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Title: Cranial Osteiology of the Hylid Frog, Smilisca baudini
Author: Trueb, Linda
Language: English
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 =====================================================================
                  UNIVERSITY OF KANSAS PUBLICATIONS
                       MUSEUM OF NATURAL HISTORY

                         -------------------

                     Volume 18, No. 2, pp. 11-35

 ------------------------- October 15, 1968 --------------------------


                  Cranial Osteology of the Hylid Frog,
                           Smilisca baudini

                                  BY

                             LINDA TRUEB



                        UNIVERSITY OF KANSAS
                               LAWRENCE
                                 1968



    UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

    Editors of this number: Frank B. Cross, William E. Duellman,
       Philip S. Humphrey

                     Volume 18, No. 2, pp. 11-35
                     Published October 15, 1968


                        UNIVERSITY OF KANSAS
                           Lawrence, Kansas



                              PRINTED BY
                ROBERT R. (BOB) SANDERS, STATE PRINTER
                            TOPEKA, KANSAS
                                 1968
                             [Union Label]
                               32-3686



                 Cranial Osteology of the Hylid Frog,
                           Smilisca baudini

                                  BY

                             LINDA TRUEB



INTRODUCTION


The paucity of descriptive cranial anatomical work dealing with hylid
frogs was pointed out by Trueb (1966) in her paper describing the
cranial osteology of _Hyla septentrionalis_. Comparative studies on
the cranial osteology of the genus _Smilisca_ (Duellman and Trueb,
1966), along with other more brief descriptions, reveal variation
among cranial characters of hylids. Since these external characters
have been useful in defining species, species groups, and genera, it
seems worthwhile to pursue correlated studies on internal cranial
structure. The following account dealing with the Neotropical tree
frog, _Smilisca baudini_ Duméril and Bibron, 1841, is the first
published description of the internal cranial anatomy of a hylid frog,
and supplements the recent account (Duellman and Trueb, 1966) of
external cranial osteology of the same species. Comparative studies of
hylid skulls are expected to yield information of taxonomic
importance.

I am grateful to Richard J. Baldauf of Texas A & M University and
William E. Duellman of the University of Kansas for critically reading
the manuscript and offering helpful suggestions. The findings reported
here result from research on Middle American hylids supported by a
grant from the National Science Foundation (GB-1441) to William E.
Duellman.


Materials and Methods

The serial sections illustrated beyond are from an adult male of
_Smilisca baudini_ (KU 89924) having a snout-vent length of 53.0 mm.
and a head width (measured at angle of jaws) of 17.0 mm. The specimen
was collected 5.2 kilometers east-southeast of Córdoba, Veracruz,
México. Transverse sections were cut at thicknesses of 10 and 15
microns on a rotary microtome and stained according to the technique
described by Baldauf (1958). Cleared and stained specimens and dried
skeletons also were used. Figure 1 is based on KU 68183-4 and Fig. 9
on KU 55614. All other drawings are made from KU 89924. In all
cross-sectional figures, bone is represented by solid black, cartilage
by stippling, and connective tissue by cross-hatching. Unless otherwise
noted all descriptions are given in an anterior-posterior sequence.

Commonly accepted English terms are used. For example, dentary is used
in preference to dentale and maxillary process instead of processus
maxillaris. If no commonly accepted English term is available for a
given structure, the Latin name is retained. For example, the
cartilaginous plate separating the cavum principale from the cavum
medium is termed the lamina superior.

 [Illustration: FIG. 1. Partially disarticulated skull (left
 frontoparietal and nasal removed) of _Smilisca baudini_, KU 68183,
 [Female] × 4. Abbreviations: _al. proc._, alary process of
 premaxillary; _ant. sq._, anterior arm of squamosal; _epi. em._,
 epiotic eminence; _exocc._, exoccipital; _fpar._, frontoparietal;
 _fpar. fon._, frontoparietal fontanelle; _max._, maxillary; _nas._,
 nasal; _pal._, palatine; _pal. proc._, palatine process; _pasph._,
 parasphenoid; _pmax._, premaxillary; _pvom._, prevomer; _post,
 sq._, posterior arm of squamosal; _pro._, prootic; _pter._,
 pterygoid; _qj._, quadratojugal; _spmax._, septomaxillary; _sept,
 nas._, septum nasi; _spheth._, spnenethmoid; _vent, sq._, ventral
 arm of squamosal.]



