point out
  no one distinguishing character, either in the shell or animal's
  body, excepting the longitudinal hyaline lines on parietes, due to
  the septa being externally visible. The presence of similar lines is
  variable in white _vars._ of _B. amaryllis_ and _amphitrite_, and
  they are seen in very young specimens of _B. eburneus_: hence it is
  impossible to consider so trifling a character as specific; moreover,
  lately I have seen a British specimen with hyaline lines, and some
  few other specimens in an intermediate condition. Under the head
  of _B. eburneus_, I have stated that although that species and _B.
  improvisus_, which in the West Indies are associated together, are
  most readily discriminated when old, yet when young, they so closely
  resemble each other that the eye requires much practice to separate
  them. On account of this species and _B. crenatus_ being sometimes
  associated together on the shores of England, I have pointed out
  under _B. crenatus_, the relative diagnostic characters of the two.
  The chief affinity of _B. improvisus_ is certainly towards _B.
  eburneus_; but in the narrow, oblique, rounded, and smooth-edged
  radii, there is a relationship shown to the species in the last
  section of the genus, such as _B. amaryllis_, and more especially to
  the fossil _B. dolosus_: so close is the resemblance in the external
  appearance of the shell, and in the structure of the opercular
  valves, to the latter species, that I for some time did not discover
  their distinctness. _Balanus improvisus_ has hitherto been overlooked
  by naturalists, and has probably been confounded with _B. crenatus_
  or _balanoides_.

  _Range and habits._--This species, as far as my experience goes,
  is commoner on the shores of Kent than on other parts of England:
  the first specimens which I met with, I owed to the kindness of Mr.
  Metcalf, they were attached to wooden stakes from Herne Bay, together
  with a single specimen of _B. crenatus_: I have seen other specimens
  from near Woolwich, from the Kentish oyster-beds, from Sandwich, and
  from Ramsgate. The only other British specimens which I have seen
  are from the River Itchen, in Hampshire, and from Loch Shieldaig,
  in Ross-shire (Mus. Jeffreys), from a depth of twenty fathoms. This
  species is often attached to wood. At Ramsgate, the specimens were
  attached to a small coasting vessel, and they must have been immersed
  five or six feet; they were associated with _B. crenatus_, and with
  a few of _B. balanoides_. In the Brit. Mus. there are specimens
  collected by Mr. Redman, from Nova Scotia, in North America. When
  her Majesty's ship Beagle was beached at Santa Cruz, in Southern
  Patagonia, numerous specimens were found adhering to her copper
  bottom, some so small as to show that the species breeds in those
  latitudes. Near Monte Video, in the estuary of La Plata, I found
  many large, but much corroded specimens, adhering to some rocks in a
  small _running_ stream of perfectly fresh water. The rise of the tide
  is here small, but at high water the specimens apparently were for
  a short time covered by the waters of the estuary, here itself only
  brackish, and occasionally almost fresh. I took home some specimens,
  and placing them in perfectly fresh water they continued for many
  hours expanding and retracting their cirri with perfect regularity
  and vigour. Here then we have a Balanus capable of living in fresh
  water, and likewise in the saltest seas: even brackish water is a
  deadly poison to several, probably to most, species of the genus; but
  this, as we have seen, is not the case with the allied _B. eburneus_.
  The water, I may add, at Woolwich, on the Thames, whence I have
  received _B. improvisus_, must at times be very brackish. I have
  already incidentally mentioned that the _var. assimilis_ was attached
  in great numbers, associated with _B. eburneus_, _tintinnabulum_, and
  _amphitrite_, on vessels from the West Indies: one specimen sent me
  by Prof. Agassiz, from Charlestown, was attached to a specimen of _B.
  eburneus_; and, lastly, I have seen three sets of the same identical
  variety attached to shells from Guayaquil (in Mus. Brit. and Cuming),
  and from West Colombia. Here, then, we have the same species with
  an enormous range, from Nova Scotia and Great Britain to South
  Patagonia; and, which is the case with scarcely a single mollusc, it
  lives both on the eastern and western tropical shores of the South
  American continent.



23. BALANUS NUBILUS. Pl. 6, fig. 2 _a_-2 _c_.

_Shell white, rugged: basis in parts imperfectly porose. Scutum with
the articular ridge minute; adductor ridge prominent, forming a deep
pit for the lateral depressor muscle: tergum with an internal patch of
purple; apex produced, purple._

  _Hab._--California, Mus. Brit. and Aug. Gould; associated with _B.
  glandula_, and attached to wood.


I have seen two specimens of this species, brought by Lady K. Douglas
from California; and two from Monterey, sent me by Dr. Aug. Gould. This
is a very distinct species, coming nearer to _B. porcatus_ than to any
other species: it is also allied to _B. cariosus_. In the basis being
in parts solid or not permeated by pores, it has claims to be placed in
the next section, in which I at one time included it.


  _General Appearance._--Shell conical, rugged, sometimes furnished
  with sharp longitudinal ribs; dirty white. Orifice not large, oval,
  toothed. Radii rather narrow, with their summits oblique, much
  jagged. Basal diameter of largest specimen 2.1; height only 1.3 of an
  inch.

  _Scuta_, broad, with the lines of growth prominent; internally,
  articular ridge very little prominent, sometimes hardly developed,
  but thick, ending downwards in a small free point. Adductor ridge
  prominent, blunt, produced straight downwards, making a deep
  longitudinal cavity for the lateral depressor muscle; in some
  specimens this cavity is almost arched over, so as to tend to be
  tubular, with a short ridge in the middle (Pl. 6, fig. 2 _a_): in
  other specimens there is no trace of this tubular structure. _Terga_,
  with the apex beaked, beak triangular, dull purple; the longitudinal
  furrow is so shallow as hardly to exist. The basal margin slopes
  down on both sides, with a nearly equable curvature towards the
  spur; hence the spur is broad in its upper part, and narrow at its
  obliquely truncated lower end. Internally, there is an elongated dark
  purple patch: the shallow articular furrow is of quite remarkable
  breadth; the articular ridge is medial, and the inflected scutal
  margin is not wide. The internal surface of the spur is formed into
  a ridge, which runs a little way up the valve, and is sometimes
  partially separated from the spur itself (fig. 2 _c_), making the
  basal extremity toothed or double. The crests for the depressores are
  pretty well developed.

  _Walls_, moderately strong: inner lamina slightly ribbed: the
  denticuli on the bases of the parietal longitudinal septa are sharp:
  I could not see any transverse septa in the parietal tubes. The
  _radii_ are rather narrow; their summits are remarkably jagged and
  very oblique; the septa are plainly denticulated on both sides, but
  chiefly on the lower side; each septum itself, towards the inner
  lamina of the radius, branches and divides: the interspaces are
  filled up nearly solidly. The _alæ_ have apparently their summits
  less oblique than those of the radii: their sutural edges are finely
  crenated. The lower edge of the sheath is hollow underneath. The
  _basis_ is flat; it is rather thin, and imperfectly porose; in parts
  it is not at all porose, in others it is traversed only by very
  minute pores: there is nevertheless, in some parts, even where the
  upper layer is not porose, an underlying, cancellated layer.

  _Animal's body_ unknown.



24. BALANUS CORRUGATUS. Pl. 6, fig. 3 _a_, 3 _b_.

_Shell white, longitudinally folded; radii narrow. Scutum internally
without an adductor ridge._

  _Fossil_, Sub-Appennine formations; Colle in Tuscany; Mus. Greenough.


I have seen only two specimens of this species attached to rock,
collected by Mr. Greenough, at Colle, and kindly given by him to me.
The species comes near to the living _B. crenatus_, also found fossil
in deposits of this same age; it differs, however, distinctly from
that species, in having its basis permeated by pores, and, in a less
degree, in the sutural edges of the radii being more plainly crenated:
the opercular valves of the two species closely resemble each other.
This may be the _B. stellaris_ of Bronn, but it is futile attempting to
identify the species of this genus merely by external characters, even
when aided, as in this case, by an excellent drawing of the shell.


  _General Appearance._--Shell conical, with broad rounded longitudinal
  folds; orifice of moderate size, oval; radii narrow, with their upper
  margins oblique; but the summits of both specimens had been much
  broken. Colour, as it appears, originally white. Basal diameter of
  largest specimen 3/4 of an inch.

  _Scuta_, with the upper portion much reflexed; the articular
  ridge is very prominent, and the articular furrow of great width;
  when the valve is viewed from the outside the articular ridge is
  very conspicuous: there is no adductor ridge. _Terga_, with the
  longitudinal furrow very slight; the spur is from one third to
  one fourth of the width of the valve, and its basal end is blunt
  and almost truncated; it stands about half its own width from the
  basi-scutal angle. Internally, the articular ridge is very prominent,
  and the articular furrow narrow and deep, extending down the valve in
  the line of the spur.

  _Parietes_: the parietal tubes are remarkably large, and I think
  this can hardly be an individual peculiarity: the tubes are crossed
  by many transverse septa, close down to the basis. The _radii_ are
  narrow, and have jagged, oblique summits: their sutural edges have
  very distinct septa, barely denticulated, with the interspaces filled
  up solidly. The _alæ_ have oblique summits; I was unable to make out
  the structure of their sutural edges. The _Basis_ is very distinctly
  permeated by pores, which are crossed by transverse septa.


The shell and opercular valves of _B. corrugatus_ so closely resemble
the same parts in _B. crenatus_, that I should not be much surprised at
seeing the two species graduating into each other, if a larger series
of specimens, from beds intermediate in age between the Sub-Appennine
formations and the present time, were obtained. If indeed the basis of
_B. crenatus_ were permeated by pores, the two species could hardly be
discriminated.



_Section_ D.

_Parietes permeated by pores. Basis and Radii not permeated by pores._



25. BALANUS PORCATUS. Pl. 6, fig. 4 _a_-4 _e_.

  BALANUS PORCATUS. _Emanuel da Costa._ Hist. Nat. Test. Brit., p. 249
        (1778).

  LEPAS BALANUS. _Linn._ Syst. Naturæ (1767).

  ---- ---- _Born._ Testacea Mus. Cæs. Desc., Tab. 1, fig. 4, (1780).

  ---- ---- _Chemnitz._ Syst. Conch., 8 Band., Tab. 97, fig. 820,
        (1785).

  BALANUS ARCTICA PATELLIFORMIS. _Ellis._ Philosoph. Transact., vol.
        50, Tab. 34, fig. 18 (1758).

  ---- SULCATUS. _Bruguière._ Encyclop. Method., Tab. 164, fig. 1
        (1789).

  LEPAS COSTATA and BALANUS. _Donovan._ British Shells, 1802-1804, Tab.
        30, fig. 1, 2.

  LEPAS SCOTICA. _W. Wood._ General Conchology, Pl. 6, fig. 3, sed non
        _Lepas balanus_, Pl. 7, fig. 3, (1815).

  BALANUS ANGULOSUS. _Lamarck_ (1818), in _Chenu_, Illust. Conch., Tab.
        11, fig. 11.

  ---- TESSELATUS. _Sowerby_ (!). Mineral Conchology, Tab. 84 (1818).

  ---- SCOTICUS. _Brown._ Illust. Conch. Great Britain, Pl. 7, fig. 2,
        sed non, Pl. 6, fig. 9 et 10 (1827); 2d edit., Pl. 53, fig.
        1-3, 22, 23 et Pl. 54, fig. 1-3.

  ---- GENICULATUS. _Conrad._ Journal Acad. Philadelphia, vol. 6, part
        2, p. 265 (1830), Tab. 11, fig. 16.

  ---- ---- _Aug. Gould_ (!). Report on the Invertebrata of
        Massachussetts, fig. 9 (1841).

_Shell white, generally sharply ribbed longitudinally: radii with their
summits almost parallel to the basis. Scutum longitudinally striated:
tergum with the apex produced and purple._

  _Var._ (_a_): _Walls without longitudinal ribs_. Mus. Brit., Cuming,
  Stutchbury, Jeffreys.

  _Hab._--South shores of England, Ireland, Scotland, Shetland Islands,
  Iceland, Davis's Straits, 66° 30′ N.; Lancaster Sound, 74° 48′ N.
  (Mr. Sutherland). Maine and Massachussetts, United States. China (?).
  In deep water, common on shells, crustacea, and rocks, sometimes
  imbedded in sponges.

  _Fossil_ in the glacial deposits of Scotland, Uddevalla, and Canada;
  in the mammaliferous and Red Crag of England; Mus. Lyell, Sowerby, S.
  Wood, &c.


  _General Appearance._--Shell conical, somewhat convex; white,
  sometimes tinted yellowish, from the thin investing membrane;
  the produced tips of the terga are purple: the parietes of each
  compartment have from two to four strong, prominent, sharp, straight
  longitudinal ribs; these are sometimes irregular, and rarely, as will
  presently be described, they are absent. The radii are smooth and of
  considerable breadth; their summits are nearly parallel to the basis
  or only slightly oblique: hence the orifice is entire; it is rather
  small and ovate, being broad at the rostral end, and very sharp and
  narrow at the carinal end.

  _Dimensions._--The largest specimens which I have seen from Great
  Britain or Ireland, have been 1.3 of an inch in basal diameter:
  in Mr. Cuming's collection, however, there was one much depressed
  specimen from the Shetland Islands, 2.1 in basal diameter: a
  regularly conical specimen from the coast of Massachussetts attained
  a nearly equal diameter; out of the glacial deposits in the Isle of
  Bute, Scotland, several specimens had this same diameter, namely, two
  inches, and were even more steeply conical, being 1.85 in height;
  some glacial specimens from Uddevalla and Canada, in Sir C. Lyell's
  collection, were 1.7 in basal diameter. Hence, it appears, as we
  shall presently see is likewise the case with _B. crenatus_ and
  _Hameri_, that northern specimens, and those from the United States
  and from the glacial deposits, often exceed in dimensions those from
  Great Britain or Ireland.

  _Scutum_: the lines or ridges of growth are broad and prominent;
  they are divided into square beads by fine striæ, radiating from the
  apex: and hence the valve is longitudinally striated. Internally,
  the articular ridge is extremely little prominent; the adductor
  ridge, or what must be called such, runs straight down under the
  articular ridge, making a deep longitudinal pit for the lateral
  depressor muscle. _Tergum_: the apex is a little produced, and
  coloured purple, as well as the upper internal surface of the valve;
  there is no longitudinal furrow, only a very slight depression: the
  spur is placed close to the basi-scutal angle; it is rather long,
  and measured across the upper part, is half as wide as the valve:
  its lower end is truncated and rounded; the basal margin slopes
  towards it. Internally, a very small portion of the scutal margin is
  inflected: the articular furrow is shallow and broad: the crests for
  the depressores are feeble. In young specimens the spur is bluntly
  pointed.

  The _Parietes_ (4 _e_) have large square parietal tubes: in the
  upper part these are filled up solidly without transverse septa:
  the longitudinal septa are finely denticulated at their bases, and
  the denticuli extend unusually close to the outer lamina. In very
  young specimens the inner lamina of the parietes is ribbed, in lines
  corresponding with the longitudinal septa, as is the case with most
  species of the genus; but in medium and large-sized specimens,
  there are between the ribs, thus produced, from one to four smaller
  ribs, which do not correspond with any longitudinal septa; they are
  finely denticulated at their bases, and may be considered as the
  representatives of longitudinal septa which have not been developed
  and reached the outer lamina. I have seen no other instance of this
  structure, namely, the presence of a greater number of ribs, on
  the inner lamina of the walls, than there are longitudinal septa.
  The _radii_ have their summits generally parallel to the surface
  of attachment, as is usual in the first section of the genus, but
  sometimes they are slightly oblique: the septa sometimes rudely
  branch a little, but they exhibit scarcely a trace of denticuli: the
  interspaces are filled up quite solidly. The _alæ_ have their summits
  very oblique; their sutural edges are finely crenated.

  _Basis_, rather thin, translucent, not permeated by pores; obscurely
  furrowed in lines radiating from the centre: the circumference is
  marked in a peculiar manner by the longitudinal septa, and by the
  tips of those intermediate, denticulated ribs, which occur on the
  inner lamina of the parietes.

  _Mouth_: labrum with six teeth: mandibles with the fourth and fifth
  teeth small and rudimentary: maxillæ, with a small notch under
  the upper pair of spines; in the lower part there is a single
  large spine. _Cirri_, dark brownish purple, making a singular
  contrast with the white operculum and shell; first pair, with one
  ramus, having twenty-six segments, and about twice as long as the
  shorter ramus, having twelve or thirteen segments, with their front
  surfaces protuberant. In the second pair the segments are but little
  protuberant: third pair about one third longer than the second pair:
  sixth pair, elongated, having in the same individual forty-six
  segments; these segments have shield-shaped fronts, bearing five
  pairs of spines, with some minute intermediate bristles. There is the
  usual point at the dorsal base of the penis.

  _Range: Geological History._--This species is common on the shores
  of Scotland and Ireland; the most southern point of Europe whence I
  have happened to see a specimen is Tenby, in South Wales: but I have
  no doubt it is found further south; and Mr. Jeffreys, who knows this
  species well, has found it common on the extreme southern shores of
  England. In the United States, it is found on the shores of Maine and
  Massachussetts: northward, I have seen specimens from Iceland, from
  Davis's Straits, and from Lancaster Sound, in lat. 74° 48′ north;
  these latter I owe to Sir J. Richardson. It is an inhabitant of deep
  water; in Mr. Thompson's collection there are several specimens from
  the Bay of Belfast, marked twenty-five fathoms, and one group said
  to have come from "about fifty fathoms, on the coast of Antrim:" one
  specimen from Cape St. Anne, Massachussetts, is marked as having come
  from only five fathoms. This species is commonly associated, on both
  sides of the Atlantic, with _B. crenatus_, and sometimes with _B.
  Hameri_ and _Verruca Strömia_: mollusca, such as pectens, modioli,
  and oysters, offer the most usual surfaces of attachment: I have,
  however, seen many specimens on crustaceans, on rocks, and even on
  the roots of the larger sea-weeds. This species is very common in the
  glacial deposits of Uddevalla, of Skien in Norway, and of Canada, and
  is associated with the same species as in the living state: I have
  seen, also, specimens from the same formation in the Island of Bute,
  Scotland. I have seen numerous specimens from the mammaliferous crag,
  and a few from the Red Crag of England. I owe to the kindness of Mr.
  J. de C. Sowerby an inspection of the original specimens of the _B.
  tesselatus_ of the Mineral Conchology, which is certainly the present
  species.

  _Affinities._--This species is very distinct from every other; it
  comes nearest, as shown in all the characters derived from its
  opercular valves, to _B. nubilus_, and in this latter species we
  have seen the basis plainly tending to lose its pores and thus
  become solid. _B. porcatus_ is perhaps allied in some degree to _B.
  trigonus_, and slightly to _B. crenatus_. The rather broad radii,
  with their summits hardly oblique, give this species a very different
  aspect from those species of the genus amongst which it must be
  placed.

  _Varieties._--A conical specimen, sent to me from the coast of
  Massachussetts, is remarkable from the radii not having been at all
  developed, being represented by mere fissures. I have seen a few
  specimens of _var._ (_a_), (one collected by Sir E. Parry in the
  arctic seas) which had a remarkably different aspect from the common
  forms, but which, after a careful examination of the opercular valves
  and of the animal's body, I feel convinced are not specifically
  distinct: they are characterised by the walls being smooth and
  absolutely destitute of the external longitudinal ribs; by the shell
  being more cylindrical, with broader radii, and with the orifice
  larger and more rhomboidal; the walls and radii are much thinner, and
  the internal lamina is less plainly ribbed: the beak of the tergum is
  not purple. As most of these specimens had grown in a group crowded
  together, the difference of shape, and perhaps the thinness of the
  walls, is thus explained. In a specimen from Davis's Straits, in
  Mr. A. Hancock's collection, most of the above characters are in an
  intermediate condition; there are only a few external longitudinal
  ribs on the parietes; and the terga have not purple apices. In Mr.
  Cuming's collection there are some fine, brilliantly white specimens
  (_without opercula_) from the coast of China; these have thin walls
  and radii, and the walls are not longitudinally ribbed, but they are
  not smooth: the orifice is not large, nor the shape of the whole
  shell cylindrical. It is just possible that these latter specimens
  may be a distinct and representative species, but I do not think so.



26. BALANUS PATELLARIS. Pl. 6, fig. 5 _a_-5 _c_.

  LEPAS PATELLARIS, (_Gmelin_). _Spengler._ Schriften der Berlin.
        Gesellschaft, &c. b. i (1780), Tab. 5; Chemnitz, Neues Syst.
        Couch., Tab. 98, fig. 839.

_Shell depressed; brown, generally with obscure longitudinal violet
stripes: radii (in full-grown specimens) with their summits rounded and
surfaces finely ribbed parallel to the basis: basis sometimes permeated
by imperfect pores. Scutum internally with an adductor ridge._

  _Hab._--Bengal, on wood, Mus. Brit.; on a shell, Mus. Stutchbury;
  Philippine Archipelago (young specimen), Mus. Cuming. According to
  Spengler, on the Coromandel and Malabar coasts.


  _General Appearance._--Shell depressed, sometimes much depressed:
  orifice elongated, rhomboidal, but little toothed; surface smooth,
  but in old specimens sometimes with the walls slightly folded
  longitudinally. The radii are rather narrow, with their summits
  oblique; in old specimens their summits are rounded, and their
  whole surface finely ribbed parallel to the basis. Colour, in old
  specimens dirty brown, tinged with violet, sometimes in longitudinal
  bands, and with whiter irregular marks in the upper parts owing to
  disintegration: in young specimens the walls are regularly banded
  longitudinally, with violet-brown and dirty white; the radii being
  generally of a paler dirty red or violet. Basal diameter of largest
  specimen .9 of an inch.

  _Scuta_, externally rather smooth; internally, articular ridge
  prominent, reflexed, with the lower edge hollowed out so as to be
  slightly hook-formed: adductor ridge small; there is a slight pit
  for the lateral depressor. _Tergum_, with the spur bluntly pointed,
  placed at about its own width from the basi-scutal angle; there is no
  longitudinal furrow, only a slight depression; carinal margin arched
  and protuberant: internally, articular ridge extremely prominent,
  running down in the direction of the middle of the spur: crests for
  the tergal depressores well developed.

  _Parietes_, with the pores rather large; the internal lamina is very
  strongly ribbed, the ribs being but slightly denticulated at their
  bases: the parietal pores do not appear to be crossed by transverse
  septa: sheath closely attached to the walls. The radii have jagged
  oblique summits forming an angle of about 45° with the horizon; in
  old specimens they become more oblique and narrow: and are then
  very remarkable from their summits being arched and rounded, with
  a crenated edge, and with their whole surface transversely ribbed
  in horizontal lines; this is likewise the case with the recipient
  furrow in the opposed compartments: in young specimens the radii are
  externally quite smooth: the septa on the sutural edges are bluntly
  denticulated; the interspaces being filled up solidly. The alæ have
  their summits oblique, but much less oblique than the summits of the
  radii; their sutural edges are very finely crenated.

  _Basis_ thin, either quite solid, that is, not permeated by pores,
  but only furrowed in lines radiating from the centre, or permeated
  by pores towards the circumference, the pores being of very small
  diameter;--so that we here have an important character variable
  within the limits of the same species. Base flat, and this holds
  good, as remarked by Spengler, even when the specimens are attached
  to cylindrical pieces of wood.

  _Animal's body_ unknown.

  _Affinities._--In the basis being sometimes permeated towards the
  circumference by pores, and by the colouring (the other species in
  this and the next section being dirty white), _B. patellaris_ has
  almost as strong a claim to be ranked in the last as in the present
  section: in the rounded summits of the radii, and in the state of
  the basis, it, perhaps, shows more affinity to _B. improvisus_ than
  to any other species; it is, however, almost equally allied to _B.
  glandula_.



27. BALANUS CRENATUS. Pl. 6, fig. 6 _a_-6 _g_.

  B. CRENATUS. _Bruguière._ Encyclop. Method. (des Vers) 1789.

  LEPAS FOLIACEA, _var. a_. _Spengler._ Skrifter af Naturhist.
        Selskabet, b. i, 1790.

  ---- BOREALIS. _Donovan._ British Shells, Pl. 160 (1802-1804).

  B. RUGOSUS. _Pulteney_ (?) Catalogue of Shells of Dorsetshire,
        1799.

  ---- ---- _Montagu_ (?) Test. Brit. 1803.

  ---- ---- _Gould_ (!). Report on Invertebrata of Massachussetts
        (1841), fig. 10.

  B. GLACIALIS (?) _J. E. Gray._ Suppl. Parry's Voyage, 1819.

  B. ELONGATUS (!), CLAVATUS (!), _Auctorum variorum_.

_Shell white: radii with their oblique summits rough and straight.
Scutum without an adductor ridge: tergum with the spur rounded._

  _Hab._--Great Britain, Scandinavia, Arctic Regions as far as
  Lancaster Sound, in 74° 48′ N. (Mr Sutherland); Behring's Straits
  (Captain Kellett); United States; Mediterranean; West Indies, (Mus.
  Brit.); Cape of Good Hope, (Mus. Krauss). Generally attached to
  shells and crustacea in deep water; sometimes to ships' bottoms. Very
  common.

  _Fossil_ in glacial deposits of Scandinavia and Canada, Mus. Lyell;
  in the mammaliferous, and Red, and Coralline Crags, Mus. S. Wood, J.
  de C. Sowerby, Bowerbank; Miocene formation, Germany, Mus. Krantz.


I find, in most collections, this species confounded with _B.
balanoides_; I have even seen the two species, placed by Leach, on
the same tablet in the British Museum: _B. balanoides_ is, moreover,
generally confounded with _Chthamalus stellatus_; nor has any one
hitherto separated the present species from _B. improvisus_. On the
other hand, trifling varieties, both of _B. balanoides_ and _B.
crenatus_, have commonly been considered as specifically distinct. From
these facts it will be seen in what confusion our commonest British
species of Balanus have been left. After due deliberation, I have
little doubt that this is the _B. crenatus_ of Bruguière, and probably
the _B. rugosus_ of Montagu, but this latter author omits all reference
to the really important diagnostic characters between this species and
_B. balanoides_. The _B. crenatus_ is certainly the _B. rugosus_ of Dr.
Aug. Gould. In various collections, I find specimens of _B. crenatus_,
when coming from the arctic regions, called _B. glacialis_, _arcticus_,
and _borealis_; though I have not met with an authentic specimen of the
_B. glacialis_ of Gray ('Supp. Parry's Voyage,' 1819, p. ccxlvi), I
have little doubt that it would prove to be the present species.


  _General Appearance._--White, usually of a dirty tint, from the
  yellowish or brownish persistent epidermis: conical, generally
  (fig. 6 _a_) with the parietes rugged and irregularly folded
  longitudinally; but sometimes much depressed and extremely smooth (6
  _b_); often cylindrical and very rugged; occasionally club-shaped (6
  _c_), the upper part being much wider than the lower: specimens in
  this latter condition sometimes have extremely narrow parietes, like
  mere ribs, and wide radii. The orifice in the cylindrical varieties
  is often most deeply toothed. The radii are generally narrow, and
  have jagged oblique summits; but not infrequently they are so narrow
  as to form mere linear borders to the compartments. The orifice is
  rhomboidal, passing into oval, either very deeply or very slightly
  toothed.

  _Dimensions._--The largest British specimen which I have seen was
  only .55 of an inch in basal diameter: specimens from Greenland
  and the northern United States frequently attain a diameter of
  three-quarters of an inch, and I have seen one single somewhat
  distorted specimen actually 1.6 of an inch in basal diameter. The
  specimens from the glacial deposits of Uddevalla and Canada appear,
  on an average, to attain as large or larger dimensions than those
  from the United States: on the other hand, the specimens from the
  mammaliferous and Red Crag are smaller, the largest being only .35 in
  basal diameter. When individuals have grown crowded together, their
  length is often twice, and even occasionally thrice, as great as
  their greatest diameter; thus I have seen a Greenland specimen 1.6 of
  an inch in length, and only .75 in diameter. In the British Museum
  there are some arctic specimens, one and a half inch in length, only
  half an inch in diameter at the summit (fig. 6 _c_), thence tapering
  downwards to a blunt point.

  _Scuta_; the lines of growth are but little prominent: the surface
  is generally covered by disintegrating membrane. The upper ends
  are usually a little reflexed, so that the tips project freely as
  small flattened points. Internally, the articular ridge is highly
  prominent and somewhat reflexed: there is no adductor ridge, but a
  very distinct impression for the adductor muscle: the depression for
  the lateral depressor muscle is small, but variable. The _terga_
  are rather small: the spur is short, and placed at rather less than
  its own width from the basi-scutal angle; the basal margin slopes a
  little towards the spur, of which the lower end is rounded or bluntly
  pointed in a variable degree. There is no longitudinal furrow, hardly
  even a depression. Internally, the articular ridge is very prominent
  in the upper part; the crests for the tergal depressores are well
  developed, but variable.

  _Compartments._--The internal carinal margin of each compartment,
  from the sheath to the basis, generally, but not invariably, projects
  a little inwards beyond the general internal surface of the shell,
  in a manner not common with the other species of the genus: the
  basal edge of this projecting margin rests on the calcareous basis,
  and is crenated like the basal edges of the longitudinal parietal
  septa. The whole internal surface of the shell is ribbed, but the
  ribs are not very prominent. The parietal tubes are large, and are
  crossed in the upper part, and often low down, by transverse thin
  septa: the longitudinal parietal septa are only slightly denticulated
  at their bases; occasionally they divide at the basis close to the
  outer lamina of the parietes, making some short outer subordinate
  pores. In the circular furrow beneath the lower edge of the sheath,
  there are sometimes little ridges, dividing it into small cells:
  sometimes, however, this furrow is filled up by irregular knobs of
  calcareous matter. The _radii_ are always rather narrow, and often
  they form mere linear ribbons of nearly uniform width along the
  edges of the compartments. Their summits or edges are always more or
  less irregular and jagged: they form an angle with the horizon of
  generally above 40°. Their septa are fine, and barely or not at all
  denticulated. The alæ have oblique summits: their sutural edges are
  rather thick and distinctly crenated. _Basis_ flat, calcareous, very
  thin, with the surface slightly marked by radiating furrows, which
  furrows answer to the radiating pores that occur in the bases of most
  species. In a club-shaped arctic specimen, one inch and a half in
  length, the summit being half an inch and the base only one fifth of
  an inch in diameter, the basis was still calcareous, thick, and not
  permeated by pores.

  _Mouth_: labrum with six teeth: mandibles with the fourth tooth
  minute or rudimentary, and the fifth generally confluent with the
  inferior angle. Maxillæ with generally, but not invariably, a small
  notch under the upper pair of great spines. _Cirri_, first pair with
  the rami very unequal in length, one ramus being nearly twice the
  length of the other; in a large specimen having a cylindrical shell
  the proportional numbers of the segments in the two rami of the first
  cirrus were ten to twenty-three; in a small conical specimen the
  numbers were only eight to thirteen. The second cirrus has only two
  or three more segments than the shorter ramus of the first pair: the
  third cirrus has one or two more segments than the second; but it is
  nevertheless decidedly longer than the second. On the dorsal surfaces
  of both segments of the pedicel of the third cirrus, there is a
  tuft of fine spines. The segments of these three pairs of cirri are
  not much protuberant in front. The segments of the posterior cirri
  have, each, four, or five, or six pairs of spines. _Penis_, with a
  straight, sharp, short point on the dorsal basis.

  _Range, habits, &c._--I have received specimens from all parts of the
  coast of Great Britain and Ireland, generally attached to crustacea
  and mollusca, and never hitherto from rocks uncovered by the tide.
  This species is also attached to floating timber, sticks, fuci, and
  occasionally to pebbles at the bottom of the sea. Mr. Thompson has
  sent me specimens from twenty-five fathoms depth in Belfast Bay:
  others on a Pinna from _about_ fifty fathoms on the coast of Antrim;
  others from between three and six fathoms attached to Laminaria
  digitata: there is a specimen in Mr. Jeffreys' collection marked
  forty-five fathoms. It is often associated, both on the coasts of
  America and Britain, with _B. porcatus_, and though these species are
  so distinct, yet when both have their surfaces similarly affected by
  being attached, as is often the case, to large Pectens, it is not at
  first easy, by external characters, to distinguish them, except by
  close inspection of the terga, which in _B. porcatus_ are beaked and
  purple. The _B. crenatus_ is sometimes associated in deep water with
  _B. Hameri_. At Ramsgate, in Kent, I saw a rudder of a ship, in which
  the two or three upper feet were thickly coated with _B. balanoides_,
  and the two or three lower feet with _B. crenatus_ and _improvisus_
  mingled, together with a few of _B. balanoides_: occasionally
  vessels are thickly encrusted with this species, but I have never
  seen an instance of its concurrence with _B. tintinnabulum_ and
  _amphitrite_--the commonest species on ships coming from the south.
  I have seen specimens from Greenland, Baffin's Bay, the coast of
  Labrador, and other specimens marked simply, "Arctic regions," and,
  again, others from the shores of Maine and Massachussetts. The arctic
  specimens, and those from the northern United States, are larger
  than the British. I have seen one single minute specimen on a crab,
  marked as having come from the Mediterranean. In the British Museum,
  amongst some specimens of _B. eburneus_, ticketed as having been sent
  from Jamaica, there was a small group of specimens, differing in no
  one essential respect from the common varieties of _B. crenatus_:
  at first I concluded that this was an erroneous habitat, and that
  the specimens had really come from the United States, where _B.
  eburneus_, is found as well as in the West Indies: for it appeared
  to me exceedingly improbable that an animal which can exist in lat.
  75° N. should inhabit the hot shores of Jamaica: but subsequently I
  have received a specimen from Prof. Krauss, collected by himself in
  Algoa Bay, which is perfectly characterised, and even has the little
  cells in the furrow under the sheath: so that I am compelled to admit
  this enormous range and capability of resisting the most extreme
  climates. That this species should live in the tropical seas is the
  more surprising, as the large size of the specimens in the northern
  seas and in the glacial deposits, might fairly have been supposed
  to have indicated special adaptation for a cold climate. The great
  geographical range of this species accords with its range in time
  from the present day to the Coralline Crag period.

  The specimens from the glacial deposits which I have examined,
  chiefly in Sir C. Lyell's collection, are very fine and large; they
  are often associated, like the now living individuals, with _B.
  porcatus_ and _Hameri_: they come from the well-known formation of
  Uddevalla and from Canada. There are well-characterised specimens
  in the mammaliferous Crag, at Bramerton and near Norwich, in Sir C.
  Lyell's collection, and from Sutton and other places in the Red Crag
  of the eastern shores of England: these specimens are decidedly not
  only smaller than the glacial, but than the recent English specimens;
  for the largest Crag specimens which I have seen had a basal diameter
  of only .35 of an inch. The specimens which I have seen from the
  Coralline Crag, and some others sent me by Krantz from the miocene
  formation of Flonheim bei Abzei, in Germany, had not their opercular
  valves, yet I cannot doubt, considering how few species there are in
  the present section of the genus, that I have rightly identified them.

  _Diagnosis._--Under the head of _B. balanoides_ I shall make a few
  remarks on the diagnosis between that and the present species; as _B.
  improvisus_ is found on the British shores, sometimes mingled with
  _B. crenatus_, I may observe that, externally, the only difference
  consists in the edges of the radii in _B. improvisus_ being much
  smoother and rounded, and in the whole shell being less rugged.
  Internally, in _B. improvisus_ the porose basis, the presence of an
  adductor ridge on the under side of the scutum, the graduated teeth
  on each side of the central notch in the labrum, and the little
  inequality in length of the rami of the first pair of cirri, are
  clearly and amply diagnostic.



28. BALANUS GLANDULA. Pl. 7, fig. 1 _a_, 1 _b_.

_Shell white; parietes with the internal lamina generally strongly
ribbed longitudinally, with the pores imperfect and small, sometimes in
part absent; radii narrow, with their summits rounded. Scutum with an
adductor ridge; tergum with the spur truncated and rounded._

  _Habitat._--California, Mus. Cuming, Aug. Gould; attached to shells
  and wood, together with _B. nubilus_. Southern Pacific ocean,
  attached to _Pollicipes polymerus_; Mus. Brit.


  _General Appearance._--Shell steeply conical, or cylindrical and
  elongated; dirty white; walls rugged, longitudinally folded; radii
  narrow, with their summits very oblique and rounded; orifice toothed.
  Basal diameter of largest specimen half an inch.

  _Scutum_, resembling externally that of _B. crenatus_; rather broad,
  surface smooth; articular ridge very prominent, and articular furrow
  very wide; hence, when the summits of the opercular valves are worn
  down, the two scuta together form a square projection indenting
  the two terga, as in _B. balanoides_. Internally, there is a small
  adductor ridge, on the lower side of which there is a pit, as if
  for a muscle. The depression for the lateral depressor muscle is
  small, but variable. _Tergum_ without any longitudinal furrow, and
  hardly a depression: spur broad, with its lower end truncated and
  rounded; internally, articular ridge very prominent; crests for the
  depressores well developed.

  _Compartments_:--The internal surface of the parietes is smooth in
  the upper part beneath the sheath, but generally very strongly ribbed
  in the lower part, the ribs being plainly denticulated at their
  bases; in other specimens, the ribs are very small, and even in parts
  quite obsolete. The parietal pores are short and imperfect, sometimes
  reduced to an extremely minute size, to be detected only when the
  walls are broken across near the basal edge, and most carefully
  examined; occasionally not even a trace of a pore exists. Hence in
  this respect, this species offers a singular case of variation. The
  _radii_ are narrow, and of nearly the same width from top to bottom;
  their very oblique summits, when well preserved, are smooth and
  rounded; their sutural edges are ribbed or crenated with extremely
  fine, smooth septa; the recipient furrow is plainly marked by these
  septa. The sutural edges of the alæ are crenated; their summits are
  less oblique than those of the radii.

  _Basis_, thin, finely furrowed in lines radiating from the centre;
  margin sometimes deeply sinuous.

  _Mouth_: labrum with the central notch rather widely open, with four
  teeth on each side of it: palpi with very short spines along their
  inner margins: mandibles with the fourth and fifth teeth forming
  mere knobs: maxillæ small, with a mere trace of a notch under the
  two great upper spines. _Cirri_; first pair with the rami unequal by
  three or four segments, the longer ramus being only one quarter of
  its own length longer than the other ramus. Second pair short, with
  the segments (and those of the shorter ramus of first pair) somewhat
  protuberant. Third pair with the rami one third longer than those of
  the second pair. Sixth pair with the upper segments elongated, and
  bearing six or seven pairs of spines.

  _Affinities._--This species in general appearance closely approaches
  _B. crenatus_ and _balanoides_, and it is related to them in many
  essential parts, such as in the opercular valves. It agrees with _B.
  balanoides_, and differs from _B. crenatus_, in the smallness and
  imperfection of the parietal pores, and in the radii having rounded
  summits; it agrees with _B. crenatus_ in the structure of its basis,
  and in the prominent longitudinal ribs on the internal surface of the
  parietes, and differs from that species in the spur of the tergum
  being squarer, and in the scutum having an adductor ridge.

  _Range._--From the appearance of the Californian specimens, I suspect
  that they had adhered to tidal shells and to wood. The specimens in
  the British Museum, adhering to _Pollicipes polymerus_, consist of
  two lots, one of unknown origin, and the other certainly brought from
  the southern half of the Pacific Ocean by Sir James Ross: it deserves
  notice, that the _Pollicipes polymerus_, the supporting object,
  ranges from California to the southern Pacific Ocean.



_Section_ E.

_Basis membranous._



29. BALANUS BALANOIDES. Pl. 7, fig. 2 _a_-2 _d_.

  LEPAS BALANOIDES. _Linn._ Fauna Succica, 1746, et Syst. Naturæ, 1767.

  ---- ---- _O. Fabricius._ Fauna Groen., p. 424, 1780.

  ---- ---- ET CLIVATUS. _Montagu_ (!). Test. Brit., 1803.

  BALANUS VULGARIS (?) _Da Costa._ Hist. Nat. Testacea, Pl. 17, fig. 7,
        1778.

  ---- OVULARIS ET ELONGATUS. _Aug. Gould_ (!). Report, Invertebrata
        of Massachussetts, figs. 7 and 8, (1841).

  ---- PUNCTATUS, CYLINDRICUS, ELONGATUS, FISTULOSUS CLAVATUS.
        _Auctorum variorum_. Sed non B. punctatus, _Bruguière_,
        Encyclop. Method., et non B. punctatus, _Montagu_, Test. Brit.

_Parietes either solid, or cancellated, or rarely formed by a single
row of pores. Tergum, with the spur bluntly or sharply pointed._

  _Var._ (_a_) _with the parietes permeated by tubes; spur of tergum
  sharply pointed; segments in the posterior pairs of cirri, bearing
  from eight to ten pairs of spines_.

  _Habitat._--Great Britain, France, Norway, Shetland Islands;
  Greenland, according to O. Fabricius; North America, in lat. 66°
  34′ N.; Labrador; Nova Scotia; Massachussetts, Delaware. Extremely
  common, attached to rocks, shells, and wood, within the tidal limits.


I have no doubt that the present species is the _Lepas balanoides_
of Linnæus; though O. Fabricius is the only author who gives, in his
"Fauna Groenlandica," a sufficient description for the species to be
recognised with certainty. I believe this also is the _B. balanoides_
of Bruguière, though he is in error, as far as my experience goes,
in stating that the basis is ever calcareous. I have little doubt,
also, that this is the _B. vulgaris_ of Da Costa. The _B. balanoides_,
in its corroded and therefore punctured state, is certainly the _B.
punctatus_ of most British collections; but I do not believe it is
the _B. punctatus_ of Montagu, which I have scarcely any doubt is
the _Chthamalus stellatus_, so often found in the southern shores of
England, and even in some of the best arranged collections, mingled
with our present species.


