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Title: The zoology of the voyage of H.M.S. Beagle [vol. 1 of 5]: Fossil mammalia
Author: Owen, Richard
Language: English
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H.M.S. BEAGLE [VOL. 1 OF 5] ***
THE
ZOOLOGY
OF
THE VOYAGE OF H. M. S. BEAGLE,
UNDER THE COMMAND OF CAPTAIN FITZROY, R.N.,
DURING THE YEARS
1832 TO 1836.
_PUBLISHED WITH THE APPROVAL OF THE LORDS COMMISSIONERS OF HER MAJESTY’S
TREASURY_.
Edited and Superintended by
CHARLES DARWIN, ESQ. M.A. F.R.S. SEC. G.S.
NATURALIST TO THE EXPEDITION.
PART I.
FOSSIL MAMMALIA:
BY
RICHARD OWEN, ESQ. F.R.S.
PROFESSOR OF ANATOMY AND PHYSIOLOGY TO THE ROYAL COLLEGE OF SURGEONS IN
LONDON; CORRESPONDING MEMBER OF THE INSTITUTE OF FRANCE, ETC. ETC.
LONDON:
PUBLISHED BY SMITH, ELDER AND CO. 65, CORNHILL.
MDCCCXL.
LONDON:
PRINTED BY STEWART AND MURRAY,
OLD BAILEY.
CONTENTS.
Page
GEOLOGICAL INTRODUCTION 3
TOXODON PLATENSIS, Description of Cranium 16
of Lower Jaw and Teeth 29
MACRAUCHENIA PATACHONICA, Cervical Vertebræ 35
Lumbar Vertebræ 40
Scapula 43
Antibrachium and Fore-foot 44
Femur 48
Tibia, Astragalus, and Metatarsal
Bone 50
GLOSSOTHERIUM, Fragment of Cranium 57
MYLODON DARWINII, Lower Jaw and Teeth 63
SCELIDOTHERIUM LEPTOCEPHALUM, Cranium and Teeth 73
Vertebral Column 84
Bones of the Extremities 88
MEGALONYX JEFFERSONII, Lower Jaw 99
MEGATHERIUM CUVIERI, Cranium and Teeth 100
TESSELATED ARMOUR AND BONES, large Edentata 106
MASTODON ANGUSTIDENS, Cuv. (?) 108
EQUUS, Molar Teeth 108
CTENOMYS PRISCUS, and a large Rodent 109
Geological contemporaneity of preceding extinct Mammalia 111
LIST OF PLATES.
PLATE I. Cranium of _Toxodon Platensis_, nat. size, basal view, with
the sixth molar, right side upper jaw, and grinding surface
of the seventh molar left side.
II. Cranium of _Toxodon Platensis_, side view.
III. Ditto, upper view.
IV. Ditto, back view, with two views of the sixth molar,
right side, upper jaw.
V. Fragment of lower jaw, and teeth of _Toxodon Platensis_.
VI. Cervical vertebræ of the _Macrauchenia Patachonica_.
VII. Ditto, ditto, and of _Auchenia Llama_.
VIII. Lumbar vertebræ of the _Macrauchenia Patachonica_.
_a._ Medium convex articular surface joining the body of the
sacrum.
_b._ Lateral concavities joining the transverse processes of the
_b._ sacrum.
_c._ Ossified ligament of an anchylosed lumbar vertebra.
IX. Fig. 1 and 2. Fragments of the Scapula.
Fig. 3. Distal end of the femur of _Macrauchenia Patachonica_.
X. Proximal ends of the anchylosed radius and ulna of
_Macrauchenia Patachonica_.
XI. Bones of the right fore-foot, _Macrauchenia Patachonica_.
XII. Femur, _Macrauchenia Patachonica_.
XIII. Anchylosed tibia and fibula, _Macrauchenia Patachonica_.
XIV. Right astragalus, _Macrauchenia Patachonica_.
XV. Fig. 1. External metatarsal bone of the right hind-foot.
Fig. 2. Proximal end of internal metacarpal bone, right
fore-foot.
Fig. 3 and 4. Proximal end of middle metacarpal bone.
Fig. 5. Proximal end of external metacarpal bone, right
fore-foot, _Macrauchenia Patachonica_.
XVI. Fragment of cranium of the _Glossotherium_.
Fig. 1. Side view.
Fig. 2. Base view.
Fig. 3. Inside view, all half natural size.
Fig. 4. Os tympanicum, natural size.
XVII. Fig. 1. _Megalonyx Jeffersonii._
Fig. 2. _Megalonyx laqueatus._
Fig. 3 and 4. _Mylodon Harlani._
Fig. 5. _Mylodon Darwinii._
XVIII. Lower jaw of the _Mylodon Darwinii_.
XIX. Fig. 1. Side view of the lower jaw.
Fig. 2 and 3. Last molar tooth.
Fig. 4. Front view of the symphysis of the lower jaw, _Mylodon
Darwinii_.
XX. Portion of the skeleton of the _Scelidotherium leptocephalum_.
XXI. Fig. 1. Side view of the cranium.
Fig. 2. Back view of the cranium.
Fig. 3, 4, and 5, anterior molar, upper jaw _Scelidotherium_.
XXII. Upper view of the cranium of the _Scelidotherium_.
XXIII. Fig. 1. Inside view of cranium.
Fig. 2. Articular surface for the lower jaw.
Fig. 3. Section of upper jaw and molar teeth.
Fig. 4. Section of lower jaw and molar teeth,
_Scelidotherium_.
XXIV. Cervical and anterior dorsal vertebræ of.
Fig. 1. _Scelidotherium._
Fig. 2. _Orycteropus._
Fig. 3. _Dasypus._
Fig. 4. _Myrmecophaga jubata_; all one-third nat. size.
XXV. Fig. 1 and 2. Humerus, radius and ulna of _Scelidotherium_.
Fig. 3. Head of humerus.
Fig. 4. Head of radius.
Fig. 5, 6, and 7. Femur of _Scelidotherium_.
XXVI. Fig. 1, 3, and 5. Astragalus of _Megatherium_.
Fig. 2, 4, and 6. Astragalus of _Scelidotherium_.
XXVII. Fig. 1 and 2. Patella and head of tibia of _Scelidotherium_.
Fig. 3, 4, and 5. Ungueal phalanx of _Scelidotherium_.
XXVIII. Fig. 1. Astragalus of _Megatherium_.
Fig. 2. Astragalus of _Scelidotherium_.
Fig. 3–6. Astragalus of _Mylodon_ or _Megalonyx_.
XXIX. Lower jaw of _Megalonyx Jeffersonii_.
XXX. Fragment of the Cranium of _Megatherium Cuvieri_.
XXXI. Section of the upper jaw and teeth of the _Megatherium
Cuvieri_.
XXXII. Fig. 1. Magnified view of structure of molar of _Megatherium_.
Fig. 2–5. Ungueal phalanx and portions of tesselated armour of
_Hoplophorus euphractus_.
Fig. 6–11. Jaws and teeth of _Ctenomys priscus_.
Fig. 12. Bones of right hind-foot of a Rodent.
Fig. 13 and 14. Fossil tooth of a Horse.
PREFACE.
His Majesty’s ship, Beagle, under the command of Captain FitzRoy, was
commissioned in July, 1831, for the purpose of surveying the southern
parts of America, and afterwards of circumnavigating the world. In
consequence of Captain FitzRoy having expressed a desire that some
scientific person should be on board, and having offered to give up part
of his own accommodations, I volunteered my services; and through the
kindness of the hydrographer, Captain Beaufort, my appointment received
the sanction of the Admiralty. I must here, as on all other occasions,
take the opportunity of publicly acknowledging with gratitude, the
obligation under which I lie to Captain FitzRoy, and to all the Officers
on board the Beagle, for their constant assistance in my scientific
pursuits, and for their uniform kindness to me throughout the voyage. On
my return (October, 1836) to England, I found myself in possession of a
large collection of specimens in various branches of natural history;
but from the great expense necessary to secure their publication, I was
without the means of rendering them generally serviceable.
The Presidents of the Linnean, Zoological, and Geological Societies,
having given me their opinion respecting the utility to be derived from
publishing these materials, I addressed a letter to the Right Honourable
the Chancellor of the Exchequer (T. Spring Rice, Esq.) informing him of
the circumstances under which I hoped that I might venture to solicit
the aid of Government. In reply, I received a communication (as below)
announcing to me that the Lords of the Treasury, from their readiness to
promote Science, were willing, under certain conditions, to give me the
most liberal assistance.
“_Treasury Chambers, August 31, 1837._
“SIR,
“It having been represented to the Lords Commissioners of Her
Majesty’s Treasury, from various quarters, that great advantage would
be derived to the Science of Natural History, if arrangements could be
made for enabling you to publish, in a convenient form, and at a cheap
rate, the result of your labours in that branch of science, my Lords
will feel themselves justified in giving their sanction to the
application of a sum, not exceeding in the whole one thousand pounds,
in aid of such a publication; upon the clear and distinct
understanding that the Work should be published, and the plates
engraved, in such a manner as to be most advantageous to the Public at
large, upon a plan of arrangement to be previously submitted to, and
sanctioned by the Board, after consultation with those persons, who,
from their attainments in this branch of science, are the most capable
of advising their Lordships thereupon; and that the payments on
account of the said sum of one thousand pounds are to be made to you
from time to time, on a certificate that such progress has been made
in the engravings, in accordance with the plan previously approved of,
as to justify the issue then applied for. My Lords have therefore
directed me to communicate to you the views they entertain upon this
subject; and to apprize you that they will be prepared to act in
conformity with their arrangement, upon learning from you that you are
ready to proceed with the Work upon the principles above laid down,
and upon receiving from you a statement of the manner in which you
think the Work should be published, and the plates engraved, so as
most effectually to accomplish the object my Lords have in view, in
sanctioning the payment from the Public Funds, in aid of the expenses
of the Work in question.
“I remain,
“Sir, Your Obedient Servant,
“A. Y. SPEARMAN.”
The object of the present Work is to give descriptions and figures of
undescribed and imperfectly known animals, both fossil and recent,
together with some account, in the one case, of their geological
position, and in the other of their habits and ranges. As I do not
possess the knowledge requisite for such an undertaking, and as I am,
moreover, particularly engaged in preparing an account of the geological
observations, made during the voyage, several gentlemen have most kindly
undertaken different portions of the Work. Besides the very great
advantage insured in thus enlisting the attainments of these Naturalists
in the several departments of science, to which they have paid most
attention, a great delay is avoided by adopting this method of
publication, which must otherwise have been incurred before the
materials could have been made known.
An Account of the Voyage, drawn up by Captain FitzRoy, (and to which I
have added a volume) being on the point of publication, I shall not in
this Work enter on any minute details respecting the countries which
were visited, but shall merely give a sketch of the geology in the
introduction to the part containing Fossil Mammalia, and a brief
geographical notice in that attached to the account of existing animals.
At the conclusion of this Work, I shall endeavour to place together the
leading results in the natural history of the different countries, from
which the collections were procured. I may here state that Mr. Owen has
undertaken the description of the Fossil Mammalia; Mr. Waterhouse, the
Recent Mammalia; Mr. Gould, the Birds; Mr. Bell, the Reptiles; and the
Rev. L. Jenyns, the Fish. Whatever assistance I may obtain in the
invertebrate classes, will be noticed in their respective places. The
specimens have been presented to the various public museums, in which it
was thought they would be of most general service: mention will be made
in each part where the objects described have been deposited.
FOSSIL MAMMALIA,
Described by
RICHARD OWEN, ESQ. F.R.S. F.G.S. F.L.S.
PROFESSOR OF ANATOMY AND PHYSIOLOGY TO THE ROYAL COLLEGE OF SURGEONS IN
LONDON; CORRESPONDING MEMBER OF THE ROYAL ACADEMY OF SCIENCES OF BERLIN;
OF THE ROYAL ACADEMY OF MEDICINE, AND PHILOMATHIC SOCIETY OF PARIS; OF
THE ACADEMY OF SCIENCES OF PHILADELPHIA, MOSCOW, ERLANGEN.
WITH
A GEOLOGICAL INTRODUCTION,
BY CHARLES DARWIN, ESQ. M.A. F.G.S. &c. &c.
CORRESPONDING MEMBER OF THE ZOOLOGICAL SOCIETY.
GEOLOGICAL INTRODUCTION.
BY MR. DARWIN.
Mr. Owen having undertaken the description of the fossil remains of the
Mammalia, which were collected during the voyage of the Beagle, and
which are now deposited in the Museum of the College of Surgeons in
London, it remains for me briefly to state the circumstances under which
they were discovered. As it would require a lengthened discussion to
enter fully on the geological history of the deposits in which these
remains have been preserved, and as this will be the subject of a
separate work, I shall here only give sufficient details, for the reader
to form some general idea of the epoch, at which these animals lived,—of
their relative antiquity one to the other,—and of the circumstances
under which their skeletons were embedded. All the remains were found
between latitudes 31° and 50° on the eastern side of South America. The
localities may conveniently be classed under three divisions, namely—the
Provinces bordering the Plata; Bahia Blanca situated near the confines
of Northern Patagonia; and Southern Patagonia.
The first division includes an enormous area, abounding with the remains
of large animals. To the eastward and southward of the great streams,
which unite to form the estuary of the Plata, those almost boundless
plains extend, which are known by the name of the Pampas. Their physical
constitution does not vary over a wide extent;—the traveller may pass
for many hundred miles on a level surface, without meeting with a single
pebble, or discovering any change in the nature of the soil. The
formation consists of a reddish argillaceous earth, generally containing
irregular concretions of a pale brown, indurated marl. This stone, where
most compact, is traversed by small linear cavities, and in several
respects resembles the less pure fresh-water limestones of Europe. The
concretions not unfrequently become so numerous, that they unite and
form a continuous stratum, or even the entire mass.
At Bajada de St^a. Fé, in the Province of Entre Rios, beds of sand,
limestone, and clay of different qualities, containing sharks’ teeth and
sea-shells, underlie the Pampas deposit. The shells, although numerous,
are few in kind. Mr. George B. Sowerby informs me that they appear to
belong to one of the less ancient tertiary epochs; they consist of
_Venus nov. spec._ near to _V. cancellata_; _Arca nov. spec._ near to
_A. antiquata_; a very large oyster, probably an extinct species; an
imperfect specimen of a second species of oyster near to _O. edulis_;
and a _Pecten_ near to _P. opercularis_. These beds pass upwards into an
indurated marl, and this again into the red argillaceous earth of the
Pampas, containing the remains of those extinct quadrupeds, which every
where characterize that deposit. To the southward of the Plata level
plains of an uniform composition, interrupted only at wide intervals by
hills of crystalline rock, extend to a distance of about three hundred
miles; and to the northward for at least an equal space, and probably
much further. As might have been expected from the perfectly level
surface, wherever a continuous section is presented on the banks of the
great rivers, very slight changes of colour show, that the deposit has
been accumulated in strata as horizontal as the land, or as the
water-line at the base of the cliffs.
In the province of Banda Oriental (to the N. and N. E. of the Plata),
and in part of that of Entre Rios, the land, though very low and level,
has a foundation of granitic and other primary rocks. These older
formations are partially covered, in most parts, by a reddish earthy
mass containing a few small calcareous concretions; while in other
parts, they are concealed by more regular strata, of indurated marl
passing into limestone, of conglomerates, and ferruginous sandstone. The
entire formation probably belongs to the same epoch with that of the
Pampas deposit. In the earthy mass, even where it is of little
thickness, and where it might readily be mistaken for detritus produced
from the underlying granites, remains of large quadrupeds have several
times been discovered.
On the shores of the Plata and in the neighbouring districts, proofs of
a change of level having taken place between the land and the water
within a recent period, may be observed. Both near Monte Video and
Colonia del Sacramiento, beds of shells are lying on the beach at the
height of several feet above the present tidal action. Near Maldonado I
saw estuary shells of recent species embedded in clay, and raised above
the level of a neighbouring fresh-water lake.
On the banks of the Parana, a shell identical with, or most closely
resembling an estuary species (_Potamomya labiata_, now living in that
part of the Plata, where the water is brackish) is accumulated in great
masses, which are found some miles inland, and are elevated several
yards above the level of the river. Sir Woodbine Parish, also, has in
his possession, shells procured from an extensive formation near
Ensenada de Barragan (south of Buenos Ayres), which is quarried for
lime. Mr. George Sowerby has examined these fossils, and says the
following are identical with living kinds; _Voluta colocynthis_,
Dillwyn: _V. angulata_, Swainson: _Buccinum globulosum_, Kiener: a
variety of _Oliva patula_: a _Cytheræa_ closely resembling or identical
with _C. flexuosa_, and a fragment of a second species, probably _C.
purpurascens_; _Potamomya labiata_; and fragments of oysters. There is,
however, a species of _Mactra_ in very great numbers, with which Mr.
Sowerby is wholly unacquainted. I may observe that I found recent shells
of the first five species inhabiting the coast, a short distance to the
southward. Some shelly limestone from the same place, which Sir Woodbine
Parish had the kindness to show me, resembles that which I saw at
Bajada, and in Banda Oriental. These beds, therefore, probably form
parts of the Pampas deposit, and are not merely indicative of the period
of its elevation. Nevertheless, on the opposite shores of the Plata,
near the mouth of the Uruguay, I found lines of sand dunes, where the
_Mactra_ and _Cytheræa flexuosa_ were lying in such quantities on the
bare surface, that the inhabitants, by merely sifting the sand, collect
them for burning into lime.
After these facts we may feel certain, that at a period not very remote,
a great bay occupied the area both of the Pampas and of the lower parts
of Banda Oriental. Into this bay the rivers which are now united in the
one great stream of the Plata, must formerly have carried down (as
happens at the present day) the carcasses of the animals, inhabiting the
surrounding countries; and their skeletons would thus become entombed in
the estuary mud which was then tranquilly accumulating. Nothing less
than a long succession of such accidents can account for the vast number
of remains now found buried. As their exposure has invariably been due
to the intersection of the plain by the banks of some stream, it is not
making an extravagant assertion, to say, that any line whatever drawn
across the Pampas would probably cross the skeleton of some extinct
animal.
At Bajada, a passage, as I have stated, may be traced upwards from the
beds containing marine shells, to the estuary mud with the bones of land
animals. In another locality a bed of the same mineralogical nature with
the Pampas deposit, underlies clay containing large oysters and other
shells, apparently the same with those at Bajada. We may, therefore,
conclude that at the period when the Arca, Venus, and Oyster were
living, the physical condition of the surrounding country was nearly the
same, as at the time when the remains of the mammalia were embedded; and
therefore that these shells and the extinct quadrupeds probably either
co-existed, or that the interval between their respective existences
was, in a geological point of view, extremely short. In this part of
South America there is reason to believe that the movements of the land
have been so regular, that the period of its elevation may be taken as
an element in considering the age of any deposit. The circumstance,
therefore, that the beds immediately bordering the Plata, contain very
nearly the same species of molluscs, with those now existing in the
neighbouring sea, harmonizes perfectly with the more ancient (though
really modern) tertiary character of the fossils underlying the Pampas
deposit at Bajada, situated at a greater height, and at a considerable
distance in the interior. I feel little doubt that the final extinction
of the several large quadrupeds of La Plata did not take place, until
the time when the sea was peopled with all, or nearly all, its present
inhabitants.
Bahia Blanca, situated in latitude 39°, and about 250 miles south of the
Plata, constitutes the second district, in which I found the remains of
quadrupeds. This large bay is nearly surrounded by very low land, on
which successive lines of sand dunes mark in many parts the retreat of
the water. At some distance inland a formation of highly indurated marl,
passing into limestone, forms an escarpment. Beyond this, rocks of the
same character extend over a wide and desolate plain, which rises
towards the flanks of the distant mountain of the Sierra de la Ventana,
composed of quartz. On the low shores of this bay, only two places
occur, where any section of the strata can be seen; and at both of these
I found fossil remains.
At Monte Hermoso, a line of cliff of about 120 feet in height, consists
in the upper part of a stratum of soft sandstone with quartz pebbles;
and in the lower of a red argillaceous earth, containing concretions of
pale indurated marl. This lower bed has the same mineralogical character
with the Pampas deposit; and possibly may be connected with it. The
embedded bones were blackened, and had undergone more chemical change
than in any other locality, which I examined. With the exception of a
few large scattered bones, the remains seemed to belong chiefly to very
small quadrupeds.
In another part of the bay, called Punta Alta, about eighteen miles from
Monte Hermoso, a very small extent of cliff, about twenty feet high, is
exposed. The lower bed seen at ebb tide, extends over a considerable
area; it consists of a mass of quartz shingle, irregularly stratified,
and divided by curved layers of indurated clay. The pebbles are cemented
together by calcareous matter, which results, perhaps, from the partial
decomposition of numerous embedded shells. In this gravel the remains of
several gigantic animals were extraordinarily numerous. The cliff, in
the part above high-water mark, is chiefly composed of a reddish
indurated argillaceous earth; which either passes into, or is replaced
by, the same kind of gravel, as that on which the whole rests. The
earthy substance is coarser than that at Monte Hermoso, and does not
contain calcareous concretions. I found in it a very few fragments of
shells, and part of the remains of one quadruped.
From the bones in one of the skeletons, and likewise from those in part
of another, being embedded in their proper relative positions, the
carcasses of the animals, when they perished, were probably drifted to
this spot in an entire state. The gravel, from its stratification and
general appearance, exactly resembles that which is every day
accumulating in banks, where either tides or currents meet; and the
embedded shells are of littoral species. But from the skeleton, in one
instance, being in a position nearly undisturbed, and from the abundance
of serpulæ and encrusting corallines adhering to some of the bones, the
water, at the time of their burial, must have been deeper than at
present. This conclusion might also have been inferred from the fact,
that in the neighbouring cliff the same bed, with its shells, has been
uplifted some yards above high-water mark. On the coast to the southward
abundant proofs occur, of a recent elevation of the continent. In the
gravel, nearly all the pebbles are of quartz, and have originally
proceeded from the lofty range of the Ventana, distant between forty and
fifty miles. Besides the pebbles of quartz, there are a few irregular
masses of the same indurated marl, of which the escarpment of the
neighbouring great plain is composed. Hence the gravel beds must have
been deposited, when the plain existed as dry land; and on it probably
those great animals once lived, of which we now find only the remains.
The indurated marl forming the plain, is the same kind of rock with that
occurring over a wide extent of the Pampas; and there is no reason to
doubt, they are parts of one great formation. Nevertheless, the gravel
bed of Bahia Blanca, although subsequent to the calcareous formation,
may be of the same age with those parts of the Pampas, which stand at a
low level near the Plata. For on this whole line of coast, I believe, as
the land has continued rising, fresh littoral deposits have been formed;
and each of these would often owe part of its materials to the
degradation of the one last elevated.
With respect to the relative age of the Monte Hermoso and Punta Alta
beds, it is not possible to speak decidedly. A certain degree of
similarity in the nature of the strata containing quartz pebbles, and
those of the reddish indurated earth; and the short distance between the
two localities, would indicate that no long interval had intervened. The
beds at Monte Hermoso, certainly were deposited more tranquilly, and
probably in a deeper sea; so that even skeletons of animals, no larger
than rats, have been perfectly preserved there. In some parts of the
surrounding country, obscure traces of a succession of step-formed
terraces may be observed; and each of these indicates a period of repose
during the elevation of the land, at which time the strata previously
existing were worn away, and fresh matter deposited. The Monte Hermoso
beds were, perhaps, formed during one such interval, anterior to the
accumulation of the shingle bank at Punta Alta.
Mr. G. Sowerby, who has been good enough to examine the shells which
were found with the remains of the quadrupeds, has given me the
following list.
1. _Voluta angulata._
2. —— _colocynthis._
3. _Oliva Brasiliensis._
4. —— Nearly related to _O. patula_, but specimen
imperfect.
5. —— Nearly related to _O. oryza_; less nearly to
small species now living at Bahia Blanca.
6. —— _Nov. spec._
7. _Buccinum cochlidium._
8. —— _globulosum._
9. —— One or two minute species, perhaps young
specimens,—unknown.
10. _Trochus_ _Nov. spec._ (?) same as one now living in
the bay.
11. —— _Nov. spec._ (?) nearly related to last;
differs in not being granular on the
surface.
12. _Assiminia_ (?) Minute species, identical with one living in
the bay.
13. _Bulinus nucleus._
14. _Fissurella_ Probably same as a kind (_nov. spec._ ?)
living in the bay.
15. _Crepidula muricata._
16. —— _Nov. spec._
17. _Cytheræa_ Closely related to, or identical with _C.
purpurascens_.
18. _Modiola_ Same as recent kind (_nov. spec._) living in
the bay.
19. _Nucula_ Near to _N. margaritacea_.
20. _Corbula_ Minute species, unknown.
21. _Cardita_ Ditto ditto
22. _Pecten_ _Nov. spec._ (?) very imperfect specimen.
23. _Ostrea_ Oysters of the same size now live in the
bay.
I may add that a fossil encrusting coralline is the same with one now
living in the bay.
Of these shells it is almost certain that twelve species (and the
coralline) are absolutely identical with existing species; and that four
more are perhaps so; the doubt partly arising from the imperfect
condition of the specimens. Of the seven remaining ones, four are
minute, and one extremely imperfect. If I had not made a collection (far
from perfect) of the shells now inhabiting Bahia Blanca, Mr. Sowerby
would not have known as living kinds, five out of the twelve fossils:
therefore, it is probable, if more attention had been paid to collecting
the small living species, some of the seven unknown ones would also have
been found in that state. The twelve first shells, as well as the four
doubtful ones, are not only existing species, but nearly all of them
inhabit this same bay, on the shores of which they are likewise found
fossil. Moreover, at the time, I particularly noticed that the
proportional numbers appeared closely similar between the different
kinds,—in those now cast up on the beach, and in those embedded with the
fossil bones. Under these circumstances, I think, we are justified
(although some of the shells are at present unknown to conchologists) in
considering the shingle strata at Punta Alta, as belonging to an
extremely modern epoch.
From the principle already adduced, namely, the regular and gradual
elevation of this part of the continent, I should have judged from the
small altitude of the beds at Punta Alta, that the formation had not
been very ancient. The conclusion here arrived at, concerning the age of
these fossil mammalia, is nearly the same, with that, inferred
respecting those entombed in the Pampas; and it will hereafter be shown,
that some of the species are common to the two districts. We may
suppose, that whilst the ancient rivers of the Plata occasionally
carried down the carcasses of animals existing in that country, and
deposited them in the mud of the estuary; other animals inhabited the
plains round the Sierra de la Ventana, and that lesser streams, acting
together with the currents of a large bay, drifted their remains towards
a point, where sand and shingle were accumulating into a shoal. The
whole area has since been elevated: the estuary mud of the former rivers
has been converted into wide and level plains; and the shoals of the
ancient Bahia Blanca now form low headlands on the present coast.
The third locality, which I have to specify, is Port St. Julian, in
latitude 49° 15′ on the coast of Southern Patagonia. The tertiary plains
of that country are modelled into a succession of broad and level
terraces, which abut one above the other; and where they approach the
coast, are generally cut off by a line of precipitous cliff. The whole
surface is thickly covered by a bed of gravel, composed of various kinds
of porphyries, and probably originating from rocks situated within the
Cordillera. The lower part of the formation consists of several
varieties of sandstone, and contains many fossil shells, the greater
number of which are not found in a living state.
The south side of Port St. Julian is formed by a spit of flat land, of
nearly a hundred feet in height; and on its surface existing species of
littoral shells are abundantly scattered. The gravel is there covered (a
circumstance which I did not observe in scarcely any other locality) by
a thin but irregular bed of a sandy or loamy soil, which likewise fills
up hollows or channels worn through it. In the largest of these channels
the remains of the single fossil quadruped, which was here discovered,
were embedded. The skeleton probably was at first perfect; but the sea
having washed away part of the cliff, has removed many of the bones,—the
remaining ones, however, still occupying their proper relative position
to each other. I am inclined to attribute the origin of this earthy
matter, to the mud which might have accumulated in channels, and on the
surface of the gravel, if this part of the plain had formerly existed as
a harbour, such as Port St. Julian is at the present day. The Guanaco,
the only large animal now inhabiting the wild plains of Patagonia, often
wanders over the extensive flats, which are left dry at the head of the
harbour during ebb tide: we may imagine that the fossil animal, whilst
in a like manner crossing the ancient bay, fell into one of the muddy
creeks, and was there buried.
I have stated that existing species of shells are scattered over the
surface of this plain; namely, _Mytilus Magellanicus_; a second and
undescribed species, now living on the beach; _M. edulis_; _Patella
deaurata_; and on another part of the coast, but having similar
geological relations, _Fusus Magellanicus_; _Voluta ancilla_; and a
_Balanus_:—these shells are among the commonest now living on this
coast. Although they must have been lying exposed to the atmospheric
changes for a very long period, they still partially retain their
different colours. From these facts we know, with certainty, that the
superficial deposit, containing the remains of the quadruped, has been
_elevated_ above the sea, within the recent period. From the structure
of the step-like plains, which front the coast, it is certain that each
step must have been modelled, subsequently to the elevation of the one
standing above it; and, as the same recent shells occur on two higher
plains, we may, with safety, conclude, that the earthy matter, forming
the surface of this lower one, together with its embedded skeleton, was
_deposited_ long after the existence of the present species, still
inhabitants of the sea. According, therefore, to the chronology, taken
from the duration of species among the molluscs, the fossil quadruped of
Port St. Julian must have been coeval, or nearly so, with those from
Bahia Blanca.
Having now briefly described the principal circumstances in the geology
of the three districts, to which I at first alluded, I will conclude, by
observing, that the fossil mammalia of La Plata, Bahia Blanca, and Port
St. Julian, must all have lived during a very modern period in the
geological history of the world. It is not the proper place in this work
to enter on any speculations, concerning the cause of the extinction of
so many gigantic animals. I will only here add, that there is the
strongest evidence against admitting the theory of a period of
overwhelming violence, by which the inhabitants of the land could have
been swept away, and destroyed. On the contrary every thing indicates a
former state of tranquillity, during which various deposits were
accumulating near the then existing coasts, in the same manner, as we
may suppose others are at this day in progress. The only physical
change, which we know has taken place, since the existence of these
ancient mammalia, has been a small and gradual rising of the continent;
but it is difficult to believe, that this alone could have so greatly
modified the climate, as to have been the cause of the utter
extermination of so many animals. Mr. Owen will mention the exact
locality where the remains of each quadruped were discovered; and, at
the conclusion, it will be easy to specify by name those, which, from
being embedded in the same deposit, are known formerly to have
co-existed on the continent of South America.
FOSSIL MAMMALIA.
BY MR. OWEN.
It may be expected that the description of the osseous remains of
extinct Mammalia, which rank amongst the most interesting results of Mr.
Darwin’s researches in South America, should be preceded by some account
of the fossil mammiferous animals which have been previously discovered
in that Continent. The results of such a retrospect are, however,
necessarily comprised in a very brief statement; for the South American
relics of extinct Mammalia, hitherto described, are limited, so far as I
know, to three species of Mastodon, and the gigantic Megatherium.
One of the above species of Mastodon (_Mast. Cordillerarum_) was
established by Cuvier[1] on remains discovered by Humboldt, in Quito,
near the volcanic mountain, called _Imbaburra_, at an elevation of 1200
toises above the level of the sea; and likewise at the Cordilleras of
Chiquitos, near Santa Cruz de la Sierra, a locality which is near the
centre of South America. A second species (_Mastodon Humboldtii_,
Cuv.[2]) is indicated by molar teeth, stated to have been discovered by
the same philosophic traveller, in Chile, near the city of Concepcion.
The third species of Mastodon appears to have once ranged in vast troops
over the wide empire of Peru: numerous teeth were brought thence to
Paris by Dombey,[3] and similar teeth, together with a humerus and tibia
from Santa Fé de Bogota were placed by Humboldt at the disposal of
Cuvier,[4] who considered them to belong to the _Mastodon angustidens_,
a species of which the fossil remains are by no means uncommon in
several localities of Europe. Cuvier is also disposed to refer to the
same species the teeth of the Mastodon from Brazil and Lima, mentioned
by Dr. W. Hunter in his observations on the _animal incognitum_ from the
Ohio.[5] The Megatherium has been scientifically described and
illustrated in the works of Bru, Cuvier, and D’Alton, whose accounts are
founded on a nearly complete skeleton of this stupendous quadruped which
has existed in the Royal Museum at Madrid for more than half a century.
The few deficiencies in its osteography have recently been supplied by
the descriptions and figures given by Dr. Buckland[6] and Mr. Clift,[7]
taken from remains of the Megatherium, brought by Sir Woodbine Parish
from Buenos Ayres, and which were discovered in the bed of the Rio
Salado, a tributary of the Rio Plata. Sir Woodbine Parish’s collection
from the same locality, includes also remains of other species of
extinct Edentata, which have not yet been described. M. D’Orbigny, in
his travels in South America (vol. i. p. 96.), states that, in the banks
of the Parana, he found the fossil remains of a large quadruped, of the
size of an Ox,—another quadruped of the size of a Cat, apparently of the
carnivorous order;—and a third, a Rodent as large as a Rat.
This meagre condition of the historical part of the subject of South
American fossils by no means arises from their actual scarcity. The
writings of some of the old Spanish authors, for instance, Torrubia,
Garcillasso, and others,[8] contain frequent allusions to the bones of
giants, who in times of old dwelt in Peru. Legentil, also, in 1728,
speaks as an eye-witness of these Peruvian remains; and his guides
pointed out to him the traces of the thunder-bolts, by which the Anaks
of the New World had been exterminated. Bones and teeth of the Mastodon
are, according to Humboldt, so abundant in a locality near Santa Fé de
Bogota in Columbia, that to this day it bears the name of the “Field of
Giants.”
But independently of these indications, the abundance and variety of the
osseous remains of extinct Mammalia in South America are amply attested
by the materials for the following descriptions, collected by one
individual, whose sphere of observation was limited to a comparatively
small part of South America; and the future traveller may fairly hope
for similar success, if he bring to the search the same zeal and tact
which distinguish the gentleman to whom Oryctological Science is
indebted for such novel and valuable accessions.
It is remarkable that all the fossils, collected by Mr. Darwin, belong
to herbivorous species of mammalia, generally of large size. The greater
part are referrible to the order which Cuvier has called Edentata, and
belong to that subdivision of the order (_Dasypodidæ_) which is
characterized by having perfect and sometimes complex molar teeth, and
an external osseous and tesselated coat of mail. The Megatherium is the
giant of this tribe; which, at the present day, is exclusively
represented by South American species, the largest (_Dasypus Gigas_,
Cuv.) not exceeding the size of a Hog. The hiatus between this living
species and the Megatherium, is filled up by a series of Armadillo-like
animals, indicated more or less satisfactorily by Mr. Darwin’s fossils,
some of which species were as large as an Ox, others about the size of
the American Tapir. The rest of the collection belongs, with the
exception of some small Rodents, to the extensive and heterogeneous
order Pachydermata; it includes the remains of a Mastodon, of a Horse,
and of two large and singular aberrant forms, one of which connects the
Pachydermatous with the Ruminant Order; the other, with which the
descriptions in the following pages commence, manifests a close affinity
to the Rodent Order.