DESCRIPTION OF INTERNAL CRANIAL OSTEOLOGY


Olfactory Region

_Alary cartilage._--The anterior end of the alary cartilage (_al. c._,
Figs. 2-5) lies within the posterior concavity of the alary process
(_al. proc._, Figs. 1-3) of the premaxillary (_pmax._). In posterior
sections the cartilage assumes a dorsolateral position (Fig. 3),
ventral and slightly lateral to the tectum nasi. The alary cartilage
remains narrowly separated from the tectum nasi but fuses
ventromedially with the septum nasi and forms a nearly complete
cartilaginous capsule around the anterior end of the cavum principale.
Posterior to the anterior end of the cavum medium and the lamina
superior, the alary cartilage separates ventrally from the lamina. In
subsequent posterior sections, the cartilage, arcuate in cross
section, becomes progressively smaller and terminates at the level of
the union of the medial and lateral recesses of the cavum inferior.

_Prenasal cartilages._--The superior prenasal cartilage is small; it
lies adjacent to the posterodorsal surface of the alary process of the
premaxillary, and anterior to the alary cartilage. The inferior
prenasal cartilage (_inf. pnas. c._, Figs. 2-6, and 8) appears
posterior to the appearance of the alary cartilage. The anterior
terminus lies at the base of the alary process; the cartilage extends
dorsally (Fig. 3) along the posterior surface of the alary process and
then curves posterodorsally and joins the solum nasi medioventral to
the posterior end of the septomaxillary (Fig. 8d).

_Tectum nasi._--The anterolateral corner of the tectum nasi (_tect.
nas._, Fig. 2) appears just posterior and dorsomedial to the anterior
end of the alary cartilage. The anterior process is short; it fuses
medially with the septum nasi forming a complete roof to the cavum
principale (Figs. 3 and 4). The oblique cartilage (_obl. c._) diverges
laterally from the tectum nasi just posterior to the terminus of the
alary cartilage (Fig. 6). Medially, the tectum nasi persists, overlaid
by the nasal bone laterally.

_Septum nasi._--Posterior to the appearance of the septum nasi (_sept,
nas._) and its union with the tectum nasi (Fig. 3), the septum
abruptly expands across the width of the skull medial to the alary
cartilage. The septum is entirely cartilaginous posterior to the level
of the olfactory eminence, except for a small amount of secondary
membranous ossification dorsomedially at a level anterior to the nasal
bones. Perichondral ossification commences in the dorsal part of the
septum nasi at the level of the olfactory eminence. Endochondral
ossification first appears dorsally in the vertical part of the septum
at the level of the internal nares. Ossification of dorsal parts of
the septum precedes ossification of ventral parts. Perichondral
ossification of the ventral part of the septum nasi is first noted at
the level of transition between the planum antorbitale and solum
nasi. Perichondral ossification gradually gives way to endochondral
ossification posteriorly.

 [Illustration: FIGS. 2-5. Transverse sections through anterior end
 of skull: 2) anterior level of inferior prenasal cartilage; 3)
 anterior level of internasal septum; 4) olfactory capsule at
 anterior level of cavum principale; 5) olfactory capsule at
 anterior level of cavum inferior. Abbreviations: _al. c._, alary
 cartilage; _al. proc._, alary process of premaxillary; _cav. med._,
 cavum medium; _cav. prin._, cavum principale; _cr. int._, crista
 intermedia; _inf. pnas. c._, inferior prenasal cartilage; _l.
 inf._, lamina inferior; _l. sup._, lamina superior; _max._,
 maxillary; _pmax._, premaxillary; _r. etx. n. f._, ramus externus
 narium foramen; _r. med. n. f._, ramus medialis narium foramen;
 _rec. lat._, recessus lateralis; _sept. nas._, septum nasi; _sol.
 nas._, solum nasi; _spmx._, septomaxilla; _tect. nas._,
 tectum nasi.]