  _General Appearance._--The shell, in middle-sized and old specimens,
  is almost invariably folded longitudinally and irregularly; it is
  either dirty white or very often pale brown, and punctured from the
  outer lamina having been corroded, to which action it is extremely
  subject. In very young specimens, the surface is usually quite white
  and smooth. The shell is sometimes much depressed; generally conical,
  but when crowded together, cylindrical or club-shaped, one specimen
  being even more than five-and-a-half times as long as wide. In Mr.
  Jeffreys' collection there is a specimen 2.5 of an inch long, .45 in
  diameter at the summit, only .2 in the middle, and rather more than
  .2 near the base. Another specimen was 1.8 in length, its greatest
  diameter being .35 of an inch at the summit. On the other hand, I
  have seen a very depressed variety, with deeply folded walls, in
  Mr. Thompson's collection from near Dublin, which was no less than
  four times as wide as high; so that the difference in proportion of
  height and greatest width, in the two extreme specimens, was nearly
  as 10 to 1. Occasionally, from some unknown cause, isolated specimens
  become cylindrical. The orifice of the shell, in the much elongated
  specimens, is generally deeply toothed. The radii are always narrow,
  sometimes extremely narrow, and have their summits smooth and rounded.

  English specimens do not usually attain half an inch in basal
  diameter; I have, however, seen one from near Yarmouth .9 of an inch
  in diameter. Specimens from Massachussetts seem rather larger than
  the average size of British specimens, many being .6 of an inch, and
  one specimen a whole inch in basal diameter.

  The _opercular_ valves so closely resemble those of _B. crenatus_,
  that the description is necessarily comparative; in some cases they
  could hardly be discriminated; generally, owing to the disintegration
  to which this species is subject, the tips of the scuta are worn off,
  and hence the articular ridges together form (Pl. 7, fig. 2 _a_) a
  square projection, indenting the two terga; but I have examined young
  specimens and others when not disintegrated, in which the opercular
  valves, viewed externally, presented no difference whatever from
  those of _B. crenatus_. The _scuta_, however, are, I think, generally
  rather thicker, with the growth-ridges more prominent, and with
  the tips certainly less reflexed than is usual with _B. crenatus_.
  Internally, the articular ridge is rather less prominent: there is
  no distinct adductor ridge. The _terga_ are often rather narrower
  in proportion, and this especially holds good in the elongated
  varieties; in these latter, there is occasionally a moderately deep
  longitudinal furrow: the spur is often exactly the same shape as in
  _B. crenatus_, but it is apt to be rather longer (Pl. 7, fig. 2 _c_)
  and more pointed: in _var._ (_a_) it is pointed (fig. 2 _d_) in a
  very remarkable manner. Internally, the articular ridge is decidedly
  more prominent than in _B. crenatus_; the crests for the tergal
  depressor muscles are either well developed or almost absent. From
  this description it will be seen, how singularly the opercular valves
  of the common varieties of these two species resemble each other. I
  may mention that in some of the much elongated specimens, the muscles
  going to the opercular valves partially lose their transverse striæ,
  and become ligamentous.

  The _Parietes_ are either quite solid, or more commonly are permeated
  by minute pores, or by small irregular square tubes (Pl. 7, fig. 2
  _b_), which only run up each successive zone of growth, for very
  short distances, giving to the shell a cancellated structure, which
  from corrosion is often externally visible. In the rather rare
  variety (_a_) the parietes are permeated by regular tubes, extending
  up to the apices of the compartments, but crossed by transverse
  septa. The longitudinal septa, when such can be said to occur, in
  no case are denticulated at their bases. The internal surface of
  the parietes is either quite smooth or is traversed (Pl. 7, fig. 2
  _b_) by very slight anastomosing ridges, but never, even in _var._
  (_a_), by regular longitudinal ribs, as in most other species. The
  carinal margin of each compartment, on the inside, projects, as in
  _B. crenatus_, inwards, beyond the general surface of the shell, and
  running down, rests on the basal membrane. The lower edge of the
  sheath is rarely hollow beneath. The walls are lined by purplish,
  or pale brown, or sometimes by almost black corium; numerous tubuli
  penetrate the under sides of the walls and opercular valves; and
  it is the intersection of these tubuli that gives the punctured
  appearance to the often corroded surface of the shell. The _radii_
  are narrow, generally very narrow; they have their upper and outer
  margins, as seen externally, very oblique, rounded and (when well
  preserved) smooth; their sutural edges are either quite smooth,
  or sometimes just perceptibly pitted, like the basal margin of
  the walls, or occasionally furnished with globular or arborescent
  little ridges. The _alæ_ are also very oblique, but to a variable
  degree, sometimes only slightly oblique: their sutural edges are
  either smooth or obscurely crenated. _Basis_, membranous; in some
  much elongated specimens, during continued growth, the basal edges
  of the compartments approach each other so closely as almost to
  touch, so that the whole shell becomes pointed at the bottom; but
  on careful inspection I have never failed to find, even in the most
  pointed specimens, a minute basal membrane; in other much elongated
  varieties, in which the shell has apparently become too large for the
  animal's body, the basal membrane, instead of being flat, becomes
  drawn up deeply inwards, so as to touch the surface of attachment
  only close round the basal edges of the shell.

  _Mouth_: labrum with the teeth on each side of the central notch
  unusually variable in number; I have seen specimens with only two
  on each side, with four on each side, with five on one side and
  four on the other, with five on one side and none on the other,
  and with six on both sides; hence the total number ranges from four
  to twelve. Mandibles, with the fourth and fifth teeth small, or
  quite rudimentary. Maxillæ, with scarcely even a trace of a notch
  under the upper pair of spines. _Cirri_; first pair, with one ramus
  one third or one fourth longer than the other; in one specimen the
  number of segments were nine and sixteen in the two rami: second
  and third cirri short, very nearly equal in length, having in the
  just-mentioned specimen respectively ten and eleven segments; the
  sixth cirrus in this same specimen had twenty-five segments, each
  segment being about as long as broad, and supporting six pairs of
  spines. In the singular variety (_a_) the posterior cirri are more
  elongated, and each segment supports seven or eight, and in one case
  even ten pairs of spines! the third pair is also in this variety
  proportionally rather longer. At the base of the third pair there
  is a tuft of fine spines. The penis has not, as in _B. crenatus_, a
  point at its dorsal basis. The branchiæ are very little plicated.


_Varieties._

  Of the varieties having much elongated, club-shaped, hour-glass
  shaped, and depressed shells, there is no necessity to say anything
  in particular. With respect to the remarkable variety (_a_), I at
  first named and described it as a distinct species: I have received
  two lots, both from North America, one being sent me by Professor
  Agassiz from Cape Cod. These agreed in having the parietes permeated
  by regular tubes; in having the spur of the tergum most sharply
  pointed; in the third pair of cirri being proportionally longer
  compared with the second pair; in the sixth pair having more numerous
  segments, namely, three times as many as in the third pair; in the
  segments of the posterior cirri being more elongated, and especially
  in the number of pairs of spines on each segment--amounting in one
  case even to ten, a number unparalleled in other cirripedes. It may
  naturally be asked why I have not retained so well marked a form as
  a distinct species? In the first place, I found the most remarkable
  character in _var._ (_a_), namely, the number of pairs of spines
  on the posterior cirri variable, there being in one lot seven or
  eight pairs, and in the other lot nine or ten pairs on each segment.
  Secondly, all the characters by which this variety differs from the
  common _B. balanoides_, are those which are variable in the latter;
  this is especially the case with the structure of the parietes, and
  in a lesser degree with the spur of the tergum. Thirdly, I found a
  specimen in Mr. Cuming's collection, from Sweden (so that this _var._
  (_a_) is not confined to North America), in which the cirri quite
  resembled those of the American specimens, but the spur of the tergum
  was in an intermediate condition as compared to that of ordinary
  varieties; and the parietal tubes were of unequal sizes, and scarcely
  more regular than sometimes in the true _B. balanoides_. And lastly,
  I have seen specimens from Ayrshire, with the parietes permeated by
  regular tubes, but with the tergum in an intermediate condition, and
  with the segments of the posterior cirri not more numerous or more
  elongated than in _B. balanoides_, supporting only six or seven pairs
  of spines, that is only one more than is common with _B. balanoides_;
  so that it was impossible to decide whether to rank the Ayrshire
  specimen under _var._ (_a_) or under the common form, so that I was
  compelled to give up _var._ (_a_) as a species.

  _Monstrous individuals, with the male organs aborted:
  Parasite._--Amongst some specimens, chiefly elongated ones, sent to
  me from Tenby, in South Wales, I found no less than seven individuals
  with some of the posterior cirri distorted, unequal on the opposite
  sides, and in an almost rudimentary condition, and in each case
  with the penis truncated, without any muscle entering the stump,
  which was _absolutely imperforate_: the vesiculæ seminales were
  much shrunk; in one case without any zoosperms; in another case
  with headless zoosperms cohering in an unusual manner; hence it is
  certain that these individuals were functionally only female, and
  could not impregnate their own ova; yet in two instances the ova
  had been impregnated, no doubt by neighbouring perfect individuals,
  for they contained well-developed larvæ. Several of these monstrous
  individuals were infested by one, two, or three curious crustaceans,
  which have been described by Mr. Goodsir,[97] as the male of the
  Balanus; but these supposed males are females, and were distended
  with ova containing almost mature larvæ; I believe that they are the
  females of the unnamed genus, belonging to the family of Ioniens,
  described by Mr. Goodsir, which live parasitic within the sack (as I
  likewise found) of the same individual Balani.

    [97] 'Edinburgh New Philosophical Journal,' July, 1843.

  _Diagnosis._--I have seen several specimens of this species and of
  _B. crenatus_, absolutely undistinguishable in external appearance. I
  may specify one of _B. balanoides_, imbedded in an alcyonidium, and
  one of _B. crenatus_, imbedded in a sponge, and therefore neither at
  all abraded. Generally, the tips of the scuta in _B. crenatus_ are a
  little reflexed, whereas in _B. balanoides_, when the shell has been
  at all disintegrated, the tips form a square projection locked into
  the terga. _Bal. crenatus_ never assumes the punctured appearance so
  common in _B. balanoides_. Very young specimens of the latter can be
  distinguished by their dead white colour and smoothness. The edges
  of the radii are almost always smoother than in _B. crenatus_, and
  they are never so wide as is sometimes the case with _B. crenatus_.
  When a specimen is disarticulated, our present species can at once
  be distinguished from _B. crenatus_ (and from _B. improvisus_), by
  its membranous basis, and by the solid or cancellated walls, which
  are rarely permeated by regular tubes or pores; and the walls when
  porose are not internally ribbed. I have already pointed out the
  few very trifling points, in which the opercula of the two species
  differ. The mouth and cirri offer likewise very few differences: in
  _B. balanoides_ there are often more teeth on the labrum than in _B.
  crenatus_; the rami of the first cirri are perhaps here rather less
  unequal; the second and third pairs of cirri are certainly in most
  cases more equal in length; and lastly, the segments of the sixth
  cirri, even in the common varieties, bear, in equal-sized specimens,
  more pairs of spines than in _B. crenatus_. We shall see that in
  habits, with regard to depth, the two species differ, _B. balanoides_
  inhabiting much shallower water than _B. crenatus_.

  _Range, Habitats, &c._--This species is extraordinarily abundant
  within the tidal limits round the shores of Great Britain, and
  apparently of the northern United States. Besides numerous specimens
  sent to me from very many English localities, the late Mr. W.
  Thompson, of Belfast, kindly placed in my hands his very large
  collection; from these materials it appears that _B. balanoides_
  is the only tidal species in the northern parts of our island; but
  in the south and south-west, it is associated with the _Chthamalus
  stellatus_ and _Balanus perforatus_. I doubt whether this species
  ever lives below the lowest tides; the case of a few specimens being
  mingled with _B. improvisus_ and _crenatus_, (mentioned under the
  latter species,) at the bottom of a rudder of a small vessel, about
  six feet deep, is hardly an exception, for the water would there
  be troubled and aërated almost as in a breaker; and on this very
  rudder the upper two or three feet were coated exclusively by the
  _B. balanoides_. This species lives on rocks at both the uppermost
  and lowest limit of the tides; I am informed by Mr. Thompson, that
  he has seen specimens attached to a spot not covered by water
  during neap-tides. As a proof of its tenacity of life, Mr. Thompson
  informs me that he accidentally kept some specimens in a box, in a
  warm sitting-room, and found them alive seven days afterwards. This
  same most accurate observer finds, however, that _B. balanoides_
  is very susceptible to brackish water; he says, "that having kept
  some specimens alive for a week in excellent health, the water being
  changed once in thirty-six hours, they were one day killed instantly
  by some water, though brought from the same part of the estuary as
  usual, having been rendered brackish by much rain having lately
  fallen." I may recall the fact, that _B. improvisus_ lives daily for
  hours in absolutely fresh running water.

  The _B. balanoides_ lives attached, often continuously coating
  many square feet of the surface, to rocks, pebbles, wooden-piers,
  littoral shells and ulvæ. The most northern point whence I have
  received specimens, is in lat. 66° 34′ in North America, collected
  by Mr. Sutherland; and the most southern point is Delaware Bay, in
  the United States, in lat. 39°; I do not believe that this species
  extends into the Mediterranean, for Ranzani (Mem. di Storia Nat.),
  who particularly attended to the nature of the basis, was not
  acquainted with any _Balanus_ having a membranous basis; and Poli
  (Test. Ut. Siciliæ,) describes only two species thus characterised,
  and these are manifestly _Chthamali_.


With respect to the rate of growth of this species, I am indebted to
Mr. W. Thompson for the following note:--

  "Sept. 29, 1848.--I examined a great number of Balani, in reference
  to the growth made by them during the present season, and found it
  to average three lines in diameter, and at most four lines. I saw a
  few minute specimens, only one line in diameter, showing that the
  species continued to breed until lately: these latter were probably
  not more than four weeks old. The young of the present year are
  plainly distinguished from the older ones, by their pure white colour
  and fresh appearance. Judging from the size of this year's specimens,
  and of the older ones on the same stones, I am of opinion that the
  term of life of the species is two years. Of the older shells, which
  I examined and found living in the spring, nine tenths are now dead,
  the walls only remaining, the opercular valves having been washed
  away."

Mr. Thompson goes on to say, that the individuals which had, on July 3,
a basal diameter of from two and a half to three lines, had attained,
by the 30th of September, a diameter of four and a half lines, this
being here the maximum size of the species.



30. BALANUS CARIOSUS. Pl. 7, fig. 3 _a_-3 _e_.

  LEPAS CARIOSA. _Pallas._ Nova Acta Acad. Scient. Petrop. tom. ii
        (1788), p. 240, Tab. 6, fig. 24.

_Parietes thick, formed by several rows of unequal-sized pores. Tergum
narrow, with the apex beaked, and spur sharply pointed._

  _Hab._--Columbia River, west coast of North America, Mus. Brit.
  and Cuming; Behring Straits (Capt. Kellett); the Kurile Islands,
  according to Pallas. Attached to shells, and to each other in groups.


  _General Appearance._--Shell steeply conical, with a rather small
  oval orifice; or cylindrical, with a large rhomboidal and little
  toothed orifice. Colour dirty white. Surface either simply rugged,
  or more commonly covered by numerous, narrow, extremely prominent,
  longitudinal plaits; from the manner in which these overlap each
  other, the shell almost appears as if thatched with straw. The
  upper corroded part of the shell usually exhibits a cancellated
  and finely punctured surface. The radii are generally very narrow,
  forming towards the base of the shell a mere narrow ribbon to each
  compartment, and often hardly distinguishable; but in one specimen
  they were of considerable width: in the former case, the alæ are
  often widely exposed. The largest specimen which I have seen was 1.5
  of an inch in basal diameter, but Pallas gives 2.2 as the measurement
  of a specimen from the Kurile Islands.

  The opercular valves are united to each other and to the shell by
  unusually strong membrane; and the upper parts of both valves, in all
  the specimens seen by me, have been much disintegrated. The _Scutum_,
  in old specimens, is faintly striated longitudinally, but in some
  there is hardly a trace of this: the occludent margin is furnished
  with a few large knobs, not corresponding with every alternate line
  of growth (as is usual with other species), but with every fourth or
  fifth line. Internally, the articular ridge is moderately prominent
  (in young specimens more prominent) and reflexed. The adductor
  ridge is sharp and prominent; in the upper part it is confluent
  with the articular ridge, but in young specimens can be seen to be
  distinct; in the lower part it borders a large deep cavity for the
  lateral depressor muscle, in the middle of which there is a very
  slight longitudinal ridge; this cavity sometimes is almost closed or
  arched over in its upper part. In one specimen, the basal margin of
  the scutum was deeply hollowed out in the middle. The _Tergum_ is
  remarkably narrow, with its apex produced into a triangular beak,
  hollow within, and sometimes faintly tinged purple. A deep, closed,
  longitudinal furrow runs down the valve. The spur is long, remarkably
  narrow, and pointed. Internally, the spur is produced up the valve
  as a ridge: the inflected scutal margin, and the prominent articular
  ridge, are both nearly straight, and parallel to the spur. The crests
  for the depressores are sharp and very prominent.

  The _Parietes_ are very thick and strong: unlike every other
  species of the genus, they consist of several very irregular rows,
  of unequally sized, round or angular tubes (3 _b_). These tubes or
  pores are generally short, and are at frequent intervals crossed by
  transverse septa; they often rather deserve to be called cells than
  tubes. New tubes are formed along the inner as well as along the
  outer lamina. They are lined by dusky purple corium. The internal
  surface of the parietes is smooth in the upper part, and in the
  lower, it is reticulated by slight, irregularly branching ridges.
  The carinal internal margin of each compartment projects a little,
  as in the case of _B. crenatus_ and _balanoides_. The lower edge of
  the sheath is either hollow beneath, or is united to the walls. The
  _radii_ in one specimen were broad, with slightly oblique, jagged
  summits; generally they are extremely narrow, forming more ribbons
  along the lower edges of the compartments, barely extending up as
  high as the sheath. They can sometimes hardly, or not at all, be
  seen, until the shell is disarticulated: in rather young specimens
  the sutural edge is sometimes quite smooth; in old specimens the
  lower part of the edge has coarse arborescent septa, with the
  interspaces filled up solidly, whilst the upper part is smooth.
  The _alæ_ are conspicuous from the outside, owing to the little
  development of the radii; but owing to the diametric growth not
  having been great, the part added during such growth is narrow;
  the summits of the alæ are only slightly oblique: the sutural edge
  is coarsely crenated, with the teeth denticulated or slightly
  arborescent.

  _Basis_ membranous.

  _Mouth_: labrum with only four very minute teeth: mandibles with four
  teeth; the third tooth broader than the first; the fourth small.
  Maxillæ, with the two upper spines placed on a slight prominence,
  beneath which there is a small notch. _Cirri_ of a very dark colour
  (much injured): the segments of the first, second, and third pairs
  very broad and short, protuberant in front, and most thickly clothed
  with spines; the third pair is very little longer than the second
  pair: the sixth pair (in a large specimen above one inch in basal
  diameter) had the segments broader than long, each furnished with
  seven pairs of spines.

  _Affinities._--This species, though very distinct, evidently comes
  near to _B. balanoides_, especially to _var._ (_a_). By merely
  doubling or trebling the irregular rows of short tubes in the
  walls of _B. balanoides_, with their reticulated inner lamina and
  longitudinally folded outer lamina, we should have the structure
  exhibited in _B. cariosus_. We have seen, also, that in _var._
  (_a_) of _B. balanoides_, the spur of the tergum is remarkably
  sharp, as in _B. cariosus_. This species, also, in a very marked
  manner approaches in many characters, especially in the opercular
  valves, in the cirri, and to a certain extent in the shell and
  basis, to _B. flosculus_, and even in external appearance to _var.
  sordidus_ of the latter--an inhabitant of the opposite extremity
  of the continent, namely, of Tierra del Fuego. Again, the tergum
  to a certain extent, and the scutum in a singular manner, resemble
  these valves in _B. nubilus_, showing an unequivocal affinity to
  that species. With respect to the most remarkable character of the
  species, namely, the several irregular rows of tubes or pores in the
  walls, it deserves notice that in _B. crenatus_, which is certainly
  closely allied to _B. balanoides_, the longitudinal septa sometimes
  divide near the outer lamina, thus giving rise to a few additional
  tubes. Of the above several species, to which our present species is
  allied, _B. flosculus_ stands in the next section, and _B. nubilus_
  and _crenatus_ in the last: hence we see that _B. cariosus_ has
  singularly divergent affinities. The peculiar structure of the
  parietes, together with the general appearance of the shell, made me
  at the first moment suppose I was examining a Tetraclita (or Conia of
  Leach); hence, also, it has arisen, that _Lepas cariosa_ of Pallas
  has often been quite erroneously given as a synonym of _Tetraclita
  porosa_.



31. BALANUS DECLIVIS. Pl. 7, fig. 4 _a_-4 _d_.

_Parietes solid; rostrum nearly twice as long as the carina or
carino-lateral compartments, hence the basis is oblique. Tergum with
the spur truncated, half as wide as the valve._

  _Hab._--West Indies; Mus. Brit.--Jamaica, imbedded in a sponge; Mus.
  Cuming.


This is a remarkable species; when first seeing it imbedded in numbers
in a sponge, I did not in the least doubt but that it was an Acasta:
on examination, however, it is found to have a membranous basis, and
therefore cannot by the definition enter into that sub-genus, to which,
however, it is very closely allied. It differs from other sessile
cirripedes very remarkably in the rostrum being nearly twice as long as
the carinal compartments, so that the basis is always very oblique, or
placed almost on one side; in this elongation of the rostrum, although
in a different direction, we are reminded of _B. calceolus_ and its
allies; and these latter we know can hardly be separated from certain
species of Acasta. Hence the position of our present species in this
section, is not natural; but I am unable to place it elsewhere, without
breaking down every definition: it should stand somewhat isolated,
on one side of a line of affinity connecting _Balanus calceolus_ and
_navicula_ with _Acasta purpurata_.


  _General Appearance._--The shell is thin, fragile, smooth, and
  white, but covered to a considerable extent by a brown membrane,
  which on the sheath and opercular valve is of a bright tint, and
  clothed with bristles. Viewed laterally, the rostrum is seen to
  be considerably bowed, and from its being nearly twice as long as
  the other compartments, with its lower end bluntly pointed, the
  basal margin of the whole shell is rendered very oblique, forming a
  slightly concave line. The lateral compartments are rather longer,
  and about one third broader than the carino-lateral compartments.
  The rostrum, from terminating downwards in a blunt point, instead
  of being square or truncated, as in all other Cirripedes, and from
  the upper end being, as is usual, pointed, has, when disarticulated
  from the other compartments, the shape of a boat. The parietes are
  not at all porose: their internal surface sometimes shows traces
  of longitudinal ribs, but sometimes there are none. The radii are
  narrow, with their summits very oblique, and their sutural edges
  smooth. The sutural edges of the alæ are likewise smooth. The largest
  of Mr. Cuming's specimens was .2 of an inch in diameter; but a
  disarticulated specimen in the British Museum must have been larger,
  having a rostrum .3 in length. The _Basis_ is membranous.

  The _Scuta_ are rather convex; they have their lines of growth
  approximate, most finely crenated, so as to be very feebly striated
  longitudinally. Internally, the articular ridge is pretty well
  developed, its lower edge being very oblique; there is barely an
  adductor ridge: the pit for the lateral depressor muscle is deep.
  The spur of the _Tergum_ is placed close to the basi-scutal angle
  of the valve; it is about half as wide as the valve, with the
  lower end truncated: sometimes it may be rather said to be bluntly
  pointed, owing to its carinal side sloping up to the basal margin.
  The articular ridge is pretty well developed. The crests for the
  depressor muscles are barely discernible.

  _Animal's body_ unknown.



_Section_ F.

_Parietes and radii not permeated by pores; basis sometimes permeated
by pores, sometimes not permeated, sometimes excessively thin and
hardly distinguishable._



32. BALANUS HAMERI. Pl. 7, fig. 5 _a_-5 _c_.

  LEPAS HAMERI. _Ascanius._ Icones rerum naturalium, Tab. 10, 1767.

  ---- TULIPA. _O. F. Müller._ Prodromus. Zoolog. Dan. 1776; sed non
        _L. tulipa_, in Poli, Test. ut. Siciliæ; neenon _B. tulipa_, in
        Bruguière, Encyclop. Method; neenon _B. tulipa_, in Sowerby,
        Genera of Shells.

  ---- TULIPA ALBA. _Chemnitz._ Syst. Conch. Tab. 93, fig. 832.

  ---- FOLIACEA. _Spengler._ Skrivter of Naturhist. Selskabet, 1 B.
        1790.

  BALANUS CANDIDUS. (Tab. emendata) _Brown._ Conch. Great Britain
        (1827), Tab. 6, figs. 9 and 10, et 2d edit. Tab. 54, figs. 9-12.

  ---- TULIPA. _Lyell._[98] In Phil. Transact., 1835, p. 37, Tab. 2,
        figs. 34-39.

    [98] Sir C. Lyell remarks that this is apparently the _B.
    Uddevallensis_ (Linn.) of Swedish lists of fossils. Prof. E. Forbes
    has shown (Mem. Geolog. Survey of England, vol. 1, p. 364) how this
    name arose, from a short description, prior to the introduction of
    the binomial system, "Lepas quæ Balanus Uddevallensis," given by
    Linnæus in his Wast-Gotha Resa, in 1747.

    For the reference to Ascanius' work, which is on the binomial
    system, and subsequent to the 10th edit. of Linnæus, in 1758, I am
    greatly indebted to Mr. Sylvanus Hanley. Had it not been for this
    gentleman, I should have used Müller's name of _B. tulipa_ as the
    first name.

_Shell white: radii with their oblique summits smooth and arched;
sutural edges smooth: basis solid. Scutum feebly striated
longitudinally: tergum with the spur narrow._

  _Hab._--Coast of Yorkshire; Scotland; Galway, Ireland; Isle of Man,
  and Anglesey, twelve fathoms. Generally in deep water; not very
  common. George's Bank, Massachussetts, United States; Mus. Aug.
  Gould. Iceland, Finmark, and the Faroe Island, according to Spengler.
  Attached to crustacea, mollusca, stems of fuci, and stones; often
  associated with _B. porcatus_ and _crenatus_.

  _Fossil._--In glacial deposits at Uddevalla in Sweden, and Beaufort
  in Canada; Mus. Lyell. Banks of the Dwina, Russia; Mus. Murchison.
  Greenland, "in blue clay," according to Spengler. Red Crag (Sutton)
  Mus. S. Wood.


  _General appearance._--Shell tubulo-conical, very smooth, white,
  generally more or less covered by yellow thin membrane: orifice
  large, sub-triangular: radii moderately broad, with their more
  or less oblique summits slightly rounded and smooth; from this
  circumstance the shell has been justly compared to the half-opened
  flower of a white tulip. Specimens often exceed an inch in basal
  diameter; I have seen one from Scarborough two inches in diameter
  and one and three quarters in height: another specimen was 1.6 in
  diameter and 3 in height. The specimens in the glacial deposits seem
  even to have acquired larger dimensions, one from Uddevalla being
  nearly four inches in height.

  _Scuta_, elongated, flat, feebly striated longitudinally:
  internally, articular ridge short, moderately prominent: adductor
  ridge, confluent in the upper part with the articular ridge,
  running straight down and forming a rather large cavity for the
  lateral depressor. _Terga_ feebly striated longitudinally, with a
  longitudinal furrow, having the sides, in old specimens, partly
  closed in: the basal margin slopes much towards the spur, which is
  rather long and narrow, with its end rounded: it is placed at about
  its own width from the basi-scutal angle. Internally, articular
  furrow narrow; crests for the depressores moderately prominent, but
  in a variable degree.

  _Compartments_: these are unusually thin, and separate easily: the
  parietes are finely ribbed longitudinally on their insides; the
  bases of these ribs being just perceptibly denticulated. _Radii_,
  with their summits oblique (usually at about an angle of 45°),
  slightly arched and quite smooth: the smoothness is produced by the
  edge being a little inflected: sutural edge quite smooth, without
  even a trace of septa or denticuli. _Alæ_ oblique, generally rather
  less oblique than the summits of the radii: sutural edges smooth,
  with an excessively fine linear furrow running along the edge, a
  little towards the inner side, and filled with a yellow ligamentous
  substance: a furrow of this kind I have seen in no other species.

  _Basis_, solid, not permeated by pores; either smooth, or slightly
  furrowed in lines radiating from the centre.

  _Mouth_: labrum with scarcely perceptible minute bead-like teeth
  thinly scattered along the edge. _Palpi_ and _outer maxillæ_ rather
  sparingly clothed with hairs. _Mandibles_ with teeth rather sharp;
  the fourth and fifth teeth small, but well developed; inferior angle
  pointed with fine spines. _Maxillæ_ with a deep notch under the two
  upper great spines. _Cirri_, the first pair is short, with rami of
  nearly equal length: the segments are not protuberant in front either
  in the first or second pairs. In the posterior cirri, the segments
  bear four pairs of spines, with a tuft of rather long intermediate
  spines: in young specimens there are only three pairs: the spines in
  the dorsal tufts are short and thin.

  When the shell is disarticulated, this species cannot be confounded
  with any other; but judging by external characters alone, it may
  sometimes be very easily confounded with _B. eburneus_, and I have
  received the two species under this one name from Massachussetts:
  generally _B. Hameri_ may be distinguished from _B. eburneus_ by
  the smoothness of the summits of its radii, and by the so-called
  epidermis being of a darker yellow.

  With respect to the fossil specimens from the glacial deposits,
  I have little to add; I have seen one from Uddevalla, as already
  remarked, four inches in height, and a lateral compartment broader by
  one fourth than the same compartment in any recent specimen. As Sir
  C. Lyell remarks (Phil. Transact.), the compartments are always found
  separated, which is accounted for by their weak union in a recent
  state. This species, when fossil, is usually associated with its
  deep-water congeners _B. porcatus_ and _crenatus_, as at the present
  day.

  I must here mention that I have examined a considerable number of
  _separated_ compartments, without opercular valves, brought from
  Barbados, in the West Indies, showing the existence there of a
  closely allied or possibly identical species. The only difference
  which I can point out in these compartments is, that the parietes
  are rather thicker, and the radii rather narrower, with more oblique
  summits: some of the compartments are two inches in length. It seems
  very improbable that the true _B. Hameri_ should extend to the West
  Indies, but after what has been seen in the case of _B. crenatus_,
  this is possible.



33. BALANUS AMARYLLIS. Pl. 7, fig. 6 _a_-6 _c_.

_Shell striped or clouded with pinkish-purple, or quite white; radii
narrow, with their oblique summits smooth or arched: basis porose.
Scutum plainly striated longitudinally: tergum with the spur narrow._

  _Var._ (_a_):[99] _bright rosy pink, not distinctly banded
  longitudinally_. Hab. North-east coast of Australia.

  _Var._ (_b_): _snow white, glossy; orifice deeply toothed_.

    [99] This variety perhaps is the _B. roseus_ of Lamarck, as
    figured in Chenu, 'Illust. Conch.' Tab. 2, fig. 9; but as Lamarck
    does not even notice such conspicuous external characters as the
    longitudinal striæ on the scuta, and the smooth rounded edges of
    the radii, it is impossible to identify his species.

  _Hab._--Mouth of the Indus; East Indian Archipelago; Philippine
  Archipelago; Moreton Bay, and the north-east coast of Australia.
  Attached frequently on ships' bottoms, associated with _B.
  tintinnabulum_ and _amphitrite_. Sometimes attached to _Gorgoniæ_
  with _B. calceolus_.


  _General Appearance._--Shell steeply conical, with the orifice
  sub-rhomboidal, moderately large, very slightly, or deeply notched:
  surface very smooth: white, longitudinally banded with pinkish or
  leaden purple, with sometimes a purplish, sometimes a yellowish tint,
  the latter owing to the persistent epidermis; the bands are pale,
  and often fade away in the lower, and sometimes in other parts of
  the shell; the epidermis is generally more persistent on the narrow
  rounded radii than on the parietes, and hence the radii are generally
  yellowish. The opercular valves are pale dull purple: the sheath is
  darker purple, with the exception of the portions of the alæ added
  during the diametric growth, which are of a dead white, and are
  externally conspicuous. The scuta are striated longitudinally. I
  may remark, that, excepting the narrowness of the radii, with their
  quite smooth, rounded and very oblique summits, some specimens are
  hardly distinguishable, in external aspect, from varieties of _B.
  amphitrite_. If the specimens from the north-east coast of Australia,
  of which I have seen many (but unfortunately only one small one had
  its opercular valves), form, as I fully believe, merely a variety; it
  is characterised by its nearly uniform beautiful rosy pink, without
  any _distinct_ longitudinal bands: of these specimens I have seen
  one two inches in basal diameter, and three in height: of ordinary
  duller-coloured striped specimens, the largest was 1.7 in basal
  diameter. Of the perfectly white _var._ (_b_), I have seen several
  specimens, the largest being .6 of an inch in diameter: these have
  a somewhat peculiar aspect, but I have met with only one specimen
  with opercular valves, and that was extremely young: I at first
  considered this form as specifically distinct; but I can point out,
  after careful examination of the whole shell, operculum, and internal
  animal of the young specimen, no sufficient diagnostic characters.

  _Scutum_, plainly striated longitudinally, with the striæ dividing
  the prominent lines of growth into squarish beads: internally, the
  upper part of the valve is roughened: the articular ridge is short,
  remarkably little prominent, and not reflexed; the adductor ridge
  is blunt and little prominent; sometimes it is almost confluent
  with the articular ridge: there is a deep but variable depression
  for the lateral depressor muscle; and in young specimens of _var._
  (_a_) it was almost absent. _Tergum_: the surface exhibits traces of
  longitudinal striæ: there is a deep longitudinal furrow, with the
  sides folded in and quite closed in full grown specimens: the scutal
  margin is considerably curved towards the scutum. The spur is long
  and narrow, with the end bluntly pointed, placed at rather above its
  own width from the basi-scutal angle; the basal margin slopes but
  little towards the spur: the crests for the depressores are feebly
  developed.

  _Parietes_: their internal surfaces are strongly ribbed
  longitudinally, with the basal ends of the ribs coarsely
  denticulated, and with the denticuli extending close to the outer
  lamina. The radii are generally narrow, but their width varies;
  their summits are very oblique, smooth, rounded, and inflected, with
  the lines of growth, in the uppermost part, curving inwards; their
  sutural edges, in the upper inflected portion, are quite smooth,
  without septa; in the lower and larger portion, the edge is crenated
  with excessively fine teeth or septa, not denticulated: the radii,
  like the parietes, have no inner lamina: the recipient grooves in
  the opposed compartments are smooth, and are in the lower part of
  the shell of unusual depth. The _alæ_, differently from the radii,
  generally have their summits very slightly oblique, but sometimes
  they are highly oblique: their sutural edges are most finely
  crenated. The _basis_ is generally flat, sometimes cup-formed; it is
  permeated by pores, crossed by transverse septa; and sometimes there
  is an underlying cancellated layer.

  _Mouth_: labrum with either six very small teeth, or with none.
  _Mandibles_ (Pl. 26, fig. 5), with the third tooth a little thicker
  than the first; fourth and fifth teeth small, but quite distinct.
  _Maxillæ_ (Pl. 26, fig. 7), with the inferior part forming a square
  step-formed projection, bearing, one behind the other, two spines
  as long as the upper pair; in a young specimen of _var._ (_a_) this
  step-formed projection was absent.

  _Cirri_: first pair with the rami unequal by about four segments: the
  shorter ramus has the segments very protuberant in front, thickly
  clothed with strongly serrated spines; the second cirrus has segments
  moderately produced; the third has them produced only in a slight
  degree. The pedicels of the second and third cirri have dorsal tufts
  of spines, but not a hairy plate prolonged over the thorax. The
  posterior cirri have segments broader than long, bearing only two
  pairs of nearly equally long spines; and between each pair there is a
  small intermediate tuft. The penis has the usual basi-dorsal point.


_B. amaryllis_ is a distinct and well-defined species, more nearly
related to _B. Hameri_ than to any other form.



34. BALANUS ALLIUM. Pl. 7, fig. 7 _a_-7 _d_.

_Shell faintly tinged with purple: radii broad, with their summits not
oblique: basis not porose. Scutum with the lines of growth crenated:
tergum with the spur extremely short, truncated, broad as half the
valve._

  _Hab._--Raine's Islet, Barrier Reef, Australia, Mus. Stutchbury. Hab.
  unknown, attached to and coated by Porites. Mus. Brit.


  _General Appearance._--Shell conical, smooth, but with the lower part
  sometimes narrowly ribbed in lines corresponding with the internal
  longitudinal ribs; tinted pale peach-blossom purple, owing to the
  sheath being finely so coloured; or wholly white. Radii broad, white,
  square on the summit, hence orifice entire, ovate passing into
  rhomboidal. The parietal portion of the carino-lateral compartments
  extremely narrow, about one eighth of the width of the parietes of
  the lateral compartments. Basis concave, partially imbedded in the
  coral. Largest specimen .35 of an inch in diameter.

  There are some specimens in Mr. Cuming's collection which appear to
  belong to this species, and are certainly very closely allied to it,
  but not having the opercula, cannot be identified positively; the
  shell is flatter, with the walls strongly ribbed up to the orifice,
  which is more rhomboidal: the basis is much more cup-formed and more
  deeply imbedded in the coral; but these differences by themselves are
  by no means sufficiently diagnostic.

  _Scutum_: the lines of growth are crenated, causing the surface to
  be very obscurely striated longitudinally: the articular ridge is
  very prominent, as can be best seen from the outside, and runs down
  the whole length of the tergal margin with a very regular curve, and
  hence differs from the articular ridge in the foregoing species. The
  adductor ridge is either absent, or very indistinct, and parallel
  to the articular ridge: there is a deep little pit for the lateral
  depressor muscle. _Tergum_ (7 _d_), with the apex somewhat produced
  or beaked, and tinged purple: external surface almost flat, without
  any longitudinal furrow: scutal margin curved. Spur very short,
  placed quite close to the basi-scutal angle of the valve; broad as
  half the valve; lower end square. Internally, the articular ridge is
  prominent only in the uppermost part of the valve: crests for the
  depressores very feeble.

  _Parietes_: their internal surface is very strongly ribbed
  longitudinally, the ribs being coarsely denticulated at their bases,
  and finely fluted along their sides. The sheath is transversely
  ribbed, and clothed with an epidermis furnished with transverse
  rows of fine hairs. The _radii_ are of a dead white, whereas the
  parietes are translucent; the summits are parallel to the basis;
  they are broad; the radii of the carino-lateral compartments appear
  extraordinarily broad, owing to the narrowness of the parietal
  portion: the sutural edges are furnished with coarse septa, which
  are sinuous, irregular, and obtusely denticulated; the interspaces
  are filled up solidly. The _alæ_ are thin, with their sutural edges
  almost smooth, and their summits oblique: in some specimens, during
  the diametric growth, a mere, almost thread-like ribbon is added
  to their sutural edges. _Basis_ slightly cup or saucer-shaped;
  moderately thick, permeated by fine pores, and generally ribbed in
  lines radiating from the centre. The walls and basis adhere together
  very firmly.

  _Mouth_: labrum with six teeth: mandibles with five teeth; the three
  upper teeth being sharp, narrow, and unusually prominent; the two
  lower teeth minute and sharp; maxillæ without a notch. _Cirri_ much
  injured: first pair with one ramus apparently one third longer than
  the other: segments not very protuberant: the posterior cirri have
  elongated segments with five pairs of spines.

  _Affinities._--This species is very distinct from all the foregoing:
  in the carino-lateral compartments being so narrow, and tending, as
  we may suppose, to become aborted; in the form and structure of the
  whole shell, and in its habits, this species shows an affinity and
  passage to the coral-inhabiting sub-genus Creusia, which has only
  four compartments. There is also a close affinity to the sub-genus
  Acasta. This species is so closely allied to the following, that
  I at one time felt some doubts whether they ought to have been
  specifically separated; it is also probably closely allied to _B.
  terebratus_, but the materials hardly suffice for judgment: it is
  also related, though less obviously, to _B. vestitus_.



35. BALANUS CEPA. Pl. 7 fig. 8 _a_-8 _c_.

_Shell dirty reddish-purple, steeply conical: radii narrow: basis
obscurely porose. Scutum with the lines of growth crenated: tergum with
the spur truncated, broad as half the valve, and depending beneath the
basi-scutal angle as much as half its own breadth._

  _Hab._--Japan, attached to an Isis, Mus. Cuming. Attached to an
  oyster, Mus. Stutchbury.


As already stated, this species comes in all essential respects very
near to the last, though differing much in appearance; I have seen
two sets of specimens, and two sets of _B. allium_, and there was no
variability or passage in the points in which they differed; hence I
must consider them as specifically distinct.


  _Shell_, steeply conical, strongly but bluntly ribbed longitudinally;
  coloured either all over dull reddish purple, or with the upper part
  only pinkish purple: in one set of specimens, the yellow epidermis
  was partially persistent. Radii narrow. Orifice small, ovate. The
  wall of the carino-lateral compartment is very narrow. The internal
  surface of the parietes is ribbed, but finely, and only in the lower
  part. The septa, on the sutural edges of the radii, are finer than in
  _B. allium_. Basis flat, obscurely permeated by pores. The largest
  specimen is .25 of an inch in basal diameter.