A DESCRIPTION OF THE CRANIUM OF
TOXODON PLATENSIS;
_A gigantic extinct mammiferous animal, referrible to the Order
Pachydermata, but with affinities to the Rodentia, Edentata, and
Herbivorous Cetacea_.
The cranium, which is the subject of the present description, was found
in the Sarandis, a small stream entering the Rio Negro, and about 120
miles to the N. W. of Monte Video: it had been originally embedded in a
whitish argillaceous earth, and was discovered lying in the bed of the
rivulet, after a sudden flood had washed down part of the bank.
The zoological characters deducible from this cranium, forbid its
association, generically, with any known Mammiferous animal, and it must
therefore be referred to an extinct genus, which I propose to call
_Toxodon_,[9] from the curved or arched form of the teeth, as will
afterwards be described. The specific name, in the absence of other
means of knowing the peculiarities of the animal than those afforded by
the skull, may be most conveniently taken from the district (La Plata),
in which its remains were first discovered.
The dimensions of the cranium of the _Toxodon Platensis_ amply attest
that the animal to which it belonged was of a magnitude attained by few
terrestrial quadrupeds, and only to be compared, in this respect, with
the larger Pachyderms, or the extinct Megatherium. The length of the
skull (of which a base view of the natural size is given in Plate I.) is
two feet four inches: the extreme breadth one foot four inches. The
other requisite admeasurements are given in the table at the conclusion
of this description.
The general form of the skull, as seen from above, is pyriform; but
viewed sideways, and without the lower jaw, it is semi-ovate; it is
depressed, elongate, of considerable breadth, including the span of the
zygomatic arches, but becoming rather suddenly contracted anterior to
them, the facial part thence growing narrower to near the muzzle, which
again slightly expands.
Among the first peculiarities which strike the observer, is the aspect
of the plane of the occipital foramen, and of the occipital or posterior
region of the cranium, the latter of which inclines from below upwards
and forwards at an angle of 50° with the basal line of the skull. This
slope of the back part of the skull is one of the characteristics of the
Dinotherium; it is common to all the Cetacea, and is met with in a
slighter degree in many Rodentia, and in the great Ant-eater and some
others of the Edentate order. The corresponding aspect of the _foramen
magnum_ presents nearly the opposite extreme to man in the occipital
scale, proposed by Daubenton to determine the diversities of the form of
the cranium, as a gage of the intelligence of different animals[10]; and
the indication of the limited capacity of the Toxodon, thus afforded, is
strengthened by the very small proportion, which the bony walls of the
cerebral cavity bear to the zygomatic and maxillary parts of the skull,
and to the size of the vertebral column, as indicated by the condyloid
processes, and foramen magnum.
The zygomatic arches are of remarkable size and strength; they commence
immediately anterior to the sides of the occipital plane, increase in
vertical extent as they pass outwards, forwards and downwards, and are
suddenly contracted as they bend inwards to abut against the sides of
the sockets of the two posterior molar teeth.
The cranial cavity is remarkably narrow at the space included by the
zygomatic arches; being, as it were, excavated on each side to augment
the space for the lodgment of the temporal muscles, so that its diameter
at this part is less than that of the anterior extremity of the upper
jaw. The upper surface of the cranium expands to form the post-orbital
processes, and again contracts anterior to these.
The muscular ridges, or other characters, at the top of the skull,
cannot be precisely determined, as a great proportion of the outer table
of the bone is broken away, exposing a coarse and thick diplöe. There
seems, however, to have been a strong ridge separating the occipital
from the coronal or upper surface of the cranium. The form of the
remaining parts, which are modified in relation to the attachment of the
muscles of the jaws, indicates that these were powerfully developed both
for the offices of mastication and prehension. The general form of the
skull, while it presents certain points of resemblance to that of the
aquatic Pachydermata, and even of the Carnivora, has much that is
peculiar to itself; but, in the facial part, it approaches the nearest
to that of the Rodentia; and the dentition of the Toxodon, as exhibited
in the upper jaw, corresponds with that which characterizes the Rodent
Order.
The teeth of the Toxodon consist of molars and incisors, separated by a
long diastema, or toothless space. In the upper jaw the molars are
_fourteen_ in number, there being seven on each side; the incisors
_four_, one very large, and one small, in each intermaxillary bone.
The general form and nature of the teeth are indicated by the sockets;
and the structure of the grinders is exhibited in a broken molar, the
last in the series on the left side of the jaw of the present cranium
(See a figure of the grinding surface restored of this tooth, fig. 2,
Pl. I.), and by another perfect molar, the last but one on the right
side of the upper jaw, which, though not belonging to the same
individual as the skull here described, undoubtedly appertains to the
same species. This latter tooth (Fig. 3, Pl. I.; figs. 2 and 3, Pl. IV.)
was found by itself, embedded in the banks of the Rio Tercero, or
Carcarana, near the Parana, at the distance of a hundred and eighty
miles from the locality where the head was discovered. Fragments of a
molar tooth of a Toxodon, apparently the seventh of the left side, upper
jaw, were also found at Bajada de St^a Fé, in the province of Entre
Rios, distant forty miles from the mouth of the Rio Tercero.
All the molar teeth are long and curved, and without fangs,[11] as in
most of the herbivorous species of the Rodent Order: in those, however,
with curved grinders, as the _Aperea_ or Guinea-pig, and _Cavia
Patachonica_, the concavity of the upper grinders is directed outward,
the fangs of the teeth of the opposite sides diverging as they ascend in
the sockets; but, in the Toxodon, the convexity of the grinders is
outward, and the fangs converge and almost meet at the middle line of
the palate, forming a series of arches, capable of overcoming immense
resistance from pressure. (See the upper view of the skull, Plate III.,
in which the fractures expose to view a part of the series of these
arched sockets.)
Of the incisors, the two small ones (the sockets of which are indicated
at _s s_, Pl. III.) are situated in the middle of the front of the upper
jaw, close to the suture between the intermaxillaries, and the two large
ones in immediate contiguity with the small incisors, which they greatly
exceed in size. The sockets of the two large incisors (_t t_, Pl. III.)
extend backwards, in an arched form, preserving a uniform diameter, as
far as the commencement of the alveoli of the molar teeth: the curve
which they describe is the segment of a circle; the position, form, and
extent of the sockets of these incisors are the same as in those of the
corresponding teeth of the Rodentia.
The matrix, or secreting pulp of the large incisors, was lodged, as in
the Rodentia, in close proximity with the sockets of the anterior
molars; and we are enabled to infer, from the form of the incisive
sockets, notwithstanding the absence of the teeth themselves, that the
pulp was persistent, and that the growth of these incisors, like those
of the Rodentia, continued throughout life.
This condition, joined with the form and curvature of the socket,
implies a continual wearing away of the crown of the tooth by attrition
against opposing incisors of a corresponding structure in the lower jaw:
and as a corollary, it may be inferred that the teeth in question had a
partial coating of enamel, to produce a cutting edge, and were, in fact,
true _dentes scalprarii_. The number of incisors in the upper jaw of
Toxodon, is not without its parallel in the Rodent Order, the genus
_Lepus_ being characterized by four, instead of two superior incisors,
which also present a similar relative size but have a different relative
position, the small incisors, in the hare and rabbit, being so placed
immediately behind the large pair, as to receive the appulse of the
single pair of incisors in the lower jaw.
In the Toxodon the position of the incisors, in the same transverse
line, might lead to the inference, that they were opposed by a
corresponding number in the lower jaw; but the numerous examples of
inequality, in the number of incisors, in the upper and lower jaws of
existing mammalia, forbid any conclusion on this point.[12] The sockets
of the small mesial incisors of the Toxodon (_s s_, Pl. III.) gradually
diminish in size, as they penetrate the intermaxillary bones, and we
may, therefore, infer that the pulp was gradually absorbed in the
progress of their development; and that, like ordinary incisors, their
growth was of limited duration, and their lodgment in the jaw effected
by a single conical fang.
I may observe, that the formation of a fang is the necessary consequence
of the gradual absorption of the matrix or pulp of a tooth; for the pulp
continues, as it diminishes in size, to deposit ivory upon the inner
surface of the cavity of the tooth from which it is receding, and the
tooth or fang thus likewise progressively diminishes in size. The
formation of the socket proceeds uninterruptedly, and the bone
encroaching upon the space left by the tooth, closely surrounds the
wasting fang, and affords it a firm support; and thus an inference may
be drawn from the form of the socket alone, as to whether the tooth it
contained had or had not one or more conical fangs, and consequently
whether its growth was temporary or uninterrupted.
Applying this reasoning to the molar teeth of the Toxodon, we infer that
their growth, like those of most of the Phytiphagous Rodents, of the
Megatherium and Armadillo, was perpetual, because their sockets are
continued of uniform size from the open to the closed extremity; and the
molar tooth which is preserved proves the accuracy of the deduction,
inasmuch as its base is excavated by a large conical cavity for the
lodgment of the pulp, the continued activity of which was the
compensation here designed to meet the effects of attrition on the
opposite or grinding surface of the tooth.
The molar tooth discovered by Mr. Darwin in the banks of the Tercero,
not only belonged to the same species as the skull under consideration,
but to an individual of the same size; it fits exactly into the socket
next to the posterior one of the right side. The figures subjoined of
this molar tooth (Fig. 3, Pl. I.; figs. 2 and 3, Pl. IV.) almost
preclude the necessity of a description. The transverse section of the
tooth gives an irregular, unequal sided, prism; the two broadest sides
of which converge to the anterior angle, which is obtusely rounded. The
outer surface of the tooth (fig. 2, Pl. IV.) is slightly concave in the
transverse direction, but undulating, from the presence of two slight
convex risings which traverse the tooth lengthwise. The inner surface
presents at its anterior part a slightly concave surface, and
posteriorly two prominent longitudinal convex ridges, separated by a
groove which is flat at the bottom, and from the anterior angle of which
the reflected fold of enamel penetrates the substance of the tooth,
advancing obliquely forwards, rather more than half-way across the body
of the tooth. A longitudinal ridge of bone projects from the internal
side of the socket, and fits into the groove above mentioned, and as a
corresponding ridge exists in all the sockets of the grinders, save the
two anterior small ones, we may infer that the five posterior grinders
on each side, had a similar structure to the tooth above described. The
external layer of enamel is uniformly about half a line in thickness; it
is interrupted for the extent of nearly three lines at the anterior
angle, and for more than double that extent at the posterior part of the
tooth, which is consequently worn down much below the level of the rest
of the grinding surface. Where the ivory is thus unprotected by the
enamel, it has a coat of cæmentum, which also fills up the small
interval at the origin of the reflected fold of enamel. On the grinding
surface of the entire tooth, and on the fractured ends of the mutilated
molars, the component fibres, or tubules, of the ivory, are readily
perceptible by the naked eye, diverging from the line which indicates
the last remains of the cavity of the pulp of the tooth, as it was
progressively obliterated during growth.
Although the complication of the grinding surface by the inflection of
simple or straight folds of enamel is peculiarly characteristic of the
Rodent type, we must regard the number of molar teeth, and their
diminution of size as they advance towards the anterior part of the jaw,
in the Toxodon, as indicative of a deviation from that order, and an
approach to the Pachyderms. The common number of grinders in the upper
jaw of Rodent animals is eight, four on each side. In some genera, as
Lemmus, Mus, Cricetus, there are only three on each side, and in
Hydromys and Aulacodus, only two on each side. In Lepus, however, we
find six on each side of the upper, and five on each side of the lower
jaw. The Toxodon, like the Tapir and Hippopotamus, has seven on each
side of the upper jaw: the first in each of these species being the
smallest. It is worthy of notice, however, that the Capybara which
adheres to the Rodent type in the number of its molars, presents in the
vastly increased size, and additional number of component laminæ of the
posterior grinders, an approximation to the pachydermatous character
just adduced, and the bony palate at the same time presents an expansion
between these molars, offering a resemblance to the Toxodon which I have
not found in any other Rodent besides the Capybara.
The most important deviation from the Rodent structure presented by the
teeth, occurs in the direction of the reflected fold of enamel, and such
a deviation might have been inferred, even in the absence of the teeth,
from the structure of the articular surface, or glenoid cavity for the
reception of the condyle of the lower jaw. As the ridge of enamel runs,
as above described, in a direction approaching that of the longitudinal
axis of the skull, it is obvious that the grinding motions of the lower
jaw should be in a proportionate degree in the transverse direction. The
glenoid cavity, therefore, instead of being a longitudinal groove, and
open behind, as in the true Rodents, is extended transversely, and is
defended behind by a broad descending bony process preventing the
retraction of the jaw, and showing marks of the forcible pressure to
which it was subject.
It is worthy of observation that, in the Wombat,—which exhibits the
Rodent type of dentition, and, like the Toxodon, has remarkably curved
molars, but in an opposite direction,—the condyle of the lower jaw is
also extended transversely, and adapted to an articular surface, which
admits of lateral motion in the trituration of the food. In the outward
span of the zygomatic arches, in which Toxodon deviates from the
Rodentia, we may trace a relation of subordinacy to the above structure
of the grinding teeth and joint of the lower jaw: the widening of the
arches giving to the masseter muscles greater power of drawing the jaw
from side to side. The depth of the zygoma bespeaks the magnitude of
these masticatory muscles, and the included space shews that the
temporal muscles were also developed to a degree, which indicates the
force with which the great incisors at the extremity of the jaws, were
used; probably, like the canines of the Hippopotamus, to divide or tear
up by the roots the aquatic plants, growing on the banks of the streams,
which the Toxodon may have frequented.
In the Rodentia, the zygoma, though sometimes as deep as in the Toxodon,
is generally almost straight, and the space included between it and the
cranium is consequently narrow. The zygoma also is placed more forwards
in all true Rodents, than in the Toxodon; and, instead of abutting
against the posterior alveoli, it terminates opposite the anterior ones.
It thus affords such an attachment to the masseter, that this muscle
extends obliquely backwards to its insertion in the lower jaw, at an
angle which enables it to act with more advantage in drawing forwards
the lower jaw,—a motion for which the joint is expressly adapted. In
many Rodents, also, there is a distinct muscle, or portion of the
masseter, which passes through the ant-orbital foramen, which is on that
account of large size. In examining the cranium of Toxodon, with
reference to this structure, it was found that the ant-orbital foramen
was not larger than might have been expected to give transmission to
nerves requisite for supplying with sensibility the large lips, and
whiskers with which the expanded muzzle of this remarkable quadruped was
probably furnished.
Having thus examined the cranium of the Toxodon in its relation, as a
mechanical instrument, subservient to the function of digestion; we next
proceed to consider the structure and composition of those cavities of
the skull which gave lodgment and protection to the organs of _special_
sense, and endeavour to deduce from their structure conclusions as to
the degree in which the organs were developed, and the circumstances
under which the senses were exercised.
The orbit of Toxodon forms the anterior boundary of the zygomatic area;
it is about as distinctly defined as in the Tapir or Dugong, having its
osseous rim less complete than in the Hippopotamus, yet more developed
than in the Capybara, Coypus, and many other Rodentia, in which the
orbit is scarcely distinguishable in the cranium from the small space
occupied by the origin of the temporal muscle.
The lower boundary of the orbit in Toxodon is formed by an excavation in
the upper and anterior part of the zygoma; the upper boundary by a
strong and rugged overarching process of the frontal bone, the posterior
angle of which (_a_, Pl. III.) descends a little way, but leaves a space
of three inches and a half between it and the opposite angle of the
malar bone below (_b_, Pl. II. and III.), the circumference of the orbit
being completed probably by ligament in the recent subject. The cavity
thus circumscribed is remarkable for the preponderance of the vertical
over the transverse or longitudinal diameter, and indicates great extent
of motion of the eyeball in the vertical direction, such as may be
supposed to be well adapted to the exigencies of an amphibious
quadruped. The orbit of the Capybara, or Water-hog, makes a near
approach to the form just described. In the elevation of the
supra-orbital boundary, and its outward projection in the Toxodon, we
perceive an approximation to the form of the orbit in the Hippopotamus,
but the size of the orbit is relatively larger in the Toxodon, which in
this respect manifests its affinity to the Rodentia.
In that part of the bony structure of the auditory apparatus, which is
visible on the exterior of the cranium, the skull of the Toxodon
presents a character in which it recedes from the Rodentia. In these,
the tympanic portion of the temporal bone is remarkably developed,
forming a large bulla ossea between the glenoid cavity and the occiput;
and it always remains disunited to the other elements of the temporal
bone. In the Toxodon the tympanic bone (_c_, Pl. II.) consists of a
rough compressed vertical osseous plate, wedged in transversely between
the occiput and the posterior part of the glenoid cavity. The internal
extremity of this plate points inwards and forwards, representing the
styloid process; behind this is seen the petrous bone, which forms a
small angular protuberance at the basis cranii, and is less developed
than in the Hippopotamus. Anterior to the petrous bone are the orifices
of the Eustachian tube, and carotid canal; external to it is the great
foramen lacerum, for the jugular vein and nervus vagus; and behind it is
the anterior condyloid foramen. The foramen auditorium externum is only
half an inch in diameter, and gives passage to a long and somewhat
tortuous meatus, which passes inwards and slightly forwards and
downwards; its direction being precisely the same as in the
Hippopotamus; it was accompanied, probably, by as small an external
auricle.
But the indications of the aquatic habits of the Toxodon, which are
presented by the osseous parts relating to the senses of sight and
hearing, are of minor import compared with those afforded by the bony
boundary of the nostrils. This boundary circumscribes a large ovate
aperture, the aspect of whose plane is upwards, and a little forwards,
as in the Herbivorous Cetaceans, and especially the Manatee (_Trichecus
Manatus_, Cuv.) In one part of the bony structure of the nasal cavity
the Toxodon deviates, however, in a marked degree from the Cetaceous
structure; I allude to the frontal sinuses, which are exposed by the
fracture of the upper part of the skull. (They are shewn in Plate III.,
and an asterisk is placed on one of the narrow canals of
intercommunication between the sinuses and the nasal passages.) The
posterior orifice of the nasal cavity is relatively larger and wider
than in the Herbivorous Cetaceans, and differs both in form and aspect
in consequence of the greater extent of the bony palate. The Toxodon
further differs from the Manatee and Dugong, in the firm nature of the
connexion of the bones of the head; and it differs from the Hippopotamus
in the strong attachment of the intermaxillary bones to the maxillaries.
There next remain to be described, as far as the shattered condition of
the skull will permit, the relative position, extent, and connexions of
the principal bones composing it.
The _occipital bone_ exhibits a complete confluence of its basilar,
condyloid, and supra-occipital elements. The basilar portion, in
connexion with the corresponding element of the sphenoid bone, describes
a curve whose convexity is downwards. The condyles are large, extended
in the transverse direction, completely terminal, and a little inclined
downwards below the level of the basilar process. The curve of the
articulating surface describes, in the vertical direction, two-thirds of
a circle, indicating that the head must have possessed considerable
extent of motion upwards and downwards upon the atlas; thus, while the
body of the Toxodon was submerged, the head probably could be raised so
as to form an angle with the neck, and bring the snout to the surface of
the water without the necessity of any corresponding inflection of the
spine. Indeed, in the form and position of the condyles, the Toxodon
more nearly resembles the true Cetacea than any other existing mammalia;
and it is only with these that it can be compared in regard to the
aspect of the plane of the occipital foramen, and of the occipital
region of the skull. This is inclined forwards from the occipital
foramen at such an angle, that on viewing the skull from above, not only
the condyles, but the entire circumference of the occipital foramen are
visible. (See Pl. III.) The upper part of the supra-occipital plate
presents a broad rugous depression, indicative of the insertion of
strong cervical muscles, and probably of a _ligamentum Nuchæ_.[13]
The ex-occipital processes advance forwards for about an inch beyond the
condyles, and then suddenly extend outwards at right angles to the
former line, and terminate in the form of vertically compressed bony
plates; the lower rugged margins of which represent or perform the
office of the mastoid processes (_d_, _d_, Pls. II. and III.). The
breadth of the entire occipital region of the skull (fig. 1, Pl. IV.)
appears to have been, allowing for the fractures, about one-third more
than the height of the same part.
The great development of the _tympanic_ bones in the Rodentia, occasions
the intervention of a considerable space between the occipital bone and
the zygomatic process of the temporal; but in the great Toxodon, in
which the sense of hearing was doubtless inferior to that enjoyed by the
small and timorous Rodents, the tympanic bone is reduced to a thin
plate, which is wedged in between the occiput and glenoid cavity. In
this structure, and the consequent posterior position of the glenoid
cavity, there is a close resemblance between the Toxodon and the
Hippopotamus, Tapir, and Rhinoceros.
The _squamous_ element of the temporal bone (N, Pl. II.) forms a small
proportion of the lateral walls of the cranium, and also enters into the
composition of the lateral and superior parts of the posterior region of
the cranium, where two deep fossæ perforated by large vascular foramina,
indicate the junction of the squamous bones with the supra-occipital
bone. The posterior surface of the skull is thus divided into three
broad and shallow depressions, the two lateral facets being slightly
over-lapped by the middle one, at their junction with it. In this
structure the Toxodon resembles the Hippopotamus, and differs
considerably from the Cetacea, in which the occipital region is rendered
convex by the extraordinary development of the brain within.
The _zygomatic_ process of the temporal bone projects boldly outwards at
its commencement, where it is of great strength, and three-sided; the
glenoid cavity extends transversely across the base or inferior surface
of this part; the lateral surfaces converge to form the ridge or upper
boundary of the zygoma. The depth of the glenoid cavity is increased by
a transverse production of bone both before and behind it: the posterior
process (_g_, Pl. II.) descends the lowest, and affords the requisite
defence against backward dislocation of the lower jaw; the pressure of
the condyle against this process is denoted by a well defined,
transversely-ovate, flattened and smooth surface, as if the bone had
been planed down at that part: the anterior transverse boundary is
convex and smooth, and probably formed part of the articulation for the
lower jaw. The lower facet of the zygoma anterior to the glenoid cavity
gradually contracts in breadth, as it advances forward, and at the
distance of three inches from the articular cavity the zygoma changes
from a prismatic to a laminar form. It is at this point that the
zygomatic suture commences, at the lower margin of the arch; whence it
extends directly forwards for more than half its length, and then bends
upwards at a right angle. The zygomatic suture has a similar course in
the Capybara, and Hippopotamus.
The remainder of the zygoma is formed externally by the _malar_ bone (G
Pl. II.), which in its position is intermediate to the Rodent and
Pachydermatous structures. It is not suspended in the middle of the
zygomatic arch, as in the former order; neither does it extend into the
region of the face so far anterior to the orbit as in the Tapir or
Hippopotamus. The exterior line of the malo-maxillary suture defines the
orbit anteriorly; but from this line the maxillary bone extends
backwards, along the inner side of the malar portion of the zygoma,
until it almost reaches the temporo-malar suture; thus abutting by an
oblique surface against nearly the whole internal facet of the malar
bone, and materially contributing to the general strength of the
zygomatic arch. The malar bone is of considerable vertical extent, and
presents a rugged and thickened inferior margin for the attachment of
the masseter. The upper margin of the malar bone is smoothly rounded,
and presents a regular semicircular excavation, forming the lower
boundary of the orbit. The relative magnitude of the zygomata to the
entire cranium far exceeds in the Toxodon that which exists in the
Hippopotamus or any other known Pachyderm. This arises from the great
vertical development of the malar bone behind the orbit, and the
vertical expansion of the temporal portion of the arch. The oblique
position of the zygoma, descending as it advances forwards, is deserving
of attention, as the Toxodon, in deviating from the Pachyderms in these
respects, makes an evident approach to the herbivorous Cetaceans, as the
Dugong and Manatee: in the latter Cetacean we observe a similar
development of the lower part of the zygomatic process of the malar
bone. It is here, also, that we may perceive an indication of a
resemblance between the Megatherium and Toxodon.
There is no discernible trace of the _lachrymal_ bone (E, Pl. II.)
having extended, as in the Hippopotamus beyond the anterior boundary of
the orbit: the lachrymal foramen is situated rather deep in the orbit,
and the bone itself appears to have been of very small size.
The surface of the supra-orbital process of the _frontal_ bone(C, Pl.
II.) is deserving of attention, as it presents a peculiar ruggedness
which is not found in any other part of the skull; the irregularity
seems, as it were, to have been produced by the impression of numerous
small tortuous and anastomosing vessels. In the skull of a Sumatran
two-horned Rhinoceros, in the Museum of the College of Surgeons (No.
816), the circumference of that part of the surface of the skull which
supported the posterior horn, and which includes precisely the same part
of the os frontis, presents the same character, the surface being broken
by numerous vascular impressions. On the supposition that this character
of the supra-orbitary arch in the Toxodon might indicate the
superincumbency of a bony case, I examined the skulls of two Armadillos,
_Dasypus Peba_ and _Das. 6–cinctus_, and found that in the Dasypus
6–cinctus, the supra-orbital ridges, which are slightly elevated, to
support the cephalic plate, presented, in a minor degree, a
corresponding rugosity. May we venture then to conjecture that the
Toxodon was defended by an ossified integument like the Armadillo, or
that it was armed with an epidermic production, analogous to the horn of
the Rhinoceros; or had the rugous surface in question as little relation
with the parts that covered it as the sculptured surface of the malar
bones in the Cavy?
After forming the rugged and prominent supra-orbital processes already
described, the frontal bone continues to send backwards a slightly
elevated ridge or _crista_, circumscribing the origin of the temporal
muscles, but the extent of this ridge, and the disposition of the
inter-orbital portion of the frontal bones cannot be determined in the
present mutilated specimen. The fractures it has sustained are not,
however, wholly unattended with advantage; they expose the structure of
the diploë, which from its coarseness of texture and thickness,
resembles that of the Cetaceous crania; and what is of still more
importance, they also demonstrate the existence and form of the frontal
sinuses.
The cavity of the nose is extensive, and the remains of the ossa
spongiosa superiora testify that the Toxodon enjoyed the sense of smell
to a degree equal at least to that of the Hippopotamus.
The _sphenoid bone_ resembles that of the Hippopotamus, but it
contributes a larger share to the formation of the internal pterygoid
processes (_p_, Pl. II.); these are of a simple form, and more developed
than in the _Hippopotamus_; they project outwards to a greater extent,
and terminate in a point. The sphenoid also sends off a short and thick
pointed process from the posterior part of the base of the internal
pterygoid processes. The ala of the sphenoid does not rise so far into
the orbit, nor does it articulate with the parietal bone, as in the
Hippopotamus; but in this part of its structure, is the same as in the
Rhinoceros. The spheno-palatine foramen is relatively larger than in the
above-named Pachyderms, and is bounded above by the descending orbital
plate of the frontal bone.
The palatal processes of the _palatine_ bones terminate anteriorly
between the last molars, and extend backwards for some distance beyond
the alveolar processes, increasing the extent of the bony roof of the
mouth posteriorly: this is a structure in which the Toxodon deviates
both from the Rodents, and Pachyderms, and resembles the Armadillos
among the Edentata; excepting that the postdental part of the bony
palate in the Toxodon is suddenly contracted in breadth. The
palato-maxillary suture is in the form of a chevron, with the angle
directed forwards, as in the Hippopotamus and Capybara, but truncated.
The _superior maxillary_ bones (F, Pl. II.) are united posteriorly to
the malar, as above described: they ascend and join the frontal and
nasal bones: their outer surface is almost vertical, smooth, and
slightly undulating; perforated at its posterior part by the ant-orbital
foramen, and joined anteriorly to the intermaxillaries by a suture
running in the sigmoid direction (as shewn in Pl. II.) from the middle
of the nasal cavity, to within four inches of the anterior boundary of
the upper jaw. We have, in the position and extent of this suture, and
the absence of tusks and their large prominent sockets, a most important
difference between the Toxodon and Hippopotamus. The chief peculiarity
in the maxillary bones, obtains in the arched form of the alveolar
processes, corresponding to the shape and position of the grinders above
described, and which are peculiar among known mammalia to the present
genus. The palatal surface of the maxillary bones is obliquely
perforated by two large foramina, from which two deep longitudinal
grooves extend forwards, and are gradually lost; we find the posterior
palatine foramina represented by similar grooves and foramina in the
Capybara.
The _intermaxillary_ bones (D, Pls. II. and III.), though large, are
relatively of less extent than in the Rodents generally. The nasal
processes do not reach the frontal bone, but are limited to the anterior
half of the nasal boundary; approaching in this respect to the
Herbivorous Cetacea. In the outward expansion of their anterior
extremities, the intermaxillaries resemble those of the Hippopotamus, in
which, however, this character is more strongly marked. The
intermaxillaries in the Hippopotamus are also much less firmly united to
the maxillary bones than in the Toxodon, and are consequently commonly
lost in the fossil crania. On the palatal surface of the intermaxillary
bones there are two grooves which diverge forwards from the line of the
suture; and anteriorly to these grooves there are the two large anterior
palatine foramina. The maxillo-intermaxillary sutures on the palate
converge as they extend backwards to a point; there appears to have been
a fissure left between this suture and the mesial suture of the
intermaxillaries; in which structure the Toxodon resembles the
Hippopotamus.
After summing up the different affinities, or indications of affinity,
which are deducible from the cranium of this most curious and
interesting fossil mammal, we are led to the conclusion, assuming it to
have had extremities cased in hoofs, that it is referrible to the Order
Pachydermata. But the structure, form, and kind of teeth in the upper
jaw, prove, indisputably, that the gigantic Toxodon was intimately
related to the Rodent Order. From the characters of this order, as
afforded by the existing species, the Toxodon, however, differs in the
relative position of the supernumerary incisors, and in the number, and
direction of the curvature, of the molars. If, moreover, the lower jaw,
next to be described, belong, as I believe, to the Toxodon, the dental
character of the genus will be _incisors_ ⁴⁄₆; _pro laniariis diastema_;
_molares_ ⁷⁄₇ ⁷⁄₇.
The Toxodon again deviates from the true Rodentia, and resembles the
Wombat, and the Pachyderms, in the transverse direction of the articular
cavity of the lower jaw.
It deviates from the Rodentia, and resembles the Pachydermata in the
relative position of the glenoid cavities and zygomatic arches, and in
many minor details already alluded to.
In the aspect of the plane of the occipital foramen, and occipital
region of the skull; in the form and position of the occipital condyles;
in the aspect of the plane of the anterior bony aperture of the
nostrils; and in the thickness and texture of the osseous parietes of
the skull, the Toxodon deviates both from the Rodentia and existing
Pachydermata, and manifests an affinity to the Dinotherium and Cetaceous
Order, especially the Herbivorous section.
At present we possess no evidence to determine whether the extremities
of the Toxodon were organized on the ungulate or unguiculate type, nor
can we be positive, from the characters which the skull affords, that
the genus may not be referrible to the _Mutica_ of Linnæus;[14] although
the development of the nasal cavity and the presence of large frontal
sinuses render it extremely improbable that the habits of this species
were so strictly aquatic, as the total absence of hinder extremities
would occasion.
Where the dentition of a mammiferous animal is strictly carnivorous,
this structure is obviously incompatible with a foot incased in a
hoof:—but where the teeth are adapted for triturating vegetable
substances the case is different. If animals so characterized are of
small size and seek their food in trees, or if they burrow for roots or
for shelter, the vegetable type of dentition must co-exist with
unguiculate extremities, as in the Edentata and Rodentia generally: but
the largest genus (Hydrochærus) of the Rodent Order, whose affinity to
the Pachydermata is manifested in its heavy shapeless trunk, thinly
scattered bristly hair, and many other particulars, has each of its toes
inclosed in a miniature hoof.
The affinity above alluded to, is too obvious to have escaped popular
notice, and the Capybara, from its aquatic habits, has obtained the name
of Water-hog. It is highly interesting to find that the continent to
which this existing aberrant form of Rodent is peculiar, should be found
to contain the remains of an extinct genus, characterized by a dentition
which closely resembles the Rodent type, but manifesting it on a
gigantic scale, and tending to complete the chain of affinities which
links the Pachydermatous with the Rodent and Cetaceous Orders.
ADMEASUREMENTS OF THE CRANIUM OF TOXODON. feet inches lines
Extreme length 2 4
Extreme breadth 1 4
Extreme height, (exclusive of the lower jaw) 10
Length of zygomatic process 1 1 6
Depth or vertical extent of do. 6
Transverse extent of zygomatic fossa 6
Transverse diameter of cranium between the zygomatic arches 5
Transverse diameter of occipital plane of the cranium 1
From the outside of one condyle to that of the opposite condyle 8 6
Length of the bony palate 1 6
Extreme breadth of ditto 6
Breadth of palate at the intermaxillary suture 2 6
Breadth of palate behind the molar alveoli 3
Longitudinal extent of the molar alveoli 9 6
Longitudinal extent of the diastema 5 6
Transverse diameter of posterior nasal aperture 3 9
Transverse diameter of occipital foramen 3
Transverse diameter of glenoid cavity 4 6
Antero-posterior do. of ditto 1
DESCRIPTION OF FRAGMENTS OF A LOWER JAW AND TEETH OF A TOXODON.
Found at Bahia Blanca, in latitude 39° on the East coast of South
America.
In looking over some fragments of jaws and teeth, forming part of Mr.
Darwin’s collection of South American mammiferous remains, and which had
been set aside with mutilated specimens referrible to species belonging
to the family of Edentata, my attention was caught by the appearance of
roots of teeth projecting, in a different direction from the grinders,
from the fractured anterior extremity of a lower jaw, and I was induced
to examine minutely the structure of the teeth in this specimen, and to
search the collection for corresponding fragments. The result was the
discovery of portions of the two rami, and the commencement of the
symphysis of a lower jaw, containing anteriorly the roots of six
incisors, and at least six molars on each side; but as the rami had been
fractured through the middle of the sixth alveolus, the number of
grinders may have corresponded with those in the upper jaw of the
Toxodon.
The most perfect of these fragments is figured in Pl. V. figures 1 and
4; figure 2 shows the form of the teeth in transverse section, and the
disposition of the enamel upon the grinding surface of the molars on the
right side, as restored from a comparison of the fractured teeth in the
two rami. From the remains of the symphysis shown at fig. 4, it will be
seen that the jaw was remarkably compressed, or narrow from side to
side; while the rami (fig. 1.) were of considerable depth, in order to
give lodgment to the matrices and bases of grinders enjoying
uninterrupted growth.
The pulps of the six incisors in this lower jaw are arranged in a pretty
regular semicircle, whose convexity is downwards; the teeth themselves
are directed forwards, and curved upwards, like the inferior incisors of
the Rodentia. The form and degree of the curvature are shown in the
almost perfect incisor (Pl. V. fig. 5) which corresponds with the left
inferior incisor of the lower jaw, and was found in the same stratum,
but belonged to another individual.