 [Illustration: FIGS. 6-7. Transverse sections through olfactory
 capsule: 6) posterior level of cavum medium; 7) anterior level of
 prevomer. Abbreviations: _cav. inf._, cavum inferius; _cav. med._,
 cavum medium; _cav. prin._, cavum principale; _cr. sub._, crista
 subnasalis; _ext. nar._, external nares; _inf._, infundibulum;
 _inf. pnas. c._, inferior prenasal cartilage; _l. inf._, lamina
 inferior; _l. sup._, lamina superior; _max._, maxillary; _nas._,
 nasal; _ncl. dt._, nasolacrimal duct; _obl. c._, oblique cartilage;
 _p. fac._, pars facialis; _p. pal._, pars palatina; _pvom._,
 prevomer; _rec. med._, recessus medialis; _sept. nas._, septum
 nasi; _sol. nas._, solum nasi; _spmax._, septomaxillary;
 _tect. nas._, tectum nasi.]

_Nasal cavities and associated structures._--The cavum principale
(_cav. prin._, Fig. 4) is the most anterior of the nasal cavities. It
first appears within the capsule bordered dorsally by the tectum nasi,
medially and ventrally by the septum nasi, and laterally by the alary
cartilage. The cavity extends posteriorly within recesses of the
sphenethmoid to the level at which the septum nasi terminates.

The cavum medium (_cav. med._, Fig. 5) lies ventral and slightly
posterior to the anterior end of the cavum principale. It appears
slightly anterior to the septomaxillary at the level of the foramen
ramus externus narium and ramus medialis narium. The appearance of the
cavum medium within the ventrolateral extension of the septum nasi
divides the latter into an upper component, the lamina superior
(_l. sup._) lying between the cavum principale and cavum medium, and a
lower part, the lamina inferior (_l. inf._) lying ventral to the cavum
medium. As the cavum medium increases in width in posterior sections,
the lamina superior and lamina inferior lose their lateral connection.
The lateral part of the cavum medium diverges in the region of the
external nares as the nasolacrimal duct (_ncl. dt._) and the cavum
medium becomes confluent with the cavum principale (Fig. 6). The
posterior end of the cavum medium lies at the level of the posterior
terminus of the septomaxillary.

Slightly posterior to the anterior end of the cavum medium the foramen
for the ramus externus narium (_r. ext. n. f._) and ramus medialis
narium (_r. med. n. f._) opens ventromedially into the floor of the
septum nasi (Fig. 4). The ventral closure of the floor of the foramen
completes the solum nasi, marks the anterior end of the recessus
medialis of the cavum inferior, and differentiates the roof of the
recess, the crista intermedia (_cr. int._), from the solum (Fig. 5).
The crista intermedia joins the laminae superior and inferior and
joins them for a short distance to the septum nasi medially. The
anterolateral part of the cavum inferior (_cav. inf._) lies
ventrolateral to the cavum medium, and extends medially to join the
medial recess. The fusion of the two recesses of the cavum inferior
completely separates the lamina inferior from the solum nasi (Fig. 6).

Near the level of the union of the recessus lateralis and recessus
medialis of the cavum inferior, the crista intermedia separates from
the septum nasi, and the lamina superior diverges at its mid-width to
accommodate the septomaxillary (_spmax._) (Figs. 5 and 6). The lateral
remnant of the lamina superior and transition zone between the lamina
superior and lamina inferior is short and is quickly replaced by
connective tissue. Slightly posterior, the distal edge of the lamina
inferior diverges laterally as a small process, which lies dorsal to
the pars facialis (_p. fac._) of the maxillary. This part of the
lamina inferior terminates posteriorly at the level of confluence
between the cavum principale and lateral recess of the cavum inferior.

Slightly posterior to the divergence of the crista intermedia from the
septum nasi, the crista terminates, thereby separating the lamina
superior and the lamina inferior from one another (Figs. 6 and 8). The
lamina superior terminates at the level of the infundibulum (_inf._,
Fig. 7), whereas the lamina inferior extends posterolaterally,
increases greatly in depth and joins the laterally ascending oblique
cartilage (_obl. c._) to form the planum terminale (_pla. ter._,
Fig. 10).

The crista subnasalis (_cr. sub._, Fig. 6) differentiates from the
lateral edge of solum nasi adjacent to the maxillary in sections just
posterior to the confluence of the recessus medialis and the recessus
lateralis. The crista persists as a rod of cartilage which gradually
diminishes in size and terminates at the posterior level of the
septomaxillary.