  _Scuta_: these are longitudinally and finely striated; the
  basi-tergal corner is more rounded off than in _B. allium_, and the
  articular ridge is not nearly so prominent: internally, the adductor
  ridge is rather more prominent. The _Tergum_ is rather broader: its
  apex is produced into a minute sharp point: the scutal margin is
  straight; the spur is broader, and measured from the basi-scutal
  angle of the valve, considerably longer; namely, as long as half
  the width of the basal margin of the spur, whereas in _B. allium_
  it is only about a quarter as long as the basal margin of the spur:
  the lower edge of the spur is not here so directly transverse to
  the longitudinal axis of the valve as in _B. allium_: the external
  surface is not so flat as in that species, and a depression runs down
  to the basi-scutal angle of the spur.


Considering the difference in the shape and appearance of the shell,
with its narrow radii and small orifice; considering the less strongly
ribbed internal lamina of the parietes, the finer septa on the sutural
edges of the radii, the slight difference in outline in the scuta and
terga, more especially the greater length of the spur, I conceive I am
right in ranking this form as a distinct species, though assuredly it
is very closely allied to _B. allium_, and even still closer to the
following _B. quadrivittatus_.



36. BALANUS QUADRIVITTATUS. Pl. 8, fig. 1.

_Shell steeply conical, having four longitudinal gray bands placed
crosswise: radii with their summits oblique: basis thin, solid. Scutum,
with the lines of growth smooth; no distinct pit for the lateral
depressor muscle: tergum as in B. cepa._

  _Hab._--East Indian Archipelago, attached to lamelliferous corals,
  and associated with _Pyrgoma grande_ and _Creusia spinulosa_, Mus.
  Brit. and Stutchbury and Darwin. Philippine Archipelago, attached to
  a Tetraclita, Mus. Cuming.


I have seen four sets of specimens of this species, taken in four
different places, one set containing above twenty individuals, and
all resembled each other exactly: nevertheless, this species comes so
close to _B. cepa_, that I am somewhat doubtful about its specific
distinctness.


  _General Appearance._--Shell smooth, or slightly folded, steeply
  conical; white, with four longitudinal bands of pale brownish-gray
  colour, namely, on the rostrum, the carina, and the two lateral
  compartments: the carino-lateral compartments are very narrow and
  almost white: the four brownish-gray bands are darkest in the upper
  part of the shell, though always rather faint, and die out towards
  the base: they can sometimes be seen to be formed of several narrow
  longitudinal stripes; the tint shows a trace of containing purple.
  The orifice of the shell is small, rhomboidal, and not quite entire,
  owing to the obliquity of the summits of the moderately broad radii.
  In structure, the shell, radii, and alæ resemble those in the last
  species. The basis, however, does not appear to be permeated by
  pores. Basal diameter of largest specimen .25 of an inch.

  The _Scuta_ most closely resemble those of _B. cepa_, but the lines
  of growth are not crenated, and internally there is only a very
  minute pit for the lateral depressor muscle, placed almost on the
  edge itself of the valve. The _Tergum_ hardly differs at all from
  that of _B. cepa_, but is perhaps of rather greater breadth.

  The _Mouth_ does not differ from that in the last two species. In the
  _Cirri_, the three posterior pairs have elongated segments, bearing
  only three pairs of spines, of which the lowest pair is minute: in
  _B. allium_, and I believe in _B. cepa_, there are five pairs of
  spines on each segment.


This species differs from the last only in the peculiar colouring,
smoother walls, more oblique radii, solid basis, and more especially
in the scuta having the lines of growth not crenated, and internally,
in the pit for the lateral depressor muscle being so very minute and
placed on the basi-tergal edge of the valve. The posterior cirri,
also, I believe, differ in the number of the spines which the segments
support; nevertheless, I cannot feel confident about the specific
distinctness of _B. quadrivittatus_.



37. BALANUS TEREBRATUS. Pl. 8, fig. 2 _a_-2 _b_.

_Shell white, strongly ribbed longitudinally, with the basal margin
produced into long points: basis concave, not permeated by pores, but
strongly ribbed externally in radiating lines; the interspaces between
the ribs being riddled by minute rounded apertures, often placed in
double rows._

  _Hab._--Unknown, Brit. Mus., attached to a lamelliferous coral.


I have in this instance broken through my rule of not describing a
Cirripede without examining the opercular valves; but the species here
named is so peculiar, that it would have been a fault to have passed it
over. There is but a single specimen in the British Museum, without, as
just stated, the operculum, and of course without the animal's body.


  _Shell_, white, depressed, conical, somewhat elongated in its
  rostro-carinal axis; orifice rather small, pentagonal, toothed,
  elongated. Parietes rather thin, with extremely prominent
  longitudinal ribs, produced at the basal edge into long spikes: the
  internal surface is also ribbed, but less strongly than the outside.
  Radii rather narrow, with oblique, not smooth summits; sutural edges
  very finely and obscurely crenated. Alæ with their summits extremely
  oblique. Lower edge of sheath closely attached to the walls. The
  carino-lateral compartments are rather narrow.

  _Basis_, slightly concave or saucer-shaped; the circumference
  is produced into long spikes, corresponding with those on the
  basal margin of the parietes: these projections equal half the
  semi-diameter of the shell. The internal surface of the basis has
  slightly prominent, rounded ridges; and the external surface has
  extraordinarily prominent, sharp ridges, radiating from the centre;
  the edges of the external ridges are irregular, notched, and knobbed.
  I have seen in no other species external ridges on the basis or
  surface of attachment; and what is more remarkable, the interspaces
  between the ridges are penetrated by small rounded apertures, of
  irregular shape and unequal sizes; and these are generally arranged
  in an irregular double row, and externally are closed by the
  membrane, which clothes the basis. In the sub-genus Acasta, the basal
  cup is sometimes penetrated by similar holes, but these seem never
  to extend over the whole basis, and are very variable; nevertheless,
  in some specimens of _Acasta spongites_ from the Cape of Good Hope,
  portions of the basis closely resembled, except in the absence of
  the radiating ridges, the structure here described, but the holes
  were not arranged in any definite order. The internal surface of
  the parietes in _Acasta sporillus_ presents a somewhat analogous
  appearance, but the pits do not penetrate through the walls. This
  species, I have no doubt, is closely allied to the sub-genus Acasta,
  and to _Balanus navicula_ with its allies, and, but much less
  closely, to _B. allium_ with its allies. Indeed, had _B. terebratus_
  inhabited a sponge, I should have been compelled to have ranked it in
  the sub-genus Acasta.



38. BALANUS VESTITUS. Pl. 8, fig. 3 _a_-3 _b_.

_Shell pinkish-purple or white, clothed by an orange-coloured membrane;
radii represented by mere fissures; basis solid. Scutum, with a sharp,
curved adductor ridge; with crests for the lateral depressor muscle:
tergum, with the spur short, truncated, one third of width of valve._

  _Hab._--New Zealand, New South Wales, Mus. Brit. and Stutchbury;
  attached to shells.


  _General Appearance._--Shape conical, often steeply conical; orifice
  small; radii not developed, represented by mere fissures. The walls
  are smooth, or slightly, or strongly ribbed longitudinally. The shell
  itself is of a fine peach-blossom pink, or nearly white, but it is
  generally covered by a thick yellow or brownish-orange epidermis.
  Opercular valves pinkish, but similarly covered. Basal diameter of
  largest specimen .7 of an inch.

  _Scuta_, with the lines of growth closely approximate; surface
  somewhat convex. Internally, the articular ridge is very little
  prominent, but runs far down the tergal margin; in some specimens,
  however, it is shorter and more prominent. The adductor ridge is
  strongly prominent, is curved towards the rostral angle, and runs
  down nearly to the basal margin. The rostral depressor muscle is
  lodged in a small cavity, formed, as usual, by the overlapping of
  the occludent margin; within this cavity there are either tolerably
  distinct little crests, or merely traces of them, for the attachment
  of the muscle. The lateral depressor muscle is attached to several
  quite distinct crests, seated in a concavity beneath the adductor
  ridge. _Tergum_ rather narrow, with the apex produced or beaked; the
  beak is purplish and flat. There is a slight rounded longitudinal
  furrow, or depression. The spur is fully one third of the width of
  the valve: it is short, with the end truncated, and placed close to
  the basi-scutal angle; the basal margin on the carinal side slopes
  gently towards the spur. Internally, the scutal margin is scarcely at
  all inflected, and the articular ridge is very little prominent: the
  crests for the tergal depressores are pretty well developed.

  From the points here enumerated, it is clear that the opercular
  valves are articulated together much less strongly than is usual with
  most species, excepting _B. allium_ and its allies. It is remarkable
  that in this species the terga are united to the sheath, not, as is
  usual, by a single opercular membrane, but by five or six, one above
  the other, the upper membranes not having been exuviated as each new
  lower one was formed. The minute spines on the membrane lining the
  sheath are rather larger and more numerous than is usual, and to the
  base of each spine a tubulus of unusual diameter runs, imbedded in
  the shell.

  The _Walls_, internally, have unusually numerous, narrow,
  approximate, strongly prominent, longitudinal ribs, denticulated at
  their bases, and inserted into the furrows on the borders of the
  basis: in old specimens these internal ribs die out in the upper
  part of the walls. The _Radii_ are not developed in any of the many
  specimens which I have seen, and the edges of the compartments on
  both sides of each suture are equally marked by slight, irregular
  ridges or knobs, answering to the septa and their recipient furrows,
  in the species with ordinarily developed radii. There is very little
  diametric growth, the orifice being gradually enlarged by the
  disintegration of the upper ends of the walls; the _alæ_, however,
  in some specimens, do grow a little along their lateral or sutural
  edges, so that some little diametric growth must be effected. The
  summits of the alæ are very oblique; their sutural edges are plainly
  crenated. The sheath descends about half way down the walls. The
  _Basis_ is flat, not permeated by pores, but deeply furrowed in lines
  radiating from the centre.

  _Mouth_: labrum sometimes with no teeth, sometimes with four minute
  teeth; mandibles with four teeth, of which the third is blunt and
  rather large; the fourth is a mere knob. Maxillæ; there is, as usual,
  an upper pair of large spines (beneath which there is sometimes
  a small notch), but all the lower spines, instead of standing as
  usual in pairs, form a single row. _Cirri_; first pair with the rami
  remarkably unequal in length, one ramus having twenty-two segments,
  and being more than twice the length of the other, having only nine
  segments: these segments, and likewise those of the second and third
  pairs, have an inverted conical shape; and they are all less thickly
  clothed with spines than is usual. The second pair is short, about as
  long as the shorter ramus of the first pair, and has ten or twelve
  segments. The third pair is above twice as long as the second pair,
  and contains twenty-four segments: this very unusual length is owing
  to the presence of numerous thin tapering upper segments, unlike
  those generally found in _Balanus_, in the third pair of cirri, and
  apparently serving as feelers. These upper tapering segments are
  of an inverted conical shape, and support on their upper margins
  two very small tufts of spines, one behind and one in front: on the
  segments lower down these tufts increase in size, and the spines are
  more spread out, so that in the basal segments, the tufts in front
  form on the upper margin two or three crowded rows of bristles. The
  three posterior pairs of cirri have elongated segments, which bear
  on their upper half three pairs of spines; of these the lowest pair
  is minute, and the middle pair is only one third of the length of
  the upper pair. The sixth cirrus, in the same individual as before,
  contained twenty-seven segments in each ramus, that is only three
  more than in the third cirrus! I must observe, that the cirri in
  all the specimens were irregular, often distorted and monstrous;
  and therefore, probably, there is considerable variation in the
  proportional numbers of the segments in the cirri.

  At the base of the penis there is a minute, knife-edged, triangular
  projection. The branchiæ are rather narrow, pointed, and not very
  large.

  _Affinities._--This is a very distinct species, as shown by the
  peculiarities in the cirri, by the absence of radii to the shell,
  and by the presence of crests for the attachment of the lateral
  scutal depressores. With the exception of this latter character, the
  opercular valves clearly show, that _B. vestitus_ is allied to _B.
  allium_, _cepa_, and _quadrivittatus_. In some respects this species
  manifests an affinity to _B. imperator_, which latter has its third
  pair of cirri nearly similar to those of _B. vestitus_.



39. BALANUS IMPERATOR. Pl. 8, fig. 4 _a_-4 _c_.

_Shell internally imperial purple; parietes thick, with their internal
basal edges rough with irregular points and ridges; radii narrow; basis
very thin, solid. Scutum, with crests for the rostral and lateral
depressor muscles; tergum, with the end of spur rounded._

  _Hab._--New South Wales, Sydney, Port Stephens, Moreton Bay; attached
  to sandstone-rocks and shells, at low-water line; Mus. Brit., College
  of Surgeons, Cuming, Stutchbury.


  _Shell_ conical, very thick and very strong; longitudinally sulcated
  more or less strongly; whole thickness of shell beautifully coloured
  rich violet, or more strictly "imperial purple;" externally the
  surface, from disintegration, is generally whitish; internally
  the colour is best developed: the narrow radii and the thin basis
  are white. The largest specimen which I have seen was one and
  three-quarters of an inch in basal diameter, the walls close to the
  basis being, in this instance, actually .3 of an inch in thickness.

  _Operculum_ thick and strong, covered by yellowish-brown epidermis;
  internally, the shelly substance is either all of the richest
  purple or yellowish-white, tinged, especially in the upper part,
  with purple. _Scuta_, with the apex beaked and somewhat reflexed;
  articular ridge very thick, little prominent; articular furrow very
  narrow, the impression made by the adductor muscle is seated very
  high up the valve; there is hardly an adductor ridge, but the surface
  of the valve is angularly prominent in a curved line, running from
  the articular ridge to near the rostral angle of the valve. At the
  rostral angle, the occludent margin is not folded inwards, as is
  generally the case, but the surface is flat, and is marked by four
  or five crests for the attachment of the rostral depressor muscle.
  There are other crests for the lateral depressor muscle. _Tergum_,
  with the apex somewhat produced and beaked, but blunt; longitudinal
  furrow shallow; spur of moderate breadth, with its lower end rounded;
  the basal margin on the carinal side of the spur slopes towards it.
  Internally, articular ridge moderately prominent. Crests for the
  tergal depressor well developed.

  _Parietes_, solid, thick, with the basal internal edge (4 _c_) formed
  of short ridges, or flattened and irregular points, which in very old
  specimens are but little prominent; inner surface, finely, closely,
  and irregularly ribbed longitudinally, but in some specimens nearly
  smooth. The _radii_ are nearly white; they are narrow, sometimes
  hardly at all developed, and have their summits very oblique and
  jagged; exteriorly, they are sulcated in a transverse direction, and
  sometimes form oblique steps, from having been formed layer over
  layer: their sutural edges are formed of irregularly branching crests
  or septa. The _alæ_ have their summits very oblique; their sutural
  edges are thick and crenated: the part added during diametric growth
  on the inner surface is smooth, and has a different appearance from
  the transversely ribbed portions of the sheath, of which the alæ
  form a portion. The lower edge of the sheath is hollow beneath. The
  carino-lateral compartments are very narrow.

  _Basis_ calcareous, thin, white, sometimes opalescent, apparently
  formed by an aggregation of very minute calcareous beads, with no
  trace of furrows radiating from the centre.

  _Mouth_: labrum hairy, with apparently some very minute teeth;
  mandibles, with the fourth and fifth teeth small and rudimentary;
  maxillæ rather broad, with a narrow and rather deep notch under the
  two great upper spines: outer maxillæ with the lower lobe very large.

  _Cirri_: first pair, with the rami unequal by several segments:
  second pair, with the rami unequal in length by about six segments:
  third pair elongated, with the segments very numerous, almost
  equalling those in the sixth cirrus; upper segments of both rami much
  elongated, each with only a circle of spines; segments in the above
  first three pairs of cirri only slightly protuberant. Posterior cirri
  elongated, with the upper segments bearing three pairs, and the lower
  segments four pairs of main spines, between which there is a small
  intermediate tuft.

  _Affinities._--This noble Balanus, in all the characters derived
  from its opercular valves, and from its cirri, is closely allied to
  the last species: in the structure, however, of the shell and of
  the basis, it comes closer to the following, _B. flosculus_. The
  crests on the under side of the scutum, for the lateral depressores,
  are confined to these three species; and the crests for the rostral
  depressores are confined to _B. imperator_ and _vestitus_, but they
  are generally rudimentary in the latter. The internal basal structure
  of the parietes is singularly like that of _Chelonobia caretta_,
  though there is no other special affinity to that genus. In the
  nature of basis; in the structure, to a certain limited extent, of
  the walls of the shell; in the narrowness of the carino-lateral
  compartments; in the elongation of the third pair of cirri; in the
  crests for the rostral and lateral scutal depressores, _B. imperator_
  comes nearer to the genus _Tetraclita_ than does any other species of
  Balanus.



40. BALANUS FLOSCULUS. Pl. 8, fig. 5 _a_-5 _f_.

_Shell purple or dirty white, with the internal basal edges of the
parietes rough with irregular points and ridges; radii narrow or
absent; basis excessively thin, in appearance absent. Scutum with
crests for the lateral depressor muscle; tergum very narrow, with the
spur pointed._

  _Var._ sordidus. (Pl. 8, fig. 5 _b_) _shell globulo-conical, dirty
  white, with numerous sharp, narrow, longitudinal folds or ridges_.

  _Hab._--Peru and Chile; generally attached to the _Concholepas
  Peruviana_, or to _Balanus psittacus_, and associated with
  _Chthamalus scabrosus_. _Var. sordidus_, inhabits Tierra del Fuego,
  attached to littoral shells, wood, and rock, associated with _Ch.
  scabrosus_.


  _General Appearance._--Shell either extremely much depressed and
  irregular, or globulo-conical, or more rarely cylindrical and
  elongated. Walls, either with a few rather broad, smooth, irregular,
  longitudinal folds, or, in _var. sordidus_, with numerous sharper
  and more prominent longitudinal ridges; basal margin very sinuous.
  Colour, either fine rich peach-blossom purple, or so pale as to be
  almost white; or in _var. sordidus_ dirty white, generally stained
  greenish from confervoid matter. Orifice small, oval, entire. Radii
  very narrow, white, or not at all developed, and with even the
  sutures not distinguishable. The purple coloured varieties, with
  the narrow white radii, the small oval orifice, and folded walls,
  have a very pretty appearance, which is far from the case with _var.
  sordidus_. The largest specimens attained a basal diameter of .6
  of an inch, but this is an unusual size: I have seen a cylindrical
  specimen of _var. sordidus_ one inch in length.

  The opercular valves are united to the sheath by unusually strong
  membrane: internally, their upper parts are stained purple. _Scuta_,
  these vary considerably in breadth, some being even broader than in
  Pl. 8, fig. 5 _c_, and others as much elongated as in fig. 5 _d_:
  these latter come from an elongated cylindrical shell. The valve
  externally is unusually convex: the apex, also, projects freely to
  an unusual degree. Internally, the articular ridge is moderately
  prominent: the adductor ridge is prominent and much curved: in the
  upper part it either lies close to, or at some little distance from
  the articular ridge. The lateral depressores are attached to several
  little crests, occupying a cavity, (often bordered above by a little
  ridge) close beneath the adductor ridge. _Tergum_, extraordinarily
  narrow and elongated; beak triangular, purple: a longitudinal furrow
  runs down the valve: spur narrow, long, bluntly pointed, lying close
  to the basi-scutal angle of the valve: the scutal margin is nearly
  straight and parallel to the spur. Internally, the articular ridge
  is prominent: the crests for the depressores are moderately well
  developed: the upper part of the valve is marked by a purple patch,
  bounded on one side by the articular ridge, and on the other side
  by a very slight special ridge. There is some variability in the
  narrowness of the whole valve, and in the sharpness of the spur.

  _Parietes._--The under surface, in the more depressed varieties, is
  roughened in a remarkable manner nearly or quite up to the sheath,
  with very irregular, projecting, and branching ridges, and sometimes
  with depending points. These ridges and points are granulated on
  their under surfaces. The roughened surface in the more conical
  varieties is confined to the basal inner margin. This structure is
  nearly the same as that in _B. imperator_, represented (Pl. 8, fig.
  4 _c_), but the little ridges are here more apt to be concentric,
  instead of radiating. The lateral edges of the compartments on the
  inside, especially the carinal edges, project inwards beyond the
  inner surface of the shell. The sheath is but little hollow on its
  under side. The diametric growth of the shell seems to be quite
  capricious; in the same group, some individuals thus increasing, and
  others not at all. When the radii are developed they are narrow and
  white, with their upper margins nearly if not quite parallel to the
  basis: their sutural edges are formed by large, irregular, branching
  teeth or septa. The _alæ_, also, have their sutural edges coarsely
  crenated; and when the shell increases by diametric growth, they are
  added to above the opercular membrane, so as to be nearly square at
  top.

  _Basis._--When a shell is removed from the surface of attachment,
  and inspected even under a lens, there appears to be no basis
  whatever, either adhering to the shell, or to the supporting surface:
  but when a more careful examination is made, with a higher power,
  an excessively thin, translucent, calcareous, irregular layer, or
  rather film, can almost always be discovered. This would be more
  easily distinguished if the specimens had adhered to rock instead of
  to the rugged shells of molluscs. At one time I thought the basis
  was partially membranous, for I have certainly scraped off small
  fragments of membrane from the supporting surface; but these, when
  examined under the compound microscope, seemed always to consist of a
  thin sheet of the yellow cementing tissue; and in some instances, a
  brittle film of shell, representing the true basis of the cirripede,
  still adhered to the upper surface of the membranous bits of cement.
  Nevertheless, so imperfect is the calcareous basis, that I should not
  be surprised if portions of a true membranous basis did really in
  some cases exist.

  _Mouth_: labrum with the notch wide, generally with a few little
  teeth; mandibles with three teeth, and some inferior knobs; maxillæ
  notched. _Cirri_, first pair with one ramus shorter by three segments
  than the other ramus. Second and third pairs short, of nearly
  equal length, thickly clothed with spines; segments very little
  protuberant. Posterior cirri, having elongated segments, supporting
  seven pairs of spines.

  _Var. sordidus._--This form is very common on the tidal shores of
  the Strait of Magellan, and of the southernmost parts of Tierra del
  Fuego, near Cape Horn: it lives attached to rocks, mytili, and logs
  of wood, and is associated with _Chthamalus scabrosus_. It almost
  certainly is the most antarctic form of the genus Balanus. If I were
  guided by external appearance alone, I should certainly separate
  this form specifically from _B. flosculus_, but, as will be seen in
  the following description, the differences consist only in _var.
  sordidus_ being much duller and rather differently coloured, in the
  longitudinal folds being sharper and more prominent, and in the whole
  shell being rather more globular, and on an average rather larger;
  but in the true _B. flosculus_ there is considerable variation in
  all these respects, as there likewise is in _var. sordidus_; thus
  some of the cylindrical varieties of the latter have less prominent
  ridges than even _B. flosculus_. In general appearance I have seen
  some _nearly_, but not exactly, intermediate forms; therefore, I do
  not feel positive that these forms may not be specifically distinct,
  but have failed, after careful examination, to find any sufficient
  diagnostic _characters_. Moreover, in the case of _Balanus lævis_,
  I was led to believe that there is an equal and somewhat analogous
  amount of variation in the specimens inhabiting Tierra del Fuego and
  northern Chile; and in this case I was enabled to show the existence
  of strictly intermediate forms in the intermediate districts.

  The shell in _var. sordidus_ is generally globulo-conical, dirty
  white, frequently with a green tinge, from the growth of confervoid
  matter. Orifice small. The exterior surface is covered with numerous
  prominent, narrow, sharp ribs or folds, the basal margin being
  serrated with projecting points where the folds terminate. When
  the radii are not developed, the sutures are very often obscure.
  Internally, the shell is faintly tinted of a port-wine purple. In
  all points of structure this form is identical with the true _B.
  flosculus_. In some few specimens the whole exterior surface was
  disintegrated and smooth; and this is generally the case with the
  upper parts of the shell. Some other specimens, which had grown
  crowded together on wood, had become cylindrical, and consequently
  the orifice was as large in diameter as the shell, namely, half an
  inch: in some of these cylindrical varieties the sheath was entirely
  soldered to the walls. The largest specimens which I have seen were
  .6 of an inch in diameter; and above one inch in height.

  _Affinities._--This species, in its opercular valves, even in such
  trifling characters as the strength of the opercular membrane,
  and in its cirri, approaches closely to _B. cariosus_. We even
  see on the under side of the scutum, in that species, a single
  little ridge, foreshadowing, as it were, the crests for the lateral
  scutal depressores, so remarkable in our present species. In the
  structure of the shell and of the basis, _B. flosculus_ is much
  more closely related to the last species, or _B. imperator_. If it
  had been possible to have arranged the species in a single line,
  _B. flosculus_ ought undoubtedly to have been placed between _B.
  cariosus_ and _imperator_.



41. BALANUS BISULCATUS. Pl. 8, fig. 6 _a_-6 _c_.

  BALANUS SULCATINUS (?) _Nyst_, apud D'Omalius (sine descript. aut
        tabulâ), Geologie de Belgique, 1853.[100]

    [100] I am indebted to M. Bosquet for a specimen, bearing this
    name and reference, found in the 'Systeme Bolderien' of Dumont,
    (miocene according to Sir C. Lyell) at Bolderberg. The specimen
    consists of a rostrum, with a portion of the base attached; and as
    these parts are in some degree characteristic, I fully believe this
    specimen to be the _B. bisulcatus_. I hope hereafter to give in
    the Palæontographical Series fuller illustrations of this and the
    following fossil species.

_Radii with their upper margins oblique and smooth; sutural edges
smooth: basis permeated by large pores. Scutum narrow, with from two to
four longitudinal furrows: tergum with the spur very short, broad as
half the valve._

  _Var._ plicatus, _with the walls deeply folded; radii narrow, with
  their upper margins very oblique_.

  _Fossil_ in Coralline Crag, Ramsholt, Gedgrave, Sutton; Mus. S. Wood,
  Bowerbank, J. de C. Sowerby. Rauville, dans le Cotentin, Mus. G.
  B. Sowerby. _Var. plicatus_, Coralline Crag, Sutton, Mus. S. Wood.
  Bolderberg, near Hasselt, Belgium, Mus. Bosquet.


  _General Appearance._--Shell conical or tubulo-conical, often rather
  globose; walls frequently thin, either very smooth, or deeply
  plicated longitudinally: occasionally the same specimen is smooth
  in the upper part, and strongly plicated in the lower. The Radii in
  the larger specimens are wide, and with their upper margins only
  slightly oblique; in the smaller, they are narrower and much more
  oblique, but in each case their upper margins are smooth and slightly
  bowed. Colour apparently originally nearly white, but with the alæ
  generally, in the smaller specimens, clouded with a dark tint:
  the radii are usually striped feebly in longitudinal lines. Basal
  diameter of largest specimen .8 of an inch; but this seems to have
  been an unusual size.

  _Scuta_: narrow, with the basal margin forming an unusually small
  angle with the occludent margin; surface slightly convex, with lines
  of growth approximate, moderately prominent; on the tergal half
  of the valve, two distinct rather broad furrows, with sometimes a
  third, and even a fourth, nearer to the occludent margin (Pl. 8,
  fig. 6 _a_), extend from the apex down the valve, and give it a very
  peculiar appearance: the furrows near the tergal margin are the
  deepest. Internally, the upper part of the valve is roughened with
  small points: the articular furrow is unusually wide: the articular
  ridge is very prominent and but little reflexed, with the lower end
  almost abruptly cut off: the adductor ridge is prominent, but short:
  there are small deepish pits for the rostral and lateral depressores.

  _Terga_ (6 _b_), broad, flat, with a slight narrow prominent rim
  along the scutal margin, which margin is slightly bowed. The basal
  margin on the carinal side of the spur slopes so gradually towards
  the spur, that the latter is barely distinct, and is very short, not
  depending nearly half its own width beneath the basi-scutal angle: it
  is broad, namely, measured across the upper part, as broad as half
  the valve; its basal end is obliquely rounded off on the carinal
  side; it is placed close to the basi-scutal angle. The carinal
  margin of the valve is just perceptibly bowed, and is formed by
  rectangularly upturned lines of growth. Internally, the upper part of
  the valve is rough; the articular ridge is prominent; the crests for
  the tergal depressor muscles are moderately well-developed.

  _Parietes_, not porose; internally, the ribs are smooth, with their
  basal edges very finely or barely denticulated. The _radii_ (as
  already stated) are of variable breadth; they have their upper
  margins either very slightly or highly oblique, but always smooth and
  rounded: their sutural edges are quite smooth, or sometimes, with
  a strong lens, traces of transverse striæ, representing septa, can
  just be discovered. The _alæ_ have their upper margins very oblique;
  their sutural edges are, in the large specimens, quite smooth; in the
  younger ones, plainly crenated; the recipient furrow being clearly
  marked with these teeth. _Basis_ plainly porose.

  _Varieties._--It is certain that there are longitudinally plicated
  specimens of this species, and that the obliquity of the upper
  margins of the radii also varies a little: nevertheless some of the
  deeply plicated specimens undoubtedly have a very different aspect
  from the ordinary varieties, and do really differ in the sutural
  edges of the alæ being crenated, and in the greater narrowness and
  obliquity of the radii; but these points are all commonly variable.
  I have not seen any large specimens of the variety, _plicatus_, so
  as to compare them with the large specimens of the normal form, yet
  I can hardly entertain any doubt, considering their agreement in
  so many important points, that I have rightly treated these forms
  as mere varieties; it is unfortunate that none of the specimens of
  the _var. plicatus_ seen by me have had opercular valves, as their
  presence would have removed all shadow of doubt.

  _Affinities_: this is a strongly characterised species, and nearly
  allied only to the following species, _B. dolosus_. The furrows on
  the scuta in some degree resemble those on _B. lævis_, but there is
  no alliance with that species. It is certain that amongst recent
  species, the chief affinity is with _B. Hameri_ and _amaryllis_.



42. BALANUS DOLOSUS. Pl. 8, fig. 7.

_Radii with their upper margins oblique and smooth; sutural edges
smooth: basis permeated by large pores. Tergum with the spur not very
short, broad as one third of valve._

  _Fossil_ in Red and Mammaliferous Crag, England; Mus. S. Wood,
  Bowerbank, Lyell, J. de C. Sowerby, Henslow, &c. Mammaliferous Crag,
  Postwick, near Norwich, Mus. Lyell.


This species so closely resembles _B. bisulcatus_, both externally and
in all the essential characters of the parietes, radii, and basis,
that it is quite superfluous to describe over again these parts. The
specific characters are derived from the opercular valves, which
present well defined distinctions, found by me constant in several
specimens of both species. _B. dolosus_, like _B. bisulcatus_, has
quite smooth and deeply plicated varieties, often adhering to the same
univalve. The ribs on the inner surfaces of the parietes are remarkably
prominent. I think the upper margins of the radii are in this species
rather more oblique than in _B. bisulcatus_. The sutural edges of the
radii are marked by the finest striæ, representing septa. The sutural
edges of the alæ are generally distinctly crenated. The basis is often
slightly cup-formed, and very plainly porose: its surface is marked by
radiating ridges. The orifice of the shell is large, and elongated,
especially in young specimens. The basal diameter of the largest
specimen is .4 of an inch.


  The _Scuta_ have no trace of the two or three longitudinal furrows so
  conspicuous on these valves in _B. bisulcatus_, and which, in that
  species, run down from the apex of the valve; this fact showing that
  the furrows occur in quite young individuals. The whole valve is not
  quite so narrow as in _B. bisulcatus_, but otherwise agrees with it
  in shape: internally, there is hardly any difference: the articular
  furrow is not so wide: the articular ridge is very prominent, and
  abruptly truncated at its lower end: the adductor ridge is also
  prominent; it here runs a little higher up the valve than in _B.
  bisulcatus_. The _Tergum_ differs more in the two species: the spur
  is not so broad; measured in its upper part, it is only about one
  third of the entire width of the valve, instead of being half as
  wide as the valve: it is considerably longer, depending beneath the
  basi-scutal angle more than half its own width: the basal margin of
  the valve on the carinal side, does not slope so gradually into the
  spur: the occludent and carinal margins are slightly arched, as in
  _B. bisulcatus_. Internally, the surface is rough, the articular
  ridge is prominent, and the crests for the tergal depressores are
  well developed,--all as in _B. bisulcatus_. It is remarkable, how
  generally the opercular valves have been preserved in this species
  in its fossil condition, as compared with most other species of the
  genus.


It is not easy to distinguish by external characters the rugged
varieties of this species from _B. crenatus_; indeed, the only
difference is that the furrows receiving the edges of the radii,
generally, exhibit in _B. crenatus_ a slight impression of the septa,
which are entirely absent in _B. dolosus_. By internal characters,
such as the non-porose parietes, and porose basis, our present species
widely differs from _B. crenatus_.



43. BALANUS UNGUIFORMIS. Pl. 8, fig. 8 _a_-8 _b_.

  BALANUS UNGUIFORMIS. _J. de C. Sowerby_ (!). Mineral Conchology (sine
        descriptione) Tab. 648, fig. 1, (Jan. 1846).

  ---- ERISMA. _J. de C. Sowerby_ (!). Ib., fig. 2.

  ---- PERPLEXUS. _Nyst_, apud D'Omalius (Sine descript. vel Tab.),
        Geologie de la Belgique, 1853.[101]

    [101] I am much indebted to M. Bosquet for specimens bearing this
    title, from Klein Spauwen, which certainly appear to me, as far as
    can be judged by the separated compartments, without the opercular
    valves, to belong to our present species.

_Parietes thin, sometimes permeated by pores; radii with their upper
margins oblique; sutural edges very finely crenated: basis solid.
Tergum with the spur narrow, bluntly pointed._

  _Var._ erisma, _with the walls longitudinally folded or ribbed_.

  _Fossil_ in the Eocene formation, Isle of Wight, Colwell Bay;
  Hordwell; Barton, (Chama Bed); Headon; Bembridge. Bergh, near Klein
  Spauwen, Belgium (?). Attached to various shells and wood. Mus. J. de
  C. Sowerby, E. Forbes, F. Edwards, Charlsworth, T. Wright, Bowerbank,
  Tennant, Bosquet.


This species, the most ancient one as yet well known in the genus,
presents to the systematist a most unfortunate peculiarity, in the
parietes being almost as often as not permeated by small pores: I have
seen no other instance, except to a limited degree in _B. glandula_, of
this character being variable, and hence it must be still considered
of high classificatory value, in so varying genus as Balanus. Owing
to this varying condition of the parietes, together with the basis
being quite solid, our present species has as good a claim to be
ranked in the last as in the present section; indeed, I think it has
more affinity to _B. crenatus_ and _glandula_ in the last section,
than to any other recent forms: I have placed it in its present place,
owing to its intimate affinity to _B. varians_, in which the parietes
seem always to be solid; and partly, I believe, because all the first
specimens examined by me exhibited no traces of parietal pores.
Owing to the kindness of Mr. F. Edwards, I have seen the original
specimens, excellently figured by Mr. J. de C. Sowerby in the 'Mineral
Conchology:' I can perceive no difference between _B. unguiformis_ and
_erisma_, excepting that the walls in the latter are longitudinally
folded,--a character we know to be variable in so many species. In both
varieties, the parietes are sometimes porose and sometimes solid. The
smaller specimens, however, figured in the 'Mineral Conchology' to the
right hand, may possibly be a distinct species, as I infer from the
narrowness of their radii.


  _General Appearance._--Shell, tubulo-conical, sometimes considerably
  elongated and sub-cylindrical: surface either very smooth, or
  slightly folded, or deeply folded so as to be strongly ribbed
  longitudinally: orifice rather large, rhomboidal, narrow at the
  carinal end, toothed, but not deeply: walls rather thin and fragile:
  radii of moderate width, with their summits oblique, not quite
  smooth. Basal diameter of largest specimen, about three quarters of
  an inch.

  _Scuta_, with the external surface smooth: there is a trace of a
  furrow running down the valve from the apex, near to the occludent
  margin, and this is only worth mentioning from the analogous furrows
  in _B. bisulcatus_. Internally, the upper surface of the valve is
  roughened: the articular ridge is very prominent, and slightly
  reflexed: there is no distinct adductor ridge; there is a slight but
  variable depression for the lateral depressor. _Tergum_, with the
  longitudinal furrow shallow; spur moderately long, about one fourth
  or one fifth of the width of the valve; placed at about its own width
  from the basi-scutal angle; basal end bluntly pointed; the basal
  margin on the opposite sides of the spur forms a nearly straight
  line; the carinal margin has an extremely narrow border formed by
  upturned lines of growth. Internally, the surface is roughened with
  little points: the articular ridge is prominent: the crests for the
  tergal depressores moderately prominent.

  _Parietes_: the longitudinal ribs on the internal surface are either
  feebly, or, in the lower part, strongly developed; their basal ends
  are only just perceptibly denticulated. As already stated, in about
  half the specimens, there were no traces of parietal pores; in the
  other half there were either distinct or obscure pores; the pores
  are circular, generally of unequal sizes, and never large; in the
  same individual they would sometimes be almost wholly absent in some
  of the compartments, and quite plain in the other compartments. The
  Radii are either moderately wide or rather narrow, and have their
  upper margins very oblique, and not distinctly arched, and not quite
  smooth: their sutural edges are very finely crenated, the teeth or
  septa not being denticulated. The upper margins of the _alæ_ are
  rather less oblique than those of the radii: their sutural edges are
  barely crenated. The _basis_ is thin, and without any trace of pores;
  the upper surface is sometimes furrowed in radiating lines.



44. BALANUS VARIANS. Pl. 8, fig. 9.

  B. VARIANS. _G. B. Sowerby_, in Darwin's Geolog. Observ. on South
        America, (Sept. 1846), Tab. 2, fig. 4, 5, 6.

_Parietes moderately thick: radii with their upper margins very
oblique; sutural edges almost smooth, or finely crenated: basis finely
porose. Tergum with the spur small, narrow, bluntly pointed._

  _Hab._--Port St. Julians, Patagonia; ancient Tertiary formation.
  Eastern plain of Tierra del Fuego (?).


This species comes so close to _B. unguiformis_, that I have some
doubt whether they ought to be specifically separated: the whole shell
is stronger, and the basis can be seen to be porose when a polished
section is made: the spur of the tergum is smaller, more pointed and
more medial, but these latter differences may be due to mere variation.
Should _B. varians_ and _unguiformis_ prove to be the same species, the
latter name has the priority.


  _General Appearance._--Shell moderately strong and thick; shape
  conical or tubular, or even inverted conical; orifice moderately
  toothed, large, sub-trigonal; walls either smooth or longitudinally
  folded; the elongated specimens are most apt to be smooth. The
  Radii are narrow and oblique. Diameter of largest specimen above
  three-quarters of an inch.

  _Scuta_, with the lines of growth moderately prominent; the internal
  surface of the valve has been ill preserved; but a very prominent,
  hardly reflexed, articular ridge, can be distinguished, as well
  as the absence of an adductor ridge. _Terga_, with no distinct
  longitudinal furrow running down the valve: spur short, bluntly
  pointed, narrow, about one fifth or one sixth of width of valve;
  placed at above its own with from the basi-scutal angle; the
  basal margin, on each side close to the spur, curves towards it.
  Internally, all that can be distinguished, is that the articular
  ridge is prominent.

  _Parietes_; their inner surfaces appear to have been nearly smooth;
  the absence of parietal pores could be made out only by polishing
  a transverse section. The Radii are narrow, and have their upper
  margins very oblique and rather smooth: in the elongated varieties
  the sutural edge appears to be almost absolutely smooth; in the
  conical specimens it is slightly crenated, the septa being apparently
  not denticulated. In living species we have a similar variation in
  the state of the sutural edges of the radii, in _B. balanoides_ and
  _crenatus_ the edges being much smoother in much elongated specimens
  than in other varieties. The _alæ_ have their upper margins less
  oblique than those of the radii, with their sutural edges barely
  crenated. The _basis_ is either flat, or, in the elongated specimens,
  deeply cup-formed; in section it can be seen to be finely and
  irregularly porose.



45. BALANUS INCLUSUS. Pl. 8, fig. 10 _a_-10 _c_.

_Shell reddish-brown: radii broad, with their upper margins not
oblique, or only moderately oblique; sutural edges with plainly
denticulated septa: basis porose. Scutum without an adductor ridge;
tergum with the spur rather narrow._

  _Var._ (_a_) (Pl. 8, fig. 10 _b_, 10 _c_), _with the shell elongated
  in its rostro-carinal axis; basis narrow, clasping the stem of a
  zoophyte; lateral compartments much broader than the almost linear
  rostrum, carina, and carino-lateral compartments_.

  _Var._ (_b_), _with rough longitudinally folded walls, and with the
  summits of the radii forming an angle of about 45° with the basis_.

  _Fossil_ in Coralline Crag, Sutton and Gedgrave; attached to
  foliaceous Bryozoa; Mus. S. Wood, Bowerbank. _Var. a_, Coralline
  Crag, Sutton, attached to cylindrical branches of corals; Mus. S.
  Wood. _Var. b_, attached to shells, Osnabruck, Hanover, Mus. Lyell;
  Bünde, Westphalia, Mus. Krantz.


My materials consist of a beautiful series of specimens in Messrs.
Wood and Bowerbank's collections; but unfortunately only a single
young specimen had its opercular valves preserved. Not one specimen
of the very curious variety (_a_) had opercular valves, yet I
cannot feel any doubt about its being only a variety, caused by its
attachment to a thin cylindrical branch of a coral, instead of to a
foliaceous Bryozoon; it will, however, be convenient to give a separate
description of this very remarkable form. With respect to var. (_b_),
both sets of specimens came to me with the name _B. stellaris_, of
Bronn; but as Bronn distinctly states, that in his species the parietes
are porose, and as such is not here the case, this cannot possibly be
that species: these specimens did not possess their opercular valves,
and therefore cannot be identified with certainty.