These incisors are nearly equal in size: they are all hollow at their
base, and the indurated mineral substance impacted in their basal
cavities well exhibits the form of the vascular pulps which formerly
occupied them. Sufficient of the tooth itself remains in four of the
sockets to show that these incisors, like the nearly perfect one (fig.
5), had only a partial investment of enamel; but though in this respect,
as well as in their curvature and perpetual growth, they resemble the
dentes scalprarii of the Rodentia, they differ in having a prismatic
figure, like the inferior incisors of the Sumatran Rhinoceros, or the
tusks of the Boar. Two of the sides, viz., those forming the anterior
convex and mesial surfaces of the incisor have a coating of enamel,
about half a line in thickness, which terminates at the angles between
these and the posterior or concave surface. In plate V. fig. 4, the
enamel of the broken incisors is represented by short lines, showing the
direction of its crystalline fibres; the white space immediately within
the enamel shows the thickness of the ivory at the base of the tooth,
the included gray substance represents a section of the formative matrix
or pulp of the tooth, which was of the usual conical form: the inferior
broken end of the incisor (fig. 5,) appears to have been distant about
one-third from the apex of the pulp.
From the relative position of the bases or roots of these incisors, we
may infer that they diverged from each other as they advanced forwards,
in order to bring their broadest cutting surfaces into line. That they
were opposed to teeth of a corresponding structure in the upper jaw is
proved by the oblique chisel-like cutting surface of the more perfect
incisor: and it is not without interest to find that the presence of
_dentes scalprarii_ at the anterior part of the mouth has not been
necessarily limited to Mammalia of small size.
The position of the pulps of these incisors, in close proximity with the
anterior grinders, corresponds with the position of the pulps of the
incisors in the upper jaw of the _Toxodon_, and indicates, in
conjunction with the size of the pulps, that a considerable extent of
the inferior incisors was lodged in the substance of the anterior part
of the jaw. It is most likely that no vertically directed tooth would be
developed in the part of the jaw so occupied by the curved bases of the
incisors, and hence a diastema or toothless space would intervene
between the molars and incisors of this lower jaw, as in the upper jaw
of the Toxodon.
It is interesting, also, to observe, that as the deviations from the
Rodent type, which occur in the cranium of the Toxodon, are the same, in
some instances, as those which obtain in the Wombat; so we find a
corresponding deviation in the size and relative position of the
inferior incisors, which, as in the Wombat, terminate anterior to the
molar teeth, instead of extending backwards beyond the last grinder, as
in most of the true Rodents. The Capybara presents the nearest approach
to this structure, the pulps of the inferior incisors being situated
opposite the interspace of the first and second grinders.
The molar teeth, in this mutilated lower jaw, like those in the upper
jaw of Toxodon, had persistent pulps, as is proved by the conical cavity
at their base, as represented in fig. 3; they consequently required a
deep socket, and a corresponding depth of jaw to form the socket and
protect the pulps. In order to economise space, and to increase the
power of resistance in the tooth, and perhaps, also, to diminish the
effects of direct pressure on the highly vascular and sensible matrix,
we find the molars and their sockets are curved, but in a less degree
than those of the upper jaw of the Toxodon. They correspond, however,
with the superior molars of the Toxodon in the antero-posterior
diameter, in being small and simple at the anterior part of the jaw, and
by increasing in magnitude and complexity as they are situated more
posteriorly. They are, however, narrower from side to side; but
supposing them to belong to the Toxodon, it would agree in this respect
with most other large herbivorous mammalia;—the fixed surface for
attrition in the upper jaw being from obvious principles more extensive
than the opposed moveable surface in the lower jaw.
The _first_ grinder, in the lower jaw here described (Pl. V. fig. 2), is
of small size and simple structure, being surrounded with a coating of
enamel of uniform thickness, and without any fold penetrating the
substance of the tooth. It is more curved than any of the other molars,
and appears to have differed from the external incisor only in its
entire coating of enamel and direction of growth; it is interesting,
indeed, to find so gradual a transition, in structure, from molar to
incisive teeth, as this jaw presents; for the robust incisors may here
be regarded as representing molars simplified by the partial loss of
enamel, and with a change in their direction.
In the _second_ molar, we find an increase in the antero-posterior
diameter, and in the length of the tooth, and the enamel at the middle
of the outer side makes a fold which penetrates a little way into the
tooth; the line of enamel, on the inner side, is slightly concave and
unbroken.
The _third_ molar presents an increase of dimensions in the same
directions as the second; the enamel on the outer side of the tooth
presents a similar fold, but it is directed a little more backwards.
In the _fourth_ molar, besides a further increase of size, and a
corresponding but deeper fold of enamel in the external side of the
tooth, we have the grinding surface rendered more complicated by two
folds of enamel entering the substance of the tooth from the inner side:
these folds divide the antero-posterior extent of the tooth into three
nearly equal parts; they are both directed obliquely forwards, half-way
across the substance of the ivory.
The _fifth_ molar presents the same structure as the fourth, which it
exceeds only slightly in size.
In the _sixth_ molar we have a proportionally greater increase of size
in the antero-posterior diameter, which measures two inches; but the
lateral diameter is but slightly augmented; its structure resembles that
of the fifth.
As these grinding teeth by no means increase in the lateral diameter in
the same proportion as in their antero-posterior diameter, the posterior
ones present, but in a greater degree, the compressed form which
characterizes the grinders of the upper jaw of the Toxodon.
It will be seen, however, that there is a difference in the structure of
the grinders in this fragment of the lower jaw and those of the upper
jaw of the Toxodon. In the lower grinders there are two folds of enamel
proceeding from the inner side of the tooth into its substance, whilst
in the upper grinders there is only one fold continued from the inner
side; in the lower grinders there is also a fold of enamel reflected
into the substance of the tooth from the outer surface, while in the
upper grinders of Toxodon we find the enamel coating on the outer side
of the tooth merely bent inwards, so as to describe, in the transverse
section, a gently undulating line; fig. 7, Pl. V. is the grinding
surface of the sixth molar, right side, upper jaw.
But this difference of structure is by no means incompatible with the
co-existence of the two series of teeth in the same animal, since we
find the grinders of the upper and lower jaws presenting differences of
structure of equal degree in existing herbivorous species. If we examine
the jaws of the Horse, for example, we shall find not only an equal
amount of difference in the structure of the upper and lower grinders,
but that they deviate from one another in a very similar manner to that
above described in the Toxodon. In this comparison attention should be
confined to the course of the external enveloping layer of enamel,
leaving out of consideration the central crescentic islands of enamel
which constitute the additional complexity of the Horse’s grinder.
Viewing then the course of the external coat of enamel on the worn
surface of the tooth, we find it describing on the outer side of the
tooth in the upper jaw an undulating line,—a middle convexity being
situated between two concavities; on the inner side of the tooth one
fold of enamel penetrates to the middle of the tooth, and on each side
of this there is a smaller fold. But in the lower jaw the line of enamel
on the outer side of the tooth, instead of merely bending outwards
midway in its course, is reflected a little way inwards; while on the
opposite, or inner side of the tooth, the enamel sends two extensive
folds into the substance of the tooth, opposite to the interspace of
which the shorter fold projects from the outer side. Now, on the
supposition that the fragment of the lower jaw here described belongs to
the Toxodon, the kind and degree of difference in the complexity of the
grinding surface of the teeth in the upper and lower jaw, are remarkably
analogous to those which exist in the Horse. I have only further to
remark that in the Horse the inflected folds of enamel, instead of being
simple and straight with the two constitutive layers in apposition, as
in the Toxodon, are irregular in their course, with cœmentum intervening
between the constitutive layers, which also diverge from each other at
their angle of reflection, so as to augment the amount of dense material
which enters into the composition of the tooth.
Many analogous examples will readily occur to the experienced
comparative anatomist. The Horse has been adduced as one to which
reference can very readily be made; but I would also cite the Sumatran
Rhinoceros, the skull of which, in the Hunterian collection, has already
been alluded to. In this species the anterior grinders, in both jaws,
are small and simple, and increase in complexity as they recede
backwards. The third superior grinder (fig. 8, Pl. V.) presents a single
fold of enamel, reflected obliquely forwards from the inner side
half-way across the tooth; the outer line of enamel describes a simply
undulating line. The opposite grinder of the lower jaw (fig. 9, Pl. V.)
has only one-half the breadth of the upper one, but has its grinding
surface further complicated by having two inflected folds of enamel from
the inner side, and one shorter and broader fold from the outer side.
This tooth, therefore, presents a close resemblance to one of the
posterior grinders of the lower jaw of the Toxodon, but differs
essentially in being of limited growth, and consequently in having
fangs.[15]
In speculating upon the nature of the organized substances which the
teeth of the Toxodon were destined to grind down, we must not only take
the structure of the tooth into consideration, but also the power of
perpetual renovation, which will compensate for the defective quantity
of enamel in the grinders of the Toxodon, as compared with those of the
existing Ruminants and Pachyderms, whose grinders, when once completed,
receive no further addition of dental substance at their base. The
Toxodon, in this character of its dentition, participated in the same
advantages with the Capybara and the Megatherium.
Although we have been enabled to observe the structure of the grinding
teeth of the upper jaw of the Toxodon in two examples only; one, an
insulated perfect grinder corresponding to the sixth alveolus on the
right side, and the other, a portion of the last grinder of the left
side remaining in the socket of the head previously described, yet from
the relations subsisting between socket and tooth, a very satisfactory
opinion may be formed of the structure of those teeth which are wanting,
as well as of their size. It thus appears, that the grinders of the
upper jaw of the Toxodon, are small and simple at the anterior part of
the jaw, and that they increase (chiefly in antero-posterior extent) in
size, as well as in complexity, as they recede backwards in the jaw. In
this respect, as well as in size, the teeth, in the fragments of the
lower jaw just described, exactly correspond. There is, however, a
slight difference in the lateral diameter of the two sets of grinders,
those of the lower jaw being narrower, as is usually the case, but not
in the same degree as in the Horse or Ruminant. A greater difference
obtains in the degree of curvature of the two sets of molars, those of
the lower jaw, especially the posterior grinders, being much less bent
than the corresponding teeth of the upper jaw. It is necessary to
observe, also, that the convexity of the curve of the inferior grinders
is directed outwards, as in the superior grinders; while in the Guinea
Pig and Wombat, which have also curved grinders, the convexity is
outwards in the lower jaw, and inwards in the upper jaw.
Nevertheless, if we take into consideration the close similarity which
exists between the teeth of the upper jaw of the Toxodon, and those of
this lower jaw in more essential points, as in their persistent pulps,
their characteristic structure and form, the depth of their sockets, and
their relative sizes and complexity; and when we consider how the depth
of this lower jaw, and its narrowness in the transverse direction,
corresponds with the characteristic form of the upper jaw of the
Toxodon, and that to these resemblances is added an apparatus of
incisors adequate to oppose the great dentes scalprarii of the upper
jaw, the conclusion seems irresistible, that the lower jaw, here
described, must be referred, if not to the same, at least to a nearly
allied species of Toxodon, as that to which the large cranium belonged.
Further researches in South America, it is hoped, will lead, ere long,
to the completion of our knowledge of the osteology of this very
remarkable and interesting genus of extinct mammiferous animals.
DESCRIPTION OF PARTS OF THE SKELETON OF
MACRAUCHENIA PATACHONICA;
_A large extinct Mammiferous Animal, referrible to the Order
Pachydermata; but with affinities to the Ruminantia, and especially to
the Camelidæ_.
In the preceding pages the nature and affinities of a large extinct
Mammal were attempted to be determined from the cranium and teeth
exclusively: we come now to consider the remains of a quadruped
consisting of bones of the trunk and extremities, without a fragment of
a tooth or of the cranium to serve as a guide to its position in the
zoological scale.
It may appear, even to anatomists and naturalists familiar with the kind
of evidence afforded by a fossil fragment, that an opinion as to the
relation of the present species to a particular family of Ruminants,
formed without a knowledge of the important organs of manducation, must
be vague and doubtful, but the evidence about to be adduced, will be
regarded, it is hoped, as more conclusive than could have been _à
priori_ expected.
The portions of the skeleton of the animal—which, in relation to the
affinity above alluded to, as well as from the length of its neck, I
propose to call _Macrauchenia_[16]—were discovered by Mr. Darwin in an
irregular bed of sandy soil, overlying a horizontal accumulation of
gravel on the south side of Port St. Julian: and independently of the
circumstances under which they were found, their correspondence with
each other in size, colour, texture and general character prove them to
have belonged to one and the same individual.
These remains include two cervical vertebræ, seven lumbar vertebræ, all
more or less fractured; a portion of the sacrum and ossa innominata;
fragments of the right scapula; of the right radius and ulna, and right
fore-foot; the right femur nearly entire, the proximal and distal
extremities of the right tibia and fibula; and a metatarsal bone of the
right hind-foot.
Before entering upon the description of these remains, a few
observations may be advantageously premised on some of the
distinguishing characters of the Camelidæ. It is well known that the
Camels and Llamas deviate in their dentition, viz., in the presence of
two incisors in the upper jaw, from the true Ruminants; and we cannot
avoid perceiving that in this particular the direction in which they
deviate tends towards the conterminous Ungulate Order, in which incisor
teeth are rarely absent in the upper jaw. They also further deviate from
the Ruminants and approach the Pachyderms in the absence of cotyledons
in the uterus and fetal membranes; having, instead thereof, a diffused
vascular villosity of the chorion, as in the sow and mare.
But besides these characters, by which, in receding from one type of
hoofed mammalia, the Camelidæ claim affinity with another, there are
many parts of their organization peculiar to themselves; of some of
these peculiarities, the relation to the circumstances under which the
animal exists, can be satisfactorily traced; in others, the connection
of the structure with the exigencies of the species, is by no means
obvious, and in this predicament stands the osteological peculiarity,
which is immediately connected with our present subject—a peculiarity in
which the Camelidæ differ not only from the other Ruminants, but from
all other existing Mammalia, and which consists in the absence of
perforations for the vertebral arteries in the transverse processes of
the cervical vertebræ, the atlas excepted.
I may observe that what is described as a perforation of a single
transverse process in a cervical vertebra is essentially a space
intervening between two transverse processes, a rudimental rib, and the
body of the vertebra. In the cold-blooded Saurians,—in which the
confluence of the separate elements of a vertebra takes place tardily
and imperfectly, if at all,—the nature of the so called perforation of
the transverse process is very clearly manifested, as in the cervical
vertebræ of the Crocodile, in which the interspace of the inferior and
superior transverse processes is closed externally by a separate short
moveable cervical rib. In the Ornithorhynchus paradoxus the vertebra
dentata also preserves throughout life this condition of its lateral
appendages: in other Mammalia it is only in the fœtal state that the two
transverse processes are manifested on each side with their extremities
united by a distinct cartilage, which afterwards becomes ossified and
anchylosed to them.
In the Hippopotamus the inferior transverse process sends downwards a
broad flat plate extended nearly in the axis of the neck, but so
obliquely, that the posterior margins of these processes, in one
vertebra, overlap the anterior ones of the succeeding vertebra below,
like the cervical ribs in the Crocodile; the same structure obtains in
many other mammalia, especially in the Marsupials. In the Giraffe, the
inferior transverse processes are represented by relatively smaller
compressed laminæ, projecting obliquely downwards and outwards from the
anterior and inferior extremity of the body of the vertebra. The
superior transverse processes in this animal are very slightly developed
in any of the cervical vertebræ, and the perforation for the vertebral
artery is above and generally in front of the rudiment of this process,
being continued as it were through the side of the substance of the body
of the vertebræ.
In the long cervical vertebræ of the Camel and Llama, the upper and
lower transverse processes are not developed in the same perpendicular
plane on the sides of the vertebræ, but at some distance from each
other; the lower transverse processes (_a_, fig. 1, Pl. VI.; _a_, fig.
1, 3, 4, Pl. VII.) being given off from the lower part of the anterior
extremity of the body of the vertebra; the upper ones (_b_, fig. 1, Pl.
VI.; _a_, fig. 1, 3, 4, Pl. VII.) from the base of the superior arch
near the posterior part of the vertebra, or from the sides of the
posterior part of the body of the vertebræ. The extremities of these
transverse processes do not become united together, but they either pass
into each other at their base, or continue throughout life separated by
an oblique groove (as in fig. 1, Pl. VI.) This groove would not,
however, afford sufficient defence for the important arteries supplying
those parts of the brain which are most essential to life; and,
accordingly the vertebral arteries here deviate from their usual course,
in order that adequate protection may be afforded to them in their
course along the neck. From the sixth to the second cervical vertebræ
inclusive in the _Aucheniæ_, and from the fifth to the second inclusive
in the _Cameli_,[17] the vertebral arteries enter the vertebral canal
itself, along with the spinal chord, at the posterior aperture in each
vertebra, run forwards on the outside of the dura mater of the chord
between it and the vertebral arch, and when they have thus traversed
about two-thirds of the spinal canal, they perforate respectively the
superior vertebral laminæ, and emerge directly beneath the anterior
oblique or articulating processes, whence they are continued along with
the spinal chord into the vertebral canal of the succeeding vertebra,
and perforate the sides of the anterior part of the superior arch in
like manner; and so on through all the cervical vertebræ until they
reach the atlas, in which their disposition, and consequently the
structure of the arterial canals, resemble those in other Ruminants.
The two cervical vertebræ of the Macrauchenia present precisely the
structure and disposition of the bony canals for the vertebral arteries
which are peculiarly characteristic of the Camelidæ among existing
Mammalia. In Plate VI. fig. 2, the groove and orifices of the canal for
the vertebral artery are shown in a section exposing the spinal canal:
in Plate VII. figures 1 and 3 exhibit the orifices at the commencement
of the arterial canals, as seen in a posterior view of the vertebræ; in
figs. 2 and 4, the terminations of the same canals are shown, in the
anterior view of the same vertebræ; the smaller figures (3 and 4) are
taken from the fourth cervical vertebra of a Llama. The vertebræ of the
Macrauchenia also closely resemble the middle cervical vertebræ of the
Vicugna and Llama in their elongated form; approaching the Auchenial
division of the Camelidæ, and deviating from the true Camels in the
relations of the length of the body of the vertebra to its breadth and
depth, and in the much smaller size of the inferior processes. Excepting
the Giraffe, there is no existing mammal which possesses cervical
vertebræ so long as the Macrauchenia; but the cervical vertebræ of the
Giraffe, differ in the situation of the perforations for the vertebral
arteries, and in the form of the terminal articular surfaces, as will be
presently noticed.
Both of the cervical vertebræ of the _Macrauchenia_ here described, are
of the same size, each measures six inches and a half in extreme length,
two inches, ten lines in breadth, and two inches, four lines in depth.
In the Giraffe and the Camelidæ, the spinous processes are thin laminæ
of considerable extent in the axis of the vertebra, but rising to a very
short distance above the level of the vertebral arch: the spinous
processes have the same form in the corresponding vertebræ of the
Macrauchenia, but present a still greater longitudinal extent; they
commence at the interspace of the anterior oblique processes, and extend
to opposite the base of the posterior oblique processes; the upper
margin describing a gentle curve, as shown in fig. 1, Pl. VI. The
transverse processes also present the form of slightly produced, but
longitudinally extended, laminæ: their disposition is essentially the
same as in the Camelidæ, but more nearly corresponds with the
modifications presented by the Aucheniæ. The inferior transverse
processes,—those which are alone developed in fish, but which are not
present in any other vertebræ save the cervical, in mammalia,—these
processes in the Macrauchenia are continued from the sides of the under
surface of the anterior part of the body of the vertebra; their
extremities being broken off, it cannot be determined how far they
extended from the body of the vertebræ, but they gradually subside as
they pass backwards: the superior transverse processes are continued
outwards from the sides of the posterior part of the body of the
vertebra, and gradually subside as they advance forwards along
three-fourths of the body of the vertebra: they are not continued into
the anterior and inferior transverse processes, as in the Vicugna, but
are separated therefrom by a narrow and shallow groove. The articular,
or oblique processes, closely resemble those of the Auchenia in form,
and in the direction of the articular surfaces; those of the anterior
processes looking inwards and a little upwards; those of the posterior,
outwards and a little downwards.
In the Macrauchenia a small longitudinal process (_c_, fig. 2, Pl. VII.)
is given off immediately below the base of the anterior oblique process;
this structure is not observable in any of the cervical vertebræ of the
Giraffe or Camelidæ.
In the form of the articulating surfaces of the bodies of the vertebræ
the Macrauchenia deviates from the Giraffe and Camel, but resembles the
Aucheniæ. In the Giraffe and Camel the anterior articulating surface is
convex and almost hemispheric, the posterior surface is proportionally
concave, so that the cervical vertebræ are articulated by ball and
socket joints; yet not, as in most Reptiles, with intervening synovial
cavities, but by the concentric ligamentous intervertebral substance
characteristic of the Mammiferous class. In the Llama and Vicugna, the
degree of convexity and concavity in the articular surface of the bodies
of the cervical vertebræ is much less than in the Camels; and in
consequence they carry their necks more stiffly and more in a straight
line. In Macrauchenia the anterior articulating surface (fig. 2, Pl.
VII.) presents a still slighter convexity than in the Llama (fig. 4, Pl.
VII.), and the posterior surface (fig. 1, Pl. VII.) presents a
correspondingly shallower concavity. The form of the extremities of the
body of the vertebræ, especially of the posterior, is sub-hexagonal, the
breadth being to the depth as eight to five. The sides and under part of
the vertebræ are slightly concave; on the inferior surface there are two
ridges, continued forwards from the posterior margin of the vertebra,
each situated about an inch distant from the middle line; they converge
as they pass forwards, and are gradually lost in the level of the
vertebra; their greatest elevation does not exceed half an inch. In the
Aucheniæ there is a longitudinal protuberance in the mesial line,
instead of the two ridges. The two long cervical vertebræ of the
Macrauchenia are also characterized by the maintenance of an almost
uniform diameter of the body, both in its vertical and transverse
extent; the cervical vertebræ of the Vicugna come nearest to them in
this respect; those of the Camel deviate further in the large excavation
at the under part of the body.
The long vertebral or spinal canal offers a slight enlargement at the
two extremities; this structure which is generally in the ratio of the
extent of motion of the vertebræ on each other is more marked in the
Camel, where the form and mode of articulation of the bodies of the
vertebræ are designed to admit of a free and extensive inflection of the
cervical vertebræ; and the result of this structure is very obvious in
the sigmoid flexure of the neck in the living animal. In the Aucheniæ,
on the contrary, the neck is carried less gracefully erect and in an
almost straight line, and the form of the vertebræ and the nature of
their joints correspond, as we have seen, to this condition. From the
length of the bodies of the cervical vertebræ of the Macrauchenia, and
the almost flattened form of their anterior and posterior articular
surfaces, I infer that the long neck in this singular quadruped must
have been carried in the same stiff and upright position as in the
Vicugna and Guanaco.
The following individual differences are observable in the two cervical
vertebræ of the Macrauchenia;—in the posterior one the superior arch is
wider and with thicker parietes, the body is more concave below, and the
inferior transverse processes have a more lengthened origin.
Not a fragment of dorsal vertebræ, ribs or sternum, is included in the
collection of the bones of the Macrauchenia; but fortunately seven
lumbar vertebræ, forming a consecutive series of the same individual as
that to which the cervical vertebræ belonged, were obtained, all more or
less fractured, but all sufficiently perfect to demonstrate their true
nature. These vertebræ, although not possessing such distinctive
characters as the cervical, contribute by no means an unimportant
element towards the illustration of the osteology of the Macrauchenia,
and support the view which I have taken of its affinities; for,
although, as will be seen from the structure of its extremities, this
animal must be referred to the Order Pachydermata, yet no existing
species of that order has more than six lumbar vertebræ; whilst among
the Ruminants it is only in the Camel, Dromedary, Llama and Vicugna,
that the lumbar vertebræ reach the number seven,—the same number which
characterizes the extinct annectant species in question. The dimensions
of the vertebræ in the Macrauchenia present the same relations to the
two cervical vertebræ above described, which the lumbar vertebræ of the
Vicugna bear to the third, fourth, or fifth of its cervical vertebræ.
But here we begin to discover modifications of form, in which the
Macrauchenia deviates from the Camelidæ, and approaches the Pachyderms,
as the Horse and Hippopotamus; and these indications become stronger as
the vertebræ approach the sacrum.
In the Camel, as well as in the Horse and Hippopotamus, the bodies of
the lumbar vertebræ diminish in vertical extent, or become flatter, as
they approach the sacrum; but this character is more strongly marked in
the Macrauchenia than in either of the above species. But in the
Camelidæ the transverse processes of the lumbar vertebræ, are elongated,
flattened, and narrow, resembling ribs, except that they are nearly
straight; and this is more particularly the case with the transverse
processes of the last lumbar vertebræ, which are the narrowest of all in
proportion to their length, and stand freely out without touching the
sacrum. The transverse processes of the lumbar vertebræ of the Giraffe
resemble those of the Camel, but are relatively smaller and shorter. In
the Hippopotamus the transverse processes of the lumbar vertebræ are
much broader in proportion to their length than in any of the Ruminants,
and they increase in breadth to the last lumbar vertebra, which presents
in addition, the following characters; each transverse process sends off
from its posterior margin a thickened and transversely elongated
protuberance, which supports a flattened articular surface adapted to a
corresponding surface on the anterior part of the transverse process of
the first sacral vertebra: it likewise presents on its anterior edge a
flattened and rough surface, which is closely attached by ligamentous
substance to the opposite part of the transverse process of the
penultimate lumbar vertebra. A similar structure exists in the last two
lumbar vertebræ of the Rhinoceros, Tapir, and Horse. In the latter
animal, anchylosis of these articulating surfaces of the lumbar and
sacral vertebræ generally takes place with age, and, judging from the
character of the same surfaces in the Hippopotamus, the motion of its
lumbar vertebræ upon the sacrum may in like manner become ultimately
arrested.
Now in the Macrauchenia, as in the Pachyderms above cited, the
transverse processes of the last lumbar vertebræ are of considerable
thickness and extent, and are joined by enarthrosis to the transverse
processes of the sacrum; but the bony structure of these joints would
indicate that they were not subject to be obliterated by anchylosis. The
articular surfaces which project from the posterior part of the
transverse processes of the last lumbar vertebræ present a regular and
smooth concavity, adapted to a corresponding convexity in the transverse
processes of the first sacral vertebra. These articulating surfaces have
evidently been covered with smooth cartilage; they present a pretty
regular transverse ellipsoid form. A view of the three joints by which,
independently of the two oblique processes, the last lumbar vertebra of
the Macrauchenia was articulated with the sacrum, is given in Plate
VIII. fig. 1. The transverse processes of the posterior lumbar vertebra,
besides their agreement with those of the Horse and Hippopotamus in the
structure just described, also correspond with them in general form, and
deviate remarkably from those of the _Camelidæ_ in their great breadth.
It will be seen that the articulations on the body and transverse
processes of the last lumbar vertebra of the Macrauchenia differ from
the corresponding articular surfaces of the Horse, inasmuch as the
middle surface is convex, while the two lateral ones are concave, and
these are moreover relatively larger than either in the Horse or
Hippopotamus: by this structure the trunk was more firmly locked to that
segment of the vertebral column, which receives and transmits to the
rest of the body the motive impetus derived from the hinder extremities,
which are in all quadrupeds the chief powers in progression; while at
the same time the shock must have been diminished by the great extent of
interposed elastic cartilages; and a certain yielding or sliding motion
would be allowed between the lumbar vertebræ and sacrum.
The anterior oblique processes of the lumbar vertebræ of the
Macrauchenia (fig. 4, Pl. VIII.) have concave articular facets turned
towards, and nearly continued into, each other at their lower
extremities; so as to form together a deep semilunar notch, into which
the corresponding convex articular surfaces of the posterior oblique
processes of the adjoining vertebra (fig. 3, Pl. VIII.) are firmly
locked. In the close approximation of the two anterior concave articular
facets, which are separated from each other only by a vertical ridge,
and a rough surface of about three or four lines in breadth, the lumbar
vertebræ of the Macrauchene resemble those of the Horse, and differ from
those of the Camel tribe and Ruminants generally, in which those
surfaces are wider apart. In the hook-like form, however, of these
articular processes the lumbar vertebræ of the Macrauchene differ from
those of the Horse; and resemble those of many Ruminant species, and of
the Anoplothere;[18] but the degree of concavity of the articulating
surface is not so great in the Macrauchene. It would be interesting to
determine the relations which the lumbar vertebræ of the Macrauchene
bear to those of the Palæothere; but the indication which Cuvier gives
of the single lumbar vertebra, of which he had cognizance in the latter
genus[19] is too slight to enable me to enter upon the comparison.
The whole length of the lumbar region in the Macrauchene is twenty
inches. When the bodies of these vertebræ are naturally adapted
together, they form a slight curve, indicating that the loins of the
Macrauchene were arched, or bent downwards towards the sacrum. That the
lumbar vertebræ were rigidly connected together, or but slightly
flexible, is evident from the flatness of the articular surfaces of the
vertebral body, and by the circumstance of ossification having extended
along the anterior vertebral ligaments, and produced an anchylosis
between the fourth and fifth lumbar vertebræ; (fig. 2, _c_, Pl. VIII.)
This kind of ossification is frequent in aged horses, and I have seen an
example of a similar anchylosis of the lumbar vertebræ, by abnormal
deposition of bone in their anterior ligaments, in the skeleton of a
Hippopotamus preserved in the Senkenbergian Museum, at Frankfort.
In preparing the preceding account of the cervical and lumbar regions of
the vertebral column of the Macrauchene, I have felt frequently a strong
desire to enter into a comparison between them and the corresponding
vertebræ of the extinct Pachyderms of the Paris Basin. Some of these, as
the _Anoplotherium gracile_, in the length and slenderness of the
cervical vertebræ, resemble both _Auchenia_ and _Macrauchenia_; others,
as the _Palæotherium minus_, and probably the rest of the genus,
resemble the _Camelidæ_ and _Macrauchenia_ in having seven lumbar
vertebræ. Cuvier points out the resemblance which the atlas of the
Anoplothere bears to that of the Camel, and especially of the Llama;[20]
but he expressly notices the existence of the canals for the vertebral
artery in the fifth or sixth cervical vertebra of the _Anoplotherium
commune_.[21] Do the cervical vertebræ—say from the third to the sixth
inclusive—of the _Palæotherium_ present an imperforate condition of
their transverse processes, or exterior part of their sides? Cuvier, who
seems not to have been aware of this peculiarity in the _Camelidæ_,
merely notices the absence of these arterial foramina in the last
cervical vertebra of the _Palæotherium minus_,[22] which, unfortunately
for the comparison I am desirous of establishing, is that which most
commonly presents this imperforate condition in the Mammalia generally.
As, however, the cervical vertebræ of the Palæothere had the anterior
articular surface of the body convex, and the transverse processes
produced into descending laminæ, it is most probable that they
corresponded with the cervical vertebræ of the typical Pachyderms in the
condition of their arterial foramina.
The sacrum and ossa innominata in the present specimen of _Macrauchenia_
are very imperfect; but sufficient is preserved to show that the sacrum
was anchylosed to the ilia: the lower boundary of this anchylosis is
marked below by an external ridge, and by vascular canals and grooves in
the substance of the bone, as in the Hippopotamus. The body of the
sacrum is lost, but the smooth articular convexities upon the transverse
processes adapted to the articular depressions of the last lumbar
vertebra are fortunately preserved.
The remains of the anterior extremity of our Macrauchenia include
fragments of a left scapula; the proximal extremities of the anchylosed
bones of the right antibrachium; the metacarpal and most of the
phalangeal bones of the right fore-foot. The first-mentioned fragments,
include the head and neck of the scapula, a small part of its body with
the beginning of the spine, the coracoid process, and the nearly entire
glenoid cavity. This articular surface (fig. 2, Pl. IX.) resembles in
its general form, and degree of concavity, that of the Camel and
Rhinoceros, and is deeper than in the Hippopotamus. The coracoid process
is represented by a slightly produced rough, thick, and obtuse
tuberosity, situated closer to the glenoid cavity than in the _Camelidæ_
or _Rhinoceros_, and having almost the same relative position and size,
as in the _Palæotherium crassum_. The superior border or costa of the
scapula presents much variety in the Ungulate quadrupeds with which we
have to compare the Macrauchenia. In the Ruminants its contour forms
behind the coracoid a concave sweep, which advances close to the spine
of the scapula. In the Camel and Horse the marginal concavity is
shallower, and the distance of the superior costa from the spine of the
scapula is greater; the extent of the supra-spinal fossa increases in
the true Pachyderms, and the Macrauchene agrees with them in this
structure. In the Tapir, however, the contour of the superior costa is
broken by a deep round notch immediately behind the coracoid: in the
Hippopotamus this process arches in a slight degree backward over a
corresponding but wider and shallower notch. In the _Palæotherium
crassum_ the concavity of the superior costa, behind the coracoid, is as
slight as in the Rhinoceros; but in the Macrauchenia the superior costa
of the scapula begins to rise or stretch away from the parallel of the
spine, immediately behind the coracoid process. The modifications of the
spine of the scapula which characterize respectively the Ruminants and
Pachyderms have been clearly and concisely set forth by Cuvier, who at
the same time points out the exceptional condition which the _Camelidæ_
present in the production of the acromial angle. It was with peculiar
interest and care, therefore, that I reunited all the fragments of the
scapula of the Macrauchene, in the hope of gaining from this part of the
skeleton as decisive evidence of an affinity to the Camel as the
cervical vertebræ had afforded. It unfortunately happens, however, that
the part of the scapula most important in this comparison is broken off;
yet from this very circumstance, combined with a slight inclination
forwards of the anterior margin of the spine immediately beneath the
fractured acromion, and from the thickness of the fractured surface, we
may infer that the acromial angle of the spine was more produced than in
the ordinary Ruminants, although evidently in a less degree than in the
Camel tribe. The Macrauchenia, however, surpasses these aberrant
Ruminants, and equals the Pachyderms in the elevation and extent of its
scapular spine: but this process commences about half an inch behind the
glenoid cavity, and rises at once to the height of three inches above
the plane of the scapula; in which structure we may trace the same
tendency to the Ruminant type, as is manifested in the scapula of the
Hippopotamus and Anoplotherium; for in most other Pachyderms the spine
increases gradually from its extremities to the middle part. The
anterior margin of the spine beneath the short acromion is perforated by
an elliptical fissure measuring ten lines, by three lines. The extent of
the spine which is preserved, measures eight inches and a half; it is a
thin and nearly straight plate of bone, expanding into a thick and
rugged upper margin, which slightly over-arches the inferior fossa.
(fig. 1, Pl. IX.) In its general form and proportions the spine of the
scapula in Macrauchenia presents the nearest resemblance to that of the
Hippopotamus; but its origin is closer to the articular surface of the
scapula than in this, or any other Pachydermal or Ruminant genus.
The portion of the antibrachium of the Macrauchenia which is preserved,
presents a condition of the radius and ulna intermediate to those which
respectively characterize the same bones in the Pachyderms and Camels.