_The septomaxillary._--The septomaxillary (_spmax._, Fig. 9a-c) is a
triradiate bone. The anterior terminus is a thin sliver of bone
oriented horizontally between the cavum principale and cavum medium
and lateral to the lamina superior (Figs. 5 and 8a). This anterior
ramus of the septomaxillary increases in size posteriorly and diverges
medially into a medial ramus (_med. r. spmax._) and lateral ramus
(_lat. r. spmax._, Fig. 9a-b) to accommodate the confluence of the
cavum principale and cavum medium (Figs. 6 and 8b-c). The small medial
ramus is associated with the distal end of the lamina superior whereas
the lateral ramus lies dorsal to the lateral margin of the cavum
medium. Just anterior to the anterior end of the nasolacrimal duct,
the ventral ramus of the septomaxillary (_vent. r. spmax._, Fig. 9c)
is present in cross-sections ventral to the cavum medium. The ventral
ramus joins the horizontal and dorsal rami of the septomaxillary at
the anterior end of the nasolacrimal duct. The medial branch terminates
posteriorly at the level at which the cavum principale joins the cavum
inferior. The lateral ramus of the septomaxillary terminates
posteriorly at the level at which the recessus medialis diverges from
the recessus lateralis posteriorly and cavum principale and recessus
lateralis are confluent.

_Planum terminale._--Posterior to the infundibulum the lamina inferior
and oblique cartilage join to form the planum terminale (_pla. term._)
which lies lateral to the cavum principale (Fig. 10). The lamina
inferior diverges ventrally from the planum terminale anterior to the
olfactory eminence. The planum terminale is restricted ventrally and
terminates at the level of the olfactory eminence (_olf. em._,
Fig. 11).

 [Illustration: FIG. 8. Transverse sections through olfactory
 capsule in region of septomaxillary: _a_) anterior terminus of
 septomaxillary; _b_) medial divergence of septomaxillary; _c_)
 dorsal ramus of septomaxillary; _d_) posterolateral terminus of
 septomaxillary. Encircled numbers represent the nasal cavities as
 follows: 1) cavum principale; 2) cavum medium; and 3) cavum
 inferius. Abbreviations: _al. c._, alary cartilage; _ant. spmax._,
 anterior end of septomaxillary; _cr. int._, crista intermedia;
 _dor. r. spmax._, dorsal ramus of septomaxillary; _ext. nar._,
 external nares; _inf. pnas. c._, inferior prenasal cartilage; _l.
 inf._, lamina inferior; _l. sup._, lamina superior; _lat. r.
 spmax._, lateral ramus of septomaxillary; _med. r. spmax._, medial
 ramus of septomaxillary; _nas._, nasal; _nlc. dt._, nasolacrimal
 duct; _obl. c._, oblique cartilage; _pvom._, prevomer; _sept.
 nas._, septum nasi; _sol. nas._, solum nasi; _spmax._,
 septomaxillary; _tect. nas._, tectum nasi.]

 [Illustration: FIG. 9. Septomaxillary drawn from cleared and
 stained specimen of _Smilisca baudini_, KU 55614: _a_) dorsal;
 _b_) ventral; _c_) lateral. In each example, the anterior end lies
 to the left. Abbreviations: _dor. r. spmax._, dorsal ramus of
 septomaxillary; _lat. r. spmax._, lateral ramus of septomaxillary;
 _med. r. spmax._, medial ramus of septomaxillary; _vent. r.
 spmax._, ventral ramus of septomaxillary.]

_Anterior and posterior maxillary processes._--The anterior end of the
anterior maxillary process (_ant. max. proc._) lies within the
maxillary at the level of the posterior terminus of the planum
terminale. The anterior maxillary process diverges medially from the
maxillary (Fig. 13) and expands dorsally along the medial face of the
pars facialis to meet the planum antorbitale just anterior to the
transition zone between the latter and the solum nasi. Posterior to
the transition zone, the planum antorbitale disappears and the
posterior maxillary process is restricted ventrally along the pars
facialis of the maxillary. Posteriorly the cartilage is associated
with the pterygoid, where it is known as the pterygoid process
(_pter. proc._, Fig. 14).

_Planum antorbitale._--The anterior terminus of the planum antorbitale
(_pla. ant._) lies medial to the ventrolateral part of the nasal and
lateral to the internal nares (Fig. 12). It abruptly expands dorsally
along the medial face of the nasal to join the tectum nasi
dorsolaterally; somewhat posteriorly the planum antorbitale joins the
anterior maxillary process ventrally at the posterior margin of the
internal nares.