  _General Appearance._--Shell conical, with the orifice rather large,
  and rhomboidal. The surface is very smooth, except in var. (_b._)
  from the Continent, in which it is rugged and longitudinally folded.
  The colour is ochreous-brown (chiefly no doubt derived from the
  imbedding substance) tinged with red. The radii often have a much
  darker and more distinct red tint; they are sometimes longitudinally
  striped with dirty white. The radii are broad, with their summits
  straight, and very slightly oblique; in _var. b_, however, they slope
  at an angle of about 45°. Basal diameter of largest specimens .6 of
  an inch; but this is an unusual size.

  _Scuta_ (from a young individual), with the growth ridges little
  prominent. Internally the articular ridge is moderately prominent,
  with its lower end very obliquely rounded off; there is no adductor
  ridge; there is a minute pit for the lateral depressor muscle.
  _Terga_, with a slight longitudinal depression extending down to the
  spur: spur short, with its lower end almost square or truncated,
  about one fourth of width of valve, and placed at about half its own
  width from the basi-scutal angle. Internally, the articular ridge is
  prominent; the crests for the tergal depressores are feebly developed.

  _Parietes_, moderately thick and generally strongly ribbed
  internally, without parietal pores. _Radii_, wide, with their upper
  margins straight, not smooth or rounded, and very slightly (or, in
  _var. b_ moderately) oblique; their sutural edges have well-developed
  septa, which are denticulated: the interspaces between the septa
  are filled up solidly. The _alæ_ have their upper margins oblique:
  they are only slightly, and sometimes not at all, added to above the
  level of the opercular membrane: their sutural edges are smooth. The
  _basis_ is thin, but plainly porose.

  _Var._ (_a_).--With respect to this remarkable variety, any one
  would at first think it specifically distinct. The shell is much
  compressed, or elongated in the rostro-carinal axis, sometimes to
  a great degree; I have seen a specimen .25 of an inch in this axis
  and only .1 in its broadest part; but this is a very unusual degree
  of elongation. The most remarkable character is the extraordinary
  narrowness of the carina, the carino-lateral compartments, and of the
  rostrum, compared with the great breadth, especially along the basal
  margin (Pl. 8, fig. 10 _b_, 10 _c_), of the lateral compartments.
  The radii are of unusual breadth. The tips of the rostrum and of the
  lateral compartments are a little arched in, tending to make the
  shell somewhat globular. The true basis is extremely narrow (fig.
  10 _c_): it is deeply grooved, from clasping the thin, cylindrical
  stem of the coral to which it had adhered; and I have seen specimens
  in which the opposite edges of the groove had met, a tube having
  been thus actually formed. From the grooved basis, and from the
  elongation of the shell in the rostro-carinal axis, this species
  presents so close a general resemblance to _Balanus calceolus_, and
  its allies, that I have seen it in a collection arranged on the same
  tablet with a fossil specimen of _B. calceolus_. Notwithstanding
  the above several strongly-marked characters, by which this variety
  differs from the ordinary form, there is a resemblance in colour and
  aspect, which though difficult to be described, made me from the
  first suspect that the two were specifically identical. In no point
  of real structure is there any difference, excepting that, perhaps,
  the pores in the basis are here rather smaller; but this might arise
  from the little development of the peculiar basis. Having come to
  this conclusion, I was interested by finding a specimen in Mr. Wood's
  collection, which had originally fixed itself (judging from the form
  of the basis) on a cylindrical stem, but which had subsequently grown
  on to an adjoining flat surface; consequently, one side of the shell
  presented all the peculiar characters of the present variety, whereas
  the other side, at the rostral end, was undistinguishable from the
  ordinary form. The unequal development of the rostrum on the two
  sides was very striking, and clearly showed how great an effect could
  be produced by the nature of the surface of attachment.

  This singular variety cannot be considered accidental, in the sense
  in which this term may be applied to some varieties: the larva
  evidently fixes itself intentionally, in a certain definite position,
  on the branch of the coral (when a branch is chosen), exactly as in
  the case of _Balanus calceolus_, or _Scalpellum vulgare_. But when
  other species of Balani occasionally fix themselves on branched
  corals, their position seems to be accidental and unsymmetrical; thus
  among the symmetrically elongated specimens of the present species, I
  found one specimen of _Balanus bisulcatus_, which had evidently been
  attached in an almost transverse position to a branch, and had thus
  become much distorted; so, again, I have seen specimens of the recent
  _B. amaryllis_ attached irregularly to a Gorgonia, in the midst of
  the symmetrically elongated shells of _Balanus navicula_, an ally of
  _B. calceolus_.

  This variety does not seem to attain so large a size as the ordinary
  form.

  _Affinities._--This species is allied to the two last described
  fossils, namely, _B. varians_ and _unguiformis_, but is perhaps
  more nearly related to the recent _B. allium_, an inhabitant of
  the Barrier Reef of Australia. The longitudinally folded variety
  (_b_) can hardly be distinguished by external aspect, or even
  by the opercular valves, from _B. crenatus_; but when the shell
  is disarticulated, the porose walls and non-porose basis of _B.
  crenatus_, allow of no mistake in the diagnosis of the two species.



2. _Sub-Genus_--ACASTA. Pl. 9.

  ACASTA. _Leach._ Journal de Physique, tom. lxxxv, 1817.

_Compartments six; parietes and basis non-porose: basis calcareous,
cup-formed, not elongated, attached to sponges, or rarely to the bark
of Isis._

  Distribution mundane; imbedded in sponges and the sponge-like bark of
  Isis.


This sub-genus, in one sense, is a very natural one, inasmuch as all
the species are closely allied in essential structure, in general
appearance, and in habit. On the other hand, in the structure of the
shell, in all the characters derived from the opercular valves and
animal's body, Acasta cannot properly be distinguished generically from
some species of Balanus; thus _B. navicula_ and _cymbiformis_ agree
in the parietes and basis not being porose and in all other essential
respects, differing only in the shell being more elongated in the
rostro-carinal axis and in being attached to Gorgoniæ instead of to
sponges; yet we shall see that _Acasta purpurata_ lives imbedded in
the bark of Isis, so that even the habit of being imbedded in sponges
fails. _Balanus terebratus_ would have been ranked as an Acasta had it
inhabited sponges. On the other hand, some species of Balanus inhabit
sponges, as is often the case with _B. spongicola_, and always with
_B. declivis_: but both these species are distinguished easily from
Acasta, the former by its porose walls and basis, and the latter by
its membranous basis; it may, however, be reasonably doubted whether
such differences ought to be considered as even sub-generic. The most
important character of Acasta probably consists in the anterior ramus
of the fourth pair of cirri, differing slightly in the arrangement
of its spines, and in some other points, from the rami of the two
posterior pairs of cirri,--a character not as yet observed in any
other cirripede. Had not the genus Acasta been already founded and
extensively admitted, certainly I should not have formed it; but
considering the close similarity in habits, aspect, and structure, of
the nine species of Acasta, and considering the already large size of
the genus Balanus, I hope I may stand excused for admitting Acasta as a
sub-genus.

_General Appearance._--The shape varies from nearly globular to that
of a somewhat flattened acorn, the orifice being often a little
contracted from the inward curvature of the tips of the parietes.
In _A. spongites_, however, the orifice is generally widely open;
and, on the other hand, in _A. sporillus_, the orifice is reduced
to a mere pore. The usual tint is pale reddish, but _A. purpurata_
is purple, and _A. sporillus_ purplish-brown. The surface is either
smooth, or is shagreened with minute points, as in _A. sporillus_,
and _fenestrata_, and in some specimens of _A. sulcata_; and in all
the species, except _A. sporillus_ and _fenestrata_, many individuals
are furnished with elongated, curved, sharp, shelly points, like
those in _var. spinosus_ of _Balanus tintinnabulum_. The summits of
the radii, which are generally of moderate breadth, are more or less
oblique; their surface is often marked by lines parallel to the basis,
instead of by vertical lines corresponding with the lines of growth,
as in most species of Balanus. The carino-lateral compartments vary in
proportional breadth in the different species: in _A. sporillus_, they
tend to become rudimentary, and in this species (Pl. 9, fig. 9 _b_)
their basal margins, or rather points, do not reach down to the basis.
The species are all small, _A. glans_ and _undulata_ are the largest,
being sometimes .55 of an inch in basal diameter.

_Opercular Valves._--These differ in no generic respect from those
of Balanus. The _Scuta_ are striated longitudinally in several of
the species: the adductor ridge is barely developed in any, being
most prominent in _A. cyathus_. The articular ridge is prominent in
_A. fenestrata_ and _purpurata_. In the _Terga_, the spur is either
truncated and very broad, or moderately narrow and bluntly pointed:
the surface of the valve is often depressed, and in _A. spongites_
and _fenestrata_ it is furrowed in the line of the spur. The articular
ridge and furrow are well developed in _A. fenestrata_ and _purpurata_.
The crests for the tergal depressor muscles are either absent or very
feebly developed.

_Structure of the Parietes and Radii._--The parietes are not porose;
internally, they are either smooth, or slightly, or strongly ribbed in
longitudinal lines; the presence of these ribs, which are homologous
with the parietal septa in Balanus, is variable even in the same
species. In _A. sporillus_ the inner surface is curiously reticulated.
The sutural edges of the radii are either smooth, or very slightly
crenated by the septa, in lines parallel to the basis. The upper
margins of both radii and alæ are always more or less oblique. The
radii sometimes do not extend down to the basis; and in this case, as
will presently be described, apertures are left in the lower half,
between the compartments. In _A. glans_ and _lævigata_ the internal
margin of the wall of each compartment, from the sheath to the basis,
projects inwards, forming inside the shell as many double ridges (Pl.
9, fig. 5 _b_), as there are compartments, namely, six: a nearly
analogous structure occurs in certain species of Balanus. The basal
edge of the sheath, in most of the species (5 _b_, 9 _b_), depends
freely, and is hollow beneath, but this is always a variable point.

_Basis._--The base is either saucer or cup-shaped, but in _A. cyathus_
it is almost flat; it is generally symmetrical and smooth, with the
lines of growth closely approximate. In _A. fenestrata_ the basis is
commonly as deep, as the shell is high. The edge, in several of the
species, is crenated with minute teeth or notches; and these are so
large in some specimens of _A. sulcata_ and _cyathus_, as to make the
edge almost pectinated. In _A. glans_, and in a lesser degree in _A.
lævigata_, there are six knob-like teeth (fig. 5 _a_), corresponding
with the points of junction, between the basal edges of the inwardly
prominent margins of the six compartments, and the basis: in those
specimens, in which the six teeth are largely developed, six ridges
produced by their successive development, extend down towards the
centre of the basal cup. When the basal cup is dissolved in acid,
there is but a little animalised tissue and an external membrane,
formed as usual in slips, and furnished with blunt little external
points (apparently representing spines), each of which has a short
tubulus extending to the corium. Although I dissolved the basis of
three specimens, I could not distinctly make out any cement; nor did I
see any cement-ducts; yet these are readily distinguished, after the
dissolution of the basis in acid, in Balanus, Elminius and Tetraclita.
There can be no doubt that the young shell must at first be cemented
to a fibre of the sponge; but I suspect that the cementing-tissue is
not subsequently formed, owing to the support afforded by the growth
of the enveloping sponge. As some species of Balanus are habitually or
occasionally imbedded in sponges, it is important to observe, that the
species of Acasta are not only imbedded, but are attached to the fibres
of the sponge: but even this character, as we have already seen, is
not sufficient to distinguish the genus Acasta from Balanus, for _B.
declivis_ is attached exclusively to sponge.

_Perforations in the Shell._--Calcification seems often to fail to a
certain extent in this genus: the basal cup in most specimens of _A.
spongites_, and in some of _A. glans_ and other species, is irregularly
perforated by numerous minute orifices, closed only by the external
membrane, and filled up inside by pulpy corium. In some specimens of
_A. spongites_, from the Cape of Good Hope, parts of the basis were
riddled like a sieve. I have seen similar perforations in the parietes
of a few specimens of _A. glans_. In some specimens of _A. sulcata_,
the radii do not extend quite down to the basal edge of the walls
(Pl. 9, fig. 2 _a_), and in consequence a small cleft, closed only
by membrane, is left between the compartments, for a little space
above the basal cup. In _A. fenestrata_ (fig. 7 _a_), and in a lesser
degree in _A. purpurata_ (8 _a_), not only do the radii not extend
to the basal cup, but the parietes either on one or both sides of
each suture are hollowed out, so that six, large or small, elongated,
membrane-covered openings are formed, which extend from beneath the
sheath down to the basal cup. These openings, which I have not seen
in any other genus, will be more fully described under the respective
species.

_Mouth._--The parts of the mouth are identical in the several species,
and present no generic differences from those in Balanus. The outer
maxillæ and palpi appear unusually prominent: the labrum is deeply
notched, with no teeth, or very obscure teeth on each side. The
mandibles have five teeth, but the fifth is sometimes confluent with
the inferior angle. The maxillæ are not notched; and carry one or two
spines, near their inferior angle, nearly as large as the upper pair.
The outer maxillæ are bilobed.

_Cirri_: in the first pair, the rami are very unequal in length, the
one ramus being from half to one-third of the length of the other.
The segments in the second and third pairs, are not so much broader,
or so much more crowded with bristles, in comparison with the three
posterior pairs, as is the case with most species of Balanus. The
three posterior pairs, except in _A. purpurata_, are much elongated,
and the long thin segments bear four, and sometimes only three, pairs
of spines, which are generally doubly and finely serrated, or even
feathered. The most remarkable fact respecting the cirri, is, that in
_A. spongites_, _sulcata_, _cyathus_, and _glans_, the fourth pair,
instead of being identical in structure, as in all other genera, with
the fifth and sixth pairs, has, on its anterior ramus, the pairs of
spines more crowded together, with the little intermediate spines, and
those in the dorsal tufts, a little longer than in the sixth cirrus;
and between the pairs of spines, there occur some straight, upwardly
pointed, very minute, and very thick spines or teeth. And, what is
still more remarkable (as will hereafter be described in detail), in
certain specimens, but not in all, of _A. sulcata_, the front surfaces
of the lower segments on the anterior ramus, are developed into thick,
small, downwardly curved, hook-like teeth; this likewise is the case
with the upper segment of the pedicel (Pl. 29, fig. 2),--a most
elegant, mandible-like organ for the prehension of prey being thus
formed. The variability of such beautifully contrived teeth is very
surprising. Some similar teeth occur on the segments of the anterior
ramus of the fourth cirrus, but not on the pedicel, in _A. cyathus_. A
few teeth resembling the above, but thicker, occur on the segments of
the anterior ramus of the same cirrus, in _A. purpurata_.

_Branchiæ, &c._--In _A. spongites_, I found the branchiæ rather small,
with transverse plications. The muscles of the sack, which run to the
opercular valves, seemed rather feeble in most of the species. The
penis in several species was remarkably long, and in _A. spongites_ I
noticed the straight projecting point at its dorsal base, as is common
in Balanus.

_Affinities, &c._--At the commencement of the description of the genus,
I gave my reasons for keeping Acasta distinct as a sub-genus from
Balanus. The species are particularly troublesome to identify, not only
from the great variability of the most obvious characters, but from
the very close general external appearance of most of the species, and
the consequent necessity for cleaning and disarticulating at least one
specimen in every group. The shape, however, of the shell and basis,
and the state of their disarticulated edges, whether smooth, crenated,
or toothed, here offer more serviceable, though still very variable,
characters for the identification of the species, than is usual with
sessile cirripedes; and this, probably, is in part due to the almost
free or unattached condition of the whole shell, suspended, as it were,
in the midst of sponges, which they inhabit. The opercular valves, on
the other hand, are less serviceable than usual.

_Range, Habitats, &c._--The species are found all over the world,
excepting in the very cold latitudes.[102] _Acasta lævigata_ ranges
from the Red Sea to the Philippines; _A. spongites_ from the south
of England and Wales to the Cape of Good Hope; and _A. cyathus_ from
Madeira to the West Indies; most of the other species seem to have
rather confined ranges. The East Indian Archipelago seems to be
the metropolis of the genus, for here _A. lævigata_, _fenestrata_,
_purpurata_, and _sporillus_, are all found. Of these four species,
_A. purpurata_ lives imbedded, not in sponges, but in the sponge-like
bark of an Isis; and I think it probable, that _A. sporillus_ may
have somewhat analogous habits. The same species often lives imbedded
in different kinds of sponge; thus, I have seen _A. lævigata_ and
_cyathus_ in apparently three kinds, and _A. spongites_ in, as I
believe, four kinds. The genus existed during the miocene period, in
the Coralline Crag, under a form closely allied to _A. spongites_.

    [102] I am greatly indebted to Mr. Bowerbank for his great
    kindness, in looking over his immense collection of sponges from
    all parts of the world, and sending to me all the specimens of
    Acasta he could find imbedded in them.



1. ACASTA SPONGITES. Pl. 9, fig. 1 _a_-1 _d_.

  LEPAS SPONGITES. _Poli._ Testacea utriusque Siciliæ, 1795, Tab. 6,
        fig. 3-6.

  BALANUS SPONGEOSUS. _Montagu._ Test. Brit. Suppl., 1808.

  ---- SPONGITES. _De Blainville._ Dict. Sc. Nat., Pl. 116, fig. 3.

  LEPAS SPONGIOSA. _Wood's_ General Conchology, p. 47.

  ACASTA MONTAGUI. _Leach_, in Lamarck, Animaux sans Vertèbres, 1818,
        et in Encyclop. Brit., Supplement, 1824, vol. 3, Pl. 57.[103]

  ---- ---- _J. E. Gray._ Annals of Philosophy, New Series, vol. 10,
        Aug. 1825.

  ---- SPONGITES. _Philippi._ Enum. Mollus. Siciliæ, vol. 2, p. 211,
        1844.

  BALANUS MONTAGUI. _Brown's_ Illustrations of Conchology (2d edit.
        1844), Pl. 53, fig. 24-26.

    [103] As the plate to the Supplement to the 'Encyclop. Brit.' is
    marked as engraved in 1817, I presume Dr. Leach gave a proof to
    Lamarck, thus enabling him to publish this species four years
    before the Supplement itself appeared.

_Carino-lateral parietes about one sixth of width of lateral parietes:
inner surface of the parietes generally ribbed feebly: scutum with the
articular ridge abruptly cut off at its lower end: tergum with the spur
rounded-truncated, about one third of width of valve._

  _Hab._--South coast of England; South Wales, (Tenby); Portugal;
  Naples; Sicily; Cape of Good Hope.


This species and the three following, have caused me much doubt and
trouble. At first I took the view here adopted, namely, that they
were distinct: more careful examination made me run them altogether
under one name. Finally, after still further deliberation, and the
examination of a few additional specimens, I concluded there was the
least chance of error in classing them separately. I may here mention,
that in some sponge from the Cape of Good Hope, this species was
mingled with _Balanus spongicola_.


  _General Appearance._--The shape is usually that of a cup, the
  orifice being, in most cases, rather large, and deeply notched, owing
  to the great obliquity of the radii and alæ. The surface is generally
  smooth, but furnished with sharp calcareous projections. The colour
  is pinkish, and chiefly in the upper part of the shell; the lower
  part is often yellowish from the preserved epidermis. The parietes
  in the carino-lateral compartments are always narrow, being only one
  sixth or one seventh of the width of the parietes in the lateral
  compartments. The radii are not very wide, never equalling in width
  the parietes. The basis is moderately deep, and sometimes very deep,
  being even occasionally curled like a horn on one side. The specimens
  from Lisbon and Naples are a little larger than any British specimen
  which I have seen; the former being .3 of an inch in basal diameter.

  _Scuta_: these are striated longitudinally in close lines, generally
  plainly, but to a variable degree. The whole valve is flat, thin,
  and rather elongated, with barely a trace of an adductor ridge:
  the articular ridge is short and rather prominent: it terminates
  downwards abruptly, and this does not appear to be the case in the
  two following species. _Terga_: these are small compared with the
  scuta, they are slightly beaked: the spur is truncated but rounded,
  more especially on the carinal side; it is rather more than one third
  of the width of the whole valve. The articular ridge and crests for
  the depressor muscles are feebly developed.

  _Structure of the Parietes and Radii._--The state of the inner
  surface of the parietes varies much; generally they are slightly
  ribbed close to the basis, the ribs sometimes extending up to the
  sheath; rarely the surface is quite smooth. The edges of the radii
  are slightly crenated. The upper internal surfaces of the radii,
  where overlying the alæ, are usually marked by feeble undulating
  lines, nearly parallel to the basis. The alæ have very oblique
  summits.

  _Basis_: this is generally of a regular cup-like form, and about two
  thirds as deep as the shell is high; sometimes it is pointed at the
  bottom and distorted. The edge is feebly crenated, and rarely quite
  smooth. It is often penetrated by small rounded irregular holes; and
  I have seen specimens from the Cape of Good Hope with parts like a
  sieve.

  _Cirri_: in the first pair, the anterior ramus is nearly thrice as
  long as the posterior ramus. The second cirrus is short, with one
  ramus longer by three or four segments than the other ramus; the
  terminal segments are truncated. The third cirrus is about one third
  longer than the second cirrus. In the anterior ramus of the fourth
  pair, the regular pairs of spines are rather crowded together in the
  upper part of each segment, and the intermediate little spines and
  dorsal tufts are rather long in comparison with those of the fifth
  and sixth pairs of cirri: moreover, amongst the regular pairs, a few
  very minute and thick spines, pointing upwards, could be perceived.
  So that we here have the very unusual case of the fourth cirrus not
  exactly resembling the fifth and sixth pairs; and we shall see, in
  the following species, that this same anterior ramus of the fourth
  cirrus presents in addition another very extraordinary character. In
  the sixth cirrus there are on each segment four pairs of spines.



2. ACASTA SULCATA. Pl. 9, fig. 2 _a_-2 _d_.

  ACASTA SULCATA. _Lamarck._ Animaux sans Vertèbres, 1818.

  ---- ---- _Deshayes_, in Guerin, Magasin de Zoologie, 1831, Tab. 24.

_Carino-lateral parietes about one sixth of width of lateral parietes:
inner surface of the parietes generally ribbed strongly: basis with the
edge strongly crenated: orifice of shell rather small: tergum with the
spur generally truncated and nearly half as wide as valve._

  _Var._ (_a_) (fig. 2 _b_): _with the walls externally ribbed_.

  _Var._ (_b_) (fig. 2 _a_): _with small membrane-covered clefts
  between the edges of the walls, close above the basal cup_.

  _Hab._--Sydney, Port Fairy, Moreton Bay in lat. 27° S., New South
  Wales; Southern Australia; and, according to Lamarck, Western
  Australia, in lat. 25° S. Mus. Brit., Cuming, Bowerbank, &c.


I am almost ashamed to admit this species, so small are its differences
compared with _A. spongites_; yet I think that it probably is a
distinct form.


  In general appearance and character this species comes very near to
  _A. spongites_. As in the latter, the parietes of the carino-lateral
  compartments are narrow.[104] The orifice seems always to be smaller.
  Internally, the parietes are generally much more strongly ribbed,
  and the edges of the basal cup more plainly crenated. The sheath is
  generally coloured of a brighter pink, sometimes tinged with orange.
  The average largest specimens (from .3 to .5 of an inch in basal
  diameter) are a little larger than the largest European specimens:
  I have seen one specimen from Moreton Bay .4 in basal diameter, and
  from the basal cup being very deep, actually .75 in height. The
  _scutum_ has the articular ridge not so prominent and not so abruptly
  cut off at the lower end, as in _A. spongites_: on the other hand,
  the adductor ridge seems rather more prominent; but these differences
  are trifling. In the _tergum_ the breadth of the spur (fig. 2 _c_, 2
  _d_) varies in specimens taken out of the same branch of sponge; some
  can hardly be distinguished from the same valves in _A. spongites_,
  but generally the spur is broader and squarer.

    [104] In Mr. Cuming's collection there is a specimen, from Sydney,
    which I fully believe to be the present species, but cannot be
    positively sure, as the opercular valves have been lost, which is
    very remarkable from the walls of the carino-lateral compartments
    being reduced to the thickness of a mere thread, not one twentieth
    of the width of the lateral compartments; we here see the structure
    of _Acasta sporillus_ prefigured.

  This species presents some remarkable varieties: in one specimen,
  from Sydney, the parietes were externally ribbed longitudinally
  (fig. 2 _b_), the ribs being roughened with minute points, giving to
  the shell an elegant appearance; and this without doubt is the _A.
  sulcata_ of Lamarck, procured by Peron at Shark's Bay, lat. 25° S.,
  on the opposite of the Australian continent: some specimens from Port
  Lincoln, in South Australia, were not ribbed, only smoothly striated
  in longitudinal lines; although both sets of specimens had almost
  smooth scuta, and were thus different from common specimens, yet
  there could be no doubt, from their similarity in all other points,
  that they did not differ specifically from them, though the latter
  had their scuta striated, but not their parietes. Hence we see that
  there is no relation between the striæ on the parietes and on the
  scuta. The Port Lincoln specimens, and some others, were remarkable
  from the radii not extending down to the basal cup, a minute cleft,
  covered only by membrane, being thus left along the sutures, low
  down between the parietes (fig. 2 _a_); we shall see this singular
  structure strongly developed in _A. perforata_. Owing apparently to
  these clefts, the edge of the basal cup, exhibited traces of six
  knob-like teeth, like those characteristic of _A. glans_.

  _Cirri._--The cirri resemble those of _A. spongites_, with the
  exception that the segments on the posterior pairs bear only
  three main pairs of spines. With respect to the anterior ramus of
  the fourth pair of cirri, the following very singular facts were
  observed:--in a specimen from New South Wales (_var._ with the tergum
  having a narrow spur), on two or three of the lower, but not on the
  lowest, segments, the front margin was produced or developed into two
  or three minute, thick teeth, slightly curved like hooks downwards:
  in other specimens from New South Wales (_var._ with the tergum
  having a broad spur, and inhabiting the same branch of sponge with
  the last-mentioned variety), there was no trace of these teeth. But
  again, in two other specimens with the tergum having a broad spur
  (collected by different persons, near Sydney), and in another from
  South Australia, this structure was carried to an extreme, for in
  these (as represented, Pl. 29, fig. 2) there were beautifully formed
  teeth on the fourteen lower segments (the twelve upper being without
  them), and likewise on the upper segment of the pedicel. These teeth
  are graduated in size on each segment; they are admirably adapted for
  securing any prey; and, in fact, they convert each segment into a
  mandible-like organ. On the segments, on which these teeth are well
  developed, some of the regular pairs of spines are aborted.

  _Diagnosis._--Finally, this species, if it be, as I believe,
  distinct, differs from _A. spongites_ only in the internal surface of
  the parietes being more strongly ribbed and brighter coloured,--in
  the edge of the basal cup being more plainly crenated,--in the
  articular ridge of the scutum being of a different shape,--in the
  spur of the tergum being often broader,--in the segments of the
  posterior cirri having only three pairs of main spines,--and, lastly,
  in the occasional presence of the hook-like teeth on the anterior
  ramus of the fourth cirrus.



3. ACASTA CYATHUS. Pl. 9, fig. 3 _a_-3 _c_.

_Carino-lateral parietes about one fourth of width of lateral parietes:
radii wider than the parietes: basis nearly flat, small: tergum with
the spur truncated, half as wide as valve._

  _Hab._--Madeira, Mus. Lowe, and Bowerbank. West Indies, Mus.
  Stutchbury. Hab. unknown, Mus. Brit.


I feel more confidence in this case, than in that of _A. sulcata_, that
we here have a distinct species, though at one time I treated it only
as a marked variety of _A. spongites_. I rely chiefly on the great
proportional width of the radii of all the compartments, and on the
width of the carino-lateral compartments, compared with the lateral
compartments, and on the general shape of the shell, which differs
considerably from that of the two previous forms: in consequence of
Acasta being attached to and imbedded in a yielding substance, such
as sponge, I believe external form to be of more value as a specific
character in this genus, than in most sessile cirripedes. I have
examined specimens taken out of the yielding _Spongia officinalis_ and
out of an unusually compact sponge, and they resembled each other in
every respect.


  _General Appearance._--Colour pale pink, or that of flesh: basis
  remarkably flat and rather small, with the walls above bulging out
  a little. The radii are very wide, being wider than the parietes
  to which they belong: the orifice is generally rather large. The
  parietes of the carino-lateral compartments vary from one third to
  one fourth of the width of the parietes of the lateral compartments.
  Basal diameter of largest specimen .35 of an inch. Internally, the
  parietes are generally more strongly ribbed than in _A. spongites_.

  The _Opercular Valves_ are large, owing to the form of the shell. The
  _Scuta_ present no particular character, and are not distinguishable
  from those of _A. sulcata_; but the adductor ridge is perhaps rather
  more developed. The _Terga_ (Pl. 9, fig. 3 _c_) are nearly as large
  as the scuta, and this is an unusual circumstance; the spur is more
  than half as wide as the valve; it is placed not quite close to the
  basi-scutal angle; on the carinal side, the basal margin of the valve
  slopes a little towards the spur. I may mention, that in several
  specimens from Madeira, the scuta and terga, on one side, had grown
  to a monstrous thickness.

  _Cirri_: these resemble, in every respect, those of _A. spongites_,
  with the remarkable exception that on the anterior ramus of the
  fourth cirrus, several segments were furnished with the beautiful
  downward curved, mandible-like teeth, as in _A. sulcata_; but
  differently from in that species, there were none on the upper
  segment of the pedicel. I should have thought this an excellent
  specific character, had not these teeth been so extremely variable in
  _A. sulcata_.

  Finally, I think this species is more nearly related to _A. sulcata_
  than to _A. spongites_.



4. ACASTA UNDULATA. Pl. 9, fig. 4.

_Shell, apparently, as in A. spongites, but larger: scutum marked by
longitudinal ridges, often in pairs, with the intermediate furrows
rather wide: spur of tergum nearly half as wide as valve._

  _Fossil_ in Coralline Crag (Sutton), Mus. S. Wood.


I owe to Mr. Wood the inspection of a fine suite of separate valves.
Owing to the shell never having been found entire, its general shape is
not known, and, what is of more consequence, the relative proportional
width of the parietes of the carino-lateral compartment is unknown. I
have (but with doubt) given it a distinct specific name, owing to the
peculiar character of the furrows on the scuta, and to the large size
of the whole shell. In its other characters it comes nearest to _A.
spongites_, excepting in the spur of the tergum, which resembles that
of _A. sulcata_.


  The compartments appear to have been rather smooth externally. The
  radii are not wide, as in _A. cyathus_; and the basis is cup-formed.
  Internally, the parietes are feebly ribbed, as in _A. spongites_.
  Judging from the dimensions of the separated valves, this species
  must have equalled and perhaps exceeded the size of the largest
  living species, namely, _A. glans_, from Australia. Hence we may
  infer, that the basal diameter probably exceeded .55 of an inch: I
  may add, that the largest European specimens of _A. spongites_, from
  Naples and Portugal, are only .3 of an inch in basal diameter.

  _Scuta._--These seem to resemble the scuta of _A. spongites_ in all
  respects, except in the longitudinal ridges standing much further
  apart, and, consequently, in the furrows being much wider: each ridge
  is generally double. Although there is a good deal of variability
  in the character of these ridges in _A. undulata_, and likewise in
  _A. spongites_, I have not seen any form intermediate between them.
  It must, however, be confessed, that this is an extremely variable
  character in many sessile cirripedes. In the _Terga_, the spur is
  about half the width of the whole valve, and therefore rather wider
  than in _A. spongites_.



5. ACASTA GLANS. Pl. 9, fig. 5 _a_-5 _c_.

  ACASTA GLANS. _Lamarck._ Animaux sans Vertèbres, 1818.

_Parietes internally quite smooth, with the lateral margins of each
compartment inwardly prominent: basis with the edge rarely crenated,
but furnished with six inwardly prominent teeth: scutum strongly
striated longitudinally._

  _Var._ (_a_) _with the edge of the basal cup finely crenated_.

  _Hab._--New South Wales, Southern Australia; Mus. Brit., Stutchbury,
  &c.


This fine species seems to be extremely common, imbedded in an open
porose sponge on the eastern and southern shores of Australia. It
is very distinct from the other species, with the exception of the
following _A. lævigata_, which, with some hesitation, I have allowed to
remain specifically separated.


  _General Appearance._--Excepting in its larger size, this species
  differs in external appearance but little from _A. spongites_; its
  colour is pale dirty reddish. The surface is generally studded
  with small calcareous points. The parietes of the carino-lateral
  compartments are about one fourth of the width of the parietes of the
  lateral compartments, and therefore proportionally of the same width
  as in _A. cyathus_. The largest specimen which I have seen, was .55
  of an inch in basal diameter.

  _Scuta._--These are slightly narrower, thicker, and more convex than
  in _A. spongites_: they are strongly striated in longitudinal lines.
  The articular ridge is very feebly developed. _Terga_: in full-grown
  specimens, the spur is half the width of the whole valve, and is
  truncated; its basal edge being parallel to the basal margin of the
  valve. The articular ridge and crests for the depressor muscles are
  very feebly developed.

  _Internal structure of the parietes._--The inner surface of the
  parietes is quite smooth, without even a trace of ribs or teeth. But
  the most important character is that the internal lateral margins on
  both sides of each compartment, from the sheath to the basis, project
  inwards and form a rim; so that when the shell is viewed from within
  (Pl. 9, fig. 5 _b_, representing the lateral and carino-lateral
  compartments, and part of the carina), the six sutures are seen to be
  strengthened by six double columns.

  _Basis._--This is moderately cup-formed. The edge, in order to
  meet the basal points of the inwardly projecting lateral margins
  of the six compartments, has six knob-like teeth. These are placed
  at unequal distances, for two on each side stand near each other,
  owing to the narrowness of the carino-lateral compartments. The
  degree of their development varies extremely; when most developed,
  as in the specimen figured (Pl. 9, fig. 5 _a_), each tooth is bifid
  and a little hollowed out, so as to receive the points of the two
  inwardly projecting margins which form each suture. Ridges, more or
  less prominent, running from each of the six marginal teeth, extend
  towards the centre of the cup. These six teeth cannot be seen from
  the outside. The edge of the cup is rarely crenated; but I have seen
  two instances in which this was the case.

  _Cirri._--In the first pair, the rami are not quite so unequal as
  in _A. spongites_; the longer ramus being about twice as long as
  the shorter. In the third pair, there are some very minute, thick,
  upwardly-pointing spines, which I did not notice in _A. spongites_.
  In the fourth pair, the spines are a little more crowded, with longer
  dorsal tufts, than in the sixth pair; and they are mingled with some
  very minute, thick, upwardly pointing spines. In young individuals,
  there are only three pairs of main spines on the segments of the
  sixth pair, instead of four pairs.



6. ACASTA LÆVIGATA. Pl. 9 fig. 6 _a_, 6 _b_.

  ACASTA LÆVIGATA. _J. E. Gray_ (!). Annals of Philosophy, (new
        series), vol. 10, Aug. 1825.

_Parietes internally quite smooth, with the lateral margins of each
compartment inwardly prominent: basis with the edge strongly crenated,
and furnished with six inwardly prominent teeth: scutum feebly striated
longitudinally, or smooth._

  _Var._ (_a_), _epidermis coloured dull orange_.--Red Sea.

  _Hab._--Red Sea, Philippine Archipelago; Mus. Brit., Cuming, &c.


This species, of which I have examined many specimens from the above
two and other unknown localities, agrees in all essential points of
structure with _A. glans_, and consequently I for some time classed
them together; but the characters, though usually of small value, by
which this form differs from _A. glans_ being apparently constant, I
have with some doubt allowed it to remain specifically distinct. These
characters are, firstly, the much smaller size of the whole shell
in _A. lævigata_; secondly, the edge of its basal cup being always
crenated, which seems to be a rare accident in _A. glans_; thirdly,
though of secondary importance, the scutum being here less plainly
striated; and, lastly, the spur of the tergum being of less breadth,
and of a more rounded outline; on the other hand, it must be confessed,
that when small specimens of _A. glans_ are taken, there is hardly any
difference in the spurs of the terga.


  _General Appearance and Structure of Shell._--The surface of the
  shell is often very smooth, but is sometimes studded with some small
  sharp calcareous points. The colour is white, or pale reddish-brown;
  but in the specimens from the Red Sea, the tint is more orange,
  with the upper part of the shell white. The orifice of the shell is
  unusually small. The largest specimen which I have seen was only .25
  of an inch in basal diameter, and therefore less than half the size
  of _A. glans_. The internal surfaces of the _parietes_ are smooth,
  with the two lateral margins inflected, as in _A. glans_. The edge of
  the basal cup has six knob-like teeth, like those in _A. glans_, but
  smaller; and, in addition, it is finely crenated.

  _Scuta_: these differ only in being less plainly striated in
  longitudinal lines; indeed, some specimens show hardly a trace of
  this structure. _Terga_; these valves, in some varieties (Pl. 9, fig.
  6 _b_) can hardly be distinguished from those of equal size from
  young individuals of _A. glans_; other varieties have the spur (Pl.
  9, fig. 6 _a_) not truncated, but broadly pointed, and therefore of
  considerably different shape.

  Neither in the mouth, nor cirri could I detect any difference with
  _A. glans_.



7. ACASTA FENESTRATA. Pl. 9, fig. 7 _a_-7 _c_.

_Shell reddish, with six large, membrane-covered apertures between the
sutures, above the basal cup: carino-lateral parietes half as broad as
lateral parietes; internally, parietes and edge of basis smooth; tergum
with the articular ridge short and prominent; spur pointed._

  _Hab._--Philippine Archipelago, Mus. Cuming.


  _General Appearance._--Shell rather elongated or tubular; with the
  upper part reddish, and the surface roughened with very minute
  points. The basal cup is generally as deep as the shell is high,
  ending downwards in a blunt point, often curved to one side. The
  summits of the radii, as usual, are oblique. The parietes of the
  carino-lateral compartments are about half as wide as the parietes of
  the lateral compartments, and are therefore of greater proportional
  width than in the foregoing or any other species of the genus.
  The large membrane-covered openings, or, as they may be called,
  windows, presently to be described, between the lower halves of the
  compartments, is much the most remarkable character of this species.
  The largest specimen which I have seen was only .23 in diameter, and
  .6 of an inch in height, measured from the basal point of the cup to
  the tips of the compartments.

  _Scuta._--These barely exhibit a trace of longitudinal striæ. The
  valve is rather thick and convex. The basi-tergal angle is much
  rounded off. Internally, the articular ridge is thick and rather
  prominent. _Terga_: the valve is furrowed in the line of the spur:
  the spur is pointed and rather long; it is distinctly separated from
  the basi-scutal angle of the valve, and the basal margin on the two
  sides of the spur forms a straight line. The articular ridge is
  prominent, and short.

  _Structure of the Parietes, Radii, and Basis._--The parietes are
  internally quite smooth down to its basis. The edges of the radii are
  also smooth, as is the edge of the basal cup. The alæ project less
  than usual. The radii are of moderate breadth, they extend downwards
  only a little below the sheath, namely, about half way down the
  shell, where they terminate, as usual, in a point. The increase in
  width, during growth, of the radii, and their not extending down to
  the basis, would necessarily cause a gap between the opposed edges
  of the walls, in the portion beneath the radii; but besides this,
  the edges of the walls themselves, beneath the radii, and on the
  opposed side beneath the alæ, are hollowed out, but on the latter
  side or beneath the alæ sometimes in a lesser degree. The result of
  this is, that the compartments, in their lower halves, are separated
  from each other by membrane-covered windows or apertures, arched at
  their upper ends, and of considerable size, namely, about as wide as
  the parietes of the carino-lateral compartments. I have only further
  to remark, that during the downward growth of the parietes, the
  apertures increase in size, but at the same time become closed up at
  their upper ends; and the arched layers of shell added at these upper
  ends, assume a very different aspect from the rest of the parietal
  surface,--appearing like two wedges, with their points upwards, let
  in, on one side of the suture, between the ordinary parietal surface
  and the radius, and, on the other side of the suture, between the
  ordinary parietal surface and the recipient furrow of the radius.

  The _animal's body_ was in a bad state of preservation; but, as far
  as I could make out, the cirri resembled those of _A. glans_.

  _Affinities._--This species differs from _A. glans_ in not having the
  internal margins of the compartments projecting inwards. It differs
  from all the ordinary varieties of _A. spongites_, in the smoothness
  of the basal edges of the parietes and of the edge of the cup; in
  the greater width of the carino-lateral compartment, though this is
  a variable point in _A. spongites_; slightly in the shape of the
  scuta and terga; and, lastly, in the large, membrane-covered openings
  between the compartments.



8. ACASTA PURPURATA. Pl. 9, fig. 8 _a_-8 _c_.

_Shell dull blueish-purple, with six small, membrane-covered apertures
between the sutures, close above the basis: tergum with the articular
ridge very short and prominent; spur very broad and rounded._

  _Hab._--Sumatra; Philippine Archipelago; imbedded in the bark of an
  Isis; Mus. Cuming, Stutchbury, Brit.


This species is perfectly distinct from the others, as shown by its
general appearance, its habits, and the structure of its opercular
valves: it is allied to _A. fenestrata_, in having membrane-covered
apertures between the compartments, and in some respects in its
opercular valves: it is also allied to _A. sulcata_ and _cyathus_ in
the parietes being often internally ribbed, in the basal cup having a
crenated edge, and in the anterior ramus of the fourth cirrus being
furnished with the minute hook-like spines.