In the former the radius and ulna are separate bones, united in the
prone position by ligament, yet so that the movement of supination
cannot be performed; in the ordinary Ruminants they are partially joined
by bony confluence, which rarely extends to the proximal extremities; in
the Camel and Llama the anchylosis of the radius and ulna is so
complete, that no trace of their original separation can be perceived,
and the olecranon appears but as a mere process of the radius.
In the Macrauchenia the anchylosis of the radius and ulna is also
complete, but the boundary line of the two originally distinct bones is
very manifest, and the proportion which each contributes to the great
articulating surface for the distal end of the humerus is readily
distinguishable. About a sixth part of this surface is due to the head
of the radius, which enters into the composition of the anterior and
outer part of the articulation, and its extent is defined by a depressed
line describing a pretty regular curve, with the concavity directed
forwards and a little outwards. (_a_, fig. 1, Pl. X.) Just below the
articular surface a strong triangular rugged protuberance projects from
the front of the head of the radius, for the attachment of the tendon of
the biceps. The line of separation of the radius and ulna is indicated
on the inner side of the head of the radius by a deep and narrow fissure
extending downwards from below the anterior part of the articulating
surface; and on the outer side by a broad groove leading upwards to a
deep pit near the proximal end of the antibrachium. We may see by the
direction of the head of the radius which is thus defined, that it
crosses obliquely in front of the ulna, as in the Elephant,
Hippopotamus, and other Pachyderms, and that the bones are anchylosed in
the prone condition: below this fissure and groove, which mark the
interosseous line, the radius and ulna become blended together into one
compact bone, which is flattened from before backwards, with a well
marked ridge on the outer side; and excavated by a single medullary
cavity, the compact walls of which present a general thickness of
one-third of an inch.
The proximal articular surface or sigmoid cavity of the antibrachium,
constituted as above described, resembles that of the Palæothere, Tapir,
and the generality of the Pachyderms in having two depressions, instead
of three, as in the Anoplothere, and Ruminants. The Hippopotamus has a
slight tendency to the latter structure, which is also less marked in
the Camel than in the ordinary Ruminants. In its general form the
sigmoid cavity of the Macrauchene resembles that of the Hippopotamus
more than that of the Camel. In the Camel this articular surface is
traversed transversely by a broad, shallow, and slightly roughened
tract, which divides the smooth surface of the joint into two parts, one
forming the anterior horizontal surface due to the conjoined radius and
ulna, the other forming the vertical concave surface on the anterior
part of the base of the olecranon. In the Hippopotamus there is, as it
were, an attempt at a similar division of the articulating surface at
the proximal end of the antibrachial bones; a deeper and rougher
depression encroaches upon the articulation from its outer side, but
stops when it has reached half-way across. In the Macrauchenia the
roughened surface, (_b_. fig. 1, Pl. X.) commencing also at the outside,
extends only one-third of the way across the articular surface: it is,
however, as shallow as in the Camel. The articular surface on the
anterior part of the base of the olecranon is broader in the
Hippopotamus than in the Camel; but in the Macrauchene it is twice as
broad as in the Hippopotamus. The size of the olecranon in the
Macrauchene exceeds that of the Hippopotamus, and _à fortiori_ that of
the Camel: indeed in its general magnitude the Macrauchenia must have
fully equalled the largest Hippopotamus; but it no doubt had a more
shapely, and less broad and bulky trunk. The olecranon of the
Macrauchenia differs in shape, both from that of the Camel and
Hippopotamus; it terminates above in a three-sided cone with an obtuse
apex; and presents a well-marked protuberance at the outer side of the
base, which is not present in either the Camel or Hippopotamus. There is
also a strong rugged ridge on the back part of the olecranon which makes
an angle before sinking into the level of the ulna below.
The confirmation of the close affinity of the Macrauchenia to the
Pachydermatous Order, which the structure of the cervical vertebræ alone
might have rendered very doubtful, is afforded by the bones of the right
fore-foot (Pl. XI.); these are fortunately in so perfect a condition, as
to make it certain that this interesting quadruped had three toes on the
fore-feet, and not more; and that the fully developed metacarpal bones
are distinct, and correspond in number with the toes, and are not
anchylosed into a single cannon bone, as in the Ruminants. The bones
preserved are the metacarpals, proximal phalanges, and middle phalanges
of each of the three toes, and the distal phalanx of the innermost toe.
The proximal end of the innermost metacarpal bone presents three
articular surfaces; the middle facet is the largest, and the two lateral
ones slope away from it at an angle of 45°. The middle facet is broad
and slightly convex in front, narrow and concave behind; the distal
articular surface of the trapezoides must have corresponded with this
surface; the outer facet is narrow, flat, extends from the fore to the
back part of the head of the bone, and must have been adapted to a
corresponding surface on the os magnum; the inner facet is the smallest,
presents a triangular form, and is situated towards the back part of the
head of the metacarpal bone; it indicates the existence of a rudimental
metacarpal bone, or vestige of a pollex. Below the outermost of the
lateral surfaces there is a crescentic articular surface with its
concavity directed outwards and downwards (fig. 2, Pl. XV.), against
which a corresponding convex articular surface of the middle metacarpal
abuts, (fig. 3, Pl. XV.) External to this surface the proximal end of
the middle metacarpal bone presents two articular surfaces for the
carpus; the larger one, which was adapted to the os magnum, is
horizontal, broad and convex before, narrow and concave behind; the
outermost facet is a small triangular surface inclined downwards to the
level of the articulating surface of the outermost metacarpal. It also
presents a posterior vertical articular surface for a sesamoid bone. The
proximal extremity of the outer metacarpal bone is joined to the middle
metacarpal, not by one semilunar surface, but by two separate
articulations of small size (fig. 4 and 5, Pl. XV.); it presents a
single large slightly convex articular surface for the os magnum, of an
irregular semicircular form, with the convexity of the curve turned
outwards.
The metacarpus increases in breadth as it approaches the phalanges; the
two lateral metacarpals bending slightly away from the middle one, and
expanding towards their distal extremities: the middle bone presents a
symmetrical figure except at its proximal extremity (fig. 2, Pl. XI.)
The distal articulating facet of each of the metacarpal bones extends so
far upon both the anterior and posterior surfaces as to describe more
than a semicircle (fig. 3, Pl. XI.); in the two lateral metacarpals it
is traversed throughout by a longitudinal convex ridge dividing it into
two equal lateral parts; the ridge is most produced on the posterior
half of the joint (fig. 4, Pl. XI.): in the middle metacarpal this ridge
subsides before it reaches the anterior part of the articular surface.
The proximal extremity of the middle proximal phalanx presents a
posterior notch corresponding to the above partially developed ridge:
the proximal extremities of the lateral phalanges are traversed by a
middle longitudinal depression, and two lateral shallow concavities
(fig. 6, Pl. XI.); but these are of such an extent as to be in contact
with only a part of the convexity above, which therefore was doubtless
adapted to a sesamoid bone on each side of the longitudinal ridge. The
structure of the above described joints proves that the motion of the
toe upon the metacarpus was much freer and more extensive than in the
Rhinoceros, which is the only existing Ungulate mammal which presents
the tridactyle structure in the fore-foot. In this species the
metacarpo-phalangeal articulations exhibit only a slight trace of the
longitudinal ridges and grooves which are confined to the posterior part
of the joint; these are more developed in the _Camelidæ_; but the Hog
and Horse in this respect approach nearer to the Macrauchene, though the
structure of the metacarpo-phalangeal joints in the Hog falls far short
of the compactness and strength combined with freedom of play in flexion
and extension which distinguish those of the Macrauchene. The
_Palæotherium medium_ most resembles the Macrauchene in the structure of
the trochlear metacarpo-phalangeal joints; but both in this species,[23]
and the _Pal. crassum_[24] the articular surface at the distal end of
the metacarpal is relatively narrower than in the Macrauchenia; moreover
all the species of the extinct Palæothere differ from the Macrauchene in
the greater size and strength of the middle as compared with the lateral
metacarpals.
The articulation at the distal extremity of the proximal phalanges (fig.
5, Pl. XI.) is simple, and not divided into two pulleys by a
longitudinal ridge; it is slightly concave from side to side; but in its
extent upon the anterior and posterior surfaces of the bone indicates a
freedom of flexion and extension of the toes, which harmonizes with the
structure of the joint above.
The proximal articulating surfaces of the second phalanges (fig. 7, Pl.
XI.) corresponds of course to those to which they are adapted; they are,
however, characterized by sending upwards an obtuse process from the
middle of their anterior margin. The distal articulating surfaces (fig.
8, Pl. XI.) resemble those of the proximal phalanges, but extend further
upon the back part of the phalanx than the front, indicating the more
horizontal position of the second phalanges.
The last phalanx, does not resemble the neatly defined ungulate
phalanges of the Ruminantia, and Solipedia, but has the irregular form
characteristic of those of the Pachydermata. It is wedge-shaped, broader
than it is long, with a rugged surface, except where it plays upon the
distal end of the second phalanx, where it is slightly concave in one
direction, and convex in the other, (figs. 7 and 9, Pl. XI.) A portion
of this phalanx extends backwards behind the articular surface, as in
the corresponding bone of the Palæothere and Rhinoceros.
The femur of the Macrauchenia (fig. 1, Pl. XII.) is full two feet in
length, and consequently longer than in any known Camel or Rhinoceros;
as compared with its transverse diameter it is much longer than the
femur of the latter animal: in the proportion of its breadth to its
length, and the expansion of its extremities as compared with the
diameter of the shaft, it more resembles that of the Camel. The femur of
the Giraffe deviates from that of the Macrauchenia in the excessive
expansion of its distal extremity. But the most striking evidence
deducible from this bone, of the affinity of the Macrauchenia to the
true Pachydermatous type is afforded by the evident traces of a third
trochanter, the outline of which is conjecturally restored in the
figure. Of the Pachyderms which have this characteristic structure, the
extinct Palæothere offers the nearest resemblance to the Macrauchene in
the general form and structure of the femur.
The head of the femur in the Macrauchene (fig. 2, Pl. XII.) presents the
form of a pretty regular hemisphere; it is less flattened above, and is
directed more obliquely inwards than in the Palæothere: the neck
supporting it does not project so far from the shaft as in the
Palæothere or Tapir, but farther than in the Camel. The great trochanter
rises above the level of the head; in which structure and in the
depression between the head and trochanter, the femur of the Macrauchene
offers a character intermediate between the Tapir or Palæothere, and the
Camel. The lesser trochanter is a slight projection from a ridge of bone
which is continued from the under part of the head of the femur to the
inner surface of the shaft. In the Palæothere the lesser trochanter is
situated more towards the posterior surface of the femur; so that, in
this particular, the Macrauchene approaches nearer to the Camel. Cuvier
makes no mention of the condition of the depression for the _ligamentum
teres_ in the Palæothere. Among existing ordinary Pachyderms the
Hippopotamus presents no trace of the insertion of a _ligamentum teres_
in the head of the femur; in the Camel the place of its insertion is
indicated by a well-marked circumscribed pit; in the Tapir a similar
circular depression is situated close to the inferior margin of the
articular convexity. The ligament was undoubtedly present in
Macrauchenia, but the place of its insertion is a broad and deep notch
leading from the under and back part of the head of the bone a little
way into its articular surface: this I regard as another of those
interesting transitional structures with which the remains of the
Macrauchenia, few and imperfect though they unfortunately are, so freely
abound.
The femur of Macrauchenia, in the flatness of the back part of its neck,
and the elongated form of the post-trochanterian depression, resembles
that of the Camel rather than that of the Palæothere; and the same
resemblance is shown in the cylindrical figure, straightness, and length
of the shaft. The depth of the trochanterian depression, and the
incurvation of the strong ridge continued downwards from the great
trochanter are individual peculiarities in the Macrauchenia.
A great part of the third trochanter is broken off; but from the remains
of its base we see that it had the same relative size as in the
Palæothere; but it is situated at the middle of the shaft of the femur,
and consequently lower down than in the Palæotheres and Tapirs. In the
general form and relative size of the condyles at the distal extremity
of the femur (fig. 3, Pl. IX. and XII.) the Macrauchene is intermediate
to the Camel and Palæothere, but resembles more the latter. In the
articular surface for the patella, it deviates somewhat from the
Palæothere, having this part longer in proportion to its breadth, more
regularly and deeply concave from side to side, and with its lateral
boundaries more sharply defined. In all these points the Macrauchene
approaches the Camel: the same affinity is shown in the protuberance
above the inner condyle; but in the extent of the posterior projection
of this condyle (fig. 3, Pl. IX.) it exceeds the Camel and Palæothere,
and displays an intermediate structure between these species and the
Hippopotamus.
There is a rough crescentic depression above the outer condyle where the
linea aspera begins to diverge; the corresponding depression is deeper
in the Hippopotamus, while in the Camel it is represented by a roughened
surface only, which is not depressed. In the fossa between the rotular
articulation and the external condyle the Macrauchene resembles the
Camel: the interspace of the condyles is relatively wider than in the
Camel, and the process above the inner condyle is more angular; in both
these respects the Macrauchene inclines towards the Palæothere.
In the structure of the bones of the leg of the Macrauchenia we find the
same transitional character which is afforded by the definable limits of
the anchylosed bones of the fore-arm. In the Pachyderma the fibula is an
entire and distinct bone. In the Ruminantia, with the exception of the
small Musk-deer, and, in an inferior degree, the Elk, the fibula appears
only as a short continuous process sent down from the under part of the
external condyle of the tibia. In the Camel tribe the only trace of the
fibula in the bones of the leg, is this process in a still more
rudimental state. In the Macrauchenia the fibula is entire, but is
confluent with the tibia through nearly its whole extent: the proximal
part of the fibula is well defined; its head is anchylosed to the outer
condyle of the tibia, but the shaft is continued free for the extent of
nearly two inches, and then again becomes confluent with the tibia,
forming apparently the outer ridge of that bone. About five inches from
the distal end of the tibia this outer ridge becomes flattened by being,
as it were, pressed against the tibia, and the anterior and posterior
edges are raised above the level of the tibia; beyond this part the
limits of the fibula begin again to be defined by deep vascular grooves.
The outer side of the distal end of the fibula is excavated by a broad
tendinous groove. The fibula and tibia are distinct bones in both the
Palæothere and Anoplothere, as in the Pachyderms. It is to the former
genus, however, especially _Pal. magnum_, that the Macrauchene presents
the nearest approach in the general form of the tibia, the principal
bone of its leg: but in the Macrauchene the tibia is relatively shorter,
and thicker, and is straighter and less expanded at its extremities,
especially the upper one, than in any of the Palæotheres.
The mesial boundaries of the two superior articulating surfaces of the
tibia are raised in the form of ridges, which are separated by a deep
groove; of these ridges the external is the highest, as in _Pal.
magnum_: but the articular surfaces in the Macrauchene slope away from
these ridges more than in the Palæotheres. The rotular or anterior
tuberosity of the tibia is more produced, and rises higher than in the
Palæotheres; the ridge continued downwards from this process is more
marked in the Macrauchene, and its limits are better defined: the shaft
of the tibia below the ridge is also more flattened in the
antero-posterior direction than in the Palæothere. The configuration of
the back part of both proximal and distal extremities of the tibia are
so clearly and accurately given in figures 2 and 3, Pl. XIII., as to
render verbal description unnecessary. Neither the text nor the figures
in the ‘Ossemens Fossiles’ afford the means of pursuing the comparison
between the Macrauchene and Palæothere in these particulars; and I
proceed, therefore, to the consideration of the inferior articulating
surface of the bones of the leg (fig. 4, Pl. XIII.)
Since, of the hind-foot, we possess in the present collection only a
single tarsal and metatarsal bone, the structure of the distal articular
surface of the tibia is attended with peculiar interest, because we are
taught by Cuvier that it reveals to us in the Ungulate animals the
didactyle or tridactyle structure of the foot. In the Ruminants this
articular surface is nearly square, and extended transversely between
two perpendicular malleoli, while in the Pachyderms with three toes to
the hind-foot the articular surface of the tibia is oblique, and is
divided into two facets between the perpendicular malleolar boundaries.
Now in the Macrauchenia, although the two bones of the leg are
anchylosed together, the extent of that part of the tarsal articular
surface which is due to the tibia is indicated, as in the case of the
radius in the joint of the fore-arm, by a groove; and we are thus able
to compare this surface with the distal articular surface of the tibia
in the Palæothere and Anoplothere. It presents in the Macrauchenia a
very close resemblance with that of the _Palæotherium magnum_,[25] being
divided into two facets by a convex rising, which traverses the joint
from behind forwards; but the ridge is narrower, the internal facet
somewhat deeper, and the external oblique surface rather flatter than in
the three-toed Palæothere. In the portion of the tarsal articular
surface due to the fibula, we find, however, a more marked deviation
from the Palæothere, and an interesting correspondence with the
Anoplothere, in the inferior truncation and horizontal articular surface
which is continued upon the lower extremity of the fibula, at right
angles with the vertical malleolar facet which forms the outer boundary
of the trochlea of the astragalus: this articular surface unerringly
indicates a corresponding articular projection in the calcaneum, which,
therefore, although the bone itself does not form part of the present
collection, we may conclude to differ from the calcaneum of the
Palæotherium, and to resemble that of the Anoplotherium, in this
particular at least.
The valuable indication which the distal articular surfaces of the
anchylosed tibia and fibula have given of the correspondence of the
hind-foot with the fore-foot of the Macrauchenia, in regard to the
number of the toes, receives ample confirmation from the astragalus,
which, of all the bones in the foot, is the one that an anatomist would
have chosen, had his choice been so limited, and which most fortunately
has been secured by Mr. Darwin, in a very perfect state, in the present
instance. I have compared this astragalus with that of the Giraffe, and
other Ruminants, the Camel, the Anoplothere, the Horse, the Hog, the
Hippopotamus, Rhinoceros, Tapir, and Palæothere: it is with the
Pachyderms having three toes to the hind-foot, that the Macrauchenia
agrees in the main distinguishing characters of this bone; its anterior
articular surface, for example, is simple, and not divided into a double
trochlea by a vertical ridge: lastly, it is with the astragalus of the
Tapir and Palæothere that it presents the closest correspondence in the
general form and the minor details of structure, and with these
Pachyderms, therefore, I shall chiefly limit the comparison of the
Macrauchenia, in regard to the bone in question. If the upper or tibial
articular surface (fig. 5, Pl. XIV.) be compared with that in the
_Palæotherium magnum_ (Ossem. Foss. Pl. LIV. fig. 2,) it will be seen,
that the general direction of that surface is more parallel with the
axis of the bone in Macrauchenia. In the Palæotherium it is turned a
little towards the outer or fibular side, and in the Tapir the general
direction of the same surface is placed still more obliquely. The
anterior border of this articulating surface is broken by a semicircular
notch in the Palæothere; in the Tapir it describes a gentle concave
curve, and the Macrauchene resembles the Tapir in this respect. The
chief difference between the astragalus of the Tapir and the Palæothere,
when viewed from above, obtains in the relative length of the bone,
anterior to the tibial articulating surface: the Macrauchene presents,
in this respect, an intermediate structure, but differs from both in the
greater extent of the tibial side of this part of the astragalus.
If we next direct attention to the anterior or scaphoid articular
surface, (fig. 3, Pl. XIV.) and compare it with that of the
_Palæotherium magnum_, (fig. 4, Pl. LIV, Ossem. Foss.) it will be seen,
that it presents in the Macrauchenia an oval, and in the Palæotherium an
irregular quadrangular form: in the Macrauchenia, this surface is
uniform or undivided, and is gently convex, except at its lower part;
while in the Palæothere it is divided by an oblique ridge into a broad
internal facet for the scaphoid bone, and a narrow internal surface for
articulation with the os cuboides; the larger surface is also concave
transversely, and slightly convex vertically: in the Tapir, the anterior
surface of the astragalus deviates still further from that of the
Macrauchenia, both in general form, and in the proportion of the
cuboidal facet. In the didactyle Anoplotherium, Camel, and true
Ruminants, where the cuboides presents a large relative size, a still
greater proportion of the anterior surface of the astragalus is devoted
to the articulation with this bone, and is separated from the scaphoid
surface by a well-developed vertical ridge. The Macrauchenia presents,
therefore, the extreme variation from this type;—and should the entire
tarsus hereafter be discovered, it will doubtless be found, that the os
cuboides is articulated posteriorly to the os calcis exclusively.
The external surface of the astragalus of the Macrauchene, (fig. 1. Pl.
XIV,) is longer in proportion to its vertical extent than in the Tapir
or Palæothere: the articular surface for the fibular malleolus is less
curved. Between this surface and the anterior facet the bone is
excavated by a deep notch, both in the Tapir and Palæothere; but in the
Macrauchenia by a gentle concavity. Beneath the malleolar articular
smooth surface in the Palæothere there is a deep pit; in the Tapir a
shallow one; but in the Macrauchenia we observe only a smooth and
slightly convex triangular surface. If we compare the inner surface of
the astragalus in these three animals, we shall find the existing Tapir
again forming a transition between the two extinct genera. In the
Palæothere, a round protuberance projects from the anterior part of this
surface: in the Tapir, we observe a gentle rising of the bone in the
same part, while in the Macrauchene (fig. 2) the surface of the bone is
level at this part. The margin of the tibial malleolar articular
surface, which is very slightly raised in the Macrauchene, is more
developed in the Tapir, and still more so in the Palæothere, where it
forms a ridge, overhanging the rough outer side of the bone. Near the
lower part of this surface we observe a small but deep depression in the
Palæothere; there is a shallower one in the corresponding part in the
Tapir; and the depression is still wider and shallower in the
Macrauchenia. In the Palæothere the astragalus articulates by three
surfaces with the os calcis, posteriorly by a large concave surface,
externally by a longitudinal sub-elliptic surface, and anteriorly by a
thin transverse facet: in the Macrauchene (fig. 4) two only of these
surfaces are present, viz. the concave and the longitudinal one, the
anterior transverse surface being wanting: in the Tapir, the transverse
surface is present, but is confluent with the longitudinal one. The
posterior surface is relatively larger and deeper in the Macrauchene
than in the Palæothere, and approaches nearer to the triangular than the
oval form: the longitudinal surface is placed more obliquely, and is
truncated anteriorly. In the Tapir this surface is confluent with the
scaphoid articular surface, but it is separated therefrom by a narrow
strip of bone in both the Palæothere and Macrauchene. It is satisfactory
to find in the bone, which marks most strongly the affinity of
_Macrauchenia_ to _Palæotherium_, so many easily recognizable
differences, because the structure of the cervical vertebræ in the
latter genus is too imperfectly known, to allow us to predicate
confidently a distinction between it and _Macrauchenia_ in that
particular; the difference, however, which they present in the condition
of the bones of the fore-arm and leg, forbids their being considered as
generically related.
There remains to be noticed only a single fractured metatarsal bone
(fig. 1. Pl. XV.) This, from its bent and unsymmetrical figure, is
evidently not a middle one, and having the side of the proximal end,
which was articulated to the adjoining metatarsal in a nearly perfect
state, it enables us to refer it with certainty to the hind-foot, since
it does not agree with any of the corresponding surfaces at the proximal
extremities of the metacarpal bones. It remains then to be determined,
whether it is an external metatarsal of the right-foot, or an internal
one of the left-foot, the general curvature of these being in the same
direction. With neither of these bones in the Tapir does our metatarsal
agree, since it has but one articular facet on the lateral surface of
its proximal end, while the outer metatarsal of the right-foot of the
Tapir, with which, in other respects, it most closely corresponds, has
two articular surfaces. In the cast of a hind-foot of a Palæothere, I
find that the outer metatarsal bone closely agrees with this metatarsal
bone of the Macrauchene, in the structure just alluded to: the
articulation with the middle metatarsal being by a single sub-oval
facet, which stands out a little way from the surface of the bone: the
articular surface in the Macrauchene presents a similar form and
condition, and is similarly situated to that in the Palæothere, being at
the posterior part of the lateral surface, and a little below the
superior or tarsal articular surface. The bone expands towards its
distal end, which corresponds in structure with those of the two lateral
metatarsals in the fore-foot, in being completely divided into two
trochlear surfaces by a well-developed median ridge, and in having the
posterior half of this ridge suddenly produced, so as to project about
two lines further from the trochlear surface than the anterior part of
the same ridge. In both the Tapir and Palæothere this anterior part of
the ridge is wholly suppressed, and the posterior is much more feebly
developed than in the Macrauchenia. The metatarsal bone here described
is of exactly the same length with the internal metacarpal bone, and
proves, in conjunction with the proportions of the astralagus, that the
fore and hind-feet of the Macrauchenia were of equal size.
Thus then we obtain evidence, from a few mutilated bones of the trunk
and extremities of a single representative of its race, that there once
existed in South America a Pachydermatous quadruped, not proboscidian,
which equalled in stature the Rhinoceroses and Hippopotamuses of the old
world. But this, though an interesting and hitherto unsuspected fact, is
far from being the sum of the information which is yielded by these
fossils. We have seen that the single ungueal phalanx bespeaks a
quadruped of the great series of _Ungulata_, and this indication is
corroborated by the condition of the radius and ulna, which are fixed
immoveably in the prone position. Now in the Ungulated series there are
but two known genera,—the Rhinoceros and Palæotherium,—which, like the
quadruped in question, have only three toes on the fore-foot. Again, in
referring the Macrauchenia to the Tridactyle family of Pachyderms, we
find, towards the close of our analysis, and by a detailed comparison of
individual bones, that the Macrauchenia has the closest affinity to the
Palæotherium.
But the Palæotherium, like the Rhinoceros and Tapir, has the ulna
distinct from the radius, and the fibula from the tibia; so that even if
the Parisian Pachyderm had actually presented the same peculiarities of
the cervical vertebræ as the Patagonian one, it would have been
hazardous, to say the least, while ignorant of the dentition of the
latter, to refer it to the genus _Palæotherium_.
Most interesting, indeed will be the knowledge, whenever the means of
obtaining it may arrive, of the structure of the skull and teeth in the
Macrauchenia. Meanwhile, we cannot but recognise, in the anchylosed and
confluent state of the bones of the fore-arm and leg, a marked tendency
in it towards the Ruminant Order, and the singular modifications of the
cervical vertebræ have enabled us to point out the precise family of
that order, with which the Macrauchenia is more immediately allied.
In first demonstrating this relationship, it was shown in how many
particulars the _Camelidæ_, without losing the essential characters of
Ruminantia, manifested a tendency to the Pachydermatous type; and the
evidence which the lost genera, _Macrauchenia_ and _Anoplotherium_, bear
to a reciprocal transition from the Pachyderms to the Ruminants, through
the _Camelidæ_, cannot but be viewed with extreme interest by the
Zoologist engaged in the study of the natural affinities of the Animal
Kingdom.
The Macrauchenia is not less valuable to the Geologist, in reference to
the geographical distribution of animal forms. It is well known how
unlooked-for and unlikely was the announcement of the existence of an
extinct quadruped entombed in the Paris Basin, whose closest affinities
were to a genus, (_Tapirus_,) at that time, regarded as exclusively
South American. Still greater surprise was excited when a species of the
genus _Didelphys_ was discovered to have co-existed in Europe with the
_Palæotherium_.
Now, on the other hand, we find in South America, besides the Tapir,
which is closely allied to the Palæothere,—and the Llama, to which the
Anoplothere offers many traces of affinity,—the remains of an extinct
Pachyderm, nearly akin to the European genus _Palæotherium_: and,
lastly, this Macrauchenia is itself in a remarkable degree a
transitional form, and manifests characters which connect it both with
the Tapir and the Llama.
ADMEASUREMENTS OF THE BONES OF THE MACRAUCHENIA.
Inches. Lines.
Length of third (?) cervical vertebra 7 9
Vertical diameter of ditto 4 0
Vertical diameter of body of ditto 2 3
Transverse diameter of ditto 3 3
Vertical diameter of spinal canal 1
Length of fourth lumbar vertebra 5 5
Vertical diameter of body of ditto 2 9
Transverse diameter of ditto 2 10
Vertical diameter of spinal canal 1 1
Transverse ditto ditto[26] 1 6
Transverse diameter of last lumbar vertebra 9
Transverse diameter of body of ditto 2 2
Vertical diameter of ditto 1 3
Entire length of lumbar region of vertebral column 20
Vertical diameter of glenoid cavity of scapula 3
Transverse ditto ditto ditto 2 10
Elevation of spine of scapula 3 5
Vertical diameter of proximal articular surface of
fore-arm 3 6
Transverse ditto ditto ditto 3 5
Height of olecranon 5 3
Greatest diameter of its base 2
Circumference of proximal end of anchylosed radius and
ulna 11 10
Entire length of inner toe of fore-foot, inclusive of
metacarpal bone 13
Breadth of proximal end of metacarpus 3 8
Breadth of distal end of ditto 5 4
Length of inner metacarpal bone 7 6
Length of middle ditto 8
Length of outer ditto 7
Length of inner proximal phalanx 3 6
Length of middle ditto 2 10
Length of outer ditto 3 4
Length of inner middle phalanx 2
Length of middle ditto 2 3
Length of inner distal phalanx[27] 1
Length of the femur 24
Diameter of base of articular surface of the head of
ditto 3 6
Greatest diameter of proximal end 7
Greatest diameter of distal end 6 3
Circumference of middle of shaft 8
Length of tibia 18
Greatest diameter of proximal end 5 7
Greatest diameter of distal end, including fibula 4 4
Circumference of middle of shaft 9
Length of metatarsal bone[28] 7 4
ERRATA.—The reader is requested to substitute the word ‘_right_’ for
‘left’ in the last line of p. 35, before the words ‘radius,’
‘fore-foot,’ and ‘femur,’ and in the first line of p. 36,
before the words ‘tibia,’ and ‘hind-foot.’
DESCRIPTION OF A FRAGMENT OF A CRANIUM OF AN EXTINCT MAMMAL, INDICATIVE
OF A NEW GENUS OF EDENTATA, AND FOR WHICH IS PROPOSED THE NAME OF
GLOSSOTHERIUM.
“La première chose à faire dans l’étude d’un animal fossile, est de
reconnaitre la forme de ses dents molaires; on détermine par-là s’il est
carnivore ou herbivore;” says Cuvier, at the commencement of that series
of splendid chapters in which the restoration of the extinct Pachyderms
of the Paris Basin is recorded. In the present case, however, as in that
of the Mammiferous animal whose fossil remains we were last considering,
the important organs, to which Cuvier directs our first attention, are
wanting. Nor are there here, as in the Macrauchenia, any remains of the
locomotive extremities to compensate for the deficiency of teeth, and
guide us into the right track of investigation and comparison. The
animal, the nature and affinities of which are the subject of the
following pages, is, in fact, represented in Mr. Darwin’s collection, by
nothing more than a fragment of the cranium.
This fragment, which was found in the bed of the same river, (see p.
16,) in Banda Oriental, with the cranium of the Toxodon, includes the
parietes of the left side of the cerebral cavity, the corresponding
nervous and vascular foramina, the left occipital condyle, a portion of
the left zygomatic process, and, fortunately also, the left articular
surface for the lower jaw. The importance of this surface in the
determination of the affinities of a fossil animal has been duly
appreciated, since the relations of the motions of the lower jaw to the
kind of life of each animal were pointed out by Cuvier; but yet we
should be deceived were we to establish, in conformity with the
generalization enunciated by Cuvier,[29] our conclusion, from this
surface, of the nature of the food of the extinct species under
consideration; for the glenoid cavity is so shaped as to allow the lower
jaw free motion in a horizontal plane, from right to left, and forwards
or backwards, like the movements of a mill-stone; and, nevertheless, I
venture to affirm it to be most probable, that the food of
_Glossotherium_ was derived from the animal and not from the vegetable
kingdom; and to predict, that when the bones of the extremities shall be
discovered, they will prove the Glossothere to be not an ungulate but an
unguiculate quadruped, with a fore-foot endowed with the movements of
pronation and supination, and armed with claws, adapted to make a breach
in the strong walls of the habitations of those insect-societies, upon
which there is good evidence in other parts of the present cranial
fragment, that the animal, though as large as an ox, was adapted to
prey.
We perceive, in the first place, looking upon the base of this portion
of skull, a remarkable cavity, situated immediately behind the tympanic
bone, of nearly a regular hemispherical form, an inch in diameter (fig.
2, _b_, Pl. XVI). The superficies of this cavity appears not to have
been covered with articular cartilage, for it is irregularly pitted with
many deep impressions; and I conclude, therefore, that it served to
afford a ligamentous attachment to the styloid element of a large _os
hyoides_. With this indication of the size of the skeleton of the
tongue, is combined a more certain proof of the extent of its soft, and
especially its muscular parts, in the magnitude of the foramen, for the
passage of the lingual or motor nerve (_c._ fig. 2 and 3). This foramen,
(the anterior condyloid,) in the present specimen, is the largest of
those which perforate the walls of the cranium, with the exception of
the foramen magnum; it is fully twice the size of that which gives
passage to the second division of the fifth nerve; its area is oval, and
eight lines in the long diameter, so that it readily admits the passage
of the little finger.
It is only in the Ant-eaters and Pangolins that we find an approximation
to these proportions of the foramen for the passage of the muscular
nerve of the tongue; and the existing Myrmecophagous species even fall
short of the larger fossil in this respect. Some idea of the size of the
lingual nerve, and of the organ it was destined to put in motion, may be
formed, when it is stated that the foramen giving passage to the
corresponding nerve in the Giraffe,—the largest of the Ruminants, and
having the longest and most muscular tongue in that order,—is scarcely
more than one-fourth the size.
With these indications of the extraordinary development of the tongue,
we are naturally led, in order to carry out a closer and more detailed
comparison of the fossil in question, to that group of mammalia in which
the tongue plays the chief part in the acquisition of the food. The
size, form, and position of the occipital condyle,—the magnitude of the
occipital foramen, (which must here have somewhat exceeded three inches
in the transverse diameter,)—the slope of the occipital surface of the
cranium from below, upwards and forwards, at an angle of 60° with the
base of the cranial cavity-each and all attest the close affinities of
the present animal to the Edentata. More decisive evidence of the same
relationship will be adduced from the organization of other parts of the
cranium. The glenoid articular surface (_a_, fig. 2, Pl. XVI.) is an
almost flattened plane, wider in the transverse than in the longitudinal
direction; and, as in the genera _Myrmecophaga_ and _Manis_, it is not
defended behind by any descending process. In its general form it
resembles the glenoid cavity of _Orycteropus_ more than that of the
preceding Edentates; but, in _Orycteropus_, the articulation is defended
posteriorly by a descending process of the zygoma, and it is also
situated relatively closer to the os tympanicum.
Had the _Glossotherium_ teeth? The extent of the temporal muscle, which
is indicated by the rugged surface of the temporal fossa, and by the
well-marked boundary, formed by a slightly elevated bony ridge, which
extends to near the line of the sagittal suture, together with the size
of the zygomatic portion of the temporal bone, and the remains of the
oblique suture by which it was articulated to the malar bone, enables me
to answer this question confidently in the affirmative. They will
probably be found to be molar teeth of a simple structure, as in the
Orycteropus.