 [Illustration: FIGS. 10-11. Transverse sections through posterior
 part of olfactory capsule: 10) region of planum terminale; 11)
 anterior region of olfactory eminence. Abbreviations: _cav. p._,
 cavum principale; _max._, maxillary; _nas._, nasal; _nlc. dt._,
 nasolacrimal duct; _olf. em._, olfactory eminence; _p. fac._, pars
 facialis; _p. pal._, pars palatina; _pla. ter._, planum terminale;
 _pvom._, prevomer; _rec. lat._, recessus lateralis; _sept. nas._,
 septum nasi; _sol. nas._, solum nasi; _tect. nas._, tectum nasi.]

 [Illustration: FIG. 12. Transverse section through the olfactory
 capsule in region of planum antorbitale. Abbreviations:
 _cav. prin._, cavum principale; _int. nar._, internal nares; _max._,
 maxillary; _nas._, nasal; _olf. em._, olfactory eminence;
 _p. fac._, pars facialis; _p. pal._, pars palatina; _pal._, palatine;
 _pla. ant._, planum antorbitale; _pvom._, prevomer; _sept. nas._,
 septum nasi; _sol. nas._, solum nasi; _tect. nas._, tectum nasi.]

_External dermal bones associated with the olfactory region._--The
association of the premaxillary (_pmax._) to the nasal cartilages is
described in preceding sections. The premaxillaries are separated from
each other medially and from the maxillaries laterally by dense
connective tissue. Anteriorly, the maxillary (_max._) bears a small
palatine process (_pal. proc._, Fig. 1) and a long, delicate pars
facialis (_p. fac._, Fig. 6), which terminates dorsally at the level
of the lamina inferior. Posterior to the transition zone between the
planum antorbitale and solum nasi the pars facialis is greatly
reduced. The pars palatina (_p. pal._, Fig. 6) persists to the
posterior part of the orbit.

The anterior end of the prevomer (_pvom._, Fig. 1) is associated with
the venter of the solum nasi at the level of the infundibulum just
posterior to the incorporation of the inferior prenasal cartilage into
the solum (Fig. 7). The prevomer expands dorsally around the distal
end of the solum to provide a bony lateral support for the olfactory
eminence (Figs. 10 and 11). A distal wing of the prevomer forms the
bony anterior and medial margins of the internal nares.

The palatine (_pal._, Figs. 1 and 12) lies in connective tissue medial
and adjacent to the pars facialis. At its maximum size the palatine
forms the bony posterior margin of the internal nares and extends
dorsomedially from the pars palatina to the distal part of the solum
nasi.

The nasal (_nas._, Fig. 1) is a thin bone overlying the tectum nasi
anteriorly (Fig. 7). It expands laterally to form a complete roof over
the cavum principale (Fig. 10). In the region of the internal nares,
the nasal forms the lateral wall of the cavum principale (Fig. 12).


Sphenethmoid Region

Posterior to the transition zone between the planum antorbitale and
solum nasi, the sphenethmoid (_spheth._, Fig. 1) is fully ossified
medially, the lateral parts of the bone at this level are only
ossified perichondrally. The septum nasi persists at the anterior
level of the orbit and terminates just anterior to the orbitonasal
foramen (_orbnas. f._) and the anterior end of the parasphenoid
(_pasph._, Fig. 13). The orbitonasal foramen is moderately large, has
a complete bony margin, and is located at the dorsolateral corner of
the braincase.

 [Illustration: FIG. 13. Transverse section through sphenethmoid
 region at level of orbitonasal foramen. Abbreviations: _ant. max.
 proc._, anterior maxillary process; _max._, maxillary; _orbnas. f._,
 orbitonasal foramen; _pasph._, parasphenoid; _spheth._, sphenethmoid.]

At the level of the orbitonasal foramen, the sphenethmoid is entirely
ossified except for a small dorsolateral extension. This distal
extension expands laterally in posterior sections as the braincase is
increased to its maximum width at the mid-length of the orbit; the
cartilaginous margin is retained throughout the length of the
sphenethmoid.

The bony dorsomedial part of the sphenethmoid diverges, forming the
anterior border of the frontoparietal fontanelle (_fpar. fon._, Figs.
1 and 14). The entire fontanelle is covered with a layer of dense
connective tissue continuous with that in which marginal bones and
cartilage of the sphenethmoid lie, and which is discrete from the
lower dermal layer of the overlying skin. At this level the braincase
is U-shaped