  _General Appearance._--Sub-globular, slightly compressed, with a
  rather small orifice; smooth, but sometimes furnished with sharp
  shelly points; dull purple, more or less dark, with the upper parts
  of the walls often white. The radii are rather narrow, and generally
  white, with their summits only slightly oblique, but variable in
  this latter respect. The parietes of the carino-lateral compartments
  are narrow, being only one sixth of the width of the parietes of the
  lateral compartments. In some specimens there are membrane-covered
  apertures of considerable size, in others mere narrow clefts, between
  the basal halves of the compartments. The basal cup is moderately
  deep. The largest specimen was only .16 of an inch in basal diameter.

  _Scuta_, rather broad, externally convex, not longitudinally
  striated: articular ridge prominent, short, not extending down above
  one third of the length of the valve. Depression for the adductor
  muscle deep. On the internal surface, near to the rostral angle, a
  rather large purple spot of corium adhered to the valve. _Terga_,
  broad, externally rather convex: scutal margin protuberant: carinal
  margin slightly inflected, or furnished internally with a rim:
  articular ridge prominent, very short, not extending down above one
  fourth of the valve. Spur very broad, rounded, confluent with the
  basi-scutal angle of the valve.

  _Internal Structure of the Parietes, Radii, and Basis._--The
  parietes, internally, are either quite smooth, or more commonly
  ribbed, with the basal edge in consequence crenated; the ribs
  are either placed at an unusual distance from each other, and
  consequently are few in number, or are pretty close together. The
  edge of the basal cup is either quite smooth, or closely crenated,
  or distantly toothed, in conformity with the state of the internal
  surface of the parietes. The radii have nearly smooth edges, with
  their summits more or less oblique. They sometimes extend down only
  three fourths, or only two thirds, of the length of the shell, and
  the margins of the parietes under the radii being a little hollowed
  out, the sutures are converted into clefts or apertures (of course
  covered by membrane) like, but not so large as, those in _A.
  fenestrata_. The margins of the parietes are hollowed out only on the
  side of the radius, and not on both sides of the sutures, as is most
  usual in _A. fenestrata_. In some specimens the radii extended down
  close to the basal cup, and only very minute clefts were left between
  the opposed edges of the parietes.

  In the _animal's body_ the only noticeable character was, that on the
  anterior ramus of the fourth pair of cirri, some of the segments were
  furnished with very broad and thick, small, downwardly curved, teeth
  or hooks, like those described in certain varieties of _A. sulcata_;
  but they are here stronger and thicker. The segments in the three
  posterior pairs of cirri are not so much elongated, as in the other
  species.



9. ACASTA SPORILLUS. Pl. 9, fig. 9 _a_-9 _d_.

_Shell purplish-brown, with the parietes internally strongly ribbed
and reticulated: carino-lateral compartments extremely narrow, not
extending down to the basis._

  _Hab._--Sooloo Islands, East Indian Archipelago; Mus. Dana.


I am indebted to Mr. Dana, the distinguished naturalist of the United
States Antarctic Expedition, for two specimens of this interesting
species, which, in the singular reticulated structure of the inner
surface of the walls, and in the almost rudimentary condition of the
carino-lateral compartments, not extending down to the basal cup, is
very distinct from the foregoing species. I have used Mr. Dana's very
appropriate MS. name of _sporillus_. The specimens were dredged up,
lying quite loose and unattached at the bottom of the Sooloo sea; the
one which I opened, must have long lain dead; but Mr. Dana assures me
that some were living, and he has sent me drawings of parts of the
mouth and cirri: I am much surprised at this circumstance; for analogy
would have made me believe that this species must have been imbedded
in some sponge-like body, such as the bark of a zoophyte, and that it
could not have lived unattached. I may add that a small fragment of a
brown leathery substance adhered to the upper end of one of the two
specimens, and this seems to indicate attachment.


  _General Appearance._--Shell shaped like a pointed acorn; slightly
  flattened; orifice extremely small; surface very finely punctured,
  covered by a purplish-brown epidermis, with transverse stripes of
  different shades, and with the apex dark; according to Mr. Dana, when
  fresh, the colour was purplish-carmine. Radii narrow, white. The
  carino-lateral compartments are extremely narrow; the wall-portion
  (fig. 9 _b_) forming a mere linear rib, terminating downwards in
  a sharp point, which does not reach the basal cup: hence this
  compartment evidently tends to become rudimentary. The basal cup is
  moderately deep and pointed. Basal diameter .16; height, from the
  bottom of the cup to the top of the shell, .24 of an inch.

  _Scuta_: narrow, with the upper part produced; not striated
  longitudinally; coloured by a pale purple, longitudinal band.
  Internally, there is scarcely a trace of an articular ridge, which,
  in the other species, is always more or less developed. _Terga_ with
  the spur bluntly pointed; nearly the whole basal margin, on the
  carinal side, slopes towards the spur.

  _Internal Structure of the Parietes, Radii, and Basal Cup._--The
  parietes are strongly ribbed internally; and these ribs are connected
  by very narrow, less prominent, transverse, slightly branched ridges,
  giving a reticulated structure to the inner surface. Between several
  of the main longitudinal ribs, in the lower part of the shell, new
  ribs may be seen in process of formation, and these tend to convert
  the reticulated structure into a double row of minute cells. I have
  not met with an exactly similar structure in any other cirripede; but
  I have no doubt that the little transverse ridges are homologous with
  the transverse calcareous septa in the parietal pores of many Balani,
  in the same manner as the internal longitudinal ribs, in this and
  other species of Acasta, are homologous with the longitudinal septa
  forming the above pores. The edge of the basal cup is pectinated with
  teeth, which lock into the teeth formed by the ends of the internal
  parietal ribs. The radii are narrow, and have smooth edges. The alæ
  project beyond the parietes to a remarkably small extent. The sheath
  is free, or hollow beneath. I have already described the almost
  rudimentary condition of the carino-lateral compartments; this is
  best exhibited in an internal view of the two compartments, as given
  in Pl. 9, fig. 9 _b_.

  _Animal's body_ unknown to me: from Mr. Dana's drawing the three
  posterior pairs of cirri seem to have been much elongated: and the
  rami of the first pair, as usual, unequal in length.


M. Deshayes has given an indifferent figure and imperfect description
of ACASTA TUBULOSA (Guerin, Magasin de Zoologie, 1831, Tab. 39; and
Guerin, Iconographie du Règne Animal, Mollusques, Tab. 38, fig. 4, but
here by a misprint called _A. spinulosa_); it is utterly impossible to
recognise the species of this genus from such materials.



3. _Genus_--TETRACLITA. Pl. 10, 11.

  TETRACLITA. _Schumacher._ Essai d'un Nouveau Syst., &c., 1817.

  CONIA. _Leach._ Journal de Physique, tom. 85, 1817.[105]

  ASEMUS. _Ranzani._ Memoire di Storia Naturale, 1820.[106]

  POLYTREMA. _De Ferussac._ Dict. Classique d'Histoire Naturelle, 1822.

  LEPAS. _Gmelin._ Systema Naturæ, 1789.

  BALANUS. _Bruguière._ Encyclop. Method., 1789.

  ---- _Lamarck._ Animaux sans Vertèbres, 1818.

    [105] From a note by the Editor, it appears that Schumacher's essay
    appeared before the number of the Journal containing Leach's paper,
    so that Schumacher's name must be adopted.

    [106] I have not seen a _complete_ copy of this work, and give the
    title from a catalogue; the running heading of the part containing
    the Cirripedia, is "Opuscoli Scientifici."

_Compartments four, sometimes externally calcified together: parietes
permeated by pores, generally forming several rows. Basis flat,
irregular, calcareous, or membranous._

  _Hab._--Throughout the tropical and warmer temperate seas.


_General Appearance._--The shell is conical, more or less depressed,
and very rarely, even when growing in crowded groups, becomes
cylindrical or elongated. The orifice is seldom large, generally
diamond-shaped or oval. The colour is either nearly white or purple,
occasionally even inky black, or very dark green, and sometimes of a
pale pink peach-blossom. The surface is sometimes smooth, but more
commonly longitudinally ribbed; the outer lamina of shell is very often
wholly corroded away, excepting close to the basis, leaving the solidly
upfilled parietal tubes exposed: these give the shell a striated
appearance, or they appear like flattened tapering points adpressed to
its surface (Pl. 10, fig. 1 _b_): Lamarck attempted to express this
appearance, by using the specific name of _stalactiferus_. The colour
of the shell depends, to a considerable extent, on the colour of the
shelly matter in these exposed parietal tubes. We shall presently see
that the corrosion and disintegration, to which some of the species
are so liable, plays an important part during their growth. The radii
are either well developed, as in most of the species; or they are
entirely absent, as in the great majority of specimens of _T. porosa_
and _serrata_. In many individuals of _T. porosa_ and _purpurascens_
not only are the radii absent, but the four compartments are calcified
together without any trace, on the external surface, of the sutures.
The largest specimen which I have seen of _T. porosa_, which is the
largest species, was two inches in basal diameter, and nearly one inch
and a half in height.

_Scuta._--These valves are sub-triangular, and either longitudinally or
transversely elongated. Externally, the growth-ridges are moderately
prominent, and in _T. costata_ and _cœrulescens_ they are crossed by
longitudinal striæ. Along the occludent margin, the inflected extremity
of each alternate growth-ridge is generally much thickened,--a set of
teeth being thus formed, by which the two valves are locked together.
In _T. porosa_, this character is variable, for sometimes every
alternate ridge, and sometimes only two or three ridges, separated
from each other by several growth-ridges, are thus developed into
teeth. The _articular_ ridge is either moderately prominent, or is
extremely prominent, as in _T. cœrulescens_; but the lower edge in no
case depends as a free, hinge-like style, as sometimes in Balanus.
The _adductor_ ridge is generally well developed and distinct from
the articular ridge: in _T. purpurascens_ it is very blunt: in _T.
serrata_ it is united to the articular ridge half way up it, thus
forming a deep tubular cavity running up to the apex of the valve: in
_T. cœrulescens_, the adductor ridge is very short, and is united to,
or almost continuous with, the lower end of the articular ridge, a
small sub-cylindrical tubular cavity being thus formed. Small crests
exist for the attachment of the rostral and lateral depressor muscles,
in most of the species, excepting _T. purpurascens_ and _costata_, in
which, however, more especially in the former, there are, instead
of crests, minute pits for the attachment. These crests vary much in
prominence in the same species.

_Terga._--These valves present no essential differences from those of
Balanus; they are sometimes beaked, and the beak is hollow and occupied
by a thread of corium, as in that genus. The external surface of the
valve is often depressed in the line of the spur, but there is never a
longitudinal furrow with the edges folded in, as in Balanus. The spur
is very short in _T. purpurascens_. In _T. radiata_, the articular
ridge is remarkably prominent. The crests for the depressor muscles are
well developed in all the species. The shape of the terga is variable
in nearly all the species, and greatly so in _T. porosa_.

_Compartments._--Owing to there being only four compartments, the
lateral pair are large; the size of the carina relatively to the
rostrum varies, according as its alæ have been added to during
diametric growth. The walls are very thick, and are composed of
numerous tubes, in some species as many as fourteen or fifteen rows
being exposed on the basal margin (Pl. 10, fig. 1 _g_). The tubes
are generally angular, and slightly elongated in the ray of the
circular shell; sometimes they are nearly circular and small. New
tubes are formed only at the basal edge of the outer lamina, by the
bifurcation of the septa which form the tubes. In very young specimens
there is only a single row of tubes; and in _T. rosea_ this holds
good throughout life: in this species (fig. 3 _d_) the tubes, in the
single row, are large and quadrangular, and the outer lamina of shell
is strengthened by numerous, small, internal, longitudinal plates.
I believe the branching septa, which separate and form the parietal
tubes, correspond with the longitudinal septa in the more simple
walls of Balanus. The tubes become solidly filled up, in their upper
parts, with hard, and generally coloured shelly matter. The degree
to which they are filled up differs in the different species; the
external side of each tube is always first thus coated. The thin outer
lamina of shell, in several of the species, commonly disintegrates and
disappears; the upfilled parietal tubes being thus exposed. The inner
lamina of the walls is generally smooth, but in _T. radiata_ it is
longitudinally ribbed, as in most species of Balanus. The sheath is
generally dark-coloured; its lower edge does not project or overhang
the inner lamina, as is usual in Balanus, excepting in _T. serrata_,
and in some few varieties of _T. porosa_.

The _Radii_, when developed, are either narrow or broad, with their
summits either oblique or extending in a straight line from the top
of one compartment to that of another. In _T. serrata_, I have not
seen a single specimen with the radii developed; in _T. porosa_, they
are very seldom developed, and then, apparently, only in quite young
specimens, in which they are narrow; in _T. purpurascens_, they seem to
be about as often developed as not, and when present they are broad;
in _T. costata_, _cœrulescens_, and _radiata_, they are always largely
developed. In some specimens of the species, in which the radii are
not developed, even the sutures do not reach the external surface;
the outer lamina of the parietes being continuous all round, so that
the shell seems formed of a single piece. Even in such specimens, the
four compartments, viewed from within, can be seen to be distinct; and
the sutures can generally be traced across the whole thickness of the
parietal tubes; in this latter case, when the sutures are broken open,
the radii are seen to be represented (Pl. 10, 1 _h_) by a few small
sinuous ridges. Owing to the disintegration of the upper and outer part
of the shell, and the consequent exposure of parts of the sheath and
alæ, the radii sometimes appear as if developed, when such is not the
case. With respect to the internal structure of the radii, they are
formed, in _T. purpurascens_ and _costata_, of tubes, like those of the
parietes, and therefore according to the normal plan; whilst in the
other species they are formed by longitudinal sinuous ridges, sending
out on each side irregular denticuli; and the interspaces between
the ridges are filled up solidly during the growth of the radii, in
all the species, except in _T. radiata_, in which they are left to a
considerable extent open. These sinuous ridges, with their denticuli,
homologically represent the branching septa which form the parietal
tubes. The edges of the alæ are crenated in all the species, except in
_T. costata_.

_Diametric growth._--When first examining groups of _T. porosa_, in
none of which the radii had been developed, and in which, consequently,
the shell could not have grown in diameter, but only at its basal
margin, I was at first unable to comprehend, how the upper part of the
shell and the orifice could have acquired their proper proportional
width. The young shell, at its first formation, starts with an orifice
so small that a pin could hardly be inserted in it; and this in many
individuals is never increased in diameter by the diametric growth
of the shell; but in place of this, as the conical shell is added to
at its base, the whole upper part disintegrates and wears away, the
orifice becoming thus enlarged. We thus see that the corrosion and
wearing away of the upper part of the shell is a necessary element in
its growth. The development of the radii, which in some of the species,
as in _T. purpurascens_, at first seems to be quite capricious, really
depends upon the fact, whether the specimens have been exposed to
disintegration; for I have almost always found that when the outer
lamina of shell has been well preserved, the radii have been developed,
and the orifice has been enlarged by their growth, instead of by the
wearing down of the upper part of the conical shell.

_Basis._--This consists of a very thin, flat, though irregular,
translucent, calcareous plate, which towards the edges is sometimes
membranous. In _T. purpurascens_, the basis is entirely membranous.
When a portion of the calcareous base is dissolved in acid, a tissue
is left, composed of several laminæ, to which numerous bifurcating
cement-ducts are attached: even before dissolution, these delicate
bifurcating ducts can just be perceived by the aid of a simple lens.

_Mouth._--The several organs present no particular characters. There
are generally three teeth on each side of the notch in the labrum.
The palpi usually have parallel sides, but are club-shaped in _T.
purpurascens_ and _costata_. The mandibles have generally four teeth,
but there are five in _T. vitiata_, and only three in _T. costata_. The
maxillæ are notched. The outer maxillæ are bilobed in front.

_Cirri._--The segments of the three posterior pairs usually support
only three pairs of main spines, but there are four pairs in _T.
vitiata_ and _costata_: between each pair, there is either a tuft of
fine spines, or a single fine spine. The rami of the first cirrus
are unequal in length. In the third cirrus, the posterior ramus is
sometimes much elongated, but sometimes both rami are short and blunt.
Some of the segments in the third cirrus often support very coarsely
and doubly pectinated spines. Under the head of _T. porosa_, it will be
seen to what a remarkable degree the relative numbers of the segments
in the several cirri vary, even in specimens taken out of the same
cluster.

The _Branchiæ_ are well developed (at least in _T. porosa_ and
_costata_), as a large, plicated, tapering fold, with a small second
fold on the inner side at the base. In _T. porosa_ the _stomach_ is
destitute of cæca. The _vesiculæ seminales_ in this same species are
large, with their broad, blunt ends reflexed. The _ovarian_ tubes
surround the sack, and cover the basal plate.

_Distribution and Habitats._--This genus is confined to the tropics,
and to the warmer parts of the temperate seas: in the southern
hemisphere, it ranges south, to the Cape of Good Hope and to Van
Diemen's Land: in the northern hemisphere it does not appear to range
so far; I do not know of any authentic case of a species having been
found in the Mediterranean, or on the shores of the United States,
north of the West Indies. _Tetraclita porosa_ is found round the whole
world; _T. radiata_, also, has a very wide range, inhabiting the West
Indies, the East Indian Archipelago, and New South Wales. This latter
species, as well as _T. cœrulescens_, is often attached to the bottoms
of ships, adhering to _Balanus tintinnabulum_. The several species live
attached to tidal rocks, to littoral shells, or to massive corals. I
have met with two instances, in the West Indies and the Philippine
Archipelago, of _T. porosa_ adhering to wood. _Tetraclita porosa_
seems to feed chiefly on crustaceans: the number and the size of the
amphipods, isopods, and entomostracans, together with an annelid,
in the stomachs of some specimens from South America, was quite
surprising. As many as five species occur in the same region, in the
eastern half of the world; thus on the shores of New South Wales, we
have _T. porosa_, _vitiata_, _radiata_, _purpurascens_, and _rosea_; in
the Philippine Archipelago, we have _T. porosa_, _vitiata_, _costata_,
and _cœrulescens_.

I have not seen any species of this genus fossil.

_Variation._--The species vary in shape nearly, but not quite as much
as in Balanus, for we very rarely here see cylindrical varieties; but
in the same species, we have extremely depressed, steeply conical, and
convex forms; the orifice varies in relative size; the state of the
surface, whether ribbed or smooth, whether well preserved or corroded,
the upfilled parietal tubes being thus exposed, varies more than in
Balanus. The colour also varies; specimens of _T. porosa_ (Pl. 10, fig.
1 _a_ to 1 _f_) being dark purple, or even inky black, white, pale
pinkish-purple, and green: as far as I have seen, the pinkish varieties
of _T. porosa_ are confined to the eastern half of the world. Even the
ridges and crests on the under side of the scutum vary in some degree;
and the tergum in _T. porosa_ varies considerably, and in some of the
other species, slightly, in general shape: I believe that the tergum
becomes narrow and elongated, when the shell is steeply conical, with
the orifice of small diameter. After having spent several days in
disarticulating and closely examining the many specimens of _T. porosa_
in my possession, I concluded that its varieties formed at least four
species, these being in external appearance extremely distinct; but
ultimately the many intermediate forms compelled me to unite all into
a single species. Again, I may instance the conical, ribbed variety
of _T. purpurascens_ (Pl. 11, fig. 1 _b_), with the outer lamina of
the shell preserved and with the radii widely developed, as having not
the smallest resemblance to the other common depressed variety (fig. 1
_a_), having a granulated surface, produced by the exposed tips of the
upfilled parietal tubes, and without a trace of the radii or even of
the sutures; I may add that, according to the characters used by some
authors, these two varieties would be classed in two separate genera.

To distinguish the species of this genus, the chief reliance must be
placed (as in the case of most other sessile cirripedes) on the general
outline of the opercular valves, and on the ridges and crests on their
under sides: the internal structure, however, of the radii, and in
a lesser degree of the parietes, must not be overlooked. I have not
found the parts of the mouth, or the differences in the cirri, of much
service; and it will be shown under _T. porosa_, that the relative
numbers of the segments in the several cirri, and even their shape, is
extraordinarily variable.

_Affinities._--All the species are pretty closely allied, and there is
no ground for making any sectional division of the eight species, more
especially not on the ground whether or no the radii are developed.
_Tetraclita purpurascens_, taking into account all the characters,
including the animal's body, is perhaps the most distinct species in
the genus, but it is closely allied to _T. costata_. In the well-marked
character of the parietes being formed of a single row of large tubes,
_T. rosea_ differs from all the other species. The genus is closely
allied to Balanus; I can point out no difference in the animal's
body, nor any constant difference in the opercular valves. The ridges
and crests on the under sides of the scuta are more prominent in
Tetraclita; and all the species, excepting two, have crests (though
sometimes very slight) for the attachment of the rostral depressor
muscle, and these occur only in two species of Balanus: crests, also,
for the attachment of the lateral depressor muscle are common in
Tetraclita, but rare in Balanus. The main character, however, of the
genus, as compared with Balanus, is derived from the existence of
only four compartments, and in a lesser degree from the several rows
of parietal pores; but, as just stated, _T. rosea_ has only a single
row, and, on the other hand, in _Balanus crenatus_, there is a slight
tendency exhibited, in the dividing of the longitudinal septa near
the outer lamina, to form a second row of parietal pores; and in _B.
cariosus_, moreover, we actually have two or three rows of irregular
and very short pores. The thin, yet solid calcareous basis which
occurs in all the species of Tetraclita, excepting _T. purpurascens_,
resembles the basis in _Balanus flosculus_ and _imperator_, but I
suspect that the structure of the cement-ducts is different in the two
genera.



1. TETRACLITA POROSA. Pl. 10, fig. 1 _a_-1 _m_.

  LEPAS POROSA. _Gmelin._ Syst. Naturæ, 1789.[107]

  BALANUS SQUAMOSUS. _Bruguière._ Encyclop. Method., 1789, Pl. 165,
        fig. 9, 10.

  LEPAS FUNGITES. _Spengler._ Skrivter af Naturhist. Selskabet, 1 B.,
        1790.

  ---- POROSA. _Wood's_ General Conchology, Pl. 9, fig. 4, 1815.

  TETRACLITA SQUAMULOSA. _Schumacher._ Essai d'un Nouveau Syst., &c.,
        1817.

  BALANUS STALACTIFERUS. _Lamarck._ Animaux sans Vertèbres, 1818.

  ---- ---- _Chenu._ Illust. Conch., Pl. 4, fig. 6, 7.

  ASEMUS POROSUS. _Ranzani._ Memoire di Storia Naturale, Tab. 3, fig.
        32-35.

  CONIA POROSA. _Sowerby._ Genera of Recent and Fossil Shells, Plate,
        1823.

  ---- ---- _Leach._ (sine descript.) Encyclop. Brit. Supplement, vol.
        3, 1824, Tab. 57.

    [107] As Gmelin and Bruguière published the same year, I do not
    know which comes first, but I have adopted the best known name.
    Most authors give, amongst their references, Chemnitz, vol. 8,
    Tab. 98, fig. 836, 837, but the accompanying description is more
    applicable to _T. serrata_ of this work than to _T. porosa_;
    without a figure or description, however, of the under side of the
    scutum it is impossible to decide. Several authors, also, give
    _Lepas cariosa_ of Pallas, as a synonym; this, however, is the
    _Balanus cariosus_ of this work.

_Radii rarely present, when present narrow; even the sutures often
absent: shell steeply conical, with the surface generally corroded, and
having a stalactiferous appearance._

  _Var._ (1) communis (Pl. 10, fig. 1 _a_): _outer lamina of shell
  almost wholly removed; the portion preserved, as well as the exposed
  parietal tubes, gray, or pale dirty brown, or dull purple_.

  _Var._ (2) nigrescens (Pl. 10, fig. 1 _c_): _outer lamina of shell
  almost wholly removed; the portion preserved, and the exposed
  parietal tubes, very dark purple or inky black_.

  _Var._ (3) viridis: _outer lamina of shell almost wholly removed;
  the portion preserved, and the exposed parietal tubes, green; under
  surface of opercular valves clouded green_.

  _Var._ (4) rubescens (Pl. 10, fig. 1 _b_): _outer lamina of shell
  almost wholly removed; the portion preserved, and the exposed
  parietal tubes, pale reddish-purple, or peach-blossom purple; under
  surfaces of opercular valves often reddish purple; terga often rather
  narrow, with the spur somewhat pointed, and with the basal margin on
  the carinal side sloping, or not making an angle with the spur_.

  _Var._ (5) elegans (Pl. 10, fig. 1 _d_): _outer lamina preserved,
  excepting sometimes near the summit of the shell; white, tinged
  with yellowish brown from the epidermis; surface strongly ribbed
  longitudinally; orifice rather small; sheath reddish-purple; terga
  narrow, with the basal margin sloping as in var._ rubescens.

  _Var._ (6) communis (young) (Pl. 10, fig. 1 _e_): _radii developed,
  very narrow; outer lamina of shell preserved, gray or dull purple;
  surface slightly ribbed longitudinally_.

  _Var._ (7) patellaris (Pl. 10, fig. 1 _f_): _radii developed,
  very narrow, white; outer lamina of shell preserved, generally
  reddish-purple; steeply conical, with the orifice extremely small;
  surface smooth, with longitudinal white ribs. Terga very narrow, with
  the spur sharply pointed, and with the basal margin on the carinal
  side sloping towards it, or not making an angle with it. Scuta, with
  the adductor ridge very prominent. Attached to a ship's bottom_.

  _Hab._--West Indies, Brazil, West Colombia, Panama, Galapagos
  Archipelago, California, Philippine Archipelago, China, East coast of
  Australia, Red Sea; generally attached to tidal rocks, sometimes to
  shells, sometimes to wooden posts. Very common.


  _General Appearance._--This, the widest-distributed and much
  the commonest species of the genus, varies greatly in external
  appearance. The usual shape is steeply conical, but some individuals
  are much depressed. In the common varieties the outer lamina of
  shell has been removed even close to the basal edge; the upfilled
  parietal tubes being thus exposed (fig. 1 _b_), as flattened
  adpressed points. These points are largest in large specimens, but
  they vary somewhat in size in specimens of equal growth. When the
  outer surface is preserved, it is generally ribbed longitudinally,
  but is sometimes quite smooth. The most general colour is dirty gray
  or dark purple; but many specimens are pale pinkish-purple, owing
  to the exposure of the parietal tubes upfilled with shelly matter
  of this tint: there are also, as given under the characters of the
  _vars._, black, white and green varieties. The sheath is always
  tinted by the prevailing colour. The radii are rarely developed,
  but generally the four sutures are distinct; sometimes these are
  externally quite obliterated, the shell, as seen from the outside,
  consisting of a single piece like a patella or fissurella. When the
  radii are developed they are very narrow, with their upper edges
  oblique: their development seems always coincident with the more or
  less perfect preservation of the shell, and their function is to
  enlarge the orifice; the enlargement being usually effected by the
  disintegration and removal of the whole upper part of the conical
  shell. The size of the orifice varies considerably; in the seventh
  variety it was extraordinarily small: in outline it varies from oval
  to rounded trigonal or rhomboidal; in some specimens, with the radii
  well developed, it was rounded pentagonal.

  _Size._--_Tetraclita porosa_ is the largest species of the genus;
  I have seen specimens attached to a large pebble of granite in the
  British Museum, which measured two inches in basal diameter, and
  nearly one inch and a half in height.

  _Scuta_: these are sub-triangular and generally a little elongated,
  but they vary slightly in relative breadth (fig. 1 _i_, 1 _l_), and
  likewise in the degree to which the basi-tergal angle is rounded off.
  The under surface is clouded with dull or pinkish-purple, or with
  green, or is nearly white. The articular ridge is not prominent, and
  the articular furrow is narrow. The adductor ridge is prominent, and
  runs upwards for some distance close and parallel to the articular
  ridge; and sometimes it extends nearly or quite up to the apex of
  the valve; in one single specimen the adductor ridge had an abraded
  appearance, and was very little developed. The crests for the rostral
  and lateral depressores are sharp and distinct. Along the occludent
  margin, the ends generally of the alternate lines of growth are
  enlarged into knobs, serving to lock the two valves together; but in
  many specimens only two or three knobs, at intervals of several lines
  of growth (fig. 1 _b_), are developed.

  _Terga_: when the upper end of the valve is not corroded, there is
  a distinct beak, hollow within for a thread of corium. The scutal
  margin is not much inflected, and the articular ridge not very
  prominent. The spur is placed quite close to the basi-scutal angle
  of the valve, so that there is no basal margin on that side of the
  valve. The width of the valve and of the spur, and the acumination of
  the extremity of the latter, varies in a remarkable manner. In the
  broad and commonest variety (fig. 1 _k_), the basal margin of the
  valve form an angle of about 130° with the carinal side of the spur,
  and the basal end of the spur is broad and truncated. In the less
  common and narrow variety (1 _m_), the basal margin in some extreme
  cases forms very nearly a straight line with the carinal side of
  the spur; and the spur itself is bluntly pointed: in _var._ 7 it is
  sharply pointed.

  _Structure of the parietes and radii._--In all cases the four sutures
  are quite distinct, from top to bottom, on the internal lamina of
  the shell, and generally they run through the whole thickness of
  the walls, and are visible externally. Often they do not quite
  reach the outer lamina, and then the shell externally consists of a
  single piece, like a patella. Sometimes the sutures can be traced
  running through the parietal tubes only for a short distance from
  the internal surface; where they cease, the two walls of the suture
  become fused together. When a perfect suture is split open, the
  radius is represented (fig. 1 _h_) by a few narrow, sinuous ridges,
  sending out on each side little branches or denticuli; these are
  received into corresponding furrows in the opposed compartment. These
  ridges run nearly parallel to each other, and somewhat obliquely,
  from the outer lamina of the shell to the basis. When the radii are
  developed, their edges are similarly formed, by sinuous denticulated
  ridges, with the interspaces between them filled up solidly. The alæ
  are but little prominent.

  The _mouth_ does not deviate from the generic type. The _cirri_
  are remarkable from the variability in the several pairs of the
  relative numbers of their segments, as shown in the following table.
  The segments do not correspond even on opposite sides of the same
  individual, as may be seen in the two lower lines of the table. I
  believe that variability to this degree is very uncommon in other
  cirripedes, though, as stated in the Introduction, the number of the
  segments always increases with the growth of the individual. The
  terminal segments in the longer rami of both the first and third
  cirrus are antenniformed,--being elongated, and of a different
  shape, with fewer bristles, compared with the basal segments of
  the same cirri. It is apparently these terminal segments which
  are particularly liable to vary in number. In both rami of the
  third cirrus, some of the segments, from the sixth to the eleventh
  inclusive, (counting from the bottom), more especially the eighth,
  ninth, and tenth, carry a few spines coarsely and doubly pectinated;
  but as some of the adjoining segments carry spines which may be
  called doubly serrated, it is not easy to draw an exact line of
  demarcation. Sometimes, though rarely, a few of the nearly terminal
  segments in the second cirrus are furnished with similar, doubly
  pectinated spines.

_Numbers of the segments in the rami of the Cirri, in different
specimens._

  +------------------+--------------+--------------+--------------+-------+
  |                  |First cirrus. |Second cirrus.|Third cirrus. |Sixth  |
  |                  |              |              |              |cirrus.|
  |                  |--------------+--------------+--------------+-------|
  |                  |Shorter|Longer|Shorter|Longer|Shorter|Longer|Either |
  |                  |ramus. |ramus.|ramus. |ramus.|ramus. |ramus.|ramus. |
  |------------------+-------+------+-------+------+-------+------+-------|
  |Specimen from     |       |      |       |      |       |      |       |
  |Pernambuco, Brazil|   9   |  19  |  10   |  11  |   15  |  27  |  26   |
  |(_var._ 1)        |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |Another specimen  |       |      |       |      |       |      |       |
  |from the same     |  12   |  23  |  12   |  14  |  14   |  19  |  23   |
  |cluster           |       |      |       |      |       |      |       |
  |(_var._ 1)        |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |From the Galapagos|       |      |       |      |       |      |       |
  |Archipelago,      |  10   |  16  |  10   | 11?  |  11   |  16  |  25   |
  |(_var._ 1)        |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |From the          |       |      |       |      |       |      |       |
  |Philippine        |  10   |  ?   |   10  |  10  |  11   |  19  |  18   |
  |Archipelago,      |       |      |       |      |       |      |       |
  |(_var._ 4)        |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |From California   |  15   |  24  |   -   |  -   |   -   |  -   |   22  |
  |(_var._ 5)        |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |From the bottom   |       |      |       |      |       |      |       |
  |of a ship         |   -   |  -   |   9   |  9   |  10   |  17  |  14   |
  |(_var._ 7),       |       |      |       |      |       |      |       |
  |young             |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |From South America|       |      |       |      |       |      |       |
  |(_var._ 1)        |   11  |  18  |  11   |  12  |  13   |  21  |  22   |
  |British Museum    |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |Another specimen  |       |      |       |      |       |      |       |
  |from the          |  16   |  25  |  13   |  18  |  15   |  23  |  27   |
  |same cluster with |       |      |       |      |       |      |       |
  |the last specimen |       |      |       |      |       |      |       |
  |------------------+-------+------+-------+------+-------+------+-------|
  |Same individual,  |       |      |       |      |       |      |       |
  |but the cirri     |  15   |  28  |  15   |  21  |  16   |  23  |   -   |
  |from the opposite |       |      |       |      |       |      |       |
  |side of the body  |       |      |       |      |       |      |       |
  +------------------+-------+------+-------+------+-------+------+-------+

  _Varieties._--Under the generic description, I have stated that after
  having spent some time in examining a very large suite of specimens
  of _T. porosa_, I concluded that at least four of the varieties were
  true species. It so happened that all the specimens which I first
  examined of the _var._ (4) _rubescens_, had the narrow sloping terga,
  and scuta with only two or three great teeth on their occludent
  margins; but ultimately, in a group thus characterised, I found one
  or two individuals with terga precisely of the shape of those in
  _var._ (1) _communis_. Again, in a group of dull purple specimens of
  _var. communis_, a few had the narrow sloping terga, and scuta with
  teeth on their occludent margins, intermediate in size and number
  between the varieties with only one or two great teeth, and those
  with every alternate growth-ridge enlarged into a tooth. Hence _var.
  rubescens_ completely broke down as a species.

  With respect to _var._ (5) _elegans_ (a M.S. specific name of Leach)
  I inferred at first, from external appearance alone, that it was
  distinct; the outer lamina of the shell and even the epidermis is
  preserved; the surface is strongly ribbed, and the whole shell,
  excepting the sheath, is nearly white; the terga are narrow, with a
  sloping basal margin as in _var._ (4) _rubescens_. Whole groups of
  specimens are thus characterised. But as _var. communis_ is often
  white, and as the surface, when the outer lamina is preserved, is
  generally, as we shall presently see, ribbed longitudinally, the
  differences in _var. elegans_ are quite unimportant.

  The (6th) _var._ differs from _var. communis_ only in the narrow
  radii having been developed, and consequently in the orifice being
  pentagonal, and in the outer, longitudinally ribbed lamina of the
  shell having been preserved. In the same group of specimens, I have
  seen every intermediate stage between this and the common form. It
  must not, however, be supposed that the young of _var. communis_
  always pass through these stages, for such is not the case. In the
  genus _Balanus_ it has been seen, how capricious in some species is
  the development of the radii.

  Of the other varieties, enumerated at the beginning, no further
  mention is required, excepting with respect to _var._ (7), the most
  singular of all. I have seen only three specimens, sent to me by Dr.
  Aug. Gould, of Boston, United States, and these from the appearance
  of their bases I have no doubt had been attached to a ship,--the
  only instance which I have met with, in the present species. The
  shell is steeply conical, with the orifice so small as to be reduced
  to a mere pore; the radii are extremely narrow and white; the shell
  is thin, with the surface smooth, but ribbed longitudinally and
  regularly; the outer lamina of the shell and the epidermis are
  perfectly preserved; the upper part of the shell is reddish purple,
  which dies away towards the base: careful examination of the apex
  shows that at the first growth the young shell was blueish-green.
  The terga are narrow, with a sloping basal margin, as in _var.
  rubescens_, but with the point of the spur sharper. I have formerly
  remarked that the shape of the terga seems influenced by the size
  of the orifice. The lower edge of the sheath depends freely: I have
  seen no other instance of this latter structure, so common, but so
  variable, in _Balanus_, in the present species, except to a partial
  extent in one distorted specimen, in Mr. Stutchbury's collection,
  adhering to _Balanus tintinnabulum_, and probably taken from a ship's
  bottom. I may add that this distorted specimen was remarkable from
  its radii being wider than in any other instance,--from its smooth
  uncoloured surface without longitudinal ribs,--and from the perfect
  preservation of the epidermis over its entire surface. Although Dr.
  Gould's specimens, in external aspect, are absolutely and entirely
  different from the common varieties of _T. porosa_, there are so many
  intermediate forms, and the differences are so little important that
  I feel no hesitation in attributing them to variation, consequent on
  the individuals having been exposed to unusual conditions, attached
  to the bottom of a ship.



2. TETRACLITA SERRATA. Pl. 10, fig. 2 _a_-2 _d_.

_Shell dark greenish-gray, with narrow, longitudinal, serrated ribs:
radii absent: scutum with the adductor and articular ridges forming a
cavity, which runs up to the apex of the valve._

  _Hab._--Cape of Good Hope; Algoa Bay; attached to sandstone and to
  Patellæ; Mus. Brit., Cuming, and Stutchbury.[108]

    [108] I have seen three separate lots of this species all from the
    Cape of Good Hope; one lot was collected by Dr. Krauss, at Algoa
    Bay, and I strongly suspect is the species described by him in his
    'Südafrikanischen Mollusken' as _Conia porosa_. If the species,
    figured by Chemnitz, and mentioned in a note (p. 329), under _T.
    porosa_, be the present species, the specimens probably did not
    come from Tranquebar, on which point Chemnitz speaks only from
    memory. I have seen one specimen ticketed New South Wales, it is
    possible, considering the case of _T. rosea_, that this may be
    correct, but I should like to have further confirmation before
    giving it as a habitat.


  _General Appearance._--Colour dark greenish gray; form steeply
  conical; surface covered, especially in the lower half of the shell,
  by numerous, narrow, sharp, longitudinal ridges, but slightly
  prominent, and serrated or transversely divided into small teeth:
  when the shell has been much disintegrated, the upper part of the
  surface consists of the exposed, smooth, rather large, upfilled
  parietal tubes. I have seen no instance of the development of
  the radii; sometimes even the sutures are with great difficulty
  distinguishable, though I believe they always reach the outer
  surface; sometimes the sutures are wide from the disintegration of
  the edges of the compartments. Orifice rounded or oval.

  _Scuta._--The scuta and carinal half of the terga are blueish-green.
  In the scuta neither the articular ridge or furrow are much
  developed: the adductor ridge is prominent, and is united to the
  articular ridge, about half way up the latter, thus forming a rather
  large, triangular cavity, which runs up to the apex of the valve.

  The _Terga_ are beaked. The spur, measured across the upper part,
  is half as wide as the valve; it is bluntly pointed; it is placed
  quite close to the basi-scutal angle of the valve, so that there
  is no basal margin on that side; it curves towards the scutum, its
  extremity extending beyond the basi-scutal angle.

  _Structure of the Shell and Radii._--The parietal tubes are rather
  large, especially those adjoining the inner lamina of the walls. The
  shell is of singularly little specific gravity, which is due to the
  parietal tubes not being filled up with shelly matter nearly to so
  great an extent as in the other species; even in the uppermost part
  the tubes are not solidly filled up, only their external sides are
  thickly coated with greenish-black shell, which by corrosion becomes
  grayish. The radii, as stated, are not developed: the shell breaks
  with singular facility along the sutures, and the radii are then seen
  to be most feebly represented by a few very small branching ridges.
  The alæ have their edges plainly crenated. The sheath is dark green,
  with the lower edge free.

  The _Mouth_ presents no particular characters. With regard to the
  _Cirri_, I am doubtful whether any confidence can be placed in the
  numbers of the segments being constant; but I may state that the
  second cirrus contained thirteen and sixteen segments in its two
  rami; the third cirrus only fourteen in both rami; and the sixth
  cirrus twenty-six segments in both rami. Whereas in every specimen
  of _T. porosa_, the longer ramus of the third cirrus contained more
  segments than either ramus of the second. About half the segments,
  namely, those in the middle of both rami of the third cirrus, are
  furnished with coarsely and doubly pectinated spines, like those in
  _T. porosa_.

  _Affinities._--Upon the whole, this species is more nearly allied
  to _T. porosa_ than to any other. In the cavity formed by the
  union of the adductor and articular ridges, it is allied to _T.
  cœrulescens_. This species differs from all, in its little specific
  gravity, consequent on the parietal tubes being only slightly filled
  up, and in the peculiarly serrated, narrow, approximate ridges on
  the external surface of the walls. The character derived from the
  adductor ridge, just alluded to, is remarkable. In the shape of the
  terga, in the absence of radii, and in the structure of the body,
  this species approaches closely to _T. porosa_.



3. TETRACLITA ROSEA. Pl. 10, fig. 3 _a_-3 _d_.

  CONIA ROSEA.[109] _Krauss_ (!). Die Südafrikanischen Mollusken, Tab.
        6, fig. 28, 1848.

  BALANUS CUMINGII. _Chenu._ Illust. Conch., Tab. 4, fig. 5.

    [109] I am greatly indebted to Professor Krauss for having sent
    me, for examination, the unique specimen collected by himself in
    Algoa Bay. There can be no doubt of the identity of the African and
    Australian specimens. It is a singular circumstance that the same
    species should occur in these two distant places, and, as far as at
    present known, not in the intermediate, more tropical coasts.

_Shell dirty white, tinged with pink; parietes formed by a single row
of large tubes: radii generally narrow: tergum with the spur rather
short and broad._

  _Hab._--New South Wales, Moreton Bay in lat. 27°, Port Jackson, and
  Twofold Bay; South Africa, Algoa Bay. Attached, in Australia, to
  littoral rocks and shells; often associated with _T. purpurascens_,
  _Chthamalus antennatus_, and _Catophragmus polymerus_; Mus. Brit.,
  Cuming, Krauss, Darwin, Stutchbury.