The evidence just alluded to of the existence of an os malæ is
interesting, because this bone is wanting in the Pangolins; and its
rudimental representative in the true Ant-eaters does not reach the
zygomatic process of the temporal bone, which consequently has no
articular or sutural surface at its anterior extremity. In the presence,
therefore, of the surface for the junction of the os malæ, and the
consequent evidence of the completion of the zygomatic arch, we learn
that the Glossothere was more nearly allied to the Armadillos and
Orycterope. That its affinity to the latter genus was closer than to the
Armadillos we have most interesting evidence in the form and loose
condition of the tympanic bone: it is represented of the natural size at
fig. 4, Pl. XVI. Through the care and attention devoted to his specimens
by their gifted discoverer, this bone was preserved _in situ_, as
represented at _d_, fig. 1; but it had no osseous connection with the
petrous or other elements of the temporal bone, and could be displaced
and replaced with the same ease as in the _Orycterope_. This bony frame
of the membrana tympani, in the Glossothere, describes rather more than
a semicircle, having the horns directed upwards; it has a groove, one
line in breadth, along its concave margin, for the attachment of the
ear-drum, and sends down a rugged process, half an inch long, from its
lower margin. In the _Dasypodes_ and _Myrmecophagæ_, the tympanic bone
soon becomes anchylosed with the other parts of the temporal; it is only
in _Orycteropus_, among the existing insectivorous _Bruta_ or
_Edentata_, that it manifests throughout life the fœtal condition of a
distinct bony hoop, deficient at the upper part. The os tympanicum of
_Orycteropus_, however, differs from that of _Glossotherium_, in forming
part of the circumference of an ellipse, whose long axis is vertical;
and in sending outwards, from its anterior part, a convex eminence,
which terminates in a point directed downwards and forwards.
Such appear to be the most characteristic features of the cranial
fragment under consideration, in which we have found, that the articular
surface for the os hyoides throws more light upon the nature of the
animal of which it is a part, than even the glenoid cavity itself. There
now remains to be described as much of the individual characters of the
constituent bones as the specimen exhibits.
The occipital bone, besides forming the posterior and part of the
inferior parietes of the cranium, extends for about half an inch upon
the sides, where the ex-occipital element is articulated by a vertical
suture with the mastoid element of the temporal: this suture is situated
in a deep and well-marked muscular depression (_e_, fig. 1), measuring
three inches in the vertical, and upwards of one inch in the transverse
direction. The other sutures, uniting the occipital to the adjoining
bones, are obliterated. The breadth of the occipital region must have
exceeded the height of the same by about one-third. The condyle extends
nearly to the external boundary of the occipital aspect of the cranium;
there is situated, external to it, only a small ovate, rounded and
smooth protuberance. The slightly concave surface of the occipital plane
of the cranium is bounded above by a thick obtuse ridge, the muscular
impressions are well sculptured upon it. It is traversed transversely at
its upper third by a slightly elevated bony crest; and the surface below
this ridge is again divided by a narrower intermuscular crest, which
runs nearly vertically, at about an inch and a half from the external
boundary of the occipital plane. As a similar crest must have existed on
the opposite side, the general character of the occipital surface in the
Glossothere would resemble that of the Toxodon. A similar correspondence
may be noticed in the terminal position of the condyle, and the slope of
the occipital plane.
Above the transverse ridge, the rough surface of the occipital plane
slopes forward, at a less obtuse angle with the basal plane, to the
first named ridge which separates the occipital from the coronal or
superior surface of the skull. The contour of this surface runs
forwards, as far as the fragment extends, in an almost straight line:
the extent of surface between the temporal muscular ridges must have
been about five inches posteriorly, but it decreases gradually as it
extends forwards: all that part which is preserved is quite smooth. The
attachment of the fasciculi of the temporal muscle, and the convergence
of their fibres as they passed through the zygoma are well-marked on the
sculptured surface of the bone. The zygomatic process is relatively
stouter than in _Orycteropus_: it is prismatic: the external facet is
nearly plane: the superior is concave, and increases in breadth
anteriorly: the inferior surface offers a slight convexity behind the
flattened articular surface for the lower jaw. The margin of the zygoma
formed by the meeting of the upper and lower facets presents a
semicircular curve, extended transversely from the cranium, and directed
forwards.
The anterior extremity is obliquely truncated from below upwards and
forwards, and presents a flattened triangular surface indicative of its
junction with an os malæ: the space between this extremity and the side
of the cranium measures one inch and nine lines across, and thus gives
us the thickness of the temporal muscle. The distance from the origin of
the zygoma to the occipital plane is relatively greater than in
_Orycteropus_; _Glossotherium_ is in this respect more similar to
_Myrmecophaga_ and _Manis_.
The sphenoid bone forms a somewhat smooth protuberance below and behind
the base of the _zygoma_. The tympanic bone is wedged in between this
protuberance in front, and the mastoid process behind. The chief
peculiarity of the broad mastoid is the regular semicircular cavity at
its under part for the articulation of the styloid bone of the tongue.
This depression is separated below by a broad rough protuberance from
the foramen jugulare, (_f_, fig. 2, Pl. XVI,) which is immediately
external to, and slightly in advance of the great foramen condyloideum,
_c_. A small rugged portion of the os petrosum separates the jugular
from the carotid canal, which arches upwards and directly inwards to the
side of the shallow sella turcica, (the external and internal orifices
of the carotid canal are shown at _g_, figs. 2 and 3). The chief
protuberance on the basis cranii is a large and rugged one, serving for
the attachment of muscles, and due chiefly to the expansion of a great
sinus in the body of the sphenoid. This protuberance is separated from
the smaller sphenoid protuberance before mentioned by a large groove
continued downwards and forwards from the tympanic cavity, and
containing the Eustachian tube, which does not traverse a complete
osseous canal. Immediately internal to the glenoid cavity is the large
orifice of the canal transmitting the third division of the fifth pair
of nerves, the principal branch of which endows the tongue with
sensibility; this foramen (_h_, fig. 2) is rather less than that for the
muscular nerve of the tongue.
The internal surface of the present cranial fragment affords a very
satisfactory idea of the size and shape of the brain of the extinct
species to which it belongs. It is evident that, as in other Bruta, the
cerebellum must have been almost entirely exposed behind the cerebrum;
and that the latter was of small relative size, not exceeding that of
the Ass; and chiefly remarkable, as in the Orycterope, Ant-eater, and
Armadillo for the great development of the olfactory ganglia. The
antero-posterior extent of the cribriform plate, as exposed in this
fragment, is three inches, and the complication of the œthmoid olfactory
lamellæ which radiate from it into the nasal cavity is equal to that
which exists in the smaller Edentata (fig. 3, Pl. XVI). The nasal cavity
is complicated in _Glossotherium_ by the great number and capacious size
of the air-cells which are in communication with it: these extend over
all the upper, lateral, and back parts of the cranial cavity, as far
even as the upper boundary of the foramen magnum: they also occupy the
anterior two-thirds of the basis cranii. The external configuration of
the skull would, therefore, afford a very inadequate or rather deceptive
notion of the capacity of the cerebral cavity, were not the existence
and magnitude of these sinuses known. The interspace of the outer and
inner tables of the cranium are separated above the origins of the
olfactory ganglia for the extent of three inches: above the middle of
the cerebrum they are an inch and a half apart; at the sides of the
cranium the interposed air-cells are from one to two inches across; at
the back part of the cranium about one inch. The sinuses have generally
a rounded form.
The foramen rotundum, (through which in figure 3 a probe is represented
as passing), and the foramen ovale are situated close together, within a
common transversely oblong depression (_i_). The carotid canal (_g_)
opens into the outer side of the commencement of this wide channel,
which conducts the great fifth pair of nerves to the outlets of its two
chief divisions.
The petrous bone projects into the cranial cavity, in the form of an
angular process with three facets: the foramen auditorium internum
(_k_), and the aqueductus vestibuli, are situated on the posterior
facet. Immediately behind the os petrosum is the foramen lacerum
jugulare (_l_), situated at the point of convergence of the vertical
groove of the lateral sinus, with a groove of similar size continued
forwards from above the anterior condyloid canal. The plane of the
internal opening of this canal (_c_, fig. 3) is directed obliquely
inwards and backwards, and the lateral wall of the foramen magnum behind
the foramen condyloideum slopes outwards to the edge of the condyle.
Immediately internal to the foramen condyloideum is a small vascular
foramen conducting a branch of the basilar artery into the condyloid
canal, for the nourishment, doubtless, of the great lingual nerve.
In the relations of the plane of the internal orifice of the anterior
condyloid foramen with that of the foramen magnum, we search in vain for
a corresponding structure in any of the Mammiferous orders, save the
Edentata:[30] and among these the Orycterope comes nearest the
Glossothere in this respect. In the degree of development of the
internal osseous ridge giving attachment to the tentorium cerebelli, the
Ant-eaters and Armadillos more resemble the Glossothere than does the
Orycterope; in which a continuous bony plate arches across the cranial
cavity: in the Manis a still greater proportion of the tentorium is
ossified, and it consequently recedes the furthest amongst the Edentata,
in this, as in most other particulars of the cranial organization, from
the Glossothere. The chief distinctive peculiarity in the cranium of the
Glossothere, so far as it can be studied in the present fragment, and
compared with that of other Edentata, is the deep, well-marked,
semicircular styloid depression, above described.
A question may arise after perusing the preceding evidence, upon which
the present fossil is referred to a great Edentate species nearly allied
to the _Orycteropus_, whether one or other of the lower jaws,
subsequently to be described, and in like manner referable, from their
dentition, either to the _Orycteropodoid_ or _Dasypodoid_ families of
Edentata, may not have belonged to the same species as does the present
mutilated cranium. I can only answer, that those jaws were discovered by
Mr. Darwin in a different and very remote locality,—that no fragments or
teeth referable to them were found associated with the present fossil;
and that, as it would be, therefore, impossible to determine from the
evidence we have now before us, which of the two lower jaws should be
associated with _Glossotherium_; and as both may with equal if not
greater probability belong to a totally distinct genus, it appears to me
to be preferable, both in regard to the advancement of our knowledge of
these most interesting Edentata of an ancient world, as well as for the
convenience of their description, to assign to them, for the present,
distinct generic appellations.
The figures in Plate XVI. preclude the necessity of a table of
admeasurements of the cranial fragment of _Glossotherium_.
DESCRIPTION OF A MUTILATED LOWER JAW AND TEETH, ON WHICH IS FOUNDED A
SUBGENUS OF MEGATHERIOID EDENTATA, UNDER THE NAME OF
MYLODON.
The genus _Megalonyx_, as is well known, owes its name and the discovery
of the fossil remains on which it was founded, to the celebrated
Jefferson,[31] formerly President of the United States. Cuvier, from an
examination of a single tooth, and the casts of certain bones of the
extremities, especially the terminal ones, determined the ordinal
affinities of this remarkable extinct quadruped.[32] But while he
retained, the name of _Megalonyx_, and used it in a generic sense,
Cuvier offered no characters whereby other fossil remains might be
generically either distinguished from, or identified with the _Megalonyx
Jeffersonii_, unless, among such remains there happened to be a tooth,
or a claw exactly corresponding with the descriptions and figures in the
_Ossemens Fossiles_; and when, of course, a specific identity, and not
merely a generic relationship would be established.
The greater part of Cuvier’s chapter on _Megalonyx_ is devoted to the
beautiful and justly celebrated reasoning on the ungueal phalanx,
whereby it is proved to belong, not to a gigantic Carnivore of the
Lion-kind, as Jefferson supposed, but to the less formidable order of
Edentate quadrupeds; and Cuvier, in reference to the tooth,—the part on
which alone a generic character could have been founded,—merely observes
that it resembles at least as much the teeth of one of the great
Armadillos, as it does those of the Sloths.[33]
In the last edition of the _Rêgne Animal_, Cuvier introduces the
_Megatherium_ and _Megalonyx_, between the Sloths and Armadillos; but
alludes to no other difference between the two genera than that of
size,—“l’autre, le _Megalonyx_, est un peu moindre.” (p. 226.) Some
systematic naturalists, as Desmarest, and Fischer, have, therefore,
suppressed the genus, and made the _Megalonyx_ a species of
_Megatherium_ under the name of _Megatherium Jeffersonii_. The dental
characters of the genus _Megatherium_ are laid down by Fischer[34] as
follows:—“_Dent. prim, et lan. ⁰⁄₀., molar es ⁴⁄₄⁴⁄₄, obducti, tritores,
coronide nunc planâ transversim sulcatâ nunc medio excavatâ marginibus
prominulis._” That _Megalonyx_ had the same number of molares as
_Megatherium_, (supposing that number in the Megathere to be correctly
stated, which it is not,) is here assumed from analogy, for neither
Jefferson, Wistar, nor Cuvier,—the authorities for _Megalonyx_ quoted by
Fischer—possessed other means of knowing the dentition of that animal
than were afforded by the fragment of a single tooth.
Now the almost entire lower jaw about to be described offers, in so far
as respects the general form and structure of the teeth, the same kind
and degree of correspondence with the _Megatherium_, as does the
_Megalonyx Jeffersonii_ of Cuvier: and, what is only probable in that
species, is here certain, viz., an agreement with the Megatherium in the
class, viz. _molares_, to which the teeth exclusively belong. The
question, therefore, on which I find myself, in the outset, called upon
to come to a decision is, as to the preference of the mode of viewing
the subject of the generic relationship of the _Megalonyx_ adopted by
Desmarest, Fischer, &c., or of that, on which Cuvier, and after him Dr.
Harlan, have practically acted: whether, in short, the genus
_Megatherium_ is to rest upon the more comprehensive characters of kind
and general structure of the teeth, or upon the more restricted ones, of
form and such modifications in the disposition and proportions of the
component textures of the tooth, as give rise to the characteristic
appearances of the triturating surface of the crown.
With respect to existing Mammalia, most naturalists of the present day
seem to be unanimous as to the convenience at least of founding a
generic or subgeneric distinction on well-marked modifications in the
form and structure of the teeth, although they may correspond in number
and kind, in proof of which it needs only to peruse the pages of a
_Systema Mammalium_ which relate to the distribution of the Rodent
Order. According to this mode of viewing the logical abstractions under
which species are grouped together, the extinct Edentate Mammal
discovered by Jefferson must be referred to a genus distinct from
_Megatherium_, and for which the term _Megalonyx_ should be retained.
This will be sufficiently evident by comparing the descriptions given by
Cuvier of one of the teeth of the _Megalonyx Jeffersonii_, and by Dr.
Harlan of a tooth of his _Megalonyx laqueatus_, with those of the
_Megatherium_ which have been published by Mr. Clift. The fragment of
the molar tooth of the _Megalonyx Jeffersonii_, described and figured in
the _Ossemens Fossiles_, seems to have been implanted in the jaw, like
the teeth of the _Megatherium_, by a simple hollow base similar in form
and size to the protruded crown: its structure Cuvier describes as
consisting of a central cylinder of bone enveloped in a sheath of
enamel.[35] The transverse section of this tooth presents an irregular
elliptical form, the external contour being gently and uniformly convex,
the internal one, undulating; convex in the middle, and slightly concave
on each side, arising from the tooth being traversed longitudinally on
its inner side by two wide and shallow depressions.
The imperfect tooth of the species called by Dr. Harlan _Megalonyx
laqueatus_, and of which a cast was presented by that able and
industrious naturalist to the Museum of the Royal College of Surgeons,
resembles in general form, and especially in the characteristic double
longitudinal groove on the inner side, the tooth of the _Megalonyx
Jeffersonii_. It is thus described by Dr. Harlan:
“The fractured molar tooth appears to have belonged to the inferior
maxilla on the right side; the crown is destroyed; a part of the cavity
of the root remains. The body is compressed transversely, and presents a
double curvature, which renders its anterior and exterior aspects
slightly convex; the posterior and interior gently concave; these
surfaces are all uniform, with the exception of the interior or mesial
aspect, which presents a longitudinal rib or ridge, one-half the
thickness of the long diameter of the tooth; with a broad, not profound
longitudinal groove or channel along each of its borders. It is from
this resemblance to a portion of a fluted column, that the animal takes
its specific appellation (_Meg^x. laqueatus_).
“The crown would resemble an irregular ellipsis widest at the anterior
portion. The tooth consists of a central pillar of bone surrounded with
enamel, the former of a dead white, the latter of a ferruginous brown
colour: the transverse diameter is more than two-thirds less than its
length, whilst that of _Meg^x. Jeffersonii_ is only one-third less—the
antero-posterior diameter is one-half its length in the former, and
two-thirds less in the latter. The proportions of this tooth are
consequently totally at variance with that of its kindred species.”
[Vide Pl. XII. fig. 7, 8, 9.][36]
Dr. Harlan describes also two claws of the fore-foot, a radius, humerus,
scapula, one rib, an os calcis, a metacarpal bone, certain vertebræ, a
femur, and tibia, of the same _Megalonyx_; these parts of the skeleton,
together with the tooth, which so fortunately served to establish the
generic relationship of the species with the _Megalonyx_ of Jefferson
and Cuvier, were discovered in Big-bone-cave, Tennessee, United States.
Dr. Harlan does not enter into the question of the generic characters of
_Megalonyx_, but it would seem that he felt them to rest not entirely on
dental modifications, for he observes that “a minute examination of the
tooth and knee-joint renders it not improbable, supposing the last named
character to be peculiar to it, that if the whole frame should hereafter
be discovered, it may even claim a generic distinction, in which case,
either Aulaxodon, or PLEURODON, would not be an inappropriate name.”[37]
There can be no doubt, as it appears to me, with respect to a fossil jaw
presenting teeth in the same number, and of the same general structure,
as in the _Megatherium_, and with individual modifications of form, as
well-marked as those which distinguish _Megatherium_ from _Megalonyx_,
that the Palæontologist has no other choice than to refer it, either as
Fischer has done with _Megalonyx_, to a distinct species of the genus
_Megatherium_, or to regard it as the type of a subgenus distinct from
both. With reference, however, to the _Pleurodon_ of Dr. Harlan, after a
detailed comparison of the cast of the tooth on which that genus is
mainly founded, with the descriptions and figures of the tooth of the
_Megalonyx Jeffersonii_, in the “Ossemens Fossiles,” they seem to differ
in so slight a degree as to warrant only a specific distinction, and
this difference even, viewing the various proportions of the teeth in
the same jaw of the _Megatherium_, is more satisfactorily established by
the characters pointed out by Dr. Harlan in the form and proportions of
the radius, than by those in the tooth itself.
The next notice of the _Megalonyx_ which I have consulted, in the hope
of meeting with additional and more precise information as to its real
generic characters, is an account given by the learned Professor
Doellinger,[38] of some fossil bones, collected by the accomplished
travellers Spix and Martius in the cave of Lassa Grande, near the
Arrayal de Torracigos, in Brazil. In this collection, however, it
unfortunately happens that there are no teeth, but only a few bones of
the extremities, including some ungueal phalanges, which Professor
Doellinger concludes, from their shape, the presence of an osseous
sheath for the claw, and the form of their articulation, to belong,
without doubt, to an animal of the Megatherioid kind, about the size of
an Ox. He particularly states that they are not bones of an immature
individual; but that they agree sufficiently with Cuvier’s descriptions
and figures of the _Megalonyx_ to be referred to that species of animal
(zu dieses thierart;) and he adds, what is certainly an interesting
fact, that the fossils in question form the first of the kind that had
been discovered out of North America.
Subsequently to the discovery of these bones, and of those of the
_Megalonyx laqueatus_ above alluded to, the remains of another great
Edentate animal were found in North America, and were deposited in the
Lyceum at New York; among these is a portion of the lower jaw with the
whole dental series of one side. It is thus described by Dr. Harlan.
“The fragment I am now about to describe is a portion of the dexter
lower jaw of the Megalonyx, containing four molar teeth; three of the
crowns of these teeth are perfect, that of the anterior one is
imperfect. These teeth differ considerably from each other in shape, and
increase in size from the front, the fourth and posterior tooth being
double the size of the first, and more compressed laterally; it is also
vertically concave on its external aspect, and vertically convex on its
internal aspect; the interior or mesial surface is strongly fluted, and
it has a deep longitudinal furrow on the dermal aspect, in which respect
it differs from the tooth of the _M. laqueatus_ previously described by
me, of which the dermal aspect is uniform, but to which, in all other
respects, it has a close resemblance. I suppose it therefore probable,
that this last may have belonged to the upper jaw. The three anterior
molars differ in shape and markings: they are vertically grooved, or
fluted, on their interior and posterior aspects, a transverse section
presenting an irregular cube. The length of the crown of the posterior
molar is two inches: the breadth about five-tenths of an inch: the
length of the tooth is three inches and six-tenths. The diameter of the
penultimate molar is eight-tenths by seven-tenths of an inch. The length
of this fragment of the jaw-bone is eight inches and four-tenths; the
height three inches and six-tenths: the length of the space occupied by
the alveolar sockets five inches and eight-tenths. The crown of the
tooth presents no protuberances, but resembles that of the Sloth; the
roots are hollow.”[39]
This fossil is referred by Dr. Harlan to his _Megalonyx laqueatus_; but,
pending the absence of other proof of the identity of species, in which,
as may be seen by comparing fig. 2, with fig. 4, in Pl. XVII., the teeth
differ widely in form, it would be obviously hazardous to adopt such an
approximation on hypothetical grounds.[40] In order, however, to obtain
more satisfactory evidence of the nature and amount of the difference
between the _Megalonyx laqueatus_, and the allied animal represented by
the above-described fragment of lower jaw, I wrote to my much respected
friend M. LAURILLARD, requesting him to send me a sketch of the teeth in
the cast of that lower jaw, which had been transmitted from New York to
the Garden of Plants. With full confidence in the characteristic
precision and accuracy of the drawing with which I have been obligingly
favoured by M. Laurillard, I am disposed to regard the amount of
difference recognizable in every tooth in the lower jaw in question
(fig, 3 and 4,) as compared with the molar tooth either of _Megalonyx
Jeffersonii_ (fig. 1,) or _Meg^x. laqueatus_ (fig. 2) to be such as to
justify its generic separation from _Megalonyx_ on the same grounds as
_Megalonyx_ is distinguished from _Megatherium_, and for the subgenus of
Megatherioid Edentata, thus indicated, I would propose the name of
MYLODON.[41] The species of which the fossil remains are described by
Dr. Harlan may be dedicated to that indefatigable Naturalist who has
contributed to natural science so much valuable information respecting
the Zoology, both recent and fossil, of the North American continent.
The fossil about to be described represents a second and smaller species
of the same genus, and I propose to call it _Mylodon Darwinii_, in
honour of its discoverer, of whose researches in the Southern division
of the New World it forms one of many new and interesting fruits.
This fossil was discovered in a bed of partly consolidated gravel at the
base of the cliff called Punta Alta, at Bahia Blanca in Northern
Patagonia: it consists of the lower jaw with the series of teeth entire
on both sides: but the extremity of the symphysis, the coronoid and
condyloid processes, and the angular process of the left ramus, are
wanting. The teeth are composed, as in _Bradypus_, _Megatherium_ and
_Megalonyx_, of a central pillar of coarse ivory, immediately invested
with a thin layer of fine and dense ivory, and the whole surrounded by a
thick coating of cement.
In the fig. 5, Pl. XVII., the fine ivory is represented by the white
striated concentric tract on the grinding surface of the teeth; it is of
a yellowish-white colour in the fossil, and stands out, as an obtuse
ridge, from that surface: both these conditions depend on the large
proportion of the mineral to the animal constituent in this substance of
the tooth. The external layer of the cement presents in the fossil the
same yellowish-brown tint as the bone itself, which it so closely
resembles, both in intimate structure and in chemical composition; the
internal layer next the dense ivory is jet black, indicating the great
proportion of animal matter originally present in this part. The central
pillar of coarse ivory, which, from its more yielding texture, has been
worn down into a hollow at the triturating surface of the tooth, also
presents, as a consequence of the less proportion of the hardening
phosphates, a darker brown colour than the external layer of the cement,
or the bone itself.
The teeth are implanted in very deep sockets; about one-sixth only of
the last molar projects above the alveolus; the proportion of the
exposed part of the tooth increases as they are placed further forwards.
The implanted part of each tooth is simple; preserving the same size and
form as the projecting crown, and presenting a large conical cavity at
the base, indicative of the original persistent pulp, and perpetual
growth of these teeth.
The extent of the whole four alveoli is four inches, eight lines; the
length of the jaw from the angle to the broken end of the symphysis is
seventeen inches and a half;[42] from the figures it will be seen that
only a small proportion of the anterior part of the jaw is lost, so that
we may regard the dentigerous part of the jaw as being limited to about
one-fourth of its entire length; the alveoli being nearly equidistant
from the two extremities. The first and second teeth, counting
backwards, are separated by an interspace of rather more than three
lines; that between the second and third is one line less; the third and
fourth are rather more than a line apart: from the oblique position,
however, of the three hinder teeth the intervals between them appear in
a side view, as in fig. 1, Pl. XIX., to be less than in reality, and the
third and fourth teeth seem to touch each other.
Each tooth has a form and size peculiar to itself, and different from
the rest, but corresponds of course with its fellow on the opposite
side. The same may be observed, but in a less degree, in the teeth of
the Megatherium itself; hence, it is obviously hazardous to found a
generic distinction upon a single tooth, unless, as in the case of the
_Glyptodon_,[43] the modification of form happens to be extremely
well-marked. The whole series of teeth, or their sockets, at least of
one of the jaws, should be known for the purpose of making a
satisfactory comparison with the previously established Edentate genera.
The first molar in the present jaw is the smallest and simplest of the
series: its transverse section is ellipsoid, or subovate, narrowest in
front, and somewhat more convex on the outer than on the inner side: the
long diameter of the ellipse is nine lines, the short or transverse
diameter six lines: the length of the tooth may be about three inches,
but I have not deemed it necessary to fracture the alveolus in order to
ascertain precisely this point.
The second tooth presents in transverse section a more irregular and
wider oval figure than the first: the line of the outer side is convex,
but that of the inner side slightly concave, in consequence of the tooth
being traversed longitudinally by a broad and shallow channel or
impression; the longitudinal diameter of the transverse section is one
inch; the transverse diameter at the widest part nine lines. There is a
slight difference in the size of this tooth on the two sides of the jaw,
the right one, from which the above dimensions are taken, being the
largest.
The transverse section of the third tooth has a trapezoidal or
rhomboidal form; the angles are rounded off; the posterior one is most
produced; the anterior and posterior surfaces are flattened, the latter
slightly concave in the middle; the external and internal sides are
concave in the middle, especially the inner side, where the concavity
approaches to the form of an entering notch. The longest diameter of the
transverse section of this tooth is thirteen lines, the shortest seven
lines and a half: in the tooth on the right side the external surface is
nearly flat; this slight difference is not indicated in the figure (Pl.
XVIII.)
The last molar, which is generally the most characteristic in the fossil
_Bruta_, presents in an exaggerated degree the peculiarities of the
preceding tooth; the longitudinal channels on both the outer and inner
surfaces encroach so far upon the substance of the tooth, that the
central coarse ivory substance is as it were squeezed out of the
interspace, and the elevated ridge of the dense ivory describes an
hour-glass figure upon the triturating surface, the connecting isthmus
being but half the breadth of the rest of the tract; the external
cæmentum preserves nearly an equal thickness throughout. Of the two
lobes into which this tooth is divided by the transverse constriction,
the anterior is the largest; their proportions and oblique position are
pretty accurately given in the figure. The longitudinal diameter of the
transverse section of this tooth is one inch, seven lines, its greatest
lateral or transverse diameter is ten lines, its least diameter at the
constricted part is three lines, the length of the entire tooth is four
inches. Judging from the form of the jaw, the length of the other teeth
decreases in a regular ratio to the anterior one. The posterior tooth is
slightly curved, as shown in fig. 2, Pl. XIX., with the concavity
directed towards the outer side of the jaw.
The general form of the horizontal ramus of the jaw, is so well
illustrated in the figures Pl. XVIII. and XIX., that the description may
be brief.
The symphysis is completely anchylosed, about four inches in length, and
extended forward to the extremity of the jaw at a very slight angle with
the inferior border of the ramus: it is of great breadth, smooth and
gently concave internally, and suggests the idea of its adaptation for
the support and gliding movements forwards and backwards of the free
extremity of a long and well-developed tongue.
The exterior surface of the symphysis is characterized by the presence
of two oval mammilloid processes, situated on each side of the middle
line, and about half-way between the anterior and posterior extremes of
the symphysis. A front view of these processes, of the natural size, is
given in fig. 4, Pl. XIX.: a side view of the one on the right side
represented in the reduced figure.
Nearly four inches behind the anterior extremity of the above process is
the large anterior opening of the dental canal: it is five lines in
diameter, situated about one-third of the depth of the ramus of the jaw
from the upper margin. The magnitude of this foramen, which gives
passage to the nerve and artery of the lower lip, indicates that this
part was of large size; and the two symphyseal processes, which probably
were subservient to the attachment of large retractor muscles, denote
the free and extensive motions of such a lip, as we have presumed to
have existed from the size of the foramina destined for the transmission
of its nervous and nutrient organs.
The angle of the jaw is produced backwards, and ends in an obtuse point,
slightly bent upwards; a foramen, one-third less than the anterior one,
leads from near the commencement of the dental canal, to the outer
surface of the jaw, a little below and behind the last molar tooth; this
foramen presents the same size and relative position on both sides of
the jaw. I find no indication of a corresponding foramen, or of
symphyseal processes in the figures or descriptions of the lower jaw of
the Megatherium, nor in the lower jaw of the Sloths, Ant-eaters,
Armadillos, or Manises, which I have had the opportunity of examining
with a view to this comparison.
In the Megatherium the inferior contour of the lower jaw is peculiarly
remarkable, as Cuvier has observed, for the convex prominence or
enlargement which is developed downwards from its middle part. In the
Mylodon the corresponding convexity exists in a very slight degree, not
exceeding that which may be observed at the corresponding part of the
lower jaw of the Ai, or Orycterope. A broad and shallow furrow extends
along the outer side of the jaw, close to the alveolar margin, from the
beginning of the coronoid process to the anterior dental foramen.
The base of the coronoid process begins external and posterior to the
last grinder: the whole of the ascending ramus of the jaw, beneath the
coronoid process is excavated on its inner side by a wide and deep
concavity, bounded below by a well-marked ridge, which extends obliquely
backwards from the posterior part of the alveolus of the last grinder to
the inferior margin of the ascending ramus, which is bent inwards before
it reaches the angle of the jaw.
The large foramen or entry to the dental canal is situated in the
internal concavity of the ascending ramus of the jaw, two inches behind
the last molar, three inches from the lower margin of the ramus, and
nearly five inches from the elevated angle of the jaw: it measures nine
lines in the vertical diameter, and its magnitude indicates the large
size of the vessels which are destined to supply the materials for the
constant renewal of the dental substance,—a substance which from its
texture must be supposed to have been subject to rapid abrasion. About
an inch behind the dental foramen a deep vascular groove, about two
lines in breadth, is continued downwards to the ridge which
circumscribes the internal concavity of this part of the jaw, and
perforates the ridge, which thus arches over the canal: this structure
is present in both rami of the jaw. The mylo-hyoid ridge is distinctly
marked about an inch and a half below the alveolar margin. Other
muscular ridges and irregular eminences are present on the outer side of
the base of the ascending ramus, and near the angle of the jaw; as shown
in fig. 1, Pl. XIX.
From the preceding descriptions it will be seen that the lower jaw of
the _Mylodon_ is very different from that of the _Megatherium_; with
that of the _Megalonyx_ we have at present no means of comparing it.
Among existing Edentata the Mylodon, in the form of the posterior part
and angle of the jaw, holds an intermediate place between the Ai and the
great Armadillo; in the form of the anchylosed symphysis of the lower
jaw it resembles most closely the Unau or two-toed Sloth; but in the
peculiar external configuration of the symphysis resulting from the
mammilloid processes above described, the Mylodon presents a character
which has not hitherto been observed in any other species of _Bruta_,
either recent or fossil.
In conclusion it may be stated, that the teeth and bones here described
offer all the conditions and appearances of those of a full grown
animal; and that they present a marked difference of size as compared
with those of the _Mylodon Harlani_, as will be evident by the following
admeasurements.
ADMEASUREMENTS OF THE LOWER JAW OF MYLODON DARWINII.
Inches. Lines.
Length (as far as complete) 17 6
Extreme width, from the outside of one ramus to that of the other 9 0
Depth of each ramus 4 9
Length of alveolar series 4 8
From first molar to broken end of symphysis 6 0
Breadth of symphysis 3 7
Longitudinal extent of symphysis 4 6
Circumference of narrowest part of each ramus 5 9
DESCRIPTION OF A CONSIDERABLE PART OF THE SKELETON OF A LARGE EDENTATE
MAMMAL, ALLIED TO THE MEGATHERIUM AND ORYCTEROPUS, AND FOR WHICH IS
PROPOSED THE NAME OF
SCELIDOTHERIUM[44] LEPTOCEPHALUM.
Of the large Edentate quadrupeds that once existed in the New World,
sufficient of the osseous remains of the gigantic Megatherium alone has
been transmitted to Europe to give a satisfactory idea of the general
form and proportions of the extinct animal.
Different bones of the Megalonyx, Mylodon, and Glyptodon have been
described, but not sufficient of the remains of any individual of these
subgenera has, hitherto, reached Europe, or been so described as to
enable us to form a comparison between them and the Megatherium, or any
of the existing Edentata, in regard to the general construction and
proportions of the entire skeleton.
This state of our knowledge of the osteology of the singular giants of
the Edentate Order renders the remains of the present animal peculiarly
interesting, since, although the extremities are too imperfect to enable
us to reconstruct the entire skeleton, a sufficient proportion of it has
been preserved in the natural position to give a very satisfactory idea
of its affinities to other Edentata, whose osteology is more completely
known.
The fossil remains here described were discovered by Mr. Darwin in the
same bed of partly consolidated gravel at Punta Alta, Northern
Patagonia, as that in which the lower jaws of the _Toxodon_ and
_Mylodon_ were imbedded. The parts of the skeleton about to be described
were discovered in their natural relative position, as represented at
Pl. XX., indicating, Mr. Darwin observes, that the sublittoral formation
in which they had been originally deposited had been subject to little
disturbance.[45] They include the cranium, nearly entire, with the teeth
and part of the os hyoides; the seven cervical, eight of the dorsal, and
five of the sacral vertebræ, the two scapulæ, left humerus, radius and
ulna, two carpal bones, and an ungueal phalanx; both femora, the
proximal extremities of the left tibia and fibula, and the left
astragalus.
The principal parts of the cranium which are deficient are the anterior
extremities of both the upper and lower jaws, the os frontis, æthmoid
bone, and the whole upper part of the facial division of the skull; but
sufficient remains to show that the general form of the skull resembled
an elongated, slender, subcompressed cone, commencing behind by a
flattened vertical base, slightly expanding to the zygomatic region, and
thence gradually contracting in all its dimensions to the anterior
extremity.
The Cape Ant-eater (_Orycteropus_), of all Edentata, most nearly
resembles the present fossil in the form of its cranium, and next in
this comparison the great Armadillo (_Dasypus gigas_, Cuv.) may be
cited: on the supposition, therefore, that the correspondence with the
above existing Edentals observable in the parts of the fossil cranium
which do exist, was carried out through those which are defective, the
length of the skull of the Scelidothere must have been not less than two
feet. If now the reader will turn to Pl. XX. he will see that this
cranium is singularly small and slender in proportion to the rest of the
skeleton, especially the bulky pelvis and femur, of which bones the
latter has a length of seventeen inches, and a breadth of not less than
nine inches; the astragalus, again, exceeds in bulk that of the largest
Hippopotamus or Rhinoceros; yet the condition of the epiphyseal
extremities of the long bones proves the present fossils to have
belonged to an immature animal. Hence, although the Scelidothere, like
most other Edentals, was of low stature, and, like the Megatherium,
presented a disproportionate development of the hinder parts, it is
probable, that, bulk for bulk, it equalled, when alive, the largest
existing pachyderms, not proboscidian. There is no evidence that it
possessed a tesselated osseous coat of mail.
I shall commence the description of the present skeleton with the
cranium. The condyles of the occiput (See Pl. XXI. fig. 2,) are wide
apart, sub-elliptic, very similar in position, form, and relative size
to those in _Orycteropus_. The foramen occipitale is transversely oval,
its plane slopes from above downwards and forwards at an angle of 40°
with that of the occipital region of the skull. This region, as before
stated, is vertical in position (see fig. 1, Pl. XXI.), of a
sub-semicircular form, the breadth being nearly one-third more than the
height; it is bounded above and laterally by a pretty regular curve; but
the superior margin is not produced so far backwards as in
_Orycteropus_. The occipital plane is bisected by a mesial vertical
ridge; there is a less developed transverse curved intermuscular crest
which runs parallel with and about half an inch below the marginal
ridge: the surface of the occipital plane on the interspaces of these
ridges is irregularly pitted with the impression of the insertion of
powerful muscles. The corresponding surface is smooth in the Orycterope
and Armadillos; in the great extinct Glossothere it resembles in
character that of the Scelidothere; but in the forward slope of the
occipital plane the Glossothere differs in a marked degree from the
present animal.
The upper surface of the cranium is smooth and regularly convex. The
extent of the origin of the temporal muscles is defined by a
slightly-raised broad commencement of a ridge, which, in the older
animal, might become more developed. There is no trace of this ridge in
the Orycterope; but it exists in the Armadillos, in which the teeth are
of a denser texture, and better organized for mastication, and
consequently are associated with better developed masticatory muscles.
It will be subsequently shown that the Scelidothere resembles the
Armadillos in so far as it possesses a greater proportion of the dense
ivory to the external cæmentum in its teeth, than does the Megatherium;
while it differs widely from the Orycterope, in the structure of its
teeth. The teeth, however, are fewer in the Scelidothere than in any
Armadillo, and relatively smaller than in most of the species of that
family. Accordingly we find that the zygomatic arches are relatively
weaker; and in this particular the _Scelidothere_ corresponds with the
Orycterope. The zygomatic process of the temporal commences posteriorly
about an inch and a half from the occipital plane, its origin or base is
extended forwards in a horizontal line fully four inches, where it
terminates as usual in a thin concave edge, as shown on the right side
in Pl. XXII. The free portion of the zygoma, continued forwards from the
outer part of this edge, is a slender sub compressed process, half an
inch in the longest or vertical diameter, and less than three lines in
the transverse; the extremity of this process is broken off; the
opposite extremity of the malar portion of the zygoma is entire, and
obtusely rounded. The bony arch may have been completed by the extension
of the temporal process to the malar one, but the two parts undoubtedly
were not connected together by so extensive a surface as in the
Orycterope. On the other hand, if the zygomatic arch be naturally
incomplete in the Scelidothere, the interspace between the malar and
temporal portions must be relatively much less than in the Sloth or
Ant-eater; for the broken end of the temporal part is separated from the
obtusely rounded apex of the malar process in the present specimen by an
interval of only one inch.
The articular surface (Pl. XXIII., fig. 2) beneath the zygoma for the
lower jaw is flat and even, with the outer and inner margin slightly
bent down, but having no definable anterior or posterior limits; its
breadth is two inches. It differs from the corresponding surface in the
Orycterope in being separated by a relatively wider interval from the
tympanic bone, and in wanting consequently the support which the bony
meatus auditorius gives in the Orycterope to the back part of the
mandibular joint. The Armadillos differ still more from the Scelidothere
in this important part of the cranial organization, inasmuch as the
glenoid cavity is not only protected behind by the descending os
tympanicum, but also in front by a corresponding vertical downward
extension of the os malæ. The Scelidothere in the general form and
relative position of the surface for the articulation of the lower jaw
resembles the Glossothere more closely than any other Edentate animal
with which I have been able to compare it.
The malar bone of the Megatherium presents, as is well known, two
characters, in which it conspicuously differs from that of the
Orycterope and Armadillos, and approximates in an equally marked degree
to the Sloths; these characters consist in a process ascending as if to
complete the posterior circumference of the orbit, and another process
descending outside the lower jaw to give advantageous and augmented
surface of attachment to the masseteric muscle, in its character of a
protractor of the jaw. Now both these modifications of the malar bone
are present in the Scelidothere, and are the chief if not the sole marks
of the affinity to the Megatherium which the structure of the cranium
affords. They are, however, the more interesting, perhaps, on that
account, and because they are associated with other and more numerous
characters approximating the species in question to the ordinary
terrestrial as distinguished from the arboreal Edentata. For if the
Scelidothere, instead of the Megathere, had been discovered half a
century ago, and if its true nature and affinities had been in like
manner elucidated by the genius and science of a Cuvier; and supposing
on the other hand that the Megatherium instead of the Scelidothere had
been one of the novel and interesting fruits of Mr. Darwin’s recent
exploration of the coast of South America, then the affinities of the
Megathere with the Sloths would undoubtedly have been viewed from a
truer point than at the time when,—the Scelidothere, and analogous
transitional forms, being unknown,—it was regarded as a gigantic Sloth.
Having indicated the principal characters of the cranium of the
Scelidothere, which determine its affinities amongst the _Edentata_,
there next remains to be considered the relative position, extent, and
connections, of the different bones composing the cranium.
The occipital bone constitutes the whole of the posterior, the usual
proportion of the inferior, and a small part of the upper and lateral
portions of the cranial cavity: there is a small descending ex-occipital
process immediately exterior to the condyle: above this part the
occipital bone is articulated to the mastoid process of the temporal,
and the supra-occipital plate is joined by a complex dentated lambdoidal
suture to the two parietals, without the intervention of interparietal
or Wormian bones; the course and form of the lambdoidal suture is shown
in Pl. XXII.; it has the same relative position as in the Orycterope; in
the Armadillos, the suture runs along the angle between the posterior
and superior surfaces of the skull. The thickness of the occipital bone,
at this angle, in the Scelidothere, exceeds an inch, and its texture
consists of a close massive diploë, between the dense outer and inner
tables, (Pl. XXIII. fig. 1.)
The squamous portion of the temporal bone has a very slight elevation,
not extending upon the side of the cranium more than half an inch above
the zygoma; it is thus relatively lower than in the _Orycteropus_; but
is similarly bounded above by an almost straight line, (Pl. XXI., fig.
1.). The mastoid process is small, compressed, with a rounded contour;
immediately internal to it is a very deep depression, corresponding to
that for the digastric muscle. But the most interesting features in this
region of the temporal bone consist in the free condition of the
tympanic bones, and the presence of a semicircular pit, immediately
behind the tympanic bone for the articulation of the styloid element of
the hyoid or tongue-bone: in these points we trace a most remarkable
correspondence with the Glossothere, and in the separate tympanic bone
the same affinity to the Orycteropus, as has been already noticed in the
more bulky extinct Edental.
This correspondence naturally leads to a speculation as to the probable
generic relationship between the Glossothere and Scelidothere: now it
may first be remarked that the styloid articular depression is
relatively much larger and much deeper in the Glossothere than in the
Scelidothere; in the former its diameter equals, as we have seen, one
inch; in the Scelidothere it measures only a third of an inch, the whole
cranium being about two-fifths smaller; if we turn next to the anterior
condyloid foramina, which in the Scelidothere are double on each side,
we obtain from them evidence that the muscular nerve of the tongue could
only have been one-third the size of that of the Glossothere. These
proofs of the superior relative development of the tongue in the
Glossothere indicate a difference of habits, and a modification,
probably, of the structure of the locomotive extremities; and when we
associate these deviations from the Scelidothere, with the known
difference in the position of the occipital plane, which in the
Glossothere corresponds with that in the _Myrmecophaga_ and _Bradypus_,
we shall be justified in continuing to regard them, until evidence to
the contrary be obtained, as belonging to distinct genera.
The parietal bones present an oblong regular quadrate figure, the
sagittal suture running parallel with the squamous, and the frontal with
the lambdoidal suture; there is scarcely any trace of denticulations in
the sagittal suture; the bones are of remarkable thickness, varying, at
this suture, from six to nine lines, and their opposed surfaces are
locked together by narrow ridges, which slightly radiate from the lower
to the upper part of the uniting surface: the substance of the bone
consists of an uniform and pretty dense diploë; and there are no sinuses
developed in it. We can hardly regard the extraordinary air-cells which
occupy the interspace of the two tables of the skull in the parietal and
occipital bones of the Glossothere (Pl. XVI., fig. 3) as a difference
depending merely on age.
The frontal and æthmoid bones are broken away in the present cranium.
The sphenoid commences two inches in front of the foramen occipitale;
the fractured state of the skull does not allow its anterior or lateral
limits to be accurately defined; its body is occupied with large
air-sinuses; the only part, indeed, of this bone which is exposed to
observation is that which forms part of the floor of the cranium; and
this we shall now proceed to describe, in connexion with the other
peculiarities of the cranial cavity, (fig. 1. Pl. XXIII.) The body of
the sphenoid is impressed on its cranial surface with a broad and
shallow sella turcica (_a_), bounded by two grooves, (_b_ _b_,) leading
forwards and inwards from the carotid foramina (_c_); the line of suture
between the sphenoid and occipital bones runs along a slight transverse
elevation (_d_), which bounds the sella posteriorly; this suture is
partially obliterated: a slight median protuberance (_e_) bounds the
sella turcica anteriorly; there are neither anterior nor posterior
clinoid processes. External to the carotid channel there is a wide
groove (_f_) leading to the foramen ovale (_g_); this foramen is about
one-third smaller than in the Glossothere, and therefore, as compared
with the anterior condyloid foramina, indicates that the tongue was
endowed with a greater proportion of sensitive than motive power in the
Scelidothere: but in reasoning on the size of this nerve, it must be
remembered that in both animals certain branches, both of the second and
third divisions of the fifth pair of nerves, are to be associated with
the persistence of large dental pulps, of which they regulate the
secreting power. Anterior to the foramen ovale, and at the termination
of the same large common groove, lodging the trunk of the fifth pair of
nerves is the foramen rotundum (_h_); this leads to a very long canal,
the diameter of which is five lines, being somewhat less than that for
the third division of the fifth pair. The anterior sphenoid is broken
away, so that no observation can be made on the optic foramina.
The basilar process of the occipital bone is perforated at its middle by
two small foramina (_i_) on the same transverse line, about half an inch
apart.
In the Armadillo these foramina do not exist: in the Orycterope they are
present, but open beneath an overhanging ridge, which is continued from
them to the upper part of the anterior condyloid foramen on each side.
The sella turcica of the Orycterope is deeper and narrower than in the
Scelidothere; and is separated from the basilar occipital process by a
transverse ridge, which sends forward two short clinoid processes; two
smaller anterior clinoid processes project backwards from the angle of
the anterior boundary of the sella turcica. The foramina ovalia and
rotunda open in the same continuous groove, as in the Glossothere and
Scelidothere, but they are relatively wider apart; and the canal for the
third division of the fifth pair is shorter, and runs more directly
outwards.
The petrous bone in the Scelidothere is relatively larger than in the
Glossothere, but this probably arises from the precocious development of
the organ of hearing in the present immature specimen in obedience to
the general law. The trunk of the fifth pair of nerves does not impress
it with so deep and well defined a groove as in the Glossothere; the
elliptic internal auditory foramen (_k_) is situated about the middle of
the posterior surface; behind this is the aqueductus vestibuli; and
immediately posterior to the petrous bone is the foramen jugulare (_l_):
the shape of the os petrosum agrees more with that of the Armadillo than
with that of the Orycterope. An accidental fracture of the right os
petrosum demonstrates its usual dense and brittle texture, and at the
same time has exposed the cochlea with part of its delicate and
beautiful lamina spiralis. The conservation of parts of the organs of
vision in certain fossils, has given rise to arguments which prove that
the laws of light were the same at remote epochs of the earth’s history
as now; and the structures I have just mentioned, in like manner,
demonstrate that the laws of acoustics have not changed, and that the
extinct giants of a former race of quadrupeds were endowed with the same
exquisite mechanism for appreciating the vibrations of sound as their
existing congeners enjoy at the present day.
The brain, being regulated in its development by laws analogous to those
which govern the early perfection of the organ of hearing, appears to
have been relatively larger in the Scelidothere than in the Glossothere:
it was certainly relatively longer; the fractured cranium gives us six
inches of the antero-posterior diameter of the brain, but the analogy of
the Orycterope would lead to the inference that it extended further into
the part which is broken away. The greatest transverse diameter of the
cranial cavity is four inches eight lines: these dimensions, however,
are sufficient to show that the brain was of very small relative size in
the Scelidothere; and, both in this respect, and in the relative
position of its principal masses, the brain of the extinct Edental
closely accords with the general character of this organ in the existing
species of the same Order. We perceive by the obtuse ridge continued
obliquely upwards from above the upper edge of the petrous bone, that
the cerebellum has been situated wholly behind the cerebrum, we learn
also from the same structure of the enduring parts that these perishable
masses were not divided, as in the Manis, by a bony septum, but by a
membranous tentorium, as in the Glossothere and Armadillos: in the
Orycteropus, as has been before remarked, there is a strong, sharp, bony
ridge extending into each side of the tentorium. The vertical diameter
of the cerebellum and medulla oblongata equals that of the cerebrum, and
is two inches three lines: the transverse diameter of the cerebellum was
about three inches nine lines; its antero-posterior extent about one
inch and a half. The sculpturing of the internal surface of the cranial
cavity bespeaks the high vascularity of the soft parts which it
contained, and there are evident indications that the upper and lateral
surfaces of the brain had been disposed in a few simple parallel
longitudinal convolutions. The two anterior condyloid foramina (_m_)
have the same relative position as the single corresponding foramen in
the Glossothere, Orycterope, and Armadillos, and the inner surface of
the skull slopes outwards from these foramina to the inner margin of the
occipital condyle.
Of the bones of the face there remain only portions of the malar,
lachrymal, palatine, and maxillaries. The chief peculiarities of the
malar bone have been already noticed: the breadth of the base of the
descending masseteric processes is two inches two lines; its termination
is broken off: the length of the ascending post-orbital process of the
malar cannot be determined from the same cause, but it is fortunate that
sufficient of this part of the cranium should have been preserved to
give this evidence of the affinities of the Scelidothere to the
Megathere. The malar bone is continued anteriorly, in a regular curve
forwards and upwards, to the lachrymal bone, and completes, with it, the
anterior boundary of the orbit: the size of the orbit is relatively
smaller than in the Orycterope, and still less than in the Ant-eaters:
here, however, we have merely an exemplification of the general law
which regulates the relative size of the eye to the body in the
mammalia. The malar bone does not extend so far forwards in front of the
orbit as in either the Orycterope or Armadillo; in the inclination,
however, with which the sides of the face converge forwards from the
orbits, the Scelidothere holds an intermediate place between the
Armadillos and Orycterope.
The lachrymal bone does not extend so far upon the face in the
Scelidothere as in the Orycterope; in which respect the Scelidothere
resembles more the Megathere. The foramen for the exit of the
infra-orbital nerve has the same situation near the orbit as in the
Megathere; its absolute distance from the anterior border of the orbit
is only half that in the Orycterope. The foramen is single in the
Scelidothere, as in the Orycterope; in the Megathere there are two or
three ant-orbital foramina. The vertical diameter of this foramen is
eight lines, the transverse diameter four lines. So much of the outer
surface of the superior maxillary bones as has been preserved, is smooth
and vertical. Each superior maxillary bone contains the sockets of five
teeth, occupying an antero-posterior extent of three inches seven lines,
(Pl. XXII. and XXIII. fig. 3). The posterior alveolus is situated just
behind the transverse line, extending across the anterior boundary of
the orbits; the remaining sockets of the molar series extend forwards
three inches in front of the orbits. In the Megatherium, the roots of
the five superior molars are all situated behind the anterior boundary
of the orbit: in the Orycteropus, on the contrary, the grinders are all
placed in advance of the orbit; so that the Scelidothere resembles that
species more than the Megathere in the relative location of the teeth.
The palatal interspace between the roots of the last molar tooth of each
series is eleven lines; the palate gradually though slightly widens, as
it advances forwards: the posterior margin of the palate is terminated
by an acute-angled notch. In the breadth of the bony palate the
Scelidothere is intermediate between the Megathere and Orycterope.
The anterior of the upper molars is represented at fig. 3, 4, and 5, Pl.
XXI., and at 1, fig. 3, Pl. XXIII.; it corresponds closely in form and
size with the opposite molar below; the base of the triangle given by
its transverse section is turned inwards and obliquely forwards.
The second molar of the upper jaw, also presents in transverse section a
triangular form, with the angles rounded off; but the inner side of the
tooth is traversed by a longitudinal groove. The largest diameter of the
transverse section, which is placed obliquely as regards the axis of the
skull, measures ten lines and a half; the opposite diameter of the tooth
is six lines.
The third and fourth molars present the same form and size, and relative
position as the second.
The fifth molar is the smallest of the series; its transverse section
gives an inequilateral triangle, with the corners rounded off; the
broadest side is turned outwards, and is slightly concave; the
antero-posterior diameter of this tooth is seven lines; the transverse
four lines. The length of the teeth in the upper jaw is about two inches
and a half.
It is almost superfluous to observe that the teeth of the Scelidothere,
as in other _Bruta_, are without fangs, and have their inserted base
excavated by large conical cavities, for the lodgment of a persistent
pulp. The tooth is composed of a small central body of coarse ivory or
‘dentine,’ traversed by medullary canals, which at the periphery of the
coarse dentine anastomose by loops, from the convexity of which the
calcigerous tubes are given off which form the fine dentine: the layer
of this substance, which immediately surrounds the coarse dentine, is
about one line and a half in thickness, and the whole is invested with a
very thin coating of cement. The teeth of the Scelidothere thus present
a more resisting structure than do those of the Mylodon; having a larger
proportion of the dense ivory composed of the minute calcigerous tubes,
and a much smaller proportion of the softer external cæmentum; in this
respect the Scelidothere recedes farther from Megathere, and approaches
nearer the Armadillos than does the Mylodon.
The lower jaw resembles, in the general form of the posterior moiety
which is here preserved, that of the Sloth and Mylodon more than that of
any other Edentate species. Its deep posterior angle is produced
backwards, and a broad coronoid process rises and nearly fills the
zygomatic space; the condyle is flat, as the glenoid surface has already
indicated; its transverse diameter is an inch and eight lines; its
antero-posterior diameter seven lines: it is principally extended
inwards beyond the vertical line of the ascending ramus. The lower
contour of the jaw describes an undulating line; which, commencing from
the posterior angle, is at first gently convex, then slightly concave,
then again convex, below the alveoli of the teeth, where it is rounded
and expanded, as in the Orycterope. The fractured condition of the right
ramus of this part fortunately exposed the roots of the four grinding
teeth, which constitute the dental series on each side of the lower jaw.
The length of the jaw occupied by these four alveoli is three inches ten
lines, which exceeds a little that of the opposed five grinders above;
the ramus of the jaw gradually diminishes in all its dimensions anterior
to the molar teeth; the dental canal passes in a gentle curve below, and
on the inner side of the alveoli, whence it gradually inclines to the
outer wall of the jaw.
The whole ascending ramus of the jaw consists of a very thin plate of
bone; it is slightly concave on the inner side, and the inferior margin
of the produced angle inclines inwards, as in the Mylodon and Sloth; it
is impressed on the outer side with two shallow depressions, and two
parallel ridges, both following the gentle curvature of the part. There
is a foramen on the outer side of the ramus at the anterior part of the
base of the coronoid process corresponding with that in the lower jaw of
the Mylodon, but the longitudinal channel which runs along the outer
side of the alveolar processes is wanting, and the expansion at the base
of those processes is more sudden and relatively greater; the general
correspondence, however, between these lower jaws is such as would lead
to the idea that they belonged to animals of the same genus, were it not
that the teeth present modifications of form in the Scelidothere, as
distinct from those of the Mylodon, as are any of the minor dental
differences on which genera or subgenera of existing Mammalia are
founded in the present state of Zoological Classification.
To make this distinction more readily intelligible, I have given a view
of the transverse section of the teeth in the right ramus of the lower
jaw (fig. 4, Pl. XXIII.), corresponding with that of the _Mylodon
Darwinii_, (Pl. XVII., fig. 5). In the present subgenus the
antero-posterior extent of the four alveoli of the lower jaw nearly
equals four inches, and is relatively greater than in the Mylodon,
although the teeth are placed closer together; this is owing to their
greater relative size. The first molar tooth presents the simplest form;
its transverse section is a compressed inequilateral triangle with the
angles rounded off; the longest diameter of this section which is
parallel with the inner alveolar border is eleven lines, the transverse
diameter almost six lines; the base or broadest side of the triangle is
turned inwards, and is slightly concave; the two smaller sides are also
slightly concave.
The second molar is placed more obliquely in the jaw; the long axis of
its transverse section intersects at an acute angle that of the jaw
itself; the transverse section presents a compressed or oblong form,
with the larger end next the outer side, and the smaller end next the
inner side of the jaw; this end is simply rounded, but the outer end
presents a sinuosity, corresponding to a broad groove which traverses
the whole length of the outer side of the tooth; the anterior, which
corresponds to the internal side or base of the transverse section of
the preceding molar, is slightly concave.
The third molar has nearly the same form and relative position as the
preceding; the long diameter of the transverse section is, in both, ten
lines and a half; the principal transverse diameter is, in the second
molar five lines, in the third nearly six; the difference of form
observable in these as compared with the two middle grinders of the
Mylodon is well-marked; in the latter these teeth are impressed with a
longitudinal groove on their inner sides; in the Scelidothere they have
a similar impression along their outer but not along the inner side.
In the last molar the resemblance is much closer, and the modification
of form by which it differs from the preceding ones is of the same kind;
the transverse section gives an irregular oblong figure with its axis
nearly parallel with that of the jaw, and constricted at the middle by
sinuosities produced by two wide channels which traverse longitudinally,
one the outer, the other the inner side of the tooth; the latter groove
is much wider and shallower in the Scelidothere than in the Mylodon. The
two lobes produced by these grooves are more equal in Scelidothere; the
anterior one is concave on its anterior surface instead of convex as in
the Mylodon; the posterior one is more compressed; the longitudinal or
antero-posterior diameter of the transverse section of this tooth is one
inch five lines; the greatest transverse diameter is nine lines; the
diameter of the isthmus joining the lobes is three lines and a half; the
entire length of this tooth is three inches three lines.[46]
VERTEBRAL COLUMN.
Of this part of the skeleton of the Scelidothere, Mr. Darwin’s specimen
includes, as is represented in Plate XX., the cervical, part of the
dorsal, and the sacral series of vertebræ in a more or less perfect
condition.
The cervical vertebræ present the ordinary mammalian number, seven, and
are free, or so articulated as to have permitted reciprocal movement
upon each other. Their transverse processes are perforated as usual for
the vertebral arteries. These processes in the atlas are remarkable for
their great breadth, length, and thickness; and indicate the muscular
forces which must have worked the head upon the spine to have been very
powerful. The axis is provided with a robust ‘processus dentatus,’
having a base equal in breadth to the body of the axis itself; and a
smooth articular convexity on the side of the apex on which the ring of
the atlas rotated. The line of union between the axis and its
characteristic process, which here resembles the body of an abortive
vertebra, is very distinct. The transverse processes of the vertebra
dentata are comparatively feeble, but this condition is amply
compensated for by the great development of the spinous process. (Pl.
XXIV. fig. 1.) This process is bent backwards at nearly a right angle,
overlaps with its reflected extremity the spine of the third cervical
vertebra, and rests by its base, on the under part of which are the
posterior articular surfaces, upon the broad and strong anterior oblique
processes of the third vertebra.
The third, fourth, fifth, and sixth cervical vertebræ have moderately
developed and pointed spinous processes: their transverse processes are
broad, and extend obliquely backwards, and slightly overlap each other.
On the under part of the transverse process of the sixth cervical
vertebra there is the fractured base of what I conjecture to have been
an expanded aliform plate, analogous to that observable in the
corresponding vertebra of the Orycterope. The seventh cervical vertebra
has part of the articular depression for the head of the first rib upon
each side of its body: the transverse process is feebly developed, but
the spine is double the height and size of those of the preceding
vertebræ.
The spinous process of the first dorsal vertebra in like manner rises to
twice the height of the preceding spine of the seventh cervical, and
preserves an equal antero-posterior diameter from its base to its
summit, which is thick and slightly bent backwards: four or five
succeeding dorsal vertebræ give evidence of having been surmounted by
spines of equal height and strength. The transverse processes of these
dorsal vertebræ present bold concavities on their inferior part for the
reception of the tubercles of the ribs, and they gradually ascend upon
the base of the spines as the vertebræ are placed further back, so as to
increase the expansiveness of the chest. The state of the fossil did not
afford further information as to the condition of this part of the
vertebral column, but the parts which have been preserved are precisely
those from which the most interesting inferences as to the affinities
and habits of the extinct quadruped can be deduced.
Whether the Megatherium be most nearly allied to the tribes of the Sloth
or Armadillo has been a question under recent discussion, and, as a
corollary of this problem, whether its habits were those of a scansorial
or of a fossorial quadruped. For, strange as it may appear at first
sight, there have not been wanting arguments, and those urged by an
anatomist to whom we owe much novel and interesting information
respecting the extinct Edentata, in support of the belief that the
Megatherium, gigantic and ponderous as must have been its frame,
actually climbed trees like a Sloth, and had claws and feet organised
for prehensile actions, and not in accordance with that type by which
they are usually adapted for digging up the soil.[47]
Now, in whatever degree the Megatherium may be involved in this
question, the smaller Megatherioid species at present under
consideration must be at least equally implicated in it. In the
adaptation of the frame of a mammiferous quadruped for especial and
peculiar actions and modes of life, such as for climbing and living in
trees, or for burrowing and seeking concealment in the earth, not only
the immediate instruments, as the feet, are modified, but the whole of
the osseous and muscular fabric is more or less impressed with
corresponding adaptations, whilst at the same time these special
adjustments are invariably subordinated to the type of organization
which characterizes the group.
The type of the order _Bruta_ or _Edentata_ is well-marked; one or more
claws of unusual length and strength, characterize the fore-feet and
sometimes the hind-feet in every genus, and the term ‘Macronykia’ would
more aptly designate them than the term which Cuvier substituted for the
good old Linnean appellation. The uniform absence of true roots to the
teeth, where these are present, is another general character; the
skeleton exhibits many well-marked peculiarities common to the whole
order; while at the same time it is modified in various modes and
degrees in accordance with the peculiar habits and exigencies of the
species.
One of the regions of the skeleton which manifests adaptive
modifications of this kind in the most remarkable degree is the cervical
division of the vertebral column. In one edentate species it is
lengthened out by two additional vertebræ more than in any other mammal;
in another it is reduced by anchylosis to as great an extent below the
regular number of moveable pieces: and these, the two most opposite
conditions of the cervical vertebræ which are to be met with in the
mammiferous class are related to equally diverse and opposite habits of
life.
With respect to the _Ai_, or three-toed Sloth, “an animal, great part of
whose life, when not engaged in eating, is spent in sleeping on
trees,—an easy attitude for repose is most essential to its comfortable
existence; and accordingly we find, that the auxiliary vertebræ at the
base of the neck contribute to produce that flexibility of this organ
which allows the head of the animal to incline forwards and rest upon
its bosom.” Dr. Buckland, from whose Paper on the “Adaptation of the
Structure of the Sloths to their peculiar Mode of Life,”[48] the
preceding judicious physiological remark is quoted, adduces the
authority of Mr. Burchell in proof that the Sloth can in a remarkable
manner and with great facility twist its head quite round, and look in
the face of a person standing directly behind it, while at the same time
the body and limbs remain unmoved. A single glance at the length and
slenderness of the cervical region of the spine, and of the feeble
condition of the transverse and spinous processes in the vertebræ
composing that part of the skeleton of the Sloth, is enough to show its
adaptation to increase the rotatory motion and flexibility of the neck.
In describing the skeleton of a species of Armadillo (_Dasypus
6–cinctus_, Linn.)[49] I was led in like manner to point out the
subserviency of the peculiarities of the cervical vertebræ to the habits
and mode of life of that animal; observing that the “anchylosis of the
cervical vertebræ obtains in the _Cetacea_, as well as in the genus
_Dasypus_, and that as in the aquatic order this firm connexion of the
cervical vertebræ assists materially in enabling the head to overcome
the resistance of the dense fluid through which they perpetually move,
so in the Armadillos a like advantage may be derived from this structure
during the act of displacing the denser material in which they excavate
their retreats.”[50]
Having in view these well-marked examples of the subserviency of the
structure of the bones of the neck to the habits of existing species of
the order _Bruta_, I proceeded to investigate the structure of the
corresponding part of the skeleton in the _Scelidotherium_, hoping
thereby to gain a new and useful element in the determination of the
problem at present under discussion, as to the affinities and habits of
the extinct Megatherioid quadrupeds.
The fossil, in its original state, yielded a view of so much of the
anterior part of the bodies of the cervical vertebræ as proved that they
were neither so numerous as in the Sloth, nor anchylosed together as in
the Armadillos: after a long and careful chiselling at the hard matrix
in which they were imbedded, the transverse and spinous processes were
exposed to view, as they are represented in Plates XX. and XXIV. The
description of these processes has already been given.
On comparing the cervical vertebræ of the Scelidotherium with those of
the existing _Bruta_, the closest resemblance to them was found in the
skeleton of the Orycterope. Now this quadruped, though not so rapid a
burrower, or so strictly a subterranean species as the Armadillos,
participates, nevertheless, to a certain extent, in their fossorial
habits, and is closely allied to them in general structure: it differs
from them, indeed, mainly in a modification of the dental system, in the
absence of dermal armour, and of anchylosis of the cervical vertebræ.
But the advantages which, as a burrower, it would have derived from the
latter structure, are compensated for by the shortness of the cervical
vertebræ, and by the great development and imbricated or interlocking
co-adaptation of the transverse and anterior spinous processes of the
cervical vertebræ. The analogous quadruped in the South American
Continent—the great ant-eater (_myrmecophaga jubata_) which uses its
powerful compressed fossorial claws for breaking through the hard walls
of the habitations of its insect prey, but which does not excavate a
subterraneous retreat for itself, presents the cervical vertebræ of a
more elongated form, and without that development of the spinous and
transverse processes which tend to fix the neck and increase the size of
the muscles which move the head: and, if we could conceive that its
fore-feet were employed to scratch up vegetable roots, instead of
disinterring termites, there would be no reason to expect any
modification of the cervical vertebræ as a direct consequence of such a
difference in the application of its fossorial extremities: when,
therefore, we find that the cervical vertebræ do actually differ in two
myrmecophagous species, to the extent observable in the Cape and South
American ant-eaters, we arrive legitimately at the conclusion that such
difference relates to fossorial habits of the one species, in which
habits the other does not participate.
Now, therefore, if this conclusion be just in regard to the Orycterope,
it must bear with more force upon the question of the habits of the
Scelidotherium as the mechanism for strengthening the connection of
cervical vertebræ, and for augmenting the surface of attachment of the
muscles which worked the head and neck, is more strongly wrought out in
that extinct species.
The great size and strength of the spinous process of the dentata, and
the mode in which it is interlocked with the spinous and oblique
processes of the third cervical, together with the imbricated
disposition of the transverse processes of this and the succeeding
vertebræ, and the remarkable height of the dorsal spines, all combine to
indicate in a very striking manner, if not to demonstrate, that the
conical head of the present species, which is comparatively small and
slender, and for its own mere support requiring therefore no such
mechanism, was used in aid of the fossorial actions of the extremities.
As the cervical vertebræ of the Megatherium have their processes
comparatively weaker than in the Scelidotherium, and the anterior dorsal
spines are relatively shorter, it may be concluded, that whatever were
the extent or nature of the fossorial labours of the enormous claws with
which it was provided, the head did not co-operate with the digging
implements in their especial task in the same degree as in the
Scelidothere and Orycterope. At the same time there is no modification
of the cervical region of the spine of the Megathere corresponding with
those which we have seen to be subservient to the arboreal habits of the
sloth, a remark which will not be deemed superfluous by those who have
perused the acute observations and arguments adduced by M. Lund in
favour of the scansorial character of the extremities of the Megatherium
and Megalonyx.
The fragments of the dorsal vertebræ and ribs of the Scelidotherium,
which are figured in Plate XX., offer no modifications which need detain
our attention; they closely conform, excepting in the greater relative
height of the anterior dorsal spines, already noticed, with the
Megatherioid type. The sacrum manifests in its vast expanse, the great
development of the posterior transverse processes to join the ischium,
the capacious medullary cavity, and wide nervous foramina, a like
conformity with the Megatherium, and a corresponding harmony with the
disproportionate bulk of the hind legs.
BONES OF THE EXTREMITIES.
The Scapula in its double spine, the osseous arch formed by the
confluence of the acromion with the coracoid process, and the
substitution of a distinct foramen for the suprascapular notch, agrees
with that of the Megatherium: but the span of the acromial arch is
relatively wider, and the surface for the articulation of the clavicle
is better marked. This articular surface, which is distinctly shewn upon
the acromion of both the scapulæ in Pl. XX. is the more interesting, as
being the only evidence of the clavicle of the Scelidothere which we at
present possess; but it is enough to prove that this quadruped enjoyed
all the advantages in the actions of the fore extremity, which arise out
of the additional fixation of the shoulder-joint afforded by the
clavicle—a bone which the extinct Megatherioids are the largest of the
mammiferous class to possess in a completely developed state. The form,
position, and aspect of the glenoid cavity for the humerus closely
correspond with the condition of the same part in the Megatherium. The
limits of the acromial and coronoid portions of the arch were still
defineable in the present skeleton, which indicates the nonage of the
individual in the unanchylosed condition of most of the epiphysiaes.
In regard to the presence of a clavicle in the Megalonyx M. Lund has
deduced certain conclusions, which, if well founded, would be equally
applicable to the present allied species, and to the great Megatherium.