  _General Appearance._--Shell steeply conical, often rather convex;
  dirty or brownish white, feebly tinted with pink; external surface
  generally much disintegrated, and having in the upper part a pillared
  appearance, owing to the exposure of the upfilled, large, square,
  parietal tubes, and, in the lower part, a striated (and sometimes
  serrated) appearance, from the exposure of the parallel, approximate
  plates, with which the outer lamina of the shell is internally
  strengthened. In only a few young specimens, the whole outer lamina
  of the shell was well preserved; and in these the surface was very
  smooth, and even glossy, giving to the specimens a quite different
  aspect; even in partially corroded specimens, the lower part of the
  shell sometimes is quite smooth. Generally, the radii are developed;
  in most specimens they are narrow, but sometimes of moderate width;
  their summits are oblique, and their edges often notched or toothed.
  The recipient furrow, in each opposed compartment, is often almost
  as wide as the radius itself, and is equally notched. In some much
  corroded specimens there were no radii. Basal diameter of largest
  specimen, 1.1 of an inch.

  _Scuta_, generally thick, sometimes very thick, with the external
  surface usually much corroded: articular furrow rather wide;
  articular ridge not very prominent; adductor ridge prominent. The
  rostral depressor muscle is attached in a small oblong pit, sometimes
  including little crests; and the lateral depressor muscle is attached
  to what may be described either as three or four parallel furrows or
  crests.

  _Terga_, with the spur placed close to the basi-scutal angle, so that
  there is no basal margin on that side; spur short, with its lower end
  truncated and rounded; broad, even exceeding, when measured across
  the upper part, half the width of the valve. Articular furrow wide.
  Apex not beaked.

  _Structure of Walls and Radii._--This species differs from all the
  others of the genus in having only a single row (fig. 3 _d_) of
  parietal tubes; these are large, quadrangular, but elongated in the
  ray of the circle. They are not filled up, even at the very top of
  the shell, but they become thickly lined all round with compact
  shelly matter. When the surface of the shell is disintegrated,
  these upfilled tubes greatly affect, as already stated, the
  external appearance. The outer lamina near the basis is internally
  strengthened by longitudinal, sharp, approximate ridges or plates,
  which, also, often affect, after corrosion, the external appearance.
  The _radii_ have their sutural edges formed by a set of narrow,
  branching ridges or septa; the ends of which, seen externally, often
  give a notched outline to this edge; the recipient furrows in the
  opposed compartments are deep, and their edges likewise are often
  notched: the interspaces between the branching ridges are filled up
  solidly. The _alæ_ have their edges coarsely crenated. The lower edge
  of the sheath is not free.

  The _mouth_ and _cirri_ present no particular characters: the third
  cirrus has both its rami elongated, with the terminal segment
  tapering. In the three posterior pairs of cirri, the tufts of little
  spines between the main pairs are rather large.

  _Affinities._--This species has no particular affinity with any
  other. The circumstance of there being only a single row of parietal
  tubes is not so important a difference as might at first be thought,
  inasmuch as in the other species, during their quite early youth, the
  walls are formed of only a single row of tubes or pores.



4. TETRACLITA PURPURASCENS. Pl. 11, fig. 1 _a_-1 _d_.

  LEPAS PURPURASCENS.[110] _Wood's_ General Conchology, p. 55, Pl. 9,
        fig. 42, 1815.

  BALANUS PLICATUS. _Lamarck._ Animaux sans Vertèbres, 1818.

  ---- ---- ET PUNCTURATUS. _Chenu._ Illust. Conch., Tab. 4, fig. 3 et
        12.

  CONIA DEPRESSA (!). _J. E. Gray._ Appendix, Dieffenbach's Travels in
        New Zealand, 1843 (sine descript. vel figurâ).

    [110] The descriptions given by Wood and Lamarck are fuller and
    more accurate than is usual in the case of Cirripedes, and I have
    no doubt regarding these two names. The _Conia depressa_ of Dr.
    J. E. Gray is, as I know from having seen the original specimens,
    the young of this same species; the name is unaccompanied by any
    description or figure.

_Shell depressed, pale purple or dirty white, with the surface
longitudinally ribbed, or corroded and granulated: radii or even
sutures none, or radii well developed and broad, with their summits
parallel to the basis: basis membranous: scutum transversely elongated:
tergum small, with the spur extremely short and rounded._

  _Hab._--Sydney, New South Wales; Flinder's Lagoon, Sir C. Hardy's
  Island, Barrier Reef; King George's Sound, Western Australia; Van
  Diemen's Land; New Zealand, adhering to _Pollicipes spinosus_;
  Mus. Brit., Cuming, Stutchbury, Darwin, &c. China (?) attached to
  _Pollicipes mitella_, Mus. Brit. and Stutchbury. Generally attached
  to tidal rocks, sometimes to shells. Very common.


  _General Appearance._--Shell generally much depressed, in a few
  cases rather steeply conical, in one single instance cylindrical,
  but not much elongated. Colour, when alive, pale, but fine purple; I
  presume, judging from some dried specimens, sometimes dirty white.
  The state of the surface varies remarkably: about half the specimens
  (fig. 1 _a_) which I have seen, had the outer lamina of shell quite
  removed, and the surface granulated, owing to the projecting and
  exposed tips of the upfilled parietal tubes; the radii are not
  developed, and often even there is no trace of the four sutures;
  the rather large orifice is somewhat rounded, and the two scuta,
  with their surfaces disintegrated, have their middle parts deeply
  indenting the terga. The shell, in the other and perhaps more common
  condition (fig. 1 _b_), has the outer lamina preserved, and is
  longitudinally ribbed with generally at least five or six ribs on
  each compartment: the radii are here very wide, and extend from tip
  to tip of the compartments, so that their summits are parallel to the
  basis; they are generally covered by a brownish epidermis, thickly
  clothed with little spines; the orifice is neatly diamond-shaped;
  the apices of the opercular valves meet at a common point: these
  specimens are almost always smaller and younger than the granulated
  specimens. Altogether the specimens in the two opposite states have,
  in their external appearance, nothing in common, and no one, without
  careful examination, would ever suspect that they were specifically
  identical; this, however, was proved by the intermediate forms,
  and, in one instance, three of the compartments had their surfaces
  granulated, and were entirely destitute of the radii, whilst the
  fourth by some chance had been preserved from corrosion, was
  longitudinally ribbed, and had its epidermis-covered radius fully
  developed. The difference in the appearance of the opercular valves,
  in the two states, is simply owing to the degradation of their upper
  parts in the granulated specimens.

  The basal diameter of the largest specimen was one inch, but the
  height only .35 of an inch.

  _Scuta_, transversely elongated, so that the basal margin is nearly
  twice as long as the tergal margin: articular ridge very little
  prominent; articular furrow wide but shallow; adductor ridge very
  blunt, slightly prominent, sometimes almost absent, almost parallel
  to the basal margin: there are no distinct crests for the rostral
  or lateral depressor muscles, but some small irregular pits for the
  latter. In one young specimen, the lines of growth were crenated,
  showing a tendency in the valve to become longitudinally striated, as
  in the allied _T. costata_. In some young and immature specimens, the
  basal margin was deeply sinuous.

  _Terga_, small in area, not above half that of the scuta: spur
  extremely short, broad, placed close to the basi-scutal angle of the
  valve, so that there is no basal margin on that side of the spur. The
  lower end and sides of the spur form one uniform curve. Articular
  ridge barely developed. Crests for the tergal depressores sharp and
  prominent.

  _Structure of the Shell and Radii._--The walls are very thick, and
  the parietal tubes small and numerous; there are sometimes from
  twelve to fifteen rows of tubes in the thickness of the wall. The
  tubes in their whole upper part are filled up solidly; and, as
  we have seen, are often exposed by disintegration. In very young
  specimens, of that size in which in _T. porosa_ there would be only
  a single row of parietal tubes, there were here two or three rows.
  The development of the radii, as we have seen, is very capricious;
  the sutures even sometimes being lost. The radii, when developed,
  are broad, square on the summit, and covered by brownish hirsute
  epidermis: internally they are formed of tubes like those forming the
  parietes; in this respect differing from all the species except the
  following, _T. costata_. The tubes in the radii run obliquely down
  towards the basis; instead of in a transverse line, directly towards
  the opposite compartment, as might have been expected from the
  structure of the radii in Balanus. The alæ have their edges finely
  crenated. The sheath in all the specimens which I have observed is
  colourless; its lower edge is not free. The corium entering the
  parietal tubes, and lining the opercular valves, the mouth, and the
  anterior cirri, is generally of an extremely dark purple colour.

  The _Basis_ is entirely membranous, in which respect this species
  differs from all the others in the genus.

  _Mouth_: all the trophi are unusually hairy or spinose. The labrum is
  deeply notched and apparently destitute of teeth on the crest. The
  palpi are club-shaped or enlarged at their extremities. The mandibles
  have the fourth tooth rudimentary.

  In the _Cirri_, the second and third pairs are remarkably short
  and blunt. In one specimen the two rami of the first cirrus had
  respectively six and sixteen segments; those of the second, six and
  seven; those of the third, seven and seven; and those of the sixth
  cirrus twenty rather elongated segments, with a small tuft of spines
  between each main pair of spines.

  _Affinities._--This species differs from all, in its membranous
  basis, and in its transversely elongated scuta. It resembles _T.
  costata_, and differs from all the other species, in its radii
  (when developed) being square on the summit, and in being formed of
  tubes,--in the smallness and number of the parietal tubes,--in the
  absence of crests on the under side of the scuta for the rostral
  and lateral depressor muscles,--in the shortness and rounded form
  of the spur to the terga,--and, lastly, in the club-shaped palpi
  and small size or absence of the fourth tooth in the mandibles. _T.
  purpurascens_ differs from _T. costata_ in those points, namely, in
  its membranous basis and transversely elongated scuta, in which it
  differs from all the other species, and, moreover, in its scuta not
  being plainly striated longitudinally, in having more ribs on the
  external surface of the parietes of its shell, and in having only
  three pairs of main spines on the three posterior cirri.



5. TETRACLITA COSTATA. Pl. 11, fig. 2 _a_-2 _c_.

_Shell depressed, whitish, generally with ten very prominent
longitudinal ribs: radii broad, with their summits parallel to the
basis: basis calcareous: scutum externally striated longitudinally:
tergum with the spur short and rounded._

  _Hab._--Philippine Archipelago, Mus. Cuming. Attached to various
  shells, within the tidal limit.


  _General Appearance._--Shell whitish, probably tinged, when alive,
  with reddish-purple; depressed; surface perfectly preserved, smooth,
  but having longitudinal very prominent ribs, almost invariably ten
  in number; namely, three on both the rostrum and carina, and two on
  the two lateral compartments, with ten corresponding projections
  round the basal margin. Orifice passing from rounded-trigonal to
  diamond-shaped. The radii are very broad and square at the summit,
  and extend from tip to tip of the compartments. Basal diameter of
  largest specimen under half an inch, generally from .3 to .4 of an
  inch.

  _Scuta_, of the usual sub-triangular shape, and not transversely
  elongated, as in _T. purpurascens_. External surface striated
  longitudinally; in many specimens there is a medial depression, or
  a row of very small pits, such as occur on the scuta of _Balanus
  trigonus_ and _lævis_. The adductor ridge is moderately developed,
  and runs nearly parallel to the occludent margin; there are no crests
  for the rostral and lateral depressor muscles.

  _Terga_: these in area equal two thirds of the scuta: the spur is
  short and rounded, and placed as described under _T. purpurascens_;
  but the articular ridge seems to be more prominent than in that
  species.

  _Structure of the Shell and Radii._--The parietal tubes are small,
  and very numerous, as in _T. purpurascens_. The radii are wide,
  square on the summits, but not so conspicuously covered by hirsute
  epidermis as in that species. Internally, the tubes forming the radii
  are smaller, and run more transversely than in _T. purpurascens_,
  that is in the normal course, as in Balanus. The edges of the alæ are
  nearly or quite smooth. The _Basis_ is as distinctly calcareous, as
  in the other species of the genus.

  _Mouth_: the trophi are not so hairy as in _T. purpurascens_; the
  labrum seems destitute of teeth; the palpi are club-shaped at their
  ends; the mandibles have only three teeth. The second and third
  cirri are not so short and blunt relatively to the others as in _T.
  purpurascens_. In the posterior cirri, the elongated segments carry
  four main pairs of spines, between which there is no intermediate
  tuft of fine spines.

  The _Affinities_ of this species have been fully pointed out under
  the last and closely related species. In external appearance, _T.
  costata_ can at first hardly be distinguished from those young and
  pale-coloured varieties of _T. purpurascens_, which have their
  external surface not corroded, and their radii well developed.



6. TETRACLITA VITIATA. Pl. 11, fig. 3 _a_-3 _e_.

_Shell white, generally tinged in the upper part with pink; surface
irregular: parietal tubes very irregular: radii moderately wide, with
their summits oblique: alæ with very thick crenated sutural edges:
tergum with the spur not joined to the basi-scutal angle; spur with its
extremity equably rounded._

  _Hab._--Philippine Archipelago; Barrier Reef, (Raine's Islet),
  Australia; Mus. Cuming and Stutchbury. Attached to massive corals, to
  coral-rock, to a Tridacna, and to _Tetraclita cœrulescens_.


  _General Appearance._--Shell conical, moderately steep: white,
  generally with a tinge of pinkish-purple in the upper part, owing to
  the exposure of the tips of the upfilled parietal tubes. The lower
  part of the surface is generally well preserved, and is formed by
  very irregular, branching, longitudinal, slightly prominent, ridges.
  Radii of moderate width, with their summits oblique. Orifice rather
  large, rounded trigonal.

  _Scuta_, rather narrow, with the upper part acuminated: the external
  surface generally much disintegrated, and marked with some irregular
  blotches of dark red. Articular ridge not prominent; articular furrow
  rather deep: adductor ridge, distinct from the articular ridge,
  pretty well developed, as are the crests for the rostral and lateral
  depressor muscles. In young and well preserved scuta, there is an
  external, medial, hyaline band, corresponding with the hollow under
  the adductor ridge, and caused by the thinness of the valve along
  this line.

  _Terga_: externally, the carinal half is longitudinally and very
  feebly striated. Internally, the articular furrow is very wide, but
  shallow, and of unusual length, owing to the preservation of the
  upper part of the valve; the articular ridge is not prominent. The
  spur does not actually join, as in all the foregoing species, the
  basi-scutal angle, but is separated from it by a short piece of basal
  margin; its two sides are more nearly parallel than is usual, and
  the end is regularly rounded. It is always rather narrow, though the
  width varies considerably (fig. 3 _d_, 3 _e_). It extends in the same
  straight line with the middle of the articular furrow. The terga,
  though not possessing any striking characters, differ considerably in
  appearance from those of the other species.

  _Structure of the Shell and Radii._--The parietal tubes are
  remarkable from their irregular shapes, and unequal sizes (fig. 3
  _b_),--hardly two resembling each other. Sometimes a single elongated
  tube will reach across the whole thickness of the walls. The septa
  between the tubes are rather thick and rugged. The tubes are
  generally darkly coloured from the adhering corium; they are solidly
  upfilled, but only in the uppermost part, by dark chocolate-red
  shelly matter. The radii are formed by irregularly branching ridges,
  with the interspaces filled up solidly. The square edges of the alæ
  are much thicker than in any other species, and are furnished with
  transverse ridges, which are sometimes slightly branched. The inner
  lamina of the walls near the basis, in most of the specimens, is
  irregularly and longitudinally ribbed, the ribs being longitudinally
  striated. The sheath and the upper part of the inner lamina of the
  parietes are clouded with chocolate-red.

  The animal's _Mouth_ and _Cirri_ were ill-preserved; but I was able
  to make out that the labrum had some strong teeth, and that the
  mandibles were furnished with five teeth, a greater number than in
  any other species. The palpi had parallel sides as usual. In the
  sixth cirrus, the segments had four pairs of main spines, instead of
  the usual number of three.

  _Affinities._--This species does not appear to be particularly
  related to any other one: perhaps it is rather nearer to the two
  following than to the foregoing species. The irregular parietal
  tubes, thick-edged alæ, form of terga, five teeth to the mandibles,
  and four pairs of spines to the segments of the posterior cirri, are
  its chief characteristics.



7. TETRACLITA CŒRULESCENS. Pl. 11, fig. 4 _a_-4 _d_.

  LEPAS CŒRULESCENS. _Spengler._ Skrivter af Selskabet, 1 Bind.,
        1790.[111]

    [111] The longitudinally folded walls, as described by Spengler,
    the blue colour, the habitat, namely, associated with _B.
    tintinnabulum_ from the East Indies, and more especially the
    expression "Valvulæ operculi cardine dentato mobilis," apparently
    referring to the highly prominent articular ridge of the scutum,
    leave little doubt on my mind that I have rightly named the present
    species.

_Shell with the upper part tinged greenish-blue, longitudinally ribbed:
radii moderately wide, with their summits oblique: scutum with a small
adductor and extremely prominent articular ridge, united together and
so forming a small sub-cylindrical cavity: tergum with the spur not
joined to the basi-scutal angle._

  _Hab._--Philippine Archipelago, attached to _Balanus tintinnabulum_;
  attached to a ship's bottom, and to _Balanus tintinnabulum_, both
  from the Pacific Ocean; attached to a massive coral, and associated
  with _T. vitiata_, and therefore from the tropical eastern seas; Mus.
  Brit., Cuming, Stutchbury.


  _General Appearance._--Shell conical, sometimes depressed; surface
  with rather broad, smooth, longitudinal ridges; whitish, with the
  upper part greenish-blue, sometimes very feebly tinted with pink;
  radii white, or mottled with blueish-green, or with pink. When the
  outer lamina of shell has been corroded, the upfilled parietal tubes,
  of a dull blueish-gray colour, are exposed. The radii are moderately
  wide, with their summits very oblique. In basal diameter, one
  specimen was 1.8, and another 1.5 of an inch.

  _Scuta_, externally furrowed very slightly in a longitudinal
  direction, causing the lines of growth to become a little sinuous.
  The valve is strong and thick; and the epidermis, when preserved,
  is hirsute with spines. The articular ridge is extraordinarily
  prominent; it projects, as measured from the external surface of
  the valve, to an amount equalling half the width of the valve in
  its widest part. The adductor ridge is very short, and is united
  to the bottom of the articular ridge, thus forming a small, nearly
  cylindrical tube, which runs up to near the apex of the valve. The
  inflected occludent margin is broad and coarsely toothed. The crests
  for both depressor muscles are not very prominent. When the scuta
  and terga are articulated together, owing to the great projection of
  the articular ridge of the scutum, its upper part is separated from
  the tergum (fig. 4 _b_), by a remarkably wide and deep, fissure-like
  hollow. The scutum, in some distorted specimens from a ship's bottom,
  was narrow in proportion to its ordinary breadth.

  _Terga._--These are large. Externally, there is a broad, longitudinal
  depression, bounded on each side by a ridge. The carinal half is
  feebly striated longitudinally. A large upper portion (fig. 4 _b_)
  projects freely, and stands, when the two valves are articulated
  together, above the apex of the scutum, but does not form a beak as
  in _T. porosa_. Internally, the tergal margin is widely inflected;
  the articular furrow is very deep, but the articular ridge is not
  prominent. The spur is not very broad; it is separated from the
  basi-scutal angle by a small space of basal margin, which forms a
  straight line with the basal margin on the opposite side of the spur.
  The end of the spur is truncated, and is parallel to the basal margin.

  _Structure of the Shell and Radii._--The walls are thick: the
  parietal tubes are rather large and regular; they become solidly
  filled up in their upper parts. The sutural edges of the radii are
  formed by unusually narrow, sinuous ridges, sending off delicate
  denticuli on each side: the interspaces between these ridges are
  solidly filled up. The crenated edges of the alæ are rather thick.
  Neither the _mouth_ nor the _cirri_ offer any noticeable character;
  but I may observe, that, in the mandibles, the third and fourth teeth
  are close together; and that, in the three posterior cirri, the tufts
  of small spines between the main pairs are small.

  _Affinities._--This species is very distinct from the others, with
  the exception of the following _T. radiata_, to which it is in
  several respects allied. The under side of the scutum, with its great
  articular ridge, and the cylindrical tube formed by the union of the
  latter with the short adductor ridge, affords the most noticeable
  character.



8. TETRACLITA RADIATA. Pl. 11, fig. 5 _a_-5 _d_.

  CONIA RADIATA.[112] _De Blainville._ Dict. Sc. Nat., 1816-1830, Pl.
        164, fig. 5, 5 _a_, (sine descript.)

  ---- LYONSII. _G. B. Sowerby._ Genera of Recent and Fossil Shells,
        Plate, 1823, (sine descript.)

    [112] The synonymy of this species is complicated. De Blainville
    gives no description under the article Conia, published in 1818, or
    in the vol. published in 1822; but I believe, from the figures of
    the opercular valves, that I have correctly identified this species
    with his _C. radiata_. Mr. Sowerby gives no description of _C.
    Lyonsii_, or any figure of the opercular valves, but his drawing
    of the shell is much better than de Blainville's, and I believe
    it is the same species. Whether de Blainville's or Sowerby's
    plate appeared first I cannot ascertain. In the second edition of
    Lamarck, _Conia radiata_ of Blainville is given as a synonym to
    _Balanus radiatus_ of that work; but this is quite erroneous. I
    may add that if de Blainville's name does not apply to the present
    species, it must to _T. cœrulescens_, and as the latter is the
    older name it will be permanent. In this case, _T. radiata_ might
    be allowed to stand as my own name, considering that Mr. Sowerby's
    figure is imperfect and is not accompanied by any description.
    At first I thought that the present species might be the _Lepas
    mitra_ of Spengler, ('Skrivter af Naturhist. Selskabet,' 1790,
    Tab. 6, fig. 5), but the parietes are not described as porose;
    and the folds on the walls are too broad; on the other hand, his
    description of the opercular valves makes me think this may be the
    same species.

_Shell white, with numerous approximate longitudinal ribs: radii broad,
with their summits slightly oblique, internally porose: tergum with the
articular ridge extraordinarily prominent, with the spur not joined to
the basi-scutal angle._

  _Hab._--West Indies, adhering to _Balanus eburneus_ and to _Lepas
  anserifera_. New South Wales, adhering to _Tetraclita porosa_.
  Attached to _Balanus tintinnabulum_, on a ship's bottom from Sumatra;
  not rarely attached to _Balanus tintinnabulum_ on ships' bottoms;
  Mus. Brit., Stutchbury, and Cuming.


  _General Appearance._--Shell white, rather steeply conical, with
  numerous, approximate, rather narrow, longitudinal, rounded ribs on
  the walls: in a specimen half an inch in diameter, there were from
  eight to twelve ribs on each compartment. The outer lamina of shell
  seems always well preserved. Orifice rounded, trigonal. Radii white,
  smooth, broad, with their summits only slightly oblique. I have
  seen one specimen 1.2 in basal diameter, but quarter of an inch is
  a common size, and very young specimens are unusually frequent in
  collections.

  _Scuta_ broad, externally not striated longitudinally. The articular
  ridge is prominent, and the furrow deep, but not in so great a
  degree as in _T. cœrulescens_. The adductor ridge is only slightly
  prominent; it extends upwards only a little way above the lower end
  of the articular ridge, and does not form with the latter a cavity.
  There are no crests for the rostral depressor muscle, but there is a
  little pit, formed by the folding over of the occludent margin.

  _Terga._--These valves, when articulated with the scuta (fig. 5 _b_),
  project above them to an extraordinary degree, and are separated
  from them by a deep, fissure-like hollow, caused by the remarkable
  prominence of the articular ridge of the terga. The upper part of
  the tergum is not beaked, and does not project freely much above the
  sack. The valve is large; externally there is a rounded longitudinal
  furrow. The tergal margin is broadly inflected. The articular furrow
  is deep, and the articular ridge far more prominent than in any other
  sessile cirripede, for it projects, as measured from the outside
  surface, more than half the width of the valve; and consequently
  the valve, when viewed vertically from above, almost appears as if
  formed by the union of three plates, viz., the articular ridge, and
  the outside surface on each side of the spur. The spur is of moderate
  width, with the corners rounded: it is placed near, but not close to
  the basi-scutal angle, so that there is on this side a small portion
  of basal margin, forming nearly a straight line with the margin on
  the carinal side. In some young specimens, about the tenth of an
  inch in diameter, from the West Indies and from New South Wales, the
  spur (the position of which I found varied a little in some other
  specimens) was placed nearly in the middle of the valve, and very
  nearly at right angles to the basal margin; it is possible that these
  may be a distinct species, but without larger specimens to judge
  from, I think it more probable that this difference in the tergum is
  due to variation and youth.

  _Structure of Shell and Radii._--The parietal tubes are commonly
  elongated in the ray of the circle: the septa are rather thick, and
  strongly crenated at their basal edges. The inner lamina of the
  walls is strongly ribbed longitudinally. The broad radii have their
  sutural edges formed by ridges, with numerous and closely approximate
  denticuli: the interspaces between the main ridges are not soon
  filled up, and at the bottom, each interspace usually terminates in
  a pore or tube; so that the radii are not solid, as in most of the
  foregoing species, but porose. The alæ have their edges crenated.

  _Basis_, calcareous, of unusual thickness; the inner, or upper
  surface, is striated from the centre in rays, corresponding with the
  ribs on the inner lamina of the walls. This striated or furrowed
  structure in the basis, shows a tendency to its becoming tubular or
  porose, as may be inferred from analogous cases in Balanus.

  _Animal's body_ unknown.

  _Affinities._--This species is rather more closely allied to the last
  than to any other. There is a close analogy in the peculiar manner
  in which the scuta and terga are articulated together in the two
  species: in this species it is effected by the great development of
  the articular ridge of the tergum, and in _T. cœrulescens_ by that
  of the scutum. The internally striated calcareous basis, and the
  internal tubular interspaces between the denticulated ridges of the
  radii, are peculiar characters. The white colour, the narrowly and
  closely ribbed parietes, and the broad radii, give this species an
  aspect, by which it can be easily recognised.



4. _Genus_--ELMINIUS.

  ELMINIUS. _Leach._ Zoological Journal, vol. 2, July, 1825.

_Compartments four: parietes not porose. Basis membranous._

  _Distribution_, Southern temperate seas.


_General Appearance._--Shell conical, with a strong tendency in most
of the species to become cylindrical: orifice generally large. Walls
either thin and smooth, or thick and plicated longitudinally. Colours
various, pale purple, greenish, white, and, in _E. plicatus_, owing to
the exposure of an intermediate lamina of shell, bright orange-yellow.
Radii, either of considerable width, with their summits oblique and
rounded, as in the first two species of the genus, or very narrow, as
in the last two species. _Elminius plicatus_ is the largest species,
and is sometimes one inch in basal diameter. The outer surface of this
latter species is occasionally much corroded.

_Scuta_: these are of the usual shape; in _E. Kingii_ and _modestus_
there is no adductor ridge and no crests for the depressor muscles;
in _E. plicatus_ and _simplex_, on the other hand, there is a well
developed adductor ridge and crests for the lateral depressor muscles;
in some individuals, also, of _E. plicatus_ there are small crests for
the rostral depressores.

The _Terga_ are remarkable for their variability in all the species;
in many specimens of _E. Kingii_ and _modestus_ the basal margin on
the carinal side of the spur is deeply hollowed out. The width and
acumination of the spur varies in all the species. In _E. plicatus_ and
_simplex_ this valve is remarkably like that of _Tetraclita porosa_. In
some specimens of _E. Kingii_ the terga and scuta are firmly calcified
together.

_Structure of the Parietes and Radii._--As in Tetraclita, the two
lateral compartments are necessarily broad. The parietes are never
porose, but consist, in appearance, of a single layer of shell. In _E.
modestus_ the basal internal edges of the parietes are smooth, but in
the other species they are striated longitudinally with short ridges,
or sometimes with sub-cylindrical projections. In those specimens of
_E. plicatus_, which have externally suffered much corrosion, the walls
have been rendered extremely thick, by the inward production of these
ridges or plates; and in this case the ridges are not confined to the
basal edges, but extend upwards close to the sheath. The basal surfaces
of the walls in these latter specimens resemble those of Chelonobia,
but the walls in that genus have an internal lamina, which here is not
the case. The radii are wide in _E. Kingii_, and of moderate width in
_E. modestus_, with their summits oblique and smoothly rounded, and
their sutural edges not in the least crenated. In _E. simplex_ they
are extremely narrow, smooth-edged, and rounded: in _E. plicatus_ they
are narrow, and in this species alone the sutural edge is sinuous, and
sends inwards short ridges or teeth. The alæ, in all the species except
this last, are likewise smooth-edged. The lower edge of the sheath
depends, more or less freely, in all the species, except in _E. Kingii_.

_Basis_, membranous in all four species. In _E. modestus_, the true
basal membrane is extremely thin, and is divided into concentric slips:
on its inner surface there are attached numerous cement-ducts, varying
from 1/3000 to 1/2000 of an inch in diameter, repeatedly trifurcating,
rarely forming hexagonal or quadrangular loops, and with the branches
placed approximately parallel to each other. Beneath the true basal
membrane there is a complicated layer of cement, in the form of a
network, or of separate tubes, or in beads and patches. In _E. Kingii_,
the basal membrane presented a wonderfully complicated appearance, in
part due to the cement forming a mass of inosculating fibres; many of
these fibres seemed to end in circular discs of cement.

Neither the _Mouth_ or _Cirri_ offer any noticeable generic characters,
as distinct from Balanus and several other genera. The _Branchiæ_,
in _E. plicatus_, are well developed and moderately plicated. In _E.
modestus_ they are small, not plicated, but with a rounded sinuous
margin: in a specimen having a basal diameter of 25/100, the branchiæ
in total length were only 4/100 of an inch. At the bottom of the sack
I observed some inwardly pointed, tapering filaments, such as occur in
Balanus. In this same species I measured the ova, which were unusually
elongated, being 19/2000 in length; I may add, that the probosciformed
penis was actually thrice the length of the animal's body in some small
but mature specimens (with ova), having a shell with a basal diameter
of 16/100 of an inch.

_Distribution and Habitats._--This genus is remarkable, inasmuch as it
is not distributed over the whole globe: three of the species occur
very commonly on the shores of New South Wales, Van Diemen's Land,
and New Zealand; not extending, as far as I can judge, much north of
Sydney: the fourth species is confined to South America, ranging
from the Falkland Islands and Tierra del Fuego, as far north as
Chiloe. Elminius, therefore, appears to be strictly a southern genus.
_Elminius Kingii_ and _modestus_ represent each other on the American
and Australian continents; so I believe _E. plicatus_, in New Zealand,
represents _E. simplex_ in New South Wales and Van Diemen's Land.
The species are all attached to tidal rocks and shells. _E. Kingii_
is sometimes attached to floating wood. At the Falkland Islands, the
last-mentioned species adhered to some rocks, in a running brook of
fresh water, at most eighteen inches under high-water mark, so that for
the greater part of each tide it was exposed to absolutely fresh water.
At Sydney I found _E. modestus_ adhering to oysters in a muddy lagoon,
almost separated from the sea, and apparently very unfavorable for
cirripedes.

_Affinities._--This genus can be distinguished from Tetraclita only
by the four compartments not being porose, and by the basis being
always membranous; whereas, in _Tetraclita purpurascens_ alone it is
membranous. _Elminius Kingii_ and _modestus_, on the one hand, are
closely allied together, as are _E. simplex_ and _plicatus_ on the
other hand. The last-named species, in the characters of its opercular
valves and in its shell, comes nearest to Tetraclita. In _T. rosea_ we
have seen that there is only a single row of parietal tubes, and the
outer lamina of the shell is strengthened (as, indeed, it is in most of
the other species of the genus, and in Balanus) by small longitudinal
plates or ridges, which are similar and homologous to those on the
internal basal edges of the parietes in three of the species of
Elminius; so that the difference in the structure of the parietes, in
Tetraclita and Elminius, is small.



1. ELMINIUS KINGII. Pl. 11, fig. 6 _a_-6 _e_.

  ELMINIUS KINGII. _J. E. Gray._ Zoological Miscellany, p. 13, 1831.

  ---- LEACHII. _King_ and _Broderip_. Zoological Journal, vol. 5,
        1832-1834, p. 334, and appendix to King and Fitzroy's Voyages.

  ---- ---- _G. B. Sowerby._ Genera of Recent and Fossil Shells,
        Plate.

_Shell smooth, gray or dirty white: radii broad, smooth-edged: scutum
without an adductor ridge; tergum with the spur distinct from the
basi-scutal angle: scutum and tergum sometimes calcified together._

  _Hab._--Tierra del Fuego, Falkland Islands, Chiloe. Attached to tidal
  rocks and sometimes to floating timber; Mus. Brit., Darwin, &c.


  _General Appearance._--Shell fragile, either steeply conical with a
  large orifice, or sub-cylindrical; surface smooth, grayish or white,
  with large portions covered by pale brown epidermis. Radii broad with
  their summits oblique, smooth, slightly arched, exhibiting a large
  surface of the alæ. The alæ usually have their summits much less
  oblique than those of the radii; the portion added during diametric
  growth is of a dead white colour. The growth ridges on the scuta
  are very little prominent, and are crossed by a very obscure band
  of blueish-gray. The largest specimen which I have seen was .8 in
  basal diameter, and the longest cylindrical variety .55 of an inch in
  height.

  The _Scuta_ are remarkable for not having any adductor ridge or
  crests for the depressor muscles; the articular ridge is prominent,
  but it is short, not extending down half the valve. I have mentioned
  under the genus, that in many specimens at the Falkland Islands the
  scuta and terga were calcified together.

  The _Terga_ are rather small: the basal margin on the carinal side
  of the spur is always hollowed out, but to a very variable degree,
  as may be seen in the three figures (6 _c_-6 _e_); this margin is
  generally dentated with one or two little points; and an inner lamina
  of shell sometimes depends beneath the outer lamina, to which the
  opercular membrane is attached, as may be seen in the figure (6 _d_)
  of the external surface of the valve. The crests for the depressor
  muscles are well developed. The tergal margin is broadly inflected,
  and the articular ridge prominent, making the articular furrow deep.
  The spur is rather narrow, and is either (6 _d_, 6 _e_) bluntly or
  sharply pointed. The basal margin on the scutal side of the spur, is
  hollowed out, but to a variable depth.

  _Structure of the Parietes and Radii._--The parietes are thin; at
  their internal basal edges they are finely striated in longitudinal
  lines. The radii are solid, with quite smooth edges; they are
  generally covered by the epidermis. The sutural edges of the alæ are
  likewise smooth, these are added to largely during the diametric
  growth; and their summits, as already stated, are much less oblique
  than the summits of the radii. The internal surface of the shell
  is smooth, and is tinted pale dull purple. The lower edge of the
  sheath can hardly be said to be free. The carinal margins of the
  compartments project a little inwards.

  _Mouth_: the labrum is deeply notched, and supports five little teeth
  on each side; the palpi are thickly clothed with spines on their
  inner sides; the mandibles have five or only four teeth: the maxillæ
  are notched, and the outer maxillæ bilobed.

  _Cirri_: the first pair has one ramus nearly twice as long as the
  other. The three posterior cirri are elongated, and each segment
  supports five or six pairs of long spines, with a few minute
  intermediate bristles.



2. ELMINIUS MODESTUS. Pl. 12, fig. 1 _a_-1 _e_.

_Shell folded longitudinally, greenish or white: radii of moderate
breadth, smooth edged: scutum without an adductor ridge: tergum narrow,
with the spur confluent with the basi-scutal angle._

  _Hab._--New South Wales; Van Diemen's Land; New Zealand; very
  commonly attached to littoral shells and rocks; associated with
  _Balanus trigonus_ and _vestitus_; Mus. Brit., Cuming, Stutchbury,
  Darwin.


  _General Appearance._--Shell conical, generally rather steep,
  occasionally depressed: walls longitudinally folded, sometimes very
  deeply, sometimes only to a slight degree: colour dull greenish or
  white. Radii of moderate width, with their summits very oblique,
  smooth and slightly arched: alæ much exposed, with their summits
  less oblique than those of the radii: the portion added to the alæ
  during the diametric growth differs much in appearance from the other
  portion. The scuta have the growth ridges but little prominent;
  they are crossed by a faint longitudinal band of gray. The largest
  specimen out of the many which I have seen, was under .4 of an inch
  in basal diameter.

  _Scuta_, destitute of an adductor ridge and of crests for the
  depressor muscles: the articular ridge is moderate; but the
  articular furrow is rather wide: the internal occludent margin is
  much thickened. The _Terga_ are narrow and small; they are somewhat
  variable in shape, caused by the degree to which the basal margin is
  hollowed out (fig. 1 _c_-1 _e_), and likewise by the extent to which
  the upper end of the valve has been worn away. No spur is apparent,
  for it is confluent with the basi-scutal angle of the valve. The
  articular ridge is very prominent, and runs down to the basi-scutal
  angle; and as the valve in this part is extremely narrow, with the
  spur not developed, it here assumes a channelled structure. The
  basi-carinal corner of the valve is furnished with rather feeble
  crests for the depressor muscles, and in those varieties in which the
  basal margin is much hollowed out, this part is remarkably narrow.

  _Structure of the Parietes and Radii._--The internal basal edges of
  the parietes and the sutural edges of the radii and alæ, are all
  smooth. The lower edge of the sheath depends freely. In the green
  varieties the colour is most distinct on the internal surface of the
  shell. The four compartments separate very easily when the shell has
  been ill preserved in spirits, or after a very short immersion in
  caustic potash.

  _Mouth_, as in _E. Kingii_, excepting that there are only three teeth
  on each side of the notch (which is deeper) on the labrum. The cirri
  resemble those of _E. Kingii_; the segments in the sixth pair are
  equally elongated, and bear five or six pairs of spines.

  _Affinities._--This species is closely allied to its South American
  representative _E. Kingii_; the differences consist in its smaller
  size, often greenish colour, more folded walls, and narrower radii:
  the internal basal edges, also, of the parietes are here smooth,
  instead of being striated, as in _E. Kingii_. The terga present even
  more obvious differences, in their narrowness, channelled under
  surface, and in the absence of the spur, or more properly in its
  confluence with the basi-scutal angle of the valve.



3. ELMINIUS PLICATUS. Pl. 12, fig. 2 _a_-2 _f_.

  ELMINIUS PLICATUS. _J. E. Gray._ Appendix to Dieffenbach's Travels in
        New Zealand, p. 269, 1843.

_Shell deeply folded longitudinally, corroded, coloured in parts
orange: radii very narrow, with their edges sinuous, and slightly
dentated: scutum having an adductor ridge._

  _Hab._--New Zealand; New South Wales(?). Attached to rocks, often
  coated by _Chamæsipho columna_; Mus. Brit. and Cuming.


  _General Appearance._--Shell tubulo-conical, or conical, rarely
  depressed; strong, rugged, coloured in parts bright orange; deeply
  plicated longitudinally, but with the upper parts corroded and
  smooth. Orifice large. The sutures are indistinct and almost
  obliterated; the radii, when most developed, are narrow. Some
  specimens have their whole surface deeply corroded; in which case
  they are finely striated longitudinally, or pitted, and are of a
  gray or brown colour. The largest specimens are one inch in basal
  diameter, but one depressed specimen was 1.3 in diameter; another was
  rather under one inch in diameter, and one inch in height.

  _Scuta_; beginning with the common tubulo-conical and not much
  corroded specimens, the valve (fig. 2 _c_) is moderately elongated,
  but in a rather variable degree. A prominent adductor ridge runs,
  from a little above a middle point of the basal margin, along
  the slightly prominent articular ridge: the articular furrow
  is moderately wide. There are distinct crests for the lateral
  depressores. In the conical, corroded specimens, the scuta (fig. 2
  _e_) are considerably broader, with the articular ridge much more
  prominent, and the furrow wider: in one such specimen, there were
  crests for the rostral depressor muscle.

  The _Terga_, in the commoner variety, resemble those of _Tetraclita
  porosa_; the spur adjoins the basi-scutal angle of the valve: the
  articular ridge is moderately prominent, and the furrow moderately
  deep. The valve is beaked, with an unusually large internal tube
  for the thread of corium: the beak, however, is often worn away. In
  the depressed much corroded specimens, the terga (fig. 2 _f_), like
  the scuta, are broader and shorter than in the commoner variety;
  and the spur more especially is broader. The scutal margin is much
  more widely inflected, and the articular ridge much more prominent;
  consequently the articular furrow is much deeper.

  _Structure of the parietes and radii._--The orange or yolk-of-egg
  colour, which is so conspicuous a character in the present species,
  is due to a layer of shell between the inner and outer lamina, and
  is exposed only by the corrosion of the latter. Hence the very base
  of the shell is not of this colour; nor are the uppermost and still
  more deeply corroded portions, for here the orange-coloured layer has
  been removed. The sheath is orange-coloured, and the operculum, to a
  certain extent, is similarly tinted. The epidermis on the parietes,
  where preserved quite close to the basis, supports remarkably strong
  spines, about 1/100th of an inch in length. The basal internal edges
  of the walls are rather coarsely striated with irregular short ridges
  and sub-cylindrical points; and the walls in most of the specimens
  are regularly and deeply folded, which, with the little ridges, gives
  the appearance represented in fig. 2 _b_, Pl. 12. I have stated,
  under the Genus, that in the corroded and depressed specimens, the
  walls are rendered extremely thick by the inward production and
  upward extension of these same ridges and points; the under surface
  of the shell acquiring almost the appearance of _Chelonobia caretta_.
  The Radii are often not developed, even the sutures being obscure;
  when most developed, they are narrow, with the outer lamina along the
  growing edge sinuous, giving to the sutures a crenated appearance.
  The sinuosities on the growing edge generally send inwards short
  ridges or septa, like those on the sutural edges of the radii in most
  Balanidæ, but of which there is no trace in the other species of
  Elminius. In very minute, colourless specimens, about the 1/20th of
  an inch in diameter, the radii are quite smooth-edged. The alæ have
  their edges strongly crenated. The lower edge of the sheath depends
  freely.