I am induced, therefore, to offer a few physiological observations on
that bone, which appear to me to lead to a more correct interpretation
of its uses and relations in the great mammiferous animals now under
consideration.
When the anterior extremities in mammalia are used simply for the
purpose of progressive motion on dry land, as in the Pachyderms and
Ruminants, or in water, as in the Cetaceans, there is no clavicle; this
bone is introduced between the sternum and acromion, in order to give
firmness and fixity to the shoulder-joint when the fore-leg is to
discharge some other office than that of locomotion. In these cases,
however, the clavicle exists in various degrees of development, and even
its rudiment may be dispensed with in some of the actions which require
a considerable extent of lateral or outward motion, and of freedom of
rotation of the fore-limb. When, therefore, we find the clavicle fully
developed in the skeleton of an extinct mammiferous animal, and so
placed as to give the humeral articulation all the benefit of this
additional mechanism, we may confidently expect that it will afford an
insight into the habits and mode of life of such extinct species. M.
Lund[51] has argued from the clavicle of the Megalonyx, that it climbed
like a Sloth. “Animals,” says Sir C. Bell,[52] “which fly or dig, or
climb, as Bats, Moles, Porcupines, Squirrels, Ant-eaters, Armadillos,
and Sloths, have this bone; for in them, a lateral or outward motion is
required.” But in regard to the present problem, we have to enquire
whether the clavicle manifests any modifications of form, of strength,
or development in relation to the special differences of these several
actions, with which its presence is asserted to be associated?
In mammals which fly, the clavicle is always complete: the rabbit, the
fox, and the badger are instances of burrowing animals in which the
clavicle is absent or rudimental. The presence of a perfect clavicle is
not more constant in climbing quadrupeds. The Ai, for example, has an
incomplete clavicle, which is attached to the acromion process, and
terminates in a point about one-fourth of the distance between the
acromion and the top of the sternum, to which the clavicular style is
attached by a long slender ligament: the advantage, therefore, which a
perfect clavicle affords in the fixation of the shoulder-joint, is lost
to this climber _par excellence_. Again, the Bears, which are the
bulkiest quadrupeds that are gifted with the faculty of climbing, and
this in so perfect a degree that the Sun-bears of the Eastern Tropics
may be termed arboreal animals,—these scansorial quadrupeds are
destitute of even the smallest rudiment of a clavicle, as I have
ascertained by repeated careful dissection.
Since, therefore, a clavicle in any degree of development is not
essential to a climbing quadruped, we must seek for some other relation
and use of that remarkably strong, and perfect bone, as it exists in the
Megathere, Megalonyx, and Scelidothere. The absence of ‘dentes primores’
or of anterior or incisive teeth in these quadrupeds at once sets aside
any idea of its connection with an action of the fore extremities, very
common in the mammals which possess clavicles, viz., that of carrying
the food to the mouth, and holding it there to be gnawed by the teeth.
Flying is of course out of the question, although our surprise would
hardly be less at seeing a beast as bulky as an elephant climbing a
tree, than it would be to witness it moving through the air. If now we
restrict our comparison to the relations of the clavicle in that order
of Mammalia to which the extinct species in question belonged, we shall
see that it is most constant, strongest, and most complete in those
species which make most use of their strong and long claws in displacing
the earth, as the Armadillos and Orycteropus: and, as the clavicle is
incomplete in one climbing Edental, we are naturally led to conclude
that its perfect development in an extinct species must have been
associated with uses and relations analogous to those with which it
coexists in other genera of the same order. Thus it will be seen, that,
in rejecting the conclusion drawn by M. Lund from the presence of a
clavicle, I concur in the opinion expressed by Dr. Buckland[53] that the
Megatherium—and with it the Megalonyx and Scelidotherium—had the
shoulder-joint strengthened by the clavicle, in reference to the office
of the fore-arm, as an instrument to be employed in digging roots out of
the ground. Not, however, that these gigantic quadrupeds fed on roots,
but rather, as the structure of the teeth would show, on the foliage of
the trees uprooted by the agency of this powerful mechanism of the
fore-legs, and of the otherwise unintelligible colossal strength of the
haunches, hind legs, and tail.
The humerus presents a large convex oval head, on each side of which is
a tuberosity for the implantation of the supra- and sub-scapular
muscles: these tuberosities do not rise above the articular convexity,
so as to restrict the movements of the shoulder-joint, as in the Horse
and Ruminants, but exhibit a structure and disposition conformable to
those which characterize the proximal extremity of the humerus in other
mammalia which enjoy rotatory movements of the upper or fore-limb. The
tuberosities are, however, relatively more developed, and give greater
breadth to the proximal end of the humerus in the Scelidothere than in
the Megathere. The distal end of the humerus, although mutilated,
clearly indicates that it had the same characteristic breadth of the
external and internal condyles, as in the Megatherium. In fig. 1. Pl.
XXV. which gives a front view of the left humerus, the broad internal
condyle, with its extremity broken off, is seen projecting to the left
hand; both in this figure and in fig. 2. in which the internal side of
the humerus is turned towards the observer, the wide groove, with its
two osseous boundaries, is shewn, which plainly indicates that the left
condyle was perforated for the direct passage of the artery or median
nerve, or of both, to the fore-arm. The groove for the musculo-spiral
nerve on the outer side of the humerus is over-arched at its upper part
by a strong obtuse process; which is comparatively less developed in the
Megatherium. The trochlear or inferior articular surface of the humerus
presents, as in the Megatherium, two well-marked convexities, with an
intervening concavity: this indication of the rotatory power of the
fore-leg is confirmed by the form of the head of the radius.
In Pl. XXV. fig. 4. a view is given of this articular surface: it
presents the form of a subcircular gentle concavity, which plays upon
the outer convexity of the humeral articular surface: immediately below
the upper concavity the radius presents a lateral smooth convex surface,
which rotates upon a small concavity on the ulna, analogous to the
‘lesser semilunar,’ in human anatomy, in which the mechanism for
rotation, so far as the upper joint of the radius is concerned, is not
more elaborately wrought out than in the present extinct edentate
quadruped. The radius expands as it proceeds to the elbow-joint, where
it attains a breadth indicative of the great power and size of the
unguiculate paw, of which it may be called the stem, and to the
movements of which it served as the pivot.
All the bones of the fore-limb just described—the scapula, the humerus,
and the radius,—indicate by the bold features and projections of the
muscular ridges and tubercles the prodigious force which was
concentrated upon the actions of the fore-paw, and the ulna, in its
broad and high olecranon (of which a side view is given in fig. 2. Pl.
XXV.) gives corresponding evidence. The great semilunar concavity is
traversed by a sub-median smooth ridge, which plays upon the interspace
of the two humeral convexities. The body of the bone is subcompressed,
straight, and diminishes in size as it approaches the carpal joint: the
immediate articulating surfaces are wanting in both the radius and ulna,
the epiphyseal distal extremities having become detached from their
respective diaphyses.
Of the terminal segment of the locomotive extremities, the only evidence
among the remains of the skeleton of the Scelidothere is the ungueal
phalanx figured at Pl. XXVII. 3, 4, and 5; but as it is uncertain
whether it belongs to the fore or hind-foot, it will be described after
the other bones of the extremities have been noticed.
Of these bones the femur is the most remarkable, both for its great
proportional size, and its extreme breadth, as compared with its length
or thickness: but in all these circumstances the affinity of the
Scelidothere with the Megathere is prominently brought into view. There
is no other known quadruped with which the Scelidothere so closely
corresponds in this respect. In proceeding, however, to compare together
the thigh-bones of these two extinct quadrupeds, several differences
present themselves, which are worthy of notice: of these the first is
the presence in the Scelidothere of a depression for a ‘ligamentum
teres’ on the back part of the head of the femur, near its junction with
the neck of the bone: this is shewn in the posterior view of the femur
given in Pl. XX. The head itself forms a pretty regular hemisphere: the
great trochanter does not rise so high as in the Megatherium, but,
relatively, it emulates it in breadth: the small trochanter is
proportionally more developed: the external contour of the shaft of the
femur is straighter in the Scelidothere than in the Megathere, and the
shaft itself is less bowed forwards at that part. The articular condyles
occupy a relatively smaller space upon the distal extremity of the femur
in the Scelidothere, and they differ more strikingly from those of the
Megathere, in being continued one into the other: the rotular surface,
for example, which is shewn in fig. 5. Pl. XXV. is formed by both
condyles, while in the Megatherium it is a continuation exclusively of
the external articular surface.
The patella, which works upon the above-mentioned surface, is a thick
strong ovate bone, with the smaller end downwards: rough and convex
externally, smooth on the internal surface, which is concave in the
vertical and convex in the transverse directions.
Of the bones of the leg only the proximal end of the tibia is preserved;
but this is valuable, as shewing another well-marked difference between
the Scelidothere and Megathere; for whereas in the latter the fibula is
anchylosed with the tibia, this bone, in the Scelidothere, presents a
smooth flat oval articular surface, which is shewn in fig. 2. Pl. XXVII.
below the outer part of the head of the bone; from the size and
appearance of which, I infer, that the fibula would not have become
confluent with the tibia, even in the mature and full-grown animal.
The relative length of the fore and hind extremities cannot be precisely
determined from the present imperfect skeleton of the Scelidothere; but
there is good evidence for believing, that the fore extremity was the
shortest. The humerus is shorter than the femur by one-ninth part of the
latter bone; and the radius, which wants only the distal epiphysis, must
have been shorter than the humerus. Now the relative development of the
fore and hind legs is one of the points to be taken into consideration
in an attempt to determine the habits and nature of an extinct mammal.
In climbing animals the prehensile power is more essential to the hinder
than to the fore parts or extremities. In the leech the principal sucker
is in the tail; and higher organized climbers, in like manner, depend
mainly on their posterior claspers in descending trees, and hold on by
means of them whilst selecting the place for the next application of
those at the fore part of the body, whether their place be supplied by
the beak, as in the Maccaws, or the fore-feet or hands in the Mammalia.
But, although we perceive the hinder limbs to be the last to lose the
advantageous structure of the hand in the Quadrumanous species, and
notwithstanding that the tail is for this purpose sometimes specially
organized to serve as a prehensile instrument, yet we find that the
power of grasping the branches of trees by either legs or tail is never
maintained at the expense of undue bulk and weight of those organs. On
the contrary, as the fore-limbs are the main instruments in the active
exertions of climbing, so they are the strongest as well as the longest
in all the best climbers, and the weight of the body which they have to
drag along is diminished by dwarfish proportions of the hinder limbs, as
in the Orangs and the Sloths.
Can those huge quadrupeds have been destined to climb that had the
pelvis and hinder extremities more ponderous and bulky in proportion to
the fore parts of the body than in any other known existing or extinct
vertebrate animals?
M. Lund argues for the scansorial character of the Megalonyx, because
its anterior extremities are longer than the posterior ones; but if they
somewhat exceed the hind legs in length, how vastly inferior are they in
respect of their breadth and thickness. The prehensile faculty of the
hinder limbs of the best climbers, as the Sloths, Orangs, and Chameleons
is by no means dependent on the superior mass of muscle and bone which
enters into their conformation, but is associated with the very reverse
conditions.
It is impossible to survey the discrepancy of size between the femur and
the humerus of the Scelidothere, as exhibited in Pl. XX., without a
conviction that it relates to other habits than those of climbing trees.
The expanse of the sacrum, the evidence of the muscular masses employed
in working the hind legs and tail, which is afforded by the capacity of
the cavity lodging the part of the spinal marrow from which the nerves
of those muscles were derived, both indicate the actions of the hind
legs and tail to have been more powerful and energetic than would be
required for mere prehension: and the association of hinder extremities
so remarkable for their bulk, with a long and powerful tail, forbids my
yielding assent to the speculation set forth by M. Lund, as to the
prehensile character of the tail of the Megalonyx.
_Astragalus._—In the examination of this characteristic bone I have kept
in view the question of the habits of the Megatherioid quadrupeds in
general, and the especial affinities of the Scelidotherium, in
illustration of which I shall notice at the same time the peculiarities
of the astragalus of the Sloth, Megatherium and Armadillo.[54]
The upper articular surface of the astragalus of the Scelidotherium (Pl.
XXVI. fig. 4.), presents, in its transverse contour, two convex pulleys,
_a_ and _b_, and an intermediate concavity, forming one continuous
articular surface. The external or fibular trochlea (_a_) is strictly
speaking convex only at its posterior part, the upper surface gradually
narrowing to a ridge, as it advances forwards from which, the inner and
outer parts slope away at an angle of 35°.
The tibial[55] convexity (_b_) is more regular and less elevated, it has
only half the antero-posterior extent of the outer pulley; its marginal
contour forms an obtuse angle at the inner side.
In the Megatherium the upper articular surface of the astragalus is also
divided into two trochleæ, of which the one on the fibular side (fig. 3,
_a_), is of much greater relative size and extent than the tibial one
(_b_), and is raised nearly four inches above the level of the latter,
although in the oblique position in which the bone is naturally placed
in the skeleton, the highest part of each convexity is on the same
level. The fibular trochlea differs also from that in the Scelidothere
in being regularly convex in the transverse as well as the
antero-posterior direction. The tibial convexity resembles that in the
Scelidothere, save in its smaller relative size; its internal margin
likewise forms an angular projection below the internal malleolus.
The upper surface of the astragalus of the Mylodon, or Megalonyx(?) (Pl.
XXVIII. fig. 5.),[56] differs from that in the Megatherium in having a
narrower fibular trochlear ridge.
The astragalus of the Ai (_Bradypus tridactylus_) differs widely from
that of either the Megathere, Mylodon(?) or Scelidothere in having a
conical cavity on the upper surface, in place of the fibular convexity,
in which concavity the distal end of the fibula rotates like a pivot.
This mechanism is closely related to the scansorial uses of the inwardly
inflected foot of the Sloth.
If the astragalus of an Armadillo[57] were placed side by side with that
of the Megathere, it would be very difficult to determine the analogous
parts, especially of the upper surface, unless guided by the
intermediate structure presented by the Scelidothere. The upper surface
of this bone, in the Armadillo, is, however, divided into two
transversely convex trochleæ, separated by a much wider transversely
concave surface. The fibular trochlea resembles that of the Scelidothere
in having its upper and outer facets sloping away at an acute angle, but
without meeting at a ridge anteriorly; this surface is not more raised
above the tibial trochlea than in the Scelidothere.
The inner trochlea differs from that of the Scelidothere in having a
greater relative antero-posterior extent, and in forming, in place of an
uniform convex surface, a trochlea similar in structure to that on the
outer side. The extent of rough surface on the upper part of the
astragalus intervening between the articular surface for the bones of
the leg, and that for the scaphoides is extremely small in the Megathere
and Mylodon(?); it is relatively greater in the Scelidothere; it is
still more extensive in the Armadillo; but is the longest in the Sloth.
The anterior extremity of the astragalus which is entirely occupied by
the scaphoid articular surface is very peculiar in the Scelidothere (Pl.
XXVI. fig. 2.): it presents one convex and two concave facets, which,
however, form part of one continuous articular surface: the convex facet
forms the internal part of the surface, and presents a rhomboidal form
with the long axis vertical. The concave facets (_c_ and _d_) are
extended transversely and placed one above the other; they are slightly
concave in the transverse, and nearly flat in the vertical directions.
In the Megatherium (fig. 1.) the scaphoid surface of the astragalus is
divided only into one concave and one convex portion, both continuous
with each other: the concave facet (_c_) corresponds with the upper
concavity in the Scelidothere, but is a pretty uniform subcircular
depression, fourteen lines in depth: the convex facet, _d_, is continued
across the whole breadth of the under part of the scaphoid surface and
corresponds with both the inner convex, and lower concave surfaces of
the scaphoid articulation in the Scelidothere.
In the Mylodon(?) (Pl. XXVIII. fig. 3.), the articular facet,
corresponding with that marked (_c_) in the astragali of the Megathere
and Scelidothere, is simply flattened, instead of being concave; the
rest of the scaphoid surface corresponds with that in the Megatherium.
In the Armadillo the scaphoid articular surface is undivided and wholly
convex: in this part of the astragalus, therefore, we find the
Scelidothere deviating from the Armadillo further than does the
Megathere; while the Mylodon or Megalonyx(?) most resembles the
Armadillo in the configuration of this part of the astragalus.
If we compare the outer surfaces of the astragalus in these quadrupeds,
we shall find, however, that the Scelidothere and Armadillo closely
agree: the outer facet of the fibular trochleæ, above described, is
continued in the Scelidothere (Pl. XXVIII. fig. 2.), upon the fibular
side of the astragalus reaching nearly half-way down the posterior part,
and down nearly the whole of its anterior.
In the Armadillo, it extends over the whole of the anterior part of the
outer side of the astragalus. In both animals the lower boundary of this
articular surface describes a strong sigmoid curve.
In the Megatherium (Pl. XXVIII. fig. 1), the corresponding surface for
the fibular malleolus on the outer side of the astragalus is formed by a
comparatively very small semicircular flattened facet, which by its
roughness indicates that the end of the fibula was attached to it by
ligamentous substance, and that the synovial bag was not continued upon
that surface as in the Scelidothere and Armadillo.
In the Mylodon(?) (Pl. XXVIII. fig. 4), even this rough facet is wanting
and the fibular trochlea is bounded by the angle which divides the upper
from the outer surface of the astragalus.
Turning now our attention to the under surface of the astragalus, we
observe that it presents in the Scelidothere (Pl. XXVI. fig. 6), an
irregular quadrate form, having the outer side occupied by an elongated
subovate articular facet, _e_, for the calcaneum, bounded externally by
a sharp edge, with its long axis and its greatest concavity in the
antero-posterior direction, and slightly convex from side to side: a
second calcaneal articular surface (_f_) is situated at the inner and
anterior angle; it is oblong and nearly flat; is continuous with the
inferior concave facet of the scaphoid articulation, but is divided from
the convex facet by a groove: the two calcaneal articulations are
separated by a deep and rough depression, traversing the under surface
of the astragalus diagonally, and increasing in breadth towards the
posterior and internal angle. The inner side of the astragalus presents
a convex protuberance.
The correspondence between the astragalus of the Scelidothere and
Megathere is best seen at the under surface of the bone: in both the two
calcaneal articulations are separated by the diagonal depression, and
the internal and anterior surface is continuous with the scaphoid
articulation. In the Megathere, however, in consequence of the absence
of the inferior concavity which characterizes the Scelidothere, the
anterior calcaneal facet (_f_) appears as a more direct backward
continuation of the scaphoidal surface; but they are divided by a more
marked angle than is represented in the figure (fig. 5, Pl. XXVI.). The
posterior and outer calcaneal surface in the Megathere (_e_) is broader
in proportion to its length, continued further upwards upon the outward
surface, is consequently more convex in the transverse direction, and is
not bounded externally by so sharp and prominent a ridge as in the
Scelidothere. The protuberance from the inner surface of the astragalus
is more compressed laterally in the Megathere than in the Scelidothere.
The correspondence between the astragali of the Mylodon(?) (Pl. XXVIII.
fig. 6) and Megathere in the conformation of the under surface is so
close, that the few differences which exist will be sufficiently
appreciated by an inspection of the figures.
In the Armadillo the astragalus, in consequence of the greater
production of its anterior part, presents more of an angular than a
quadrate figure; and the scaphoid articular surface, being
proportionally carried forwards, is altogether separated from the
anterior calcaneal surface. The posterior and inner calcaneal surface
resembles that in the Scelidothere, but is less inclined upwards; and is
continuous with the posterior part of the tibial articular surface.
Thus the astragalus in the structure of its two most important
articulations, viz. that which receives the superincumbent weight from
the leg, and that which transmits it to the heel, presents a closer
correspondence in the Scelidothere with that of the Dasypus, than with
that of the Megathere or Mylodon.
The ungueal phalanx of the Scelidothere before alluded to, is
represented of the natural size in Pl. XXVII. The side view, fig. 3.
shows the position of the articular surface on the proximal end, sloping
obliquely towards the under surface, and overtopped by an obtuse
protuberance, calculated to impede any upward retraction of the claw:
the present joint, in fact, illustrates in every particular the argument
by which Cuvier established the true affinities of the allied extinct
genus Megalonyx.[58]
The present phalanx is, however, less compressed, and less incurved than
those of the Megalonyx, which have been hitherto described; but it more
resembles in these proportions one of the smaller, and presumed hinder,
ungueal phalanges of the Megatherium. The upper and lateral parts of the
bone are rounded, and it gradually tapers to the apex, which is broken
off. The osseous sheath for the claw is developed only at the under part
of the bone: it presents the form of a thick flat plate of bone, with
the margin very regularly and obliquely bevelled off, and having a
vertical process of bone attached lengthwise to the middle of its under
surface. This process must have served for the insertion of a very
powerful flexor tendon. The figures of this bone preclude the necessity
of any further verbal description.
M. Lund lays most stress upon the argument founded on the inward
inflection of the sole of the foot in the Megalonyx, and appeals with
greatest confidence to this structure in support of his hypothesis of
the scansorial habits of that extinct Edental.[59]
It is quite true that the Quadrumana derive advantage from this position
of the foot in climbing trees, and that it is carried to excess in the
Sloths, which can only apply the outer edge of the foot to the ground.
But we may ask, was the inversion of the sole of the foot actually
carried to such an extent in the _Megalonyx_? And, admitting its
existence in an inferior degree, is it then conclusive as to the
scansorial habits of that species?
M. Lund expressly states that it is produced by a different structure
and arrangement of the tarsal bones, from that which exists in the
Sloth, but he does not specify the nature of this difference.
If the astragalus, which I have referred with doubt to the _Megalonyx_,
do not actually belong to that genus, it is evidently part of a very
closely allied species. Now this astragalus, as we have seen, resembles
most closely that of the Megatherium; and since we may infer that the
calcaneum, scaphoides, and cuboides had a like correspondence, the
inclination of the sole of the foot inwards must have been very slight,
as I have determined from examination of the structure and co-adaptation
of those bones in the incomplete skeleton of the Megatherium in the
London College of Surgeons. Such an inclination of the foot may be
conceived to have facilitated the bending of the long claws upon the
sole, during the ordinary progressive movements of the animal, but it is
quite insufficient to justify the conclusion, that it related to an
application of the hind-feet for the purposes of climbing.
It is not without interest again to call to mind the deviation of the
structure of the astragalus of the Scelidothere from the Megatherioid to
the Dasypodoid type of structure. For if the Megatherioid type of
structure had really been one suitable to the exigencies of climbing
quadrupeds, it might have been expected to have exhibited the scansorial
modifications more decidedly, as the species diminished in stature; but
as regards the instructive bone of the hind-foot, the modifications of
which we have just been considering, this is by no means the case.
DESCRIPTION OF A MUTILATED LOWER JAW OF THE
MEGALONYX JEFFERSONII.
In the preceding section an astralagus was described, which was regarded
as belonging possibly to the same Edentate species as the jaw figured
and described, p. 69, Pl. XVIII. and XIX., under the name of _Mylodon
Darwinii_; but the same correspondence,—that of relative size,—renders
it equally possible that this astragalus may belong to the species of
_Megalonyx_ to which the lower jaw now under consideration appertains.
There could be no doubt, from its structure, that it was the astragalus
of a gigantic species of the order _Bruta_, and of the _Megatherioid_
family, and more nearly allied to the Megathere than is the
Scelidothere, but sufficiently distinct from both.
The lower jaw, figured in Pl. XXIX., is the only fossil brought home by
Mr. Darwin that could be confidently referred to the genus _Megalonyx_;
but the form of the tooth in place on the right side of the jaw fully
justifies this determination. The jaw itself is deeply and firmly
imbedded in the matrix, so that only the upper or alveolar border is
visible. The coronoid and condyloid processes are broken away, and the
texture of the remaining part of the jaw was too friable, and adhered
too firmly to the surrounding matrix to admit of more of its form being
ascertained than is figured.
There were four molars on each side of this jaw; the large oblique
perforation near the fractured symphysis is the anterior extremity of
the wide dental canal. The forms of the alveoli are best preserved in
the right ramus: the first is the smallest, and seems to have contained
a tooth, of which the transverse section must have been simply
elliptical: the second tooth is likewise laterally compressed, but the
transverse section is ovate, the great end being turned forwards: the
third socket presents a corresponding form, but a larger size: the
fourth socket is too much mutilated to allow of a correct opinion being
formed as to the shape of the tooth which it once contained. The natural
size of the tooth _in situ_, and of the adjoining socket, is given in
Pl. XXIX., fig. 2. The difference of form which the jaw of the Megalonyx
presents, as compared with that of the Mylodon, especially in the
greater recedence of the two horizontal rami from each other, will be
appreciated by comparing Pl. XVIII. with Pl. XXIX.
DESCRIPTION OF A FRAGMENT OF THE SKULL AND OF THE TEETH OF THE
MEGATHERIUM CUVIERI.
Notwithstanding the full, accurate, and elaborate accounts of the
skeleton of the Megatherium given by Bru,[60] Cuvier,[61] Pander and
D’Alton,[62] and Mr. Clift,[63] the fragments of this most gigantic of
quadrupeds brought home by Mr. Darwin, possess much interest, and have
added, what could hardly have been anticipated, important information as
to the dental system, whereby an error in the generic character of the
Megatherium has been corrected.
The fragments here alluded to are portions of the skull of three
full-grown Megatheres: the most perfect part of which affords a view of
the posterior, and of part of the basal surface, which regions of the
cranium have not hitherto been elsewhere figured or described, (Pl.
XXX.)
The plane of the occipital foramen forms with that of the base of the
skull an angle of 140°, the plane of the posterior surface of the skull
forms with the basal plane an angle of 68°. The occipital condyles are
therefore terminal, or form the most posterior parts of the cranium. The
extent of their convex curvature in the antero-posterior direction,
which equals that of a semicircle, indicates that the Megatherium
possessed considerable freedom and extent of motion of the head. The
condyles are not extended in the lateral direction so far as in the
Toxodon; their axis is more oblique than in the Glossotherium, and their
internal surface is more parallel with the axis of the skull, the
foramen magnum not presenting that infundibuliform expansion which is so
characteristic of the Glossotherium. The occipital condyles resemble
most in form and position those of the Scelidotherium; but in the angle
of the occipital plane the Megatherium is intermediate between the
Scelidothere and Glossothere. The ex-occipitals terminate laterally and
inferiorly, each in a short, but strong obtuse process. The posterior
plane of the skull is traversed by a strong arched intermuscular crest,
which forms the upper boundary of a pretty deep fossa, which is divided
by a median vertical ridge, extending downwards to within an inch of the
upper margin of the foramen magnum. A second strong obtuse transversely
arched ridge curves over the first, and forms the upper boundary of the
posterior or occipital region of the skull: the interspace between the
two transverse ridges is very irregular, and indicates the firm
implantation of powerful nuchal muscles or ligaments, (Pl. XXX. fig. 1.)
In the configuration and angle of the occipital plane the Megatherium
indicates the same general correspondence with the Edentate type, which
has been pointed out in the descriptions of the crania of the
Glossothere and Scelidothere: and the resemblance to the Scelidothere is
not less striking in the small proportional size of the cranium in this
quadruped, which surpasses the rest of its class in so great a degree in
the colossal proportions of its hinder parts.
Having detected in the base of the skull of the Scelidothere an
articular semicircular pit for the head of the styloglossal bone,
similar to, but relatively smaller than, that remarkable one in the
skull of the Glossothere, it became a matter of interest to determine
whether this structure, which does not exist in any of the existing
Edentals, should likewise be present in the gigantic type of the
Megatherioid family. The result of a careful removal of the matrix from
the basal region of one of the cranial fragments of the Megatherium was
the detection of this articular cavity, in each temporal bone in the
same relative position as in the Glossothere and Scelidothere. The
styloid articular cavity is relatively smaller, and shallower, than in
the Glossothere, its proportions being much the same as those of the
Scelidothere. The cranial or posterior extremity of the stylo-hyoid bone
in the Scelidotherium is bent upwards at an obtuse angle (Pl. XXI.), and
terminates in an articular ball which rotates in this cavity. The size
of this bone, and its mode of articulation, indicates great power and
muscularity of tongue in the Megatherioids, and calls to mind the
importance of that organ in the Giraffe, which subsists on the same kind
of food as that which I have supposed to have supported the
Megatherioids, although the general organization of these animals and
the mode in which the foliage was brought within reach of the tongue are
as opposite as can well be imagined.
The anterior condyloid foramen presents scarcely one-half the absolute
size of that of the Glossothere, whence we may infer a correspondingly
inferior development of the tongue in the Megathere. The fractured
parietes of the cranial cavity of the Megatherium every where exhibit
evidences of the great extent of the air-cells or sinuses continued from
the nasal cavity: on the basilar aspect of the cranium they extend as
far back as the jugular foramina: the whole of the basi-sphenoid being
thus excavated, and permeable to air, derived from the sphenoid sinuses,
(Pl. XXX. fig. 2.). The vertical diameter of the cranial cavity is four
inches, eight lines; its transverse diameter, which is greatest in the
posterior third part of the cavity, corresponding with the posterior
part of the cerebrum is six inches: from the indications afforded by the
remains of the cranial cavity in Mr. Darwin’s specimens, I conclude that
the brain of the Megatherium was more depressed, and upon the whole,
smaller by nearly one-half than that of the Elephant; but with the
cerebellum relatively larger, and situated more posteriorly with
relation to the cerebral hemispheres: whence it may be concluded that
the Megatherium was a creature of less intelligence, and with the
command of fewer resources, or a less varied instinct than the Elephant.
It has been usual to characterize the Megatherium, in conformity with
the concurrent descriptions of Bru, Cuvier, and D’Alton, by the dental
formula of _molares_ ⁴⁄₄ ⁴⁄₄, i. e. by the presence of four grinding
teeth on each side of the upper, as of the lower jaw. It was the
agreement of the excellent authorities above cited in this statement,
which induced Mr. Clift and myself to regard a single detached tooth,
which formed part of the valuable collection of remains of the
Megatherium deposited in the Hunterian Museum by Sir Woodbine Parish, as
being, from its comparatively small size, the tooth of either a younger
individual or of a smaller species of Megatherium. Upon clearing away
the matrix from the palatal and alveolar surface of one of the cranial
fragments of the Megatherium in Mr. Darwin’s collection, I was gratified
by the detection of the crown of a fifth molar, corresponding in size
and form with the detached tooth, above alluded to: its small size, and
its position have doubtless occasioned its being over-looked in the
cranium of the great skeleton at Madrid.
The anterior molar of the upper jaw presents a nearly semicircular
transverse section, with the angles rounded off; the three succeeding
teeth are four-sided, with the transverse somewhat exceeding the
antero-posterior diameter: they are rather longer and larger than the
first: the last molar is likewise four-sided, but presents a sudden
diminution of diameter, and is relatively broader. The following are the
respective dimensions of the upper maxillary teeth.
First Second Third Fourth Fifth
Molar. Molar. Molar. Molar. Molar.
In. Lines. In. Lines. In. Lines. In. Lines. In. Lines.
Length 8 6 9 4 9 4 8 7 5 2
Transverse 1 9 2 4 2 3 2 0 1 4
diameter
Antero-posterior 1 5 2 0 2 0 1 11 0 10
diameter
Besides the differences in size, the upper molars vary as to their
curvature: this difference is exhibited in the vertical section of these
teeth figured in Pl. XXXI. The convexity of the curve of the first,
second and third molars is directed forwards; the fourth is straight,
its anterior surface only describing a slight convexity in the vertical
direction; the fifth tooth is curved, but in a contrary direction to the
others; and the bases of the five molars thus present a general
convergence towards a point a little way behind the middle of the
series.
The next peculiarity to be noticed in these remarkable teeth is the
great length of the pulp-cavity (_d_), the apex of which is parallel
with the alveolar margin of the jaw: a transverse fissure is continued
from this apex to the middle concavity of the working surface of the
tooth, which is thus divided into two parts. Each of these parts
consists of three distinct substances,—a central part analogous to the
body or bone of the tooth or ‘dentine,’ a peripheral and nearly equally
thick layer of _cæmentum_, and an intermediate thinner stratum of a
denser substance, which is described in Mr. Clift’s memoir on the
Megatherium as ‘enamel,’ and to which substance in the compound teeth of
the Elephant, it is analogous both in its relative situation, and
relative density to the other constituents.
Microscopic examinations of thin and transparent slices of the tooth of
the Megatherium prove, however, that the dense layer separating the
internal substance from the cæmentum is not enamel, but presents the
same structure as the hard ‘dentine’ or ivory of the generality of
Mammalian teeth; and corresponds with the thin cylinder of hard
‘dentine’ in the tooth of the Sloth. No species of the Order _Bruta_ has
true enamel entering into the composition of its teeth; but the
modifications of structure which the teeth present in the different
genera of this order are considerable, and their complexity is not less
than that of the enamelled teeth of the Herbivorous Pachyderms and
Ruminantia, in consequence of the introduction of a dental substance
into their composition corresponding in structure with that of the teeth
of the _Myliobates_, _Psammodus_, and other cartilaginous fishes.
The microscopic investigation of the structure of the teeth of the
Megatherium was undertaken chiefly with the view of comparing this
structure with that of the teeth of the Sloth and Armadillo, and of thus
obtaining an insight into the food, and an additional test of the real
nature of the disputed affinities of the Megatherium. The central part
of the tooth (_c._ Pl. XXXI.) consists of a coarse ivory, like the
corresponding part of the tooth of the Sloth. It is traversed throughout
by medullary canals ¹⁄₁₅₀₀ of an inch in diameter, which are continued
from the pulp-cavity, and proceed, at an angle of 50°, to the plane of
the dense ivory, parallel to each other, with a slightly undulating
course, having regular interspaces, equal to one and a half diameters of
their own areæ, and generally anastomosing in pairs by a loop of which
the convexity is turned towards the origin of the tubes of the fine
dentine, as if each pair so joined consisted of a continuous reflected
canal, (_c._ fig. 1, Pl. XXXII.) The loops are generally formed close to
the fine dentine. In a few situations I have observed one of the
medullary canals continued across the fine dentine, and anastomosing
with the corresponding canals of the cæmentum. The interspaces of the
medullary canals of the coarse dentine are principally occupied by
calcigerous tubes which have an irregular course, anastomose
reticularly, and terminate in very fine cells. The more regular and
parallel calcigerous tubes, which constitute the thin layer of hard
dentine, are given off from the convexity of the terminal loops of the
medullary canals. The course of these tubes (_b._ fig. 1, Pl. XXXII.) is
rather more transversely to the axis of the tooth than the medullary
canals from which they are continued. They run parallel to each other,
but with minute undulations throughout their course, in which they are
separated by interspaces equal to one and a half their own diameter. As
they approach the cæmentum they divide and sub-divide, and grow more
wavy and irregular: their terminal branches take on a bent direction,
and form anastomoses, dilate into small cells, and many are seen to
become continuous with the radiating fibres or tubes of the cells or
corpuscles of the contiguous cæmentum. This substance enters largely
into the constitution of the compound tooth of the Megatherium: it is
characterized, like the cæmentum of the Elephant’s grinder, by the
presence of numerous radiated cells, or purkingian corpuscles, scattered
throughout its substance, but may be distinguished by wide medullary
canals which traverse it in a direction parallel with each other, and
forming a slight angle with the transverse axis of the tooth. These
canals are wider than those of the central coarse dentine, their
diameter being ¹⁄₁₂₀₀th of an inch; they are separated by interspaces
equal to from four to six of their own diameters, divide a few times
dichotomously in their course, and finally anastomose in loops, the
convexity of which is directed towards, and in most cases is in close
contiguity with, the layer of dense dentine.
Fine calcigerous tubes are every where given off at right angles from
the medullary canals of the cæmentum, which form a rich reticulation in
their interspaces, and a direct continuation between the loops of the
medullary canals and the calcigerous tubes of the dense dentine. The
cæmentum differs from the coarse dentine in the larger size and wider
interspaces of its medullary canals, and by the presence of the
bone-corpuscles in their interspaces; but they are brought into organic
communication with each other, not only by means of the tubes of the
dense dentine, but by occasional continuity of the medullary canals
across that substance. The tooth of the Megatherium thus offers an
unequivocal example of a course of nutriment from the dentine to the
cæmentum, and reciprocally. Retzius observes with respect to the human
tooth, that “the fine tubes of the cæmentum enter into immediate
communications with the cells and tubes of the dentine (zahnknochen), so
that this part can obtain from without the requisite humours after the
central pulp has almost ceased to exist.” In the Megatherium, however,
those anastomoses have not to perform a vicarious office, since the pulp
maintains its full size and functional activity during the whole period
of the animal’s existence. It relates to the higher organized condition,
and greater degree of vitality of the entire grinder in that extinct
species.
The conical cavities (_d._ Pl. XXXI.) attest the size and form of the
persistent pulp; the diameter of its base is equal to the part of the
crown of the tooth which is formed by the coarse and fine dentine. From
the gradual thinning off and final disappearance of these substances as
they reach the base of the tooth, I conclude that they were both formed
at the expense of the pulp. The fine tubes and cells must have been
excavated in its peripheral layer for the reception of the hardening
salts of the dense dentine, and the rest converted into the parallel
series of medullary canals with their respective systems of calcigerous
tubes, in a manner closely analogous to the development of the entire
tooth of the Orycteropus. The coarser dentine of the tooth of the
Megatherium differs, in fact, from the entire tooth of the
_Orycteropus_, only in that the parallel medullary canals and their
radiating calcigerous tubes are not separated from the contiguous canals
by a distinct layer of cæmentum, and that the medullary canals
anastomose at their peripheral extremities. The wide spaces, (_e._ Pl.
XXXI.) indicate the thickness of the dental capsule by the ossification
of which the exterior stratum of cement was formed. It was not until I
knew the true structure of the tooth of the Megatherium, that I could
comprehend the mode of its formation. The parallel layers of enamel in
the Elephant’s grinder are formed, as is well known, by membranous
plates passing from the coronal end of the closed capsule towards the
base of the tooth; but a certain extent of enamel can only thus be
formed, and when the crown of the grinder has once protruded, and come
into use, the enamel cannot be added to. The modification of the
structure of the tooth of the Megatherium readily permits the
uninterrupted and continuous formation of the dense substance which is
analogous to the enamel of the Elephant’s grinder.
With respect to the question of the respective affinities of the
Megatherium to the Bradypodoid or Dasypodoid families, the result of
this examination of the teeth speaks strongly for its closer
relationship with the former group: the Megalonyx, Mylodon, and
Scelidotherium, in like manner correspond in the structure of their
teeth with the Sloth, and differ from the Armadillo.
If from a similarity of dental structure we may predicate a similarity
of food, it may reasonably be conjectured that the leaves and soft
succulent sprouts of trees may have been the staple diet of the
Megatherioid quadrupeds, as of the existing Sloths. Their enormous
claws, I conclude, from the fossorial character of the powerful
mechanism by which they were worked, to have been employed, not, as in
the Sloths, to carry the animal to the food, but to bring the food
within the reach of the animal, by uprooting the trees on which it grew.
In the remains of the Megatherium we have evidence of the frame-work of
a quadruped equal to the task of undermining and hauling down the
largest members of a tropical forest. In the latter operation it is
obvious that the immediate application of the anterior extremities to
the trunk of the tree would demand a corresponding fulcrum, to be
effectual, and it is the necessity for an adequate basis of support and
resistance to such an application of the fore extremities which gives
the explanation to the anomalous development of the pelvis, tail, and
hinder extremities in the Megatherioid quadrupeds. No wonder, therefore,
that their type of structure is so peculiar; for where shall we now find
quadrupeds equal, like them, to the habitual task of uprooting trees for
food?
DESCRIPTION OF FRAGMENTS OF BONES, AND OF OSSEOUS TESSELATED DERMAL
COVERING OF LARGE EDENTATA.
It is now determined that there once existed in South America, besides
the Megatherium, the Megalonyx, and the allied genera described in the
preceding pages of the present work, gigantic species of the order
_Bruta_ belonging to the Armadillo family, and defended, like the small
existing representatives of that family, by a tesselated bony dermal
covering. The largest known species of these extinct _Dasypodidæ_ is the
_Glyptodon clavipes_, of which the armour and parts of the skeleton have
been described by MM. Weiss and D’Alton in the Berlin Transactions for
1827 and 1834: and the generic and specific characters and name, with an
account of the dental system, and bones of the extremities, were
recorded in the Geological Proceedings for March 1839. It would seem
that parts of the same, or a nearly allied gigantic species were
described in the same year by M. Lund; under the name of _Hoplophorus_.
Of the valuable and interesting discoveries of this able Naturalist I
regret that I was not aware until the appearance of a notice of them in
the Comptes Rendus for April, 1839.[64] Amongst the fragments of bony
tesselated armour in Mr. Darwin’s collection are a few pieces which were
found by him, associated with remains of Toxodon and Glossotherium near
the Rio Negro in Banda Oriental.[65] These fragments, if we may judge
from their thickness, must have belonged to an animal at least as large
as the _Glyptodon clavipes_; but the pattern differs in the greater
equality of size of the component tesseræ. The thickness of the largest
fragment is one inch and a half, the tesseræ vary in diameter from one
inch to half an inch, and are separated by grooves about two lines in
depth, and two in diameter. The pattern formed by the anastomosis of
these grooves is an irregular net-work; the contour of the tesseræ is
either unevenly subcircular, hexagonal, pentagonal, or even four-sided;
with the sides more or less unequal. In those portions of this armour,
where one of the tesseræ exceeds the contiguous ones in size, the
imagination may readily conceive it to be the centre of a rosette,
around which the smaller ones arrange themselves, but there is no
regular system of rosettes, as in the portions of the dermal armour of
the Glyptodon figured by Weiss, and those brought to England by Sir
Woodbine Parish, in which the central piece is double the size of the
marginal ones.
The portions of the tesselated bony dermal covering of a Dasypodoid
quadruped, figured in Pl. XXXII. figs. 5 and 4, of the natural size,
were discovered folded round the middle and ungueal phalanges, figs. 2
and 3, at Punta Alta, in Bahia Blanca, in an earthy bed interstratified
with the conglomerate containing the remains of the fossil Edentals.
In one of these fragments, measuring six inches long by five broad, the
tesseræ are arranged in rosettes, and so closely correspond in size and
pattern with the bony armour described by M. Lund, as characterizing his
species, _Hoplophorus euphractus_, that I feel no hesitation in
referring them to that animal. One of the pattern rosettes is figured at
fig. 4, together with the thickness of the armour at this part, and the
coarse tubulo-cellular structure of the bone. Another portion of dermal
armour from the same locality, gives the pattern shown in fig. 5, formed
by square or pentagonal tesseræ, arranged in transverse rows; it is
certain that this portion of armour belonged to the same animal as the
preceding piece; and probably that it constituted part of the transverse
dorsal bands of the _Hoplophorus_.
The middle and ungueal phalanx, as well as the portions of armour, are
given of the natural size, in Pl. XXXII. The upper and outer surface of
the phalanx, is shown in fig. 2. It is smooth and flat; joins the inner
surface by a sharp edge, which runs along the upper and inner side of
the bone; and passes by a gradual convexity to the under surface; the
ridge corresponding with the base of the claw, is feebly developed at
the under and lateral parts of the base of the claw. Below the double
trochlear joint for the middle phalanx, there are two articular surfaces
for two large sesamoid bones.
The middle phalanx corresponds in its small antero-posterior diameter
and wedge-shape, with that of the great Glyptodon: but the terminal
phalanx is longer and deeper, in proportion to its breadth.
Among the collection of fossils from Punta Alta, in Bahia Blanca, there
is an interesting fragment of the head of a gigantic animal of the
Edentate order, including the glenoid cavity, and part of the zygomatic
process of the left side. The articular surface for the lower jaw,
exhibits, in its flatness, extent, and the absence of a posterior ridge,
the well-marked characteristics of this part of the Edental structure.
It measures two inches four lines in the transverse, and two inches two
lines in the antero-posterior diameter. The commencement of the
zygomatic process presents a vertical diameter of two inches, and a
transverse diameter of eight lines at the thickest part. It is slightly
concave at its lower border, and convex above. The small portion of the
cranial parietes, which is preserved, exhibits the cellular structure
consequent upon the great extension and development of the nasal
air-sinuses: this condition of the cranial parietes, has already been
noticed in the description of the more perfect skulls of the large
extinct Edentata.
NOTICE OF FRAGMENTS OF MOLAR TEETH OF A
MASTODON.
Of the remains of this gigantic extinct Pachyderm, observed by Mr.
Darwin at Santa Fé, in Entre Rios, and on the banks of the Tercero, the
fragments of the teeth and portions of the skeleton which reached
England, are not sufficient to lead to a determination of the species;
but sufficiently prove it to have been nearly allied, if not identical,
with the _Mastodon angustidens_ of Cuvier, and unquestionably distinct
from the _Mastodon giganteum_ of the United States.
NOTICE OF THE REMAINS OF A SPECIES OF
EQUUS,
_Found associated with the extinct Edentals and Toxodon at Punta Alta,
in Bahia Blanca, and with the Mastodon and Toxodon at Santa Fé, in Entre
Rios._
The first of these remains is a superior molar tooth of the right side;
it was embedded in the quartz shingle, formed of pebbles strongly
cemented together with calcareous matter, which adhered as closely to
the tooth in question, as the corresponding matrix did to the associated
fossil remains. The tooth was as completely fossilized as the remains of
the Mylodon, Megatherium, and Scelidothere; and was so far decomposed,
that in the attempt to detach the adherent matrix, it became partially
resolved into its component curved lamellæ. Every point of comparison
that could be established proved it to differ from the tooth of the
common _Equus Caballus_ only in a slight inferiority of size.
The second evidence of the co-existence of the horse with the extinct
Mammals of the tertiary epoch of South America reposes on a more perfect
tooth, likewise of the upper jaw, from the red argillaceous earth of the
Pampas at Bajada de Santa Fé, in the Province of Entre Rios.[66]
This tooth is figured at Pl. XXXII. fig. 13 and 14, from which the
anatomist can judge of its close correspondence with a middle molar of
the left side of the upper jaw.
This tooth agreed so closely in colour and condition with the remains of
the Mastodon and Toxodon, from the same locality, that I have no doubt
respecting the contemporaneous existence of the individual horse, of
which it once formed part.
This evidence of the former existence of a genus, which, as regards
South America, had become extinct, and has a second time been introduced
into that Continent, is not one of the least interesting fruits of Mr.
Darwin’s palæontological discoveries.
DESCRIPTION OF REMAINS OF RODENTIA, INCLUDING THE JAWS AND TEETH OF AN
EXTINCT SPECIES OF
CTENOMYS.
The fragment of the upper jaw, figured in Pl. XXXII. fig. 6, exhibits
the first and second molar _in situ_, and the socket of the third and
fourth molar, of a Rodent, which by the form and number of the upper
maxillary teeth is referable to the genus Ctenomys. The molars are a
little larger, the longitudinal groove on their external surface is
somewhat deeper, and the last molar is relatively wider than in the
existing subterraneous species,—the Tucutucu (_Ctenomys Brasiliensis_,
Bl.), of whose habits so interesting an account is given in the
description of the Mammalia of the present Collection (No. IV. p. 79).
The form of the grinding surface of the first and second upper molar is
shown below the fig. 6, and three views of the second grinder are given
at figs. 7, 8, and 9. The fragment of the lower jaw of the same fossil
Rodent is figured at fig. 10 and 11. The long anterior incisor is
relatively narrower than in the _Ctenomys Brasiliensis_. I have not had
the means of comparing this fossil with the _Ctenomys Magellanicus_; but
since it is probable that the _Ct. Magellanicus_ may not be specifically
different from the _Ct. Brasiliensis_, it may be concluded that the
present fossil is equally distinct from both.
The portion of the right hind-foot of the Rodent figured at fig. 12,
includes the calcaneum, astragalus, cuboides, external and middle
cuneiform bones, and the metatarsals and proximal phalanges of the toes
corresponding with the three middle toes of five-toed quadrupeds. The
metatarsals are chiefly remarkable for the well-developed
double-trochlear articular surface, and intermediate ridge. These
remains, as well as the jaws and teeth of the Ctenomys, were discovered
at Monte Hermoso in Bahia Blanca.
In the same reddish earthy stratum of that locality, Mr. Darwin
discovered the decomposed molar of a Rodent, equalling in size, and
closely resembling in the disposition of its oblique component laminæ,
the hinder molar of the Capybara (_Hydrochærus_). The fossil differs,
however, in the greater relative breadth of the component laminæ.
I have, lastly, to notice the head of a femur, and some fragments of
pelvic bones from the same formation which bear the same proportion to
the tooth above alluded to, as subsists between the teeth and bones of
the Capybara, and which are sufficient to prove that there once has
existed in South America a species of the family _Caviidæ_, as large as
the present _Capybara_, but now apparently extinct.
This fact, together with the greater part of those which have been
recorded in the foregoing pages of the present work, establishes the
correspondence, in regard to the characteristic type, which exists
between the present and extinct animals of the South American Continent:
we have abundant evidence likewise of the greater number of generic and
specific modifications of these fundamental types which the animals of a
former epoch exhibited, and also of the vastly superior size which some
of the species attained.
At the same time it has been shewn that some of the present laws of the
geographical distribution of animals would not have been applicable to
South America, at the period when the Megatherioids, Toxodon, and
Macrauchenia existed: since the Horse, and according to M. Lund, the
Antelope and the Hyæna, were then associated with those more strictly
South American forms. The Horse, which, as regards the American
continent, had once become extinct, has again been introduced, and now
ranges in countless troops over the pampas and savannahs of the new
world. If the small Opossums of South America had been in like manner
imported into Europe, and were now established like the Squirrels and
Dormice in the forests of France, an analogous case would exist to that
of the Horse in South America, as the fossil Didelphys of Montmartre
proves.
With respect to the geological contemporaneity of the fossils collected
by him, Mr. Darwin subjoins the following observations:—
“The remains of the following animals were embedded together at Punta
Alta in Bahia Blanca:—The _Megatherium Cuvierii_, _Megalonyx
Jeffersonii_, _Mylodon Darwinii_, _Scelidotherium leptocephalum_,
_Toxodon Platensis (?)_ a Horse and a small Dasypodoid quadruped,
mentioned p. 107; at St. Fé in Entre Rios, a Horse, a Mastodon,
_Toxodon Platensis_, and some large animal with a tesselated osseous
dermal covering; on the banks of the Tercero the Mastodon, Toxodon,
and, according to the Jesuit Falkner, some animal with the same kind
of covering; near the Rio Negro in Banda Oriental, the _Toxodon
Platensis_, Glossotherium, and some animal with the same kind of
covering. To these two latter animals the _Glyptodon clavipes_,
described by Mr. Owen in the Geological Transactions, may, from the
locality where it was discovered, and from the similarity of the
deposit which covers the greater part of Banda Oriental, almost
certainly be added, as having been contemporaneous. From nearly the
same reasons, it is probable that the Rodents found at Monte Hermoso
in Bahia Blanca, co-existed with the several gigantic mammifers from
Punta Alta. I have, also, shown in the Introduction, that the
_Macrauchenia Patachonica_, must have been coeval, or nearly so, with
the last mentioned animals. Although we have no evidence of the
geological age of the deposits in some of the localities just
specified, yet from the presence of the same fossil mammifers in
others, of the age of which we have fair means of judging, (in
relation to the usual standard of comparison, of the amount of change
in the specific forms of the invertebrate inhabitants of the sea,) we
may safely infer that _most_ of the animals described in this volume,
and likewise the Glyptodon, were strictly contemporaneous, and that
_all_ lived at about the same very recent period in the earth’s
history. Moreover, as some of the fossil animals, discovered in such
extraordinary numbers by M. Lund in the caves of Brazil, are identical
or closely related with some of those, which lately lived together in
La Plata and Patagonia, a certain degree of light is thus thrown on
the antiquity of the ancient Fauna of Brazil, which otherwise would
have been left involved in complete darkness.”
LONDON:
PRINTED BY STEWART AND MURRAY,
OLD BAILEY.
[Illustration:
Pl. I.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Base of the Skull of Taxodon Platensis._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. II.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Side View of the Skull of Taxodon._
_One-third the Natural Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. III.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Top View of the Skull of the Taxodon._
_One-third the Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. IV.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Taxodon Platensis._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. V.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Fragments of the lower Jaw and Teeth of a Taxodon._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. VI.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Cervical Vertebræ of Macrauchenia._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. VII.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Cervical Vertebræ of
1, 2. Macrauchenia 3,4 Auchenia._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. VIII.
_G. Scharf del et lithog._
_Lumbar Vertebræ, Macrauchenia._
_Fig. 1. Posterior View of last lumbar. Fig: 2,3&4. Fourth lumbar
Vertebra._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. IX.
_Lithog. from Nature by G. Scharf._
_Macrauchenia._
_Fig. 1_2. Scapula. Fig. 3. Femur._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. X.
_G. Scharf del et lithog._
_Proximal Extremity of anchylosed Ulna and Radius Macrauchenia._
_⅔ Nat. Size._
_London, Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XI.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Bones of the right fore-foot, Macrauchenia._
_Fig. 1, ⅔, 2_9, Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XII.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_⅔ Nat. Size._
_Right Femur. Macrauchenia._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XIII.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Macrauchenia._
_Right Tibia and Fibula.—Fig. 2_4. ⅔ Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XIV.
_Lithog. from Nature by G. Scharf._
_Right Astragalus., Macrauchenia._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XV
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Macrauchenia._
_Fig. 1. Metatarsal. 2_5. Metacarpals. Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XVI.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Fragment of the Cranium of the Glossotherium_
_½ Nat. Size._
]
[Illustration:
Pl. XVII.
_Fig. 3. 4. Laurillard del. Fig. 5. G. Scharf del et lithog._ _Printed
by C. Hallmandel._
_1. Megalonyx Jeffersoni. 2. Meg. laqueatus. 3. 4. Mylodon Harlani. 5.
Myl. Darwinii._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XVIII
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Mylodon. ⁵⁄₉ Nat Size._
]
[Illustration:
Pl. XIX
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Mylodon._
_Fig 1. ⁵⁄₉ Nat. Size. Fig. 2.3.4. Nat. Size._
]
[Illustration:
Pl. XX.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Scelidotherium._
_⅔ Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXI.
_G. Scharf del et lithog._ _Printed by C. Hallmandel._
_Scelidotherium._
_Fig 1 & 2 ⅔ Nat. Size. Fig 3_5. Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. XXII.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Scelidotherium._
_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]
[Illustration:
Pl. XXIII.
_Lithog. from Nature by G. Scharf._
_Cranial Cavity and Dentition of Scelidotherium._
_Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXIV.
_Lithog. from Nature by G. Scharf._
_Cervical and Anterior dorsal Vertebræ_
_Fig: 1. Scelidothere. Fig: 2. Orycterope. Fig: 3. Armadillo. Fig:4.
Great Ant-eater._
_One-third Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXV.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Scelidotherium ⅓ Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXVI.
_Printed by C. Hallmandel._
_Left Astragalus_
_Fig. 1.3.5. Megatherium: ⅓ Nat. Size. 2.4.6 Scelidotherium. ⅔ Nat.
Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXVII.
_Lithog. from Nature by G. Scharf._
_Scelidotherium._
_Fig. 1.2. ⅔ Nat. Size. 3.4.5 Nat. Size._
_Published by Smith, Elder & Cº͈ 65, Cornhill._
]
[Illustration:
Pl. XXVIII.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Left Astragalus._
_Fig. 1. Megatherium. ⅓ Nat. Size. Fig: 2. Scelidotherium. ⅔ Nat.
Size. Fig. 3–6, Mylodon.? ⅔ Nat. Size._
]
[Illustration:
Pl. XXIX.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Lower Jaw of Megalonyx._
_Fig. 1. ⅔. Fig. 2. Nat. Size._
]
[Illustration:
Pl. XXX.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Megatherium ½ Nat. Size_
]
[Illustration:
Pl. XXXI.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_Section of the superior maxillary teeth_,
_Megatherium._
_¾ Nat. Size._
]
[Illustration:
Pl. XXXII.
_Lithog. from Nature by G. Scharf._ _Printed by C. Hallmandel._
_1. Megatherium. 2_5. Hoplophorus. 6_12. Ctenomys. 13_14. Equus._
]
-----
Footnote 1:
See Ossemens Fossiles, Ed. iv. tom. ii. p. 368. Pl. 27. fig 1. 12.
Footnote 2:
_Ibid._ p. 370. Pl. 27. fig. 5.
Footnote 3:
_Ibid._ p. 347, 367.
Footnote 4:
_Ibid._ p. 337. Pl. 26. fig. 7.
Footnote 5:
Philosophical Transactions, vol. lviii. p. 34. (1768.)
Footnote 6:
Bridgewater Treatise, p. 139.
Footnote 7:
Geological Transactions, vol. iii. p. 437. pl. 44, 45, 46.
Footnote 8:
Quoted by Cuvier, Ossem. Foss. Ed. iv. tom. ii. p. 351.
Footnote 9:
Τοξον, arcus; οδους, dens.
Footnote 10:
Mem. de l’Acad. des Sciences de Paris, 1764, p. 568.
Footnote 11:
True fangs exist only in teeth of temporary growth, they may be one or
more in number, but always diminish in size as they recede from the
crown of the tooth, and are either solid, or with a very small canal.
Footnote 12:
This was written before an examination of the fragment of a lower jaw,
forming part of Mr. Darwin’s collection of Fossil Remains, had led me
to suspect that it was referrible to the genus Toxodon; should this
suspicion prove correct, the four unequal incisors of the upper jaw
are opposed to six equal sized ones in the lower.
Footnote 13:
I have ascertained that this elastic ligament exists in the neck of
the Dugong.
Footnote 14:
The German Translator (See _Frorieps Notizen_., 1837, p. 119) of the
abstract of my description of the Toxodon, published in the
Proceedings of the Geological Society, asks, what is the _Mutica_
(misprinted _Muticata_), of Linnæus? The term is quoted from the
Systema Naturæ, Ed. xii. p. 24. Linnæus first divides Mammalia into
three groups, according to modifications of the locomotive organs,
viz. _Unguiculata_, _Ungulata_, _Mutica_, and subdivides these,
according to modifications of the dentary organs, into the orders,
_Bruat_, _Glires_, _Primates_, &c.
Footnote 15:
Besides the relation to _food requiring much comminution_, which teeth
with persistent pulps bear, they are also connected with the
_longevity of the individual_. The term of life in a herbivorous
animal, with grinders of temporary growth, is, of necessity, dependent
on the duration of these essential aids to nutrition; thus, a sheep
generally wears down its grinders in twelve years, and its natural
term of life is consequently limited to about that period.
Footnote 16:
Μακρος _longus_, αυχην _cervix_: from the latter word Illiger derived
_Auchenia_, his generic name of the Llama, Vicugna, &c.
Footnote 17:
In the seventh cervical vertebra of the Camel, as in many other
Mammalia, there is no perforation in any part for the vertebral
arteries. In a Vicugna, I find the same structure; but in a Llama, the
side of the body of the seventh cervical vertebra is perforated
longitudinally on the right side. In the Camel, the vertebral arteries
pierce the sixth cervical vertebra, immediately below the superior
transverse processes, and pass obliquely to the anterior aperture of
the cervical canal, where they emerge beneath the anterior oblique
processes, and then enter the spinal canal of the fifth cervical
vertebra, as described in the text.
Footnote 18:
Cuvier, Ossemens Fossiles, iii. p. 238.
Footnote 19:
Loc. cit. p. 234.
Footnote 20:
Loc. cit. p. 235.
Footnote 21:
Loc. cit. p. 237.
Footnote 22:
Loc. cit. p. 232.
Footnote 23:
See Ossem. Fossiles, Pl. XX. fig. 3.
Footnote 24:
Loc. cit. Pl. XXII. fig. 6.
Footnote 25:
See Ossem. Foss. iii. Pl. XXVI. fig. 5.
Footnote 26:
This diameter increases rapidly in the posterior lumbar vertebræ, in
correspondence with the enlargement of the spinal chord, which gives
off the great nerves of the hinder extremities.
Footnote 27:
The relative breadth of these bones is shown in the figures of the
fore-foot, Pl. XI.
Footnote 28:
The figures in Pl. XIV. preclude the necessity of giving the
admeasurements of the astragalus.
Footnote 29:
“Comme le genre de vie de chaque animal est toujours en rapport avec
les mouvements dont sa mâchoire est susceptible, on retrouve dans la
conformation des surfaces destinées à l’articulation, les
particularités qui semblent le déterminer d’avance. Ainsi dans les
animaux qui vivent de chairs, substances filamenteuses qui ne peuvent
être écrasées, mais seulement coupées et dechirées, le mouvement de la
mâchoire inférieure ne peut s’exécuter que de haut en bas. Dans les
herbivores, les frugivores et les granivores, comme le principal
mouvement est celui de broiement pour écraser, comprimer les herbes et
les fruits, pour briser les grains et les réduire, en pâte, le
mouvement des mâchoires se fait encore de droite à gauche, et
réciproquement, on en même temps, de devant en arrière, en un mot,
dans un plan horizontal autant que dans un vertical: les uns
représentent des ciseaux, les autres des meules de moulin.”
Footnote 30:
In the monotrematous Echidna, the large canal for the lingual nerve
has a widely different direction and course from that in the placental
Edentata.
Footnote 31:
Transactions of the Philosophical Society of Philadelphia, vol. iv. p.
246.
Footnote 32:
Its relations to the Edentata, previously conjectured by Dr. Wistar,
are proved in the Annales du Muséum, tom. v. p. 358; its more
immediate affinities as an annectant form in that group are discussed
in the edition of the Ossem. Fossiles, of 1833, tom. v. pt. 1. p. 160.
Footnote 33:
Speaking of this tooth, Cuvier observes, “Je l’avois cru d’abord
nécessairement _de paresseux_; mais aujourdhui que je connois mieux
l’ostéologie des divers _tatous_, je trouve qu’elle ressemble au moins
autant à une dent de l’un des grands _tatous_.”—Loc. cit. p. 172.
Footnote 34:
Synopsis Mammalium.
Footnote 35:
It is most probable that the substance which is here termed “enamel,”
is similar to that which forms the dense prominent ridges in the tooth
of the Megatherium, and which I have shown to be composed of minute
parallel calcigerous tubes, similar to the ivory or bone of the human
tooth.
Footnote 36:
Medical and Physical Researches, pp. 323–4.
Footnote 37:
Loc cit. p. 330.
Footnote 38:
Spix and Martius, Reise in Brazil, Band ii. p. 5.
Footnote 39:
Harlan’s Medical and Physical Researches, 1835, p. 334. M. de
Blainville speaks of a cast of a fragment of a lower jaw “portant
encore _cinq_ dents en série;” as having been transmitted to the
Museum of the Garden of Plants from North America, together with other
bones, all of which he refers to the genus _Megalonyx_; M. de
Blainville does not describe these teeth, which is to be regretted,
inasmuch as, if he be correct in regard to their number, which can
hardly be doubted, and if he wrote with any clear and definite ideas
of the generic characters of _Megalonyx_, this would indicate that
_Megalonyx_ differed generically both from _Megatherium_ and _Mylodon_
in a more important dental character than has hitherto been suspected
(See “Comptes Rendus, &c.” 1839, No. V. p. 142.)
Footnote 40:
Dr. Harlan also indicates differences in certain parts of the skeleton
of the New York fossils as compared with his _Meg^x. laqueatus_; but
thinks them probably due to a difference in the age of the
individuals: he says “There is also in Mr. Graves’ collection, in New
York, a tibia, nearly perfect from the right leg; the segment of a
flattened sphere, on which the external condyle of the femur moves, is
rather more depressed, than in the specimen from Big-bone-cave. Other
marks and peculiarities are observable on this bone, not found on that
of the _Megalonyx laqueatus_ of Big-bone-cave, but they are probably
due to a difference in the age of the individuals.” Loc. cit. p. 335.
Footnote 41:
Μυλη, _mola_; οδους, _dens_.
Footnote 42:
If the lower jaw of _Mylodon Harlani_, bears the same proportion to
its teeth as does that of _Mylodon Darwinii_, it must be about two
feet in length.
Footnote 43:
See Proceedings of the Geological Society, March 1839, and Parish’s
Buenos Ayres, p. 178, _b_, Pl. 1, fig. 2 and 3.
Footnote 44:
Σκελις, _femur_; Θηριον, _bellua_; in allusion to the disproportionate
size of the thigh-bone.
Footnote 45:
This beach is covered at spring tides; many parts of the skeleton were
encrusted with recent _Flustræ_, and small marine shells were lodged
in the crevices between the bones.
Footnote 46:
It requires little stretch of imagination to conceive that this more
complex posterior tooth (Pl. XXIII., fig. 4, 4) in the lower jaw is
the representative of the two smaller posterior teeth (ib. fig. 3, 4,
and 5) of the upper jaw conjoined.
Footnote 47:
Lund, Videnskabernes Selskabs, Natur.: og Mathem. Afhandlinger,
Kiöbenhavn, vol. viii.
Footnote 48:
Linn. Trans. vol. xvii. (1833) p. 17.
Footnote 49:
Zool. Proceedings, 1832, p. 134.
Footnote 50:
The anterior prolongation of the sternum in front of the neck and the
corresponding anterior position of the clavicles and scapulæ occasions
a transference of such a proportion of the moving powers of the head
from the cervical vertebræ to these bones in the mole, as renders any
modifications of these vertebræ, like those in the Armadillo, uncalled
for.
Footnote 51:
Loc. cit.
Footnote 52:
Bridgewater Treatise, p. 46.
Footnote 53:
Bridgewater Treatise, p. 152.
Footnote 54:
_Dasypus 6–cinctus_, L., is the species of which I have the astragalus
separate, so as to be able to follow out the comparison.
Footnote 55:
In distinguishing these trochleæ as fibular and tibial, it is to be
understood that the terms relate only to aspects corresponding to the
position of those bones, and not that the fibula is articulated to the
whole of the trochlea so called: it probably rested only upon the
outer facet in the Scelidothere.
Footnote 56:
This astragalus was found at Santa Fé, in Entre Rios, associated with
the remains of the Mastodon and Toxodon; but from its size and form I
entertain little doubt that it belonged to a Megatherioid quadruped as
large as the Mylodon or Megalonyx. The brief allusion to the
astragalus of the Megalonyx in M. Lund’s Memoir does not afford the
means of determining with certainty this point.
Footnote 57:
See the figures of this bone, given by Cuvier in Pl. x. and xi.
Ossemens Fossiles, vol. v. part i.
Footnote 58:
Ossemens Fossiles, vol. v. part i. p. 163.
Footnote 59:
For the translation of the following passage, and of others alluded to
in the present work, from the original Danish Memoir of M. Lund, loc.
cit., I am much indebted to the Rev. W. Bilton, M.A. &c. &c.:—
“Thus in every point of comparison we have instituted between the
organization of burrowers and climbers; we have seen that the
Megalonyx constantly differs from the former and resembles the latter;
but the point to which I last alluded (the obliquity of foot), I
consider to be quite decisive.
“There is one other point in its organization, which is not quite
without weight in reference to our present inquiry,—I mean its
unusually powerful tail. Now, it is certainly true that many animals
which are not climbers have a powerful tail, as e. g. Armadillos,
while the others that climb well, have none, as Sloths and Apes. But
when we find a remarkably powerful tail attached to an animal that
according to all probability was a climber, we are led to infer that
this organ must have served for that purpose: in other words, that the
Megalonyx was furnished with a prehensile tail.
“How far the Megatherium is to be considered in the same light as the
Megalonyx cannot be decided without an accurate and scientific
examination of its skeleton at Madrid. Pander and D’Alton do not
mention any distortion of the hind-foot, neither does their figure
exhibit any. It is nevertheless quite possible that such may exist,
but that it is disguised by the faulty manner in which the skeleton is
put up. It strikes me as little probable that two animals which agree
so well in the principal particulars of their organization should
differ so much in one of the most important. The Megatherium has been
proved by later discoveries to possess the same powerful tail as the
Megalonyx, and as it corresponds also with the latter entirely in the
conformation of its extremities, the same difficulties present
themselves against the supposition of its having been a burrower. But
if the Megatherium was really a climber, it must have had still more
occasion (on account of its greater size), for that peculiar
arrangement of the hind-feet which we have described in the
Megalonyx.”
Footnote 60:
Descripcion del Esqueleto de un quadrupedo muy corpulento y raro, que
se conserva en el Real Gabinete de Historia Natural de Madrid. Folio,
Madrid, 1796.
Footnote 61:
Ossemens Fossiles, tom. v. pt. i. p. 179.
Footnote 62:
“Das Riesen Faulthier, Bradypus giganteus, von Dr. Chr. Pander und Dr.
E. D’Alton.” Folio, Bonn, 1821.
Footnote 63:
Transactions of the Geological Society, 1835, p. 438.
Footnote 64:
An excellent translation of the description of the Brazilian fossils
found by M. Lund, is published in the Annals of Natural History, July
and August, 1839.
Footnote 65:
At the distance of a few leagues from the locality here mentioned,
other fragments were found by Mr. Darwin; also near Santa Fé, in Entre
Rios; also on the shores of the Laguna, near the Guardia del Monte,
South of Buenos Ayres; also, according to the Jesuit Falkner, on the
banks of the Tercero.
Footnote 66:
Mr. Darwin has more particularly described the circumstances of the
embedment of this tooth in his Journal of Researches, p. 149, during
the Voyage of the Beagle.
------------------------------------------------------------------------
TRANSCRIBER’S NOTES
1. The Errata was corrected.
2. Silently corrected obvious typographical errors and variations in
spelling.
3. Retained archaic, non-standard, and uncertain spellings as printed.
4. Re-indexed footnotes using numbers and collected together at the end
of the last chapter.
5. Enclosed italics font in _underscores_.
6. Denoted superscripts by a caret before a single superscript
character, e.g. M^r.
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