  _Mouth_: the labrum shows some tendency to be bullate; the notch is
  broad and shallow: the palpi have a thick brush of bristles on their
  inner sides. The mandibles have four or five teeth. In the maxillæ,
  the upper spines above the broad notch, are very strong. In the outer
  maxillæ, the two lobes are widely separated.

  _Cirri_: in the first pair, one ramus is about one fifth longer
  than the other. In the third pair, the posterior ramus is one
  fourth longer than the anterior ramus, and its terminal segments
  are tapering, each having a single circle of bristles: the other
  segments, and those of the shorter ramus, support many coarsely
  pectinated spines. In the sixth cirrus, the segments are protuberant
  in front, and carry four pairs of stout spines, with a tuft of fine
  bristles between them.

  _Affinities._--This species differs considerably from the first two
  of the genus. In several characters it approaches nearer than the
  other species to Tetraclita, especially to _T. porosa_;--namely,
  in the scutum having an adductor ridge and crests for the lateral
  depressores, in the whole form of the tergum, in the thick walls
  liable to much corrosion, in the narrow radii, and in their edges,
  as well as those of the alæ, being crenated; and, lastly, in the
  character of the cirri, more especially of the third pair, with
  its coarsely pectinated spines. It also approaches, in all its
  characters, _Balanus imperator_ and _flosculus_.



4. ELMINIUS SIMPLEX. Pl. 12, fig. 3.

_Shell ribbed longitudinally, dirty white; radii extremely narrow,
smooth-edged; scutum having an adductor ridge._

  _Hab._--New South Wales (Sydney and Twofold Bay); Van Diemen's Land;
  tidal rocks, often attached to other Cirripedes, and associated
  with _Balanus nigrescens_, _Tetraclita purpurascens_, _Catophragmus
  polymerus_; Mus. Brit., Cuming, Stutchbury, and Darwin.


This species, of which I have seen specimens from the above three
localities, all exactly agreeing with each other, is perhaps the
Australian representative of _E. plicatus_, which seems to be confined
to New Zealand.[113] In all essential points it comes so near that
species, that I shall make the greater part of my description
comparative.

    [113] I am bound to state that I have seen two specimens of _E.
    plicatus_ marked Sydney, and one marked Moreton Bay, but in both
    cases the collectors had visited New Zealand, so that a wrong
    habitat by mistake might easily have got attached to the specimens
    in question.

  _General Appearance._--In external appearance there is considerable
  difference from _E. plicatus_, for _E. simplex_ is generally of a
  regular conical shape, of a dirty-white colour, with the surface well
  preserved, having moderately wide, not very prominent longitudinal
  ribs. The orifice is rather small and pentagonal. The radii are
  extremely narrow or linear, with quite smooth edges; the sutures,
  however, are always very distinct, and in the upper part, the alæ
  are generally rather widely exposed, as viewed from the outside.
  The largest specimen which I have seen was .7 of an inch in basal
  diameter.

  The _opercular valves_ are closely similar to those of _E. plicatus_,
  but the _scutum_ is generally a little more elongated, and the
  articular furrow not so deep: in accordance with this last fact,
  the articular ridge in the _tergum_ is not so prominent as in _E.
  plicatus_; but we have seen that these several characters are highly
  variable in _E. plicatus_. The slope of the basal margin of the
  tergum towards the spur varies in the present species, in a strictly
  analogous manner, as it does in _Tetraclita porosa_.

  _Structure of the Parietes and Radii._--The parietes are not so thick
  as in _E. plicatus_; internally they are tinted pale purple; when
  broken transversely, a row of microscopically minute orange-coloured
  dots can generally be distinguished between the outer and inner
  laminæ; and these evidently represent the orange-coloured layer in
  _E. plicatus_. The sheath also exhibits a faint tinge of orange. The
  radii are very narrow, and are quite smooth-edged, differently from
  in _E. plicatus_. The edges of the alæ barely exhibit a trace of
  being crenated.

  In the body I could perceive no difference from _E. plicatus_,
  excepting that in the third pair of cirri the two rami are like each
  other, and do not support any coarsely pectinated, only serrated,
  spines; but after what we have seen on the variability of these very
  same characters in _Tetraclita porosa_, I dare not trust to them. The
  three posterior pairs of cirri, also, seem here to be more elongated
  in proportion to the others, than in _E. plicatus_.

  _Affinities._--It is certain that this species is most closely allied
  to _E. plicatus_; but as I have seen many specimens of the latter
  brought by different persons from New Zealand, and as I have observed
  in them no approach to the characters of _E. simplex_, which, in
  specimens from three localities, also appear to be constant, I have
  considered the two forms as specifically distinct. The present
  species differs from _E. plicatus_, in its white, conical, moderately
  ribbed, well preserved, smaller shell; and more especially in the
  orange-coloured intermediate lamina of _E. plicatus_ being here
  represented only by microscopically minute dots. But the radii being
  smooth-edged, is the most important differential character, though in
  _E. plicatus_, during its earliest growth, whilst still immature and
  colourless, the radii are likewise smooth-edged.



5. _Genus_--PYRGOMA.

  PYRGOMA. _Leach._ Journal de Physique, tom. 85, 1817.

  BOSCIA. _Ferussac._ Dict. Classique d'Hist. Naturelle, 1822.

  SAVIGNIUM. _Leach._ Zoological Journal, vol. 2, July, 1825.

  MEGATREMA. _Ib._ Ib.

  ADNA. _Ib._ Ib.

  DARACIA. _J. E. Gray._ Annals of Phil. (new series), August, 1825.

  CREUSIA. _De Blainville._ Dict. Sc. Nat., Pl. 116, 1816-30.

  NOBIA. _G. B. Sowerby, junr._ Conchological Manual,[114] 1839.

    [114] The name, Nobia, is given in this work on the authority of
    Leach, but this must be a mistake, probably caused by some MS.
    name, (that fertile source of error in nomenclature), having been
    used.

_Shell formed of a single piece; basis cup-formed, or sub-cylindrical,
attached to corals._

  _Distribution_, imbedded in corals, chiefly in the tropical seas,
  round the world.


I feel no hesitation in including the above several genera in one
genus. In external appearance the _P. monticulariæ_ (Pl. 13, fig. 5
_a_), which forms the genus Daracia of Gray, is the most distinct, but
it is so intimately allied to the two, indeed to the three, foregoing
species, that it cannot be separated from them. Of the first five
species, _P. grande_, _conjugatum_, and _cancellatum_, form a graduated
series, but with the steps very distinct, the chief difference being
in the length of the spur of the tergum; for the fact of the scutum
and tergum being calcified together in _P. grande_ and _conjugatum_,
and distinct in _P. cancellatum_, is certainly unimportant, as may
be inferred from what we shall see in comparing together the last
four species of the genus, and from what we shall see in _Creusia
spinulosa_. The three above-mentioned graduated species are connected
with the last four species of the genus, by several points of
resemblance between _P. grande_ and _crenatum_. The first two species,
namely, _P. Anglicum_ and _Stokesii_, are the most closely related
together, and may indeed possibly be identical; these two, in all the
characters derived from the opercular valves, resemble Balanus and
other ordinary forms, and for this very reason they have some claims
to be generically separated from the other species of Pyrgoma; for in
these latter, the opercular valves seem to have broken loose from all
law, presenting a greater diversity in character than do all the other
species of Balaninæ and Chthamalinæ taken together.

_General Appearance._--The shell consists of a single piece, generally
without any suture, even on the internal surface; and this is the case,
at least in _P. Anglicum_, in extremely young colourless specimens:
nevertheless, in some specimens of this very species, and of _P.
conjugatum_, there were traces of two, but only two, sutures on the
sheath, one on each side towards its carinal end. The shell is much
depressed or actually flat; and I have seen specimens even slightly
concave; in _P. Anglicum_, however, the shell is steeply conical. The
outline is generally oval; but in _P. monticulariæ_ it is extremely
irregular. The surface is generally furnished with more or less
prominent ridges, radiating from the orifice, which is oval and small;
sometimes, as in _P. monticulariæ_, excessively small. The colour is
white, or pinkish-purple. Most of the species are small, but I have
seen specimens of _P. grande_ three quarters of an inch in diameter in
the longer axis, and, including the deep, almost tubular, basis, more
than three inches in length or depth.

_Opercular Valves._--In three species, viz., _P. conjugatum_, (Pl. 12,
fig. 7 _c_), _grande_, and _monticulariæ_, the scuta and terga, on
each side, are calcified together so perfectly, that there is no trace
of a suture or line of junction: in _P. milleporæ_, these valves are
generally slightly calcified together, but with the suture distinct.
The _Scuta_ differ so much in shape in the different species, that
little can be said of them in common: in _P. Anglicum_ and _Stokesii_
they resemble those of Balanus; but in the other species they are
much more elongated than is usual, and this is carried to an extreme
in the last four species; this elongation is due to a great increase
in breadth, as may be inferred from the position of the apex of the
valve, and from the direction of the lines of growth. But the two most
remarkable characters are, first,--the extraordinary development of
the adductor ridge, so that, in _P. conjugatum_, _cancellatum_ (Pl.
12, fig. 5 _c_), _grande_, and _crenatum_, it extends considerably
beneath the basal margin, being produced, in the first two species, at
the rostral angle, into a point; at the tergal end of the valve, the
adductor ridge, when thus much developed, blends into the articular
ridge. The second very remarkable character is the addition of a
special ledge along the occludent margin of the scutum, and along the
carinal margin of the tergum, which I will call the occludent ledges
(_limbus occludens_), and which serve to close the orifice leading
into the sack. The occludent ledge is small in _P. grande_, and is
clothed with thick yellow spines, giving it a brush-like appearance:
in _P. crenatum_ and _dentatum_ it is largely developed, the ledge on
the scutum being articulated with that on the tergum, as shown in Pl.
13, fig. 4 _a_, 4 _b_,--the ledges being here and elsewhere marked
by little bristly points. In _P. monticulariæ_, however, this ledge
arrives at its maximum development (Pl. 13, fig. 5 _f_), the rest of
the valve (the scuta and terga being here, as in _P. grande_, calcified
together) being reduced to a mere basal edge or border. Excepting for
the adductor muscle, the depressions or crests for the other muscles,
both on the scuta and terga, are hardly developed.

_Terga_: these, as in the case of the scuta, differ so much in shape
in the several species, that little can be said of them in common.
In _P. Anglicum_ and _Stokesii_, they are of the normal shape; in
_P. cancellatum_ (Pl. 12, fig. 5 _d_) this, to a certain extent, is
likewise the case, but the spur is produced to a quite extraordinary
length. In _P. grande_ (Pl. 13, fig. 1 _b_) there is no distinct spur,
and the whole valve is square. In _P. milleporæ_ (2 _f_) there is no
spur, and the valve is arched and triangular. In _P. crenatum_ the spur
is broad, rounded, and depressed (fig. 4 _b_), with the carino-basal
portion of the valve reduced to a mere border, barely distinguishable
from the great occludent ledge. In _P. monticulariæ_ there is no spur,
and the whole valve forms a mere border to the occludent ledge; and,
lastly, in _P. dentatum_, the valve is extremely variable in shape
(fig. 3 _c_, 3 _d_, 3 _f_), and on its internal surface (fig. 3 _g_)
there is an inwardly projecting, most singular and anomalous, tooth.
Hence we see how wonderfully variable the terga are in this genus.

_Structure of the Walls._--The shell consisting, as has been stated,
of a single piece, is generally thick. From the close alliance between
this genus and _Creusia_, it is probable that the shell, if examined
immediately after the metamorphosis, would be found to show traces
of being formed of four compartments. The walls are either solid
or porose; their basal margin is formed by strong crenated ridges,
answering to the longitudinal septa in Balanus; but these in _P.
monticulariæ_ are modified into a very irregular surface. The internal
surface of the shell is generally smooth, or only slightly ribbed. The
sheath has its lower edge free in _P. Anglicum_ and _Stokesii_, and
in a slight degree in _P. milleporæ_, but in the other species it is
closely attached to the walls. In _P. monticulariæ_ the sheath might
easily be overlooked. In _P. Anglicum_, _grande_, and _crenatum_, it
descends almost to the basal margin of the depressed shell, and as the
opercular valves and membrane are attached to the lower edge of the
sheath, the animal's body necessarily comes to be almost exclusively
lodged in the cup-formed basis. In _P. grande_ and _conjugatum_, the
lines of growth in the sheath are bent upwards on each side, at points
corresponding with the line of union between the scutum and tergum, in
a manner I have not seen in any other cirripede; and this sometimes
gives the appearance of two lateral sutures. I may here remark, that
the manner of growth in Pyrgoma is almost the converse of that in
Balanus, Tetraclita, and other allied genera; for in these latter, the
basis increases in diameter, and the shell chiefly in height; whilst in
Pyrgoma, the shell, from being so flat, increases almost exclusively in
diameter, whereas it is the basal cup which is added to in height or
depth.

_Basis._--This in all the species is more or less regularly cup-formed
or sub-cylindrical. In _P. grande_ it penetrates the coral to a
surprising depth; in _P. monticulariæ_ it is irregular in outline,
corresponding with the shell. The basis is generally almost wholly
imbedded in the coral; but this is not the case with _P. Anglicum_,
in which the basis is generally exserted, as it is in a slight degree
in _P. grande_. The shelly layer forming the basis, in most of the
species, is very thin, and is finely plicated owing to its edge
folding between the ridges or septa that form the basal edges of the
shell; this is very conspicuous in _P. cancellatum_. The basis is not
permeated by pores, except in _P. Anglicum_. In some sessile cirripedes
a cleft, covered only by membrane, may be observed all round between
the lower edge of the shell and the basis; a cleft of this nature
is rather conspicuous in _P. crenatum_, so that small portions of
the septa on the internal surface of the walls can be seen from the
outside. In _P. monticulariæ_, an analogous structure, developed to an
extreme degree, presents a very different and unique appearance; the
shell is nearly flat, and the smooth outer lamina does not nearly reach
to the circumference, a wide border being thus left exposed, which is
roughened (Pl. 13, fig. 5 _a_, 5 _c_, 5 _d_, 5 _e_) by the exposure of
the irregular septa. I have not seen a fresh specimen, but there cannot
be a doubt that this border is properly covered by membrane.

_Animal's Body._--From some cause, perhaps from the corals in which
the species of Pyrgoma are imbedded, long remaining damp, the internal
organs are generally badly preserved. I have received, in spirits of
wine, only _P. Anglicum_, but I have examined dry specimens, in a
tolerable condition, of _P. milleporæ_ and _crenatum_. As neither
the mouth nor cirri, in these three species, offer any noticeable
characters, distinct from those in Balanus or Acasta, my ignorance of
these organs in the other six species is not important. In the above
three species the labrum is deeply notched, with about three teeth on
each side of the notch, except in _P. milleporæ_, in which the number
is six. In all, the mandibles have five teeth, the two lower ones being
small: the maxillæ are not notched: the outer maxillæ are bilobed. In
the _Cirri_, the rami in the first pair are very unequal in length,
the segments being slightly protuberant in the shorter ramus. On the
segments of the posterior cirri there are four pairs of spines in _P.
Anglicum_, and three pairs (of which the second and third are short)
in _P. milleporæ_. At the dorsal basis of the penis there is a small
straight projecting point.

_Affinities._--The species (with the exception of the first two) are
much more distinct from each other, and more easily determined than
is usual with sessile cirripedes; it is, however, quite useless to
attempt naming the species without disarticulating and cleaning the
opercular valves. Although these valves differ so greatly in some of
the species from those of Balanus and the allied forms, the genus
itself, as a whole, does not differ much, except in the shell not being
divided into compartments, and in the basis being cup-formed and not
generally permeated by pores,--these latter characters being in common
with Acasta. With respect to the absence of separate compartments,
it should be remembered, that in the same species of Tetraclita we
have individuals with the four compartments distinct and furnished
with radii, and other individuals without any trace of a suture
externally,--the outer lamina of shell (though not the inner) having
become completely confluent all round. At the commencement of this
description, when giving my reasons for uniting the several proposed
genera into one genus, I gave a sketch of the affinities of the
species: I have only to add, that the following sub-genus Creusia is
closely, perhaps too closely, allied to Pyrgoma.

_Geographical and Geological Distribution._--Most of the species are
inhabitants of the hot coral-growing zones, in both the eastern and
western hemispheres, but more especially, as it would appear, in the
East Indian Archipelago. From the habits of the corals, most of the
species must be inhabitants of shallow water. _Pyrgoma Anglicum_,
however, is an inhabitant of deep water on the southern shores of
England, whereas at St. Jago, in the Cape de Verde Islands, I myself
collected it adhering to a Caryophyllia, within the tidal limits. This
same species existed on the shores of England during the Coralline Crag
period; and at this epoch it attained a larger size than at present.
Two species of the genus, according to Sismonda, are found in the
tertiary beds of Piedmont. Mr. Stutchbury, who is so well acquainted
with recent cirripedes, informs me that he has for many years examined
fossil Secondary corals, in the expectation of finding imbedded,
species of this genus or of some allied form, but without success. The
same species of Pyrgoma is by no means always confined to the same
coral: I have seen _P. crenatum_ on four or five different corals,
and _P. Anglicum_ on at least three kinds: on the other hand, I have
seen _P. milleporæ_ only on the _Millepora complanata_ (a member,
as I believe, of the vegetable kingdom), and _P. monticulariæ_ on a
_Monticularia_ from near Singapore.



1. PYRGOMA ANGLICUM. Pl. 12, fig. 4 _a_-4 _c_.

  PYRGOMA ANGLICA. _G. B. Sowerby._ (sine descript.) Genera of Recent
        and Fossil Shells, fig. 7, No. 18, Sept. 1823.

  MEGATREMA (ADNA) ANGLICA. _J. E. Gray._ Annals of Philosoph. (new
        series), vol. x, Aug. 1825.

  PYRGOMA SULCATUM. _Philippi._ Enumeratio Molluscorum Siciliæ, Tab.
        12, fig. 24, (1836).

  ---- ANGLICA. _Brown._ Illustrations of Conchology, (2d edit.,
        1844), Tab. 53, fig. 27-29.

_Shell steeply conical, purplish red: orifice oval, narrow: basis
permeated by pores, generally exserted out of the coral: scutum and
tergum sub-triangular._

  _Hab._--South coast of England and of Ireland, (12 to 45 fathoms,
  Forbes and MacAndrew); Sicily; Madeira; St. Jago, Cape de Verde
  Islands; generally attached to the edge of the cup of a Caryophyllia,
  in deep water, but at St. Jago within the tidal limits; Mus. Brit.,
  Cuming, Lowe, &c.

  _Fossil_ in the Coralline Crag, Ramsholt; Mus. S. Wood.


  _General Appearance._--Shell steeply conical, slightly compressed,
  the lower part with rounded, approximate, radiating ribs: colour
  dull purplish-red: orifice oval, small, and narrow. The basis is not
  deeply conical, and occasionally is even flat. Generally it stands
  exserted; but in the Coralline Crag specimens, it is almost wholly
  imbedded. Externally it is furnished with ribs corresponding with
  those on the shell. The largest recent specimen which I have seen,
  from St. Jago, was .22 of an inch in basal diameter; but some few of
  the British specimens are nearly as large, and one of the fossils
  from the Coralline Crag a little larger.

  The _Scuta_ and _Terga_ are of the ordinary shape of these valves in
  Balanus and its allies. _Scuta_ triangular, with the basal margin a
  little curved and protuberant: adductor and articular ridges distinct
  from each other, moderately prominent: there is a small hollow for
  the lateral depressor muscle. _Terga_ triangular, with the spur
  rather narrow, moderately long, placed near, but not confluent with
  the basi-scutal angle of the valve. The basal margin forms an angle
  rather above a right angle with the spur. Internally, the articular
  ridge and crests for the depressor muscles, feebly developed.

  _Internal Structure of the Shell and Basis._--Internally, the shell
  is ribbed more or less prominently. The lower edge of the sheath,
  which is reddish, and extends far down the walls, seems always to
  project freely. In several specimens there were on each side, at
  the carinal end of the shell, a trace of a suture, which could
  be perceived only on the sheath. The basis appears always to be
  permeated by minute tubes or pores, though these are sometimes rather
  difficult to be seen.



2. PYRGOMA STOKESII. Pl. 12, fig. 6.

  MEGATREMA STOKESII. _J. E. Gray._ (sine descript. aut figurâ) Annals
        of Philosophy, (new series), vol. 10, Aug. 1825.

_Shell moderately conical, pale-purplish red; orifice oval: basis not
permeated by pores, deeply imbedded in the coral: scutum and tergum
sub-triangular._

  _Hab._--Imbedded in the Mycedia (Agaricia) agaricites; therefore from
  the West Indies;[115] Brit. Mus. and Stutchbury.

    [115] I am greatly indebted to Mr. Dana for having named for me the
    coral in which this species was imbedded, and informing me that it
    is a West Indian species.


This species comes so close to the last, that I am not sure that I have
acted rightly in retaining it, but I think that it is distinct; and in
this case, it is the representative, on the other side of the Atlantic,
of _P. Anglicum_ of our own side. It will be sufficient to point out
the few points of difference. The shell is much more depressed, with
the orifice oval, larger, and not so narrow. It is apparently of a
paler red, and the radiating ribs perhaps not so prominent. The basis
offers the most important difference, being deeply imbedded in the
coral; and there is not the least appearance of the thin shelly layer,
of which it is composed, being permeated by pores, as, we have seen,
is always the case with _P. Anglicum_. As in this latter species,
the sheath here depends freely. The opercular valves are closely
similar; but in the scutum, the adductor ridge occupies a rather more
central position; and in the tergum, the basal margin is more inclined
towards, or forms a greater angle with, the spur: these differences, by
themselves, I consider quite insufficient to characterise a species;
but conjoined with the flatter shell, the larger orifice, the more
deeply imbedded and non-porose basis, they may, I think, be admitted as
specific. In dimensions, this species seems to attain a slightly larger
size than _P. Anglicum_, for several specimens were .22 of an inch in
diameter.



3. PYRGOMA CANCELLATUM. Pl. 12, fig. 5 _a_-5 _f_.

  PYRGOMA CANCELLATUM. _Leach_ (!). Encyclop. Brit., Supplement, vol.
        3, Pl. 57, 1824.

  ---- LOBATA. _J. E. Gray_ (!). Annals of Philosophy, (new series),
        vol. 10, 1825.

  CREUSIA RAYONNANTE. _De Blainville._ Dict. Sc. Nat. (sine descript.),
        Pl. 116, fig. 7, _a_, _b_.

_Shell with the circumference generally lobed: scutum elongated, with
the adductor ridge descending far below the basal margin, and produced
at the rostral end into a square point; tergum with the spur four times
as long as the upper part of the valve._

  _Hab._--Imbedded in a Gemmipora, probably from the West Indies;[116]
  Mus. Brit. and Cuming.

    [116] In this case I am again indebted to Mr. Dana for naming the
    coral; he informs me that the genus is found in the Pacific and
    East Indies; the specimen sent he believes is the _G. cinerascens_.


  _Appearance and Structure of Shell._--Shell nearly flat, sometimes
  tinted dull dirty purple, with the surface marked by slight, broad,
  approximate ridges, the ends of which form considerable projections,
  giving to the shell, when not too much encrusted by coral, a lobed
  border. In young specimens (.15 of an inch in diameter) some of
  the points projected half as much as the semi-diameter of the
  shell, giving it a radiating appearance. The orifice is oval and
  rather small, but of variable size. The shell is thick, and, near
  the outer lamina, is penetrated by pores: the internal surface is
  smooth. The sheath (fig. 5 _b_) extends down but a short distance
  from the orifice; it is closely attached to the walls: the lines of
  growth, at a point on each side, bend a little downwards (instead of
  upwards, as in the two following species), and hence the lower edge
  of the sheath irregularly projects downwards on each side. The basal
  cup, internally, is plicated, the hollows corresponding with the
  projecting, longitudinal, parietal septa, which form the lobed border
  of the shell. The largest specimen which I have seen, was rather
  above .4 of an inch in diameter.

  The _Scuta_ and _Terga_ are not calcified together: they are both
  much elongated. _Scutum_ (fig. 5 _c_, 5 _e_).--For the first time in
  the genus, or indeed in the family, this valve presents a remarkable
  character in the adductor ridge being immensely developed, so as to
  project far below the ordinary basal margin. At the rostral end, it
  at first appears to project even more than it really does, for the
  toothed occludent margin is in fact a prolongation of the true valve,
  as distinct from the adductor plate. Excluding this very narrow,
  prolonged, occludent margin, the adductor plate projects for a
  length equalling the rest of the valve. Along the tergal margin, the
  adductor plate is united to the articular ridge; and at the rostral
  end, it is produced into a square tooth, whence a square-edged ridge
  extends on the surface of the plate upwards to the ordinary basal
  margin of the valve. The exact shape of this adductor plate varies
  a little, as does the degree to which it is closely attached to the
  ordinary basal edge of the valve. The valve, as distinct from the
  adductor plate, is narrow, with the basal margin regularly curved.
  The articular ridge is very prominent.

  _Terga_ extremely narrow, linear, consisting in chief part of the
  spur, which is fully four fifths of the entire length. Externally
  (fig. 5 _d_) the valve is furrowed, with the edges more or less
  folded in along the spur. The upper or ordinary part of the valve is
  about one third wider than the spur. The basi-carinal angle is sharp,
  owing to the basal margin being a little hollowed out. A special
  plate of shell (fig. 5 _f_), hollow under its basal edge, runs from
  the carinal margin to the articular ridge, which latter is situated
  in the middle of the valve, and against which the articular ridge of
  the scutum abuts. The spur is central; its end is bluntly pointed.
  The total length of the tergum rather exceeds that of the scutum, the
  produced adductor ridge being included in the latter.

  _Affinities._--Under _P. grande_ I shall make a few remarks, showing
  that in several characters _P. cancellatum_ and _grande_ are at
  opposite ends of a short series, with _P. conjugatum_ intermediate
  between them.



4. PYRGOMA CONJUGATUM.[117] Pl. 12, fig. 7 _a_-7 _c_.

    [117] Dr. Gray thinks this is the _Pyrgoma stellata_, of Chenu,
    ('Illust. Conch.'); it may be so; but the figure given of the
    shell will do equally well or rather better for the _Pyrgomum
    dentatum_ of this work, and for some varieties of _P. crenatum_.
    Without a careful description of the opercular valves, it is really
    impossible to recognise, with any approach to certainty, the
    species of this genus.

_Shell nearly flat with approximate radiating ridges: scutum and tergum
calcified together without any suture: scutum with the adductor ridge
descending below the basal margin, and produced at the rostral end
into a point: tergum with the spur about as large as the upper part of
valve._

  _Hab._--Red Sea; Brit. Mus. and Cuming.


  _Appearance and Structure of Shell._--Shell white, or with a tinge of
  purple; nearly flat, with moderately prominent, narrow, approximate
  ridges, radiating from the orifice, which is oval, rather narrow,
  and not very small. In the largest specimens the ridges are less
  prominent than in the figure given. The walls are thick, and not at
  all porose: the sheath extends down almost to the base of the shell,
  and its lower edge is closely attached to the walls: on each side,
  towards the carina, there is a trace of a suture, and the lines of
  growth on the sheath are here a little upturned. The basis is deeply
  imbedded and internally furrowed; the calcareous layer forming it, is
  thin. The largest specimen was .4 of an inch in the longer diameter.

  The _Scuta_ and _Terga_ (fig. 7 _b_, 7 _c_), in all the specimens
  which I have seen, are calcified together, with no trace of a
  suture. There is, however, a slight furrow, which, I believe, marks
  the normal line of separation between the two valves; and in the
  following description this is assumed to be the case. The _Scutum_
  has the adductor ridge greatly developed, so as to project below
  the ordinary basal margin to a distance as great as the height of
  the valve. At the rostral end, this adductor ridge or plate is
  produced into a point; and at the tergal end, it is blended with the
  articular ridge, and united to the inner face of the tergum. That
  portion of the scutum which corresponds with the valve in ordinary
  cases, and alone is externally visible as long as the operculum is
  united by the opercular membrane to the sheath, is narrow, with the
  basal margin considerably hollowed out: the occludent edge is formed
  into thick teeth. The _Tergum_ is elongated, rather exceeding in
  length the scutum, the latter being measured from the apex to the
  rostral projection of the adductor plate. The surface of the valve is
  depressed in the line of the spur, with the basal end of the latter
  bluntly pointed. A very slight flexure (fig. 7 _c_) on the basal
  margin indicates where we may believe the spur to commence, showing
  that it rather exceeds in length the whole upper part of the valve.
  The lines of growth obscurely indicate a tendency to the formation of
  a slight "occludent ledge" along the carinal margin. Traces are just
  visible of crests for the attachment of the tergal depressor muscles.



5. PYRGOMA GRANDE. Pl. 13, fig. 1 _a_-1 _d_.

  NOBIA GRANDIS. _G. B. Sowerby, junr._ (sine descript.) Conchological
        Manual, fig. 29, 1839.[118]

  CREUSIA GRANDIS. _Chenu._ Illust. Conch., Tab. 1, fig. 2 _a_, sed
        non, fig. 2.

    [118] It is quite possible that this may be the _Balanus
    duploconus_ of Lamarck, but with such a character as the following,
    who can recognise a species? "_B. testæ parte supremâ univalvi,
    indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ
    ellipticâ._ L'exemplaire est sans opercule et incomplet."

_Shell moderately convex, nearly smooth: scutum and tergum calcified
together without any suture: scutum furnished with a small occludent
ledge, with the adductor ridge descending below the basal margin:
tergum square without a spur._

  _Hab._--Singapore and East Indian Archipelago; Mus. Brit., Cuming,
  Stutchbury; imbedded in two kinds of coral.


  _Appearance and Structure of Shell and Basis._--The shell is conical,
  though to a variable degree, and sometimes is much depressed. The
  surface is smooth, with only traces of narrow approximate ridges.
  The colour is white, often with a tinge of dark purple. The orifice
  is oval, and moderately large. The shell and a small portion of
  the basis usually stand exserted above the coral. The walls are
  of variable thickness; when thick, the pores, by which they are
  permeated, are but little apparent; sometimes there is more than a
  single row of pores. The points of the septa on the basal edge of the
  shell are small. The internal surface of the shell is smooth. The
  sheath is closely attached to the walls, and descends nearly to the
  basis; on each side its lines of growth are slightly upturned. The
  basis is deeply cup-formed or cylindrical, and in section oval like
  the shell; it penetrates the coral to a very remarkable depth,--in
  one instance to three inches. The shelly layer forming it, is thin,
  finely furrowed, and not permeated by pores. This is the largest
  species in the genus; one specimen was three quarters of an inch in
  its longer diameter, and above three inches in length.

  The _Scuta_ and _Terga_ are calcified together, without any trace
  of a suture; the line of junction can be inferred only from the
  analogy of _P. conjugatum_, in which species the valves have a more
  normal character, and are separated by a slight furrow. It may be
  seen in the figure (Pl. 13, fig. 1 _d_) of the right and left hand
  opercular valves, viewed from vertically above in their proper
  relative positions, how abnormal their appearance is, which is partly
  caused by the spinose occludent ledges, presently to be described,
  but chiefly from the carinal margins of the two terga not being
  straight and parallel, as in all other cirripedes, and therefore not
  meeting each other, as is usual. In other genera, only the upper
  part of the carinal margin of the two terga can be opened for the
  exsertion of the cirri, the lower portion being united by membrane;
  but here, I have little doubt, from the position of attachment of
  the adductor muscle (fig. 1 _c_), so close to and almost on the
  terga, that the whole length of the carinal margin of the two terga
  is free or disunited for the protrusion of the cirri. This opening
  between the two terga evidently cannot be closed, but is probably
  filled up, and the animal thus protected, by the dorsal surfaces of
  the curled-up cirri; such, I believe, being likewise the case with
  some pedunculated cirripedes, as with Conchoderma. The _scutum_ has
  a large adductor plate, which extends some little way (namely, about
  one quarter of the height of the valve), below the ordinary basal
  margin. This latter margin is slightly sinuous, and a little hollowed
  out towards the tergal corner of the valve. I believe that the ridge,
  which runs down to the basi-scutal corner of the tergum, though
  appearing to be part of the scutum, really belongs to the tergum.
  The adductor scutorum plate is not, as in the last two species,
  produced into a point at the rostral angle; at the tergal end it
  blends into the under surface of the tergum. The occludent margin
  is coarsely toothed. Rather on the under side of this margin and in
  the upper part, there is a narrow occludent ledge, which extends up
  beyond the apex of the valve, and thence runs a little way along the
  carinal margin of the tergum. This ledge is thickly clothed with
  strong, yellowish-brown spines, and hence appears like a brush. It
  is remarkable that the cavity for the adductor scutorum muscle is
  situated almost on the tergum.

  The _tergum_ is of large size, and nearly square; it is, in
  appearance, separated from the scutum by a ridge running up to the
  apex. The basal margin forms a right angle with the carinal margin,
  along which latter margin the lines of growth are upturned, and blend
  into the occludent ledge, which is common to the two valves. There
  cannot be said to exist any spur, the whole basal margin being almost
  straight; nevertheless, on close examination, the ridge which in
  appearance separate the scutum and tergum, may, I think, be safely
  considered as one side of the spur (which, it should be remembered,
  has in all ordinary cases a longitudinal furrow or depression), and
  the other side of the spur is, apparently, very feebly indicated by a
  slight flexure in the middle of the basal margin. Hence, if the spur
  had been developed, it would probably have been half as wide as the
  valve. There are no crests for the tergal depressor muscles.

  _Affinities._--The present species, with the last two, form an
  interesting series. _Pyrgoma grande_ and _conjugatum_, however,
  are more closely allied to each other than to _P. cancellatum_. In
  the scutum, the whole valve is least elongated, with the adductor
  plate least developed, in _P. grande_, and most elongated, with
  the adductor plate most developed, in _P. cancellatum_. In the
  outline of the tergum the range of shape is quite remarkable; in _P.
  conjugatum_, which stands between the other two species, the spur is
  rather long, whereas in _P. grande_ there is no spur at all--a very
  unusual circumstance--and in _P. cancellatum_, at the other end of
  this short series, the spur attains a length wholly unparalleled in
  any other cirripede.



6. PYRGOMA MILLEPORÆ.[119] Pl. 13, fig. 2 _a_-2 _f_.

    [119] From external aspect I suspect this species to be the
    _Creusia madreporarum_, Leach (?), as given in Chenu, 'Illust.
    Conch.,' Tab. 1, fig. 6. But I feel sure that Leach has nowhere
    published this name; and it may be observed that Chenu gives it
    with a mark of doubt. The shell in its imbedded state is only
    figured; the opercular valves are not given; and no descriptive
    details are added. Under these circumstances I have not adopted
    this name; I have, perhaps, been in some degree influenced by the
    fact that this species, judging from the many specimens examined by
    me, is never imbedded in madrepores, but exclusively in millepores,
    so that Chenu or Leach's specific name of _Madreporarum_ is
    singularly inappropriate.

_Shell with the orifice narrowly ovate: sheath dark purple: Scutum much
elongated: tergum triangular, convex, without a spur._

  _Hab._--Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus.
  Brit., Stutchbury, &c. Imbedded in _Millepora complanata_, sometimes
  associated with _Balanus Ajax_.


  _Appearance and Structure of Shell._--Shell oval, flat, coloured pale
  dull purple, or white, with slight and narrow ridges, radiating, from
  the orifice, which is not quite central, but is placed rather nearer
  to the carinal than to the rostral end of the shell. The orifice is
  small and narrow; the carinal end being rounded, and the rostral end
  narrower and sharper,--this being the exact reverse of the usual
  shape of the orifice in the Balanidæ. The walls are thick, and are
  formed of large square tubes. The internal surface of the shell is
  smooth. The sheath (fig. 2 _b_) is much more elongated than the
  shell, for at the rostral end it extends to the basal margin, and at
  the carinal end nearly to it, whereas on the two sides it is some way
  distant from the base. The orifice is considerably out of the centre
  of the sheath, being placed nearest to the carinal end. The sheath
  has its lower edge slightly prominent or free; the lines of growth
  are neither turned up nor down on the two sides, as in the last three
  species. When nearly full grown the sheath is coloured dark purple,
  but when young it is white, hence the upper part is white, surrounded
  by an oblong purple ring, and this is surrounded by the white shell.
  The basal cup is deep, and internally nearly smooth. The largest
  specimens were .3 of an inch in their longer diameter. Great masses
  of the Millepora are absolutely studded with this Pyrgoma, with
  usually more specimens on one side of the plate or branch than on the
  other. They stand in approximately parallel positions, the broad or
  carinal ends of the orifices pointing upwards.

  _Scuta._--The scuta and terga are closely united, and _are_ often
  (perhaps always) in some slight, though variable degree, calcified
  together; and hence they often break, rather than separate, at the
  line of articulation. The external fissure or line of junction
  between them (fig. 2 _c_) is oblique to the longer axis of the
  scutum; in the uppermost part of the valve it is sometimes almost
  obliterated. The two valves together are nearly as long as the
  sheath, and consequently much longer than the orifice of the shell.
  The scutum is much elongated, being fully four times as broad as
  high. The valve narrows towards the rostral end, but in a variable
  degree: the basal margin is hollowed out a little (but to a variable
  amount), close to the basi-tergal corner. Along the occludent margin
  a slip of the valve, widening downwards, is a little bent inwards,
  and this inflected portion is separated from the rest of the surface
  by a slightly angular ridge, running from the apex to near the
  rostral angle. Internally, at the basi-tergal corner, a slight ridge,
  parallel and close to the basal margin (and which can be seen only
  when the basal edge of the valve is held upwards), represents the
  adductor ridge, which we have seen so largely developed in the last
  three species, and shall again see in _P. crenatum_. The articular
  ridge (fig. 2 _d_) is extremely prominent, consisting of a more or
  less rectangular shoulder.

  _Terga_: these are rather small compared to the scuta: they are
  triangular and much arched: there is no trace of a spur. Internally
  (fig. 2 _f_), the articular ridge is central: there are some vestiges
  of crests for the depressor muscle.

  _Affinities._--This well-marked species, in the tendency of the
  opercular valves to be soldered together, and in the remarkable
  absence of a trace of a spur to the tergum, is allied to _P. grande_,
  but it is more closely allied to the three following species.



7. PYRGOMA DENTATUM. Pl. 13, fig. 3 _a_-3 _g_.

_Scutum much elongated, with a tooth-like articular projection: tergum
convex, irregularly triangular, sometimes with an imperfect spur, and
on the internal surface with an inwardly projecting tooth; scutum and
tergum furnished with an occludent ledge._

  _Var._ (1), 3 _c_, 3 _g_: _tergum, with a sharp internal tooth,
  projecting rectangularly inwards_.

  _Var._ (2), 3 _d_: _tergum, with a broad blunt internal tooth,
  depending beneath the spur-like portion of the valve_.

  _Var._ (3), 3 _f_: _tergum, with the basi-carinal end of the valve
  truncated, with a small blunt internal tooth projecting rectangularly
  inwards_.

  _Hab._--Red Sea; Mus. Brit. and Cuming. Also associated with _Pyrgoma
  crenatum_, and attached to _Meandrina spongiosa_.[120]

    [120] Mr. Dana informs me that he believes that this coral comes
    from the West Indies; though the specimens originally described
    by him had no label. If this be so, both _Pyrgoma dentatum_ and
    _crenatum_ have very wide ranges.


  _Appearance and Structure of Shell._--Shell nearly flat, oval,
  white or pink, with rather distant prominent ridges radiating from
  the moderately large (for the genus) oval orifice. The ridges are
  often obscured, and apparently sometimes almost obliterated by the
  encrusting coral. Shell permeated near the outer lamina by short
  imperfect pores: internal surface smooth: sheath inconspicuous,
  descending rather more than half way down the walls; lower edge
  closely attached to the walls. Basis deep. Diameter of largest
  specimen .3 of an inch.

  _Scuta_: these are elongated, but to a very variable degree, some
  specimens being quite three times, and some barely twice as broad
  as high. I observed this same variability in two sets of specimens,
  differing, as we shall presently see, in the form of their terga:
  it depends in part, but not wholly, on the varying width of the
  occludent ledge, which is sometimes only a fourth, and sometimes
  half as high as the rest of the valve. This is the first species
  in the genus in which the occludent ledge--a structure peculiar
  to the genus--has been amply developed. The basal margin of the
  valve is slightly sinuous, and a very little hollowed out near the
  basi-tergal corner; it is also very slightly reflexed, the reflexed
  portion being separated from the upper part of the valve by a very
  slight depression or even furrow. I notice this slightly reflexed
  portion, simply as indicating a well marked feature in the basal
  margin of the following closely allied species. Internally (3 _g_),
  the adductor ridge is thick and slightly prominent, but does not
  descend beneath the basal margin: it blends into the articular ridge,
  which here projects in a remarkable manner and degree (3 _b_, 3 _e_,
  3 _g_), like a rounded tooth. This tooth is in part a development of
  the occludent ledge; it varies much in shape. The line of junction
  between the scutum and tergum is nearly straight, and nearly at right
  angles to their longer axes. In some specimens the scuta and terga
  are partially calcified together.

  _Terga_: in three sets of specimens the terga differed considerably,
  but as in every other respect there was the closest resemblance,
  I do not doubt that these are merely varieties. In all three, the
  valve is rather small, irregularly sub-triangular in shape, and
  externally somewhat convex; in all three, there is an occludent
  ledge, of variable width as in the scuta; and in all three, there is
  an internal tooth-like projection, of variable form, unlike anything
  I have seen in any other cirripede. In the _first variety_ (Pl. 13,
  fig. 3 _c_, 3 _g_), the basi-carinal corner of the valve is bluntly
  pointed, and a slight flexure separates this portion of the valve
  from the other and scutal half, which latter thus exhibits some
  tendency to be converted into a spur: on the internal surface (3
  _g_) of this spur-like portion of the valve, there is a rather long,
  sharp tooth, which projects rectangularly inwards; it is flattened
  in a plane at right angles to the longer axis of the scutum and
  tergum together: it cannot be seen from the outside. In the _second
  variety_, the shape of the valve is not very different (Pl. 13, fig.
  3_d_), excepting that the flexure, separating the basi-carinal corner
  of the valve from the spur, is deeper; but on the internal face of
  the spur, the tooth is far broader than in the first variety, and
  is flattened quite differently, viz., in a plane nearly parallel to
  the surface of the valve, and instead of projecting rectangularly
  inwards, it depends beneath the basal edge of the so-called spur, and
  can be seen from the outside. In the _third variety_ (fig. 3 _f_),
  the whole carinal end of the valve is cut off, and there can hardly
  be said to be any trace of a spur, yet a slight furrow apparently
  marks the line of separation between the basi-carinal portion of
  the valve, here become very narrow, and the broad, irregular part,
  which would have formed the spur had such been developed: on the
  internal surface of the latter portion of the valve, there is a very
  small, blunt, slightly flattened tooth, projecting inwards, and more
  resembling that in the first than that in the second variety.

  _Affinities_: under _P. crenatum_ I will point out the diagnosis and
  relationship between this and that species.



8. PYRGOMA CRENATUM. Pl. 13, fig. 4 _a_, 4 _b_.

  PYRGOMA CRENATUM. _G. B. Sowerby._ Genera of Recent and Fossil
        Shells, (No. 218, Sept. 1823), fig. 1 to 6.

_Scutum much elongated, with the adductor ridge descending below the
reflexed basal margin: tergum with a broad depressed spur: scutum and
tergum furnished with a wide occludent ledge._

  _Hab._--Philippine Archipelago; Singapore; Mus. Brit., Cuming,
  Stutchbury; sometimes associated with _Creusia spinulosa_.


  _Appearance and Structure of Shell._--Shell not distinguishable
  from that of _P. dentatum_; nearly flat, oval, white, sometimes
  pale pink, with rather distant prominent ridges radiating from the
  moderately large oval orifice. Shell solid, or permeated near the
  outer lamina by short imperfect pores: internal surface smooth. The
  sheath descends nearly to the base of the walls; it is but little
  conspicuous, and its lower edge is closely attached to the inner
  surface of the shell. Basis deep. Diameter of largest specimen under
  .3 of an inch.

  _Scuta_: in this species the scutum is more abnormal than in any
  other Cirripede in the whole family: this is owing both to the
  adductor ridge descending far beneath the basal margin, and to the
  great development of the occludent ledge; hence the middle and very
  narrow portion of the valve alone answers to the scutum, as seen in
  other genera. The whole valve, including the adductor ridge and the
  occludent ledge, is narrow, being more than twice as broad as high;
  but the proportional width varies, owing chiefly (as in the last
  species) to the varying width of the occludent ledge. This ledge
  commences a little way from the rostral point of the valve, and
  gradually widening, extends to the apex, where it is either as high
  or twice as high as the rest of the valve. It is articulated by a
  convex surface, and by a hollow on its under side with the occludent
  ledge of tergum. The scutum cannot be said to have any tergal margin;
  without, indeed, the articular surface of the occludent ledge be
  thus called. The basal margin is curved, and considerably reflexed,
  of which peculiarity we have seen a vestige in the last species:
  the reflexion is not well shown, owing to the foreshortening of the
  reflexed edge, in fig. 4 _a_: this reflexed edge not being shown,
  causes the lines of growth to appear as if they ran more transversely
  to the longer axis of the valve, than they really do; for they run
  nearly as in the scutum (fig. 3 _e_) of _P. dentatum_. The direction
  of these lines of growth is of importance, for they show that the
  elongation of the scuta is due to an inordinate increase in their
  breadth, as compared to the same valves in ordinary species. The
  adductor ridge, having a sinuous margin, runs from near the apex to
  near the rostral angle: it descends below the basal margin about as
  far as the height of the true valve, excluding the occludent ledge.
  Of course this adductor ridge or plate lies beneath the membrane
  connecting the opercular valves with the sheath, and is concealed by
  it, as long as these valves remain within the shell. The edge of the
  occludent ledge is straight, but the edge of the middle portion of
  the valve, that is of the true valve, is much bowed.

  _Terga_ (fig. 4 _b_): these are of so irregular a shape that they
  can hardly be described; they may, however, be said to consist of
  two portions joined together, of which the lower portion is a little
  elongated transversely, of somewhat variable shape, with part of its
  surface considerably depressed (compared with the rest of the valve),
  sometimes being even concave: this concave portion apparently answers
  to the spur in other cirripedes. At the scutal corner of the valve
  there is a shoulder (perhaps answering to the inflected scutal margin
  in an ordinary tergum), which locks into a hollow on the under side
  of the occludent ledge of the scutum. The internal surface of the
  so-called spur is rounded and convex. The upper part of the tergum
  is in main part formed by a great occludent ledge; but this, on its
  lower side, is bordered by a narrow irregular slip, which, as shown
  by the lines of growth, represents the whole of the ordinary valve,
  excepting, of course, the spur already described. The occludent
  ledges of both valves support some fine spines.

  _Affinities._--Observing how extraordinarily the terga varied in _P.
  dentatum_, and that the shells were identical in that and the present
  species, it occurred to me at first that they might, perhaps, be both
  extreme varieties of one form: but in the scutum of _P. crenatum_,
  the invariably great development of the adductor plate,--the marked
  manner in which its basal margin is reflexed,--the absence of a
  tooth-like articular projection,--and again in the tergum of _P.
  crenatum_, the invariably large size of the concave spur, without
  any internal tooth, altogether convince me that the two species must
  be considered as distinct. This species is allied to _P. grande_, in
  the scutum of that species having an occludent ledge, though small,
  and a great adductor plate. I have only further to remark, that the
  figure of the opercular valves, given in Sowerby's Genera of Recent
  and Fossil Shells, is so good, that there can not be the least doubt
  about the present identification.



9. PYRGOMA MONTICULARIÆ. Pl. 13, fig. 5 _a_-5 _f_.

  PYRGOMA (DARACIA) MONTICULARIÆ. _J. E. Gray_ (!). Zoological
        Miscellany, p. 6, 1831.

_Shell of an irregular shape, with a roughened exterior border: orifice
minute, circular: scutum and tergum both much elongated, calcified
together without any suture, both furnished with a broad occludent
ledge._

  _Hab._--Singapore; Mus. Brit., Cuming, and Stutchbury. Sometimes
  associated with _Creusia spinulosa_.


  _Appearance and Structure of Shell._--Shell dull white, very
  irregular in outline, sometimes rounded, more often unequally
  elongated, and frequently star-shaped,--the projections being
  quite irregular. Whole shell nearly flat, but with the central
  part saddle-backed, or formed into a more or less prominent ridge,
  extending in the line of the longitudinal axis of the animal's
  body: the circumferential portions of the shell not unfrequently
  are a little recurved upwards. Orifice extremely minute, circular.
  The outer lamina of shell, which is smooth, does not extend to the
  circumference, and consequently a rather broad, nearly equal border,
  which is rough, surrounds the whole shell. I have no doubt that,
  when the shell was alive, this border was covered by a membrane,
  which, in drying, has curled up and been lost, in the same manner
  as the strictly analogous but narrow open seam between the basal
  edges of the shell and the basis in some cirripedes (as in the last
  two species of _Pyrgoma_) is protected. The roughened border can
  sometimes be plainly seen to be formed of normal (Pl. 13, fig. 5
  _e_) longitudinal septa having crenated edges, with shorter septa
  between the longer ones; but more often the septa are so irregular,
  and so much branched (5 _d_), that the whole resembles a mass of
  moss. Why the outer lamina of the shell in this one species does not
  nearly reach the circumference of the walls, I cannot conjecture. The
  extremely irregular, depressed shape of the shell, with the minute
  circular orifice, and the singular rough circumferential and often
  slightly reflexed border, together give to this species so peculiar
  an aspect, that until close examination I did not believe that it
  was a cirripede. The extreme irregularity of shape depends in great
  part upon the irregular growth of the Monticularia, in which it is
  imbedded.

  Internally (fig. 5 _b_) the walls are smooth, but they are perforated
  by many quite irregular, small orifices, which have the appearance
  of having been formed by some boring animal, but really serve, as
  I believe, to admit threads of corium into certain irregular pores
  which penetrate the shell. The sheath descends but a very short
  distance from the orifice: it is closely attached to the walls,
  and might easily be overlooked. The basis is deep, of an irregular
  outline, like that of the shell, and is formed by a very thin shelly
  layer. The largest specimen which I have seen, measuring from the
  extreme projecting points, was .4 of an inch in diameter.

  The _Scuta_ and _Terga_ (fig. 5 _f_) are calcified together without
  any trace of a suture; together they form a bow with the two ends
  curled rather abruptly inwards; they are both extremely narrow, but
  furnished with an occludent ledge, twice or thrice as high as the
  proper valves themselves. This occludent ledge, which is finely
  hirsute, begins at about one third of the length of the scutum
  from the rostral angle, and runs to near the basi-carinal angle of
  the tergum. The scutum itself is curved, with a slip, along the
  true occludent margin (best seen at the rostral end), lying in a
  different plane from the rest of the valve, much in the same way as
  in the scutum of _P. milleporæ_. The adductor ridge descends a very
  little below the basal margin of the valve, and extends for nearly
  its entire length: this adductor ridge makes the proper valve even
  narrower than it at first appears. The _Tergum_ is extremely narrow,
  forming merely a border to the occludent ledge; but it is not short,
  being about two thirds of the length of the scutum. There is no trace
  of a spur; indeed, the valve is rather narrower where the spur should
  have stood, than it is at the basi-carinal end. The scuta and terga
  are calcified together by their apices.

  _Affinities._--Although this species, as above stated, differs so
  remarkably in external appearance from the other species of the
  genus, and, indeed, of the whole family, yet the shell in no one
  essential point of structure materially differs from its congeners;
  and if we compare the opercular valves with those of the three
  last species, we shall be struck with their close, yet graduated,
  affinity: in _P. milleporæ_ the scuta and terga tend to become
  calcified together, and the rostro-occludent end of the scutum is
  bent into a different plane from the rest of the valve. In _P.
  dentatum_ we have an occludent ledge exactly as in the present
  species; but in that species the adductor plate is less developed
  than in _P. monticulariæ_; on the other hand, in _P. crenatum_, the
  adductor plate is more developed than in _P. monticulariæ_. If in
  _P. crenatum_ we were to remove the spur from the tergum (and it is
  much less developed in _P. dentatum_; and in _P. milleporæ_ it is
  entirely absent) this valve would be almost identical with that of
  _P. monticulariæ_. Under these circumstances I consider it impossible
  to separate the present species as a distinct genus.



_Species Dubiæ._


The _Daracia Linnæi_ of J. E. Gray (Annals of Philosophy, new series,
vol. 10, 1825), was published without description or figure.

The _Megatrema semicostata_ of G. B. Sowerby, junr. (Conch. Manual,
fig. 33, 1839), is not described, and is very indifferently figured
without the opercular valves, and therefore can never be recognised.

There is an admirable figure of a Pyrgoma, without any specific name,
in the great 'Description d'Egypte,' but from the want of some details,
I cannot positively recognise the species; I am inclined to believe
that it is the _P. dentatum_ of this work.

M. Chenu, in his grand Illustrations Conchyliologiques, has given most
beautiful figures of several species of Pyrgoma, and of Creusia, but
unfortunately, from the opercular valves not having been figured, I
find it impossible to recognise them. The new species are _Pyrgoma
stellata_ (on which I have appended a note under _P. conjugatum_), and
_P. spongiarum_ and _P. corymbosa_ of Valenciennes.



6. _Sub-Genus_--CREUSIA. Pl. 13, 14.

  CREUSIA. _Leach._ Journal de Physique, tom. 85, July, 1817.

_Compartments four, furnished with radii; basis cup-formed: attached to
corals._

  _Distribution_, imbedded in corals throughout the tropical seas.


Creusia is closely allied to Pyrgoma; and had not this genus already
been adopted by several authors, I should not, I think, myself have
formed it; though, no doubt, it harmonises well with some of the other
genera of the family, which are perhaps all too intimately related.
Creusia differs from Pyrgoma only in the shell being separated by
sutures, into four compartments, with well developed radii: in other
respects, such as in general habit, in the cup-formed imbedded basis
(not permeated by pores), in the opercular valves, in the characters
derived from the mouth and cirri, there are no generic differences.
This affinity is more particularly evident when Creusia is compared
with the first few species of Pyrgoma: indeed, for a short time, I was
inclined to consider _var._ 10 of Creusia as identical with _Pyrgoma
conjugatum_. With respect to the species of Creusia, I confess I have
been much perplexed in determining whether there be only one, or half
a dozen. The latter conclusion would almost certainly be arrived at if
only a few specimens were examined; and it might naturally be thought
that some of the species were extremely well marked; the difficulty
of drawing any line between varieties and species, begins only when
some hundreds of specimens, from various parts of the world, are
disarticulated, cleaned, and carefully examined. Creusia, in this
respect, offers a striking contrast with Pyrgoma, in which nearly all
the species are strongly characterised. The shell differs very little
in the several varieties or species of Creusia; and the most marked
difference, namely, whether the walls are permeated by irregular
pores or not, seems certainly quite variable. It is in the opercular
valves, which in other genera offer by far the most reliable character,
that we encounter the chief cause of perplexity; for the characters
thus derived, though at first appearing very distinct, blend into
each other, and are not accompanied by any well marked differences
in the shell. Only a few of my specimens have any habitat; but the
geographical range, as far as it does go, throws no light on the
question which forms to regard as species and which as varieties. As is
generally the case with cirripedes, the variations are local, so that
the greater number of specimens imbedded in the same coral resemble
each other. Under these circumstances I have thought it best, after
repeated examinations of a very large suite of specimens, to describe
separately each variety, without attaching any name to it; but I will
first make a few general remarks on the structure of the shell. If
I do not thus throw much light on the subject, I shall at least not
burden it with error. I believe that the species will be definitely
made out only by persons resident in the coral-bearing zones. I have
given copious illustrations of the opercular valves; for, if my view be
correct, this genus offers a curious and striking case of variation;
if, on the other hand, I am wrong, the drawings, I hope, will aid
others in coming to a more correct conclusion.



1. CREUSIA SPINULOSA. Pl. 13, fig. 6 _a_-6 _h_: Pl. 14, 6 _i_-6 _u_, 6
U.

  CREUSIA SPINULOSA. _Leach_ (!). Encyclop. Brit. Suppl., vol. 3, Pl.
        57, 1824.

  CREUSIA SPINULEUSE. _De Blainville._ Dict. Sc. Nat., Pl. 116, fig. 6.

  CREUSIA GREGARIA. _G. B. Sowerby_ (!). Genera of Recent and Fossil
        Shells, No. 18, Sept. 1823.

  ---- GRANDIS. _Chenu._ Illust. Conch. Tab. 1, fig. 2, sed non fig. 2
        _a_ and _b_.

  _Hab._--Philippine Archipelago, China, Singapore, Java, Red Sea, West
  Indies; imbedded in various corals; Mus. Brit., Cuming, Stutchbury,
  Dunker, &c.


  _General Appearance._--The shell is oval, generally flat, sometimes
  conical, with narrow and approximate ridges radiating from the
  orifice (fig. 6 _a_). The ridges, however, are sometimes distant from
  each other, and considerably prominent, projecting round the basal
  border. The orifice is either neatly diamond-shaped or oval. The four
  compartments are quite distinct; the radii are generally white, of
  considerable width, and with their summits not oblique. The colour
  is either white, or pale pinkish-purple; but in _var._ 11, bright
  pink. Even in the white specimens, when well preserved, the sheath is
  generally, but not always, either pale or dark purple. The largest
  specimen which I have seen, from the West Indies, was above half an
  inch in diameter; but from .3 to .4 of an inch is the more usual full
  size. I believe that the size, as well as the great variability of
  the present species, is partly determined by the rate of growth of
  the various zoophytes in which the specimens are imbedded, for the
  shell has to keep on nearly a level with the surface of the coral.

  _Structure of Shell and Basis._--The walls are internally ribbed;
  the ribs being usually prominent, sometimes to such a degree as
  to deserve to be called plates. The outer lamina is of variable
  thickness, and the prominence of the internal ribs appears in
  considerable part to depend on the extent to which the outer lamina
  has been thickened from within. In many specimens, instead of the
  interspaces between these internal ribs or longitudinal septa
  being solidly filled up, separate and successive laminæ have been
  deposited, by which the shell is rendered cancellated or porose; but
  the pores are very irregular; and sometimes they form two or three
  irregular rows one behind the other: this structure seems eminently
  variable. The edges of the radii are formed by crenated, and
  occasionally branched, septa. That part of the alæ, which is added
  during the diametric growth of the shell, is very thin. The lower
  edge of the sheath seems always to be free. The shelly layer, forming
  the basis, which is deeply cup-formed, is thin, more or less finely
  furrowed in radiating lines, and not permeated by pores.

  The opercular valves will be best described under the following
  eleven varieties.

  _Var._ (1), Pl. 13, fig. 6 _a_, 6 _b_, 6 _c_, 6 _d_.--_Hab._ Java,
  and probably several other districts.--I will first describe a
  typical sub-variety, by which I mean a sub-variety not presenting
  any extreme character. The scutum is of a sub-triangular shape, with
  the basi-tergal corner much rounded off (6 _d_), and generally but
  not always hollowed out in a rather remarkable manner. The adductor
  ridge is considerably prominent, and extends high up, parallel to
  the articular ridge, which latter is rounded and prominent, but
  to a variable degree. Near the rostral angle there is sometimes a
  small tooth, or only a trace of one, depending beneath the basal
  margin; this tooth we shall hereafter see much more developed. The
  _Tergum_ is about two thirds of the width of the scutum. It is
  often slightly beaked, but this is more conspicuous in some of the
  following varieties. The spur is about half the width of the valve,
  and its basal end is truncated, and nearly parallel to the basal
  margin of the valve, but the truncated form passes insensibly into
  a rounded outline. The shell in this variety is generally thick and
  is not permeated by pores; the orifice is diamond-shaped. But in
  another sub-variety the walls of the shell are always, or nearly
  always, permeated by pores, and the tergum is very much narrower,
  with the spur sharper, so that at first I concluded that these two
  sub-varieties were specifically distinct: we shall, however, soon
  see in _var._ 2 and in _var._ 4, that no confidence whatever can be
  placed in the exact breadth of the tergum, or in the porosity of the
  walls; hence I have been driven to consider the two varieties just
  mentioned as merely sub-varieties.

  _Var._ (2), fig. 6 _e_, 6 _f_, 6 _g_.--_Hab._ China; Red Sea.--The
  shell is almost invariably permeated by pores, sometimes arranged
  in two or three very irregular rows. In some specimens the scutum
  exactly resembles that in _var._ 1, but with the tooth near the
  rostral angle often rather larger: in other specimens the scutum is
  much more elongated transversely (fig. 6 _e_), with the adductor
  ridge more medial, and the basal margin not at all hollowed out
  at the basi-tergal corner of the valve. The tergum, here, is the
  remarkable feature, being sometimes excessively narrow, with a long
  sharp spur, which often, but not always, terminates in a needle-like
  point. In other specimens, from the same coral and certainly
  belonging to this same variety, the valve is not so narrow (6 _g_),
  and the spur not so pointed; consequently (as in several analogous
  cases in other cirripedes) it is impossible to draw any line of
  distinction between the specimens with the narrow and broad terga.

  _Var._ (3), fig. 6 _h_ [_Creusia gregaria of G. B. Sowerby!_]--_Hab._
  Unknown.--The scutum presents here exactly the same considerable
  range of variation as in _var._ 2. The tergum is broad, as in _var._
  1, but the spur is rounded, and from not being placed so immediately
  close to the basi-scutal angle of the valve, gives to it a rather
  different aspect. The breadth of the spur varies; an extreme variety
  is given in fig. 6 _h_.

  _Var._ (4), Pl. 14, fig. 6 _i_, 6 _k_, 6 _l_.--_Hab._ Philippine
  Archipelago; West Indies.--The _scutum_ here presents the same
  sub-varieties as heretofore, excepting that I have not seen any so
  much elongated transversely. The shell is covered either with slight,
  closely approximate ribs, as in the foregoing varieties, or with more
  distant and more prominent ribs. In specimens taken out of the same
  branch of coral the walls were either porose or solid. Sometimes the
  sheath is bright purplish-pink. It is the _tergum_, again, which
  presents a remarkable range of difference; for the longitudinal
  depression or furrow which in the former varieties was quite open,
  here has its edges more or less folded inwards, and is sometimes
  quite closed. This same variation has been commonly observed in many
  species of Balanus, in which it appears to be dependent on the age
  of the individual; but this does not appear to be the case in the
  present genus. As a consequence of the greater or less folding in of
  the two sides of the furrow, the spur is rendered more or less narrow
  and pointed, and thus becomes removed to a greater or less distance
  from the basi-scutal angle of the valve. Further, as a consequence of
  this folding in, the internal surface of the valve along the line of
  the external furrow, is raised into a longitudinal ridge. The length
  of the spur varies considerably. In some very young individuals, the
  basal margin descends lower on the scutal than on the carinal side
  of the spur. In one set of specimens (fig. 6 _l_), a plate extended
  from the carinal margin to near the central longitudinal ridge
  just mentioned: a similar structure was described under _Pyrgoma
  cancellatum_.

  _Var._ (5), Pl. 14, fig. 6 _m_.--_Hab._ Unknown.--We have seen in
  _vars._ 2 and 3 that the scutum varies considerably in shape: here
  it is unusually narrow, with the adductor ridge almost touching the
  articular ridge. There is no little tooth near the rostral angle, and
  the basi-scutal corner is not hollowed out. The tergum also varies;
  in some individuals it is truncated and like that figured of _var._
  1, but rather more rounded; in other specimens (from the same branch
  of coral) the basal margin so blends into the spur that the latter
  can hardly be discriminated (fig. 6 _m_); in other respects the
  outline resembles pretty closely that of one of the _sub-vars._ (fig.
  6 _f_) of _var._ 2. The shell is not porose; it is thick, with strong
  internal ribs, and resembles that of _var._ 1; but it is of a pale
  purplish colour.

  _Var._ (6), _an. spec._? fig. 6 _n_-6 _q_.--_Hab._ Philippine
  Archipelago.--This is a very remarkable variety; we have, imbedded
  in the same coral, and with shells absolutely identical, specimens
  with the scutum having three distinct but graduated forms. Firstly,
  a scutum transversely elongated, in all external respects like some
  of the varieties mentioned under _vars._ 2 and 3, with no rostral
  tooth, and not hollowed out at the basi-tergal corner, but with the
  adductor ridge more prominent. Secondly, a scutum of the same general
  shape, but with the adductor ridge so much developed (fig. 6 _n_) as
  to descend slightly beneath the basal margin, and to be seen when
  the valve is viewed externally; there is a very slight tooth near
  the rostral angle (as in some former sub-varieties), and which can
  be here rather more clearly seen than hitherto, to be formed by the
  adductor ridge (closely united to the external surface of the valve)
  extending thus far, and being here produced a little downwards.
  Thirdly (6 _p_), we have the adductor ridge immensely developed,
  descending far below the basal margin of the ordinary valve; and
  the basal margin at the basi-tergal corner is angularly and deeply
  hollowed out. The appearance of the valve is widely different from
  that in the first sub-variety, yet it is impossible to separate the
  first and second sub-varieties, and it is almost equally certain that
  the third sub-variety is only an exaggeration of the second. The
  lower edge of the adductor ridge, in the third sub-variety, varies
  a little in outline; it is deeply sinuous, and is produced at the
  rostral angle into a point, of which we have heretofore seen only a
  feeble representation. It would appear as if the great development of
  the adductor plate had caused the exterior ordinary surface of the
  valve to shrink or be less developed. There is a striking resemblance
  in the structure here described with that in _Pyrgoma cancellatum_
  and _conjugatum_. The terga belonging to the above scuta, also,
  vary; the spur being sometimes square (6 _o_), and sometimes bluntly
  pointed: when the spur runs in the same exact line (6 _q_) with the
  scutal margin of the valve, a peculiar aspect is given to it, but
  this is by no means always the case. Both opercular valves are often
  partially coloured pinkish-purple. The shell is not porose; it is
  thin, with remarkably prominent internal plates; it is apparently
  always of small size, which I attribute to this variety inhabiting
  a hard thin plate-like coral. The sheath is bright pinkish-purple,
  of which we have had instances in some of the other varieties; and
  the shell itself is sometimes pinkish. Taking the scutum of the
  first sub-variety, together with the commonest accompanying variety
  of tergum, I find it quite impossible to assign to it a specific
  character; if, on the other hand, we consider the scutum of the
  third sub-variety by itself, nothing can appear more distinct; but
  I must repeat, there can be hardly a shadow of doubt that the three
  sub-varieties of scutum here described, graduate into each other, and
  are specifically identical.

  _Var._ (7), fig. 6 _r_.--_Hab._ Probably Philippine Archipelago,
  associated with _Balanus quadrivittatus_.--There can be hardly any
  question of this being specifically identical with the last variety.
  It inhabits a different coral. All the specimens were of small size.
  The walls are not so thin, and the internal ribs not so prominent as
  in _var._ 6. The sheath is either white or dull purple; I can, in
  short, point out no difference in the shell from the typical _var._
  1. The scutum is not so much elongated transversely as in _var._
  6, and the basi-tergal corner is more cut off,--in which respect
  it resembles the common varieties. The adductor ridge is largely
  developed, so as to be just visible when the valve is viewed from the
  outside, in a degree between the first and second sub-varieties of
  _var._ 6: but the most singular character is the larger development
  of the tooth near the rostral angle, and this was the case in the
  same degree in all the specimens which I examined. The tergum
  resembles that rather unusual sub-variety of _var._ 6 (fig. 6 _q_),
  which has the scutal margin and the one side of the spur forming a
  straight line. It appears to me that it would be absurd to consider
  these slight differences, in parts unquestionably subject to much
  variation, as specific, when we are almost forced to admit that the
  much greater differences in the three sub-varieties of _var._ 6, are
  not of specific value.

  _Var._ (8).--_Hab._ Unknown, Mus. Cuming.--I have seen only a single
  specimen of this, and refer to it on account of _var._ 11. The shell
  is rather steeply conical, with distant and prominent ribs; the radii
  are narrow; the walls are not permeated by pores; the colour is pale
  purple. Altogether its external appearance is very different from
  that of the foregoing varieties; but the scuta are identical with
  those of _var._ 1, excepting that the rostral tooth is rather larger,
  being nearly as large as in the last, _var._ 7. The tergum precisely
  resembles that in some specimens of _var._ 2. Hence this variety
  differs from the first two varieties only in the shade of colour, the
  external shape, and the greater prominence of the external radiating
  ribs of its shell. All these characters are variable in the several
  foregoing varieties, and they have been found, as yet, insufficient
  to discriminate species in any genus of sessile Cirripedes.


_Varieties With the Scuta and Terga Calcified Together._

  _Var._ (9), Pl. 14, fig. 6 _s_ [_C. spinulosa_, of Leach (!)]--_Hab._
  Unknown.--The shell is undistinguishable by a single character from
  many specimens of the first, third, and fourth varieties; it is not
  permeated by pores. The scutum and tergum, with the exception of the
  one striking difference of their being calcified together without
  any trace of a suture, are identical with those of _var._ 3, as may
  be seen by comparing the figures 6 _h_ and 6 _s_. Hence to separate
  this form specifically from _var._ 3, we should have to rely solely
  on the calcification or union of the scuta and terga; but we have
  seen this is a point which is variable in _Elminius Kingii_, _Pyrgoma
  milleporæ_, and in some species of Balanus. The serial affinities,
  moreover, in Pyrgoma, clearly show that this is a character of no
  great importance. I must add that in several specimens of several of
  the varieties, the scuta and terga were so closely joined, that until
  careful examination, I was unable to detect the suture separating
  them; such being the case it must be quite unimportant for any
  functional purpose, whether or not the valves are calcified together.
  I feel, consequently, hardly any doubt that I have acted right in
  treating the present form as a mere variety.

  _Var._ (10) _an. spec._? fig. 6 _t_.--_Hab._ Unknown.--This variety
  bears nearly the same relation to _var._ 6, as the last variety did
  to _var._ 3. The shell is rather stronger than in _var._ 6, with the
  internal ribs not so prominent; and except in being tinted pale dull
  purple, it differs in no respect from the shell of _var._ 1. If we
  imagine the scutum and tergum in the third sub-variety of _var._ 6
  (6 _p_, 6 _q_), in which the adductor ridge descends far beneath the
  true basal margin of the valve, to be calcified together, without any
  suture, we shall produce almost the identical valves of the present
  variety. The scutum, however, here is not quite so much elongated
  transversely, and the occludent margin is spinose and is furnished
  with large teeth; these two characters give the valve a somewhat
  different aspect, and hence I am more doubtful than in the foregoing
  case, whether this form may not be specifically distinct. I must,
  however, state that in _Tetraclita porosa_, I ascertained that the
  teeth on the occludent margin of the scuta were even more variable
  than here is supposed to be the case; and as for the shape of the
  valve we have seen what wonderful variation there is in _var._ 6. The
  tergum in this variety is about intermediate between the two common
  forms, in the sub-varieties of _var._ 6. As for the calcification of
  the two valves together, we have seen, under the last variety, how
  little important a character it is.

  _Var._ (11) fig. 6 U, 6 _u_ [_Creusia grandis_, of Chenu, Tab. 1,
  fig. 2, but not fig. 2 _a_ and _b_].--_Hab._ Singapore, associated
  with _Pyrgoma monticulariæ_.--This variety is very closely related
  to the last. The shell, however, has a very peculiar aspect, which
  made me for some time think it must be specifically distinct. It is
  of a much brighter pink than in any of the foregoing varieties; the
  surface is marked with very prominent, distant ribs, and the radii
  are narrow, in which latter points, together with the tint (though
  here brighter), this variety cannot be distinguished from _var._ 7.
  The shell, however, is permeated by several rows of pores, in which
  respect it resembles the shell in _var._ 2, and some specimens of
  _vars._ 3 and 4. In the opercular valves there is a close general
  resemblance with those of the last _var._ 10; the tooth, however,
  near the rostral angle, is not so prominent; and in the tergum, the
  spur is more truncated, shorter and broader than in _var._ 10, and
  closely resembles that in _var._ 1 and 2. But I cannot consider any
  of the points here specified of much weight.


  The foregoing descriptions show how singularly the affinities of the
  several varieties interlock in the most complicated manner. Hereafter
  some one may, perhaps, succeed in grouping several of these forms as
  species; but I am sure he ought not to attempt it without possessing
  a very large suite of specimens, or without the great advantage of
  comparing some two or three of the forms, fresh in their native site.


  _Species dubiæ._--Under Pyrgoma, I have stated that though Chenu,
  in his 'Illust. Conch.,' has given beautiful external figures of
  the shells, imbedded in the coral, yet from the want of details on
  the opercular valves and on the structure of the shell, I cannot
  recognise his species. So it, likewise, is with Creusia. Chenu gives
  the following new species; _C. radiata_, _multistriata_, _decorata_,
  and _striata_. The _C. madreporarum_, I suspect to be the _Pyrgoma
  milleporæ_ of this work, as there stated. The _C. grandis_ no doubt
  is the _P. grande_ of this work, the _Nobia grandis_ of Sowerby. The
  name _Creusia Childreni_ is given by Dr. Gray, without description or
  figure, in the 'Annals of Philosophy,' vol. 10, new series, 1825.



7. _Genus_--CHELONOBIA. Pl. 14: Pl. 15, fig. 1.

  CHELONOBIA. _Leach._ Journal de Physique, tom. 85, (1817).

  CORONULA. _Lamarck._ Animaux sans Vertèbres, 1818.

  ---- _Ranzani._ Memoire di Storia Naturale, 1820.

  ---- _De Blainville._ Dict. des Sciences Naturelles.

  ASTROLEPAS. _J. E. Gray._ (Klein) Annals of Philosophy, (new series),
        vol. 10, (1825).

_Compartments extremely thick, six; but one of them, the rostrum,
internally is composed of three rudimentary compartments, united
together: basis membranous: scuta narrow, united to the terga by a
horny articular ridge._

  _Distribution_, throughout the tropical, and warmer temperate seas of
  the whole world; attached to turtles, crustacea, or smooth gasteropod
  molluscs.


This is a distinct and well-defined genus. Several authors have
confounded it with Coronula, but this has been owing to an entire
misapprehension of the structure of the shell in the two genera. In
Coronula, the parietes are very thin, and are so deeply folded as to
appear like rays or septa connecting the outside of the shell (the
expanded ends of the folds) and the internal surface of the shell, but
the open spaces between the folds are occupied by the epidermis of the
Whale, and are external to the cirripede. In Chelonobia, the parietes
are remarkably thick; hence the plates or septa, connecting the outer
and inner lamina, are of unusual length; and the spaces between them,
though of course internal with regard to the cirripede and occupied by
the ovaria, have been compared to the spaces external to and between
the folded walls of Coronula. There is but little special affinity
between these genera; and I regret that they have come to be placed one
after the other in this work: but the elongated opercular valves,--the
thick and double opercular membrane,--the weak depressor muscles,--and
the peculiar manner in which the scutum is articulated by the aid of
a horny projection to the tergum, may indicate some real but slight
affinity to Coronula; the many points of difference, however, in the
structure of the shell and of the opercular valves, and especially in
the cementing apparatus of the basal membrane, and in the branchiæ,
all prove that the genera are very distinct. The singular structure of
the rostrum, which, in fact, consists of three compartments externally
blended together, and which three correspond in all essential respects
to the rostrum and two rostro-lateral compartments in the Chthamalinæ,
offers a very striking point of identity with that sub-family; but
neither in the mouth, cirri, or other part, can I detect any other
evidence of this relationship. Having so far discussed the affinities
of the genus, I may add, that the three species, though decidedly
distinct, are closely and nearly equally related to each other.

_General Appearance._--The shell is generally depressed, and broadly
oval or almost circular; in _C. testudinaria_ and _caretta_, it
has a massive appearance: the surface is generally smooth, or,
when disintegrated, finely striated: the colour is white. The six
compartments do not differ much in size: the rostrum is rather larger
than the carina, and the lateral compartments, than the carino-lateral
compartments. It is remarkable that in _C. caretta_ (Pl. 14, fig.
2), even in specimens which have not grown crowded together, the
compartments are almost invariably placed rather unsymmetrically,
the rostrum and carina not exactly facing each other.[121] The shell,
though so thick and massive, yields easily along the lines of suture.
The radii are moderately wide, or narrow, or not at all developed,
being represented by mere sutures: in this latter case, in _C.
caretta_, the orifice of the shell is enlarged, in the same manner as
we have seen in some species of Balanus and Tetraclita, by the gradual
wearing away of the upper part of the shell. In _C. testudinaria_,
the radii have a singular notched structure (fig. 1 _a_), and the
whole shell a star-like appearance. The orifice is not filled up by
the elongated opercular valves,--a considerable extent of opercular
membrane being visible on the two sides. The largest specimen which
I have seen, namely, of _C. testudinaria_, was nearly two and a half
inches in its longer diameter.

    [121] In Mr. Stutchbury's collection there is a specimen of _C.
    testudinaria_ in which there are only five compartments, one of the
    lateral compartments having been aborted; of this I have seen no
    other instance in any genus.

_Structure of the Parietes._--The parietes are of unequalled thickness,
especially in the first two species of the genus. From the outer lamina
(see Pl. 14, fig. 4, and the section in Pl. 15, fig. 1), numerous
vertical plates extend inwards, alternately to a less or greater
distance, some of them reaching to the inner lamina: these answer to
the longitudinal parietal septa in other genera, and the elongated
cavities between them (which occur in _C. testudinaria_ and _patula_)
answer to the parietal tubes or pores. The radiating plates or septa
have their sides finely channelled, and their basal edges generally
slightly sinuous and always finely toothed. The interspaces between
the plates in the uppermost part of the shell are filled up solidly,
and, in _C. caretta_, even down to near the basis: in this latter
species, the plates are irregular and much broken up, so as in parts
to consist of little, separate, flattened points. In _C. patula_, the
inner lamina of the parietes (_b_, in fig. 4. Pl. 14) can be best
made out to be distinct from the sheath (_e c e_ in fig. 4, and _c′_
in the section of _C. testudinaria_, fig. 1, Pl. 15). The sheath in
this genus descends in a very remarkable manner to the basal membrane,
and has its basal edge toothed like the basal edges of the radiating
septa. The inner lamina itself does not descend to the basal membrane.
In _C. testudinaria_, the inner lamina is of great thickness; but in
the section, (fig. 1) owing to its having been taken high up, the
inner lamina, (_b_), is not distinct from the shelly matter deposited
between the septa. In _C. caretta_, the line of separation between the
inner and outer laminæ can in no part be distinguished, owing to the
interspaces between the septa having been solidly filled up, close down
to the basis.

_Sheath._--The layer of shell surrounding the internal cavity (_e c
e_, fig. 4, Pl. 14), and extending down to the basal membrane, I have
called the sheath, owing to its being distinctly continuous with the
innermost layer in the upper part of the shell, to which the opercular
membrane is attached: this can be best seen by examining the alæ in
the separated compartments of _C. patula_. The sheath is not only
remarkable from thus descending to the basal membrane, but in _C.
testudinaria_ and _patula_ from its lower edge being perforated by
arched channels (under _c_, in fig. 4), allowing thick ribbons of
corium to reach the interspaces between the radiating septa. There
is one central arch or channel in the middle of the lower part of
the sheath of each compartment, and one on each line of suture, the
sheath being a little hollowed out on both sides of the sutures. As the
rostrum, as far as its internal structure is concerned, consists of
three compartments, we have altogether in the shell eight compartments
and eight sutures, and consequently altogether sixteen arches through
the lower part of the sheath, allowing sixteen thick ribbons of corium
to penetrate the parietes, and thus likewise reach the radii. There is,
however, sometimes a little variation in the number of these arched
channels. The upper part of the sheath is transversely marked by
zones of growth, to the lower one of which the opercular membrane is
attached. The line of attachment is not low down the sheath.

_Radii and Alæ._--The radii, when the compartments are disarticulated,
present a remarkable structure, from appearing to consist of a distinct
inner and outer portion. The radius normally consists of an inner and
outer lamina, united by septa parallel to the basis; but here the inner
portion is formed by a central ridge (_a a_, fig. 5, Pl. 14), sending
off on both sides little septa, often sub-branched; it is of nearly
uniform width; and there is no distinct inner lamina. The outer portion
(_b_), which often equals or exceeds in thickness the inner portion,
is, in fact, the normal outer lamina, developed to an unparalleled
degree. In most Cirripedes the edge of the radius is received in a
slight furrow in the opposed compartment, the lid of which furrow is
narrow, and matches the outer lamina of the radius; here the lid of the
recipient furrow is very broad, and resembles the outer lamina of the
radius in all its characters. In order to allow of growth between the
_thick_ opposed edges of the outer lamina of the radius and the lid of
the recipient furrow, the two surfaces are finely dentated (look in
fig. 5, under the pits, marked _b_), almost like the crown of a molar
tooth; thus allowing films of corium to enter. The structure here
described is common, in a greater or less degree, to all three species,
but is best seen in _C. testudinaria_. In this species, moreover, (fig.
5, _b_), the outer lamina, instead of being smooth and of either equal
or gradually increasing thickness from top to bottom, is generally, but
not always, (fig. 1 _a_), deeply pitted or notched in transverse lines,
the outer lamina being thus rendered alternately thicker and thinner,
and so formed into transverse ridges and valleys. Hence the lines of
suture become toothed, the points of the teeth facing each other, and
not interlocking. In the transverse section, fig. 1, Pl. 15, of the
same species, taken high up across the shell, (_f_) is the pitted outer
lamina, and (_e_) the inner portion of the radius. Although the radii
are thus specially added to in thickness, they are not so thick as the
very thick walls, and hence the lines of suture form furrows more or
less deep. The _alæ_ are of moderate thickness, and have their sutural
edges crenated by fine transverse septa.

_Rostrum._--I have already alluded to the peculiar compounded structure
of this compartment, unlike anything we have as yet seen.[122] The
thin outer lamina is quite continuous, and shows no trace of the
triple nature of the compartment; as may be seen by comparing the
drawing (Pl. 14, fig. 1 _a_) of the shell of _C. testudinaria_,
with the transverse section (Pl. 15, fig. 1) of the same species:
in this latter figure, _a_ is the outer lamina, and B A B the three
compartments of the rostrum. But when the outer lamina is worn away,
as is always the case with the upper part of the walls in _C. caretta_
(Pl. 14, fig. 2), the two fissures separating the three compartments
of the compounded rostrum, are plainly exhibited on the outside of
the upper part of the shell. On the internal surface, the sutures
separating the three compartments are always open, except in the upper
part of the sheath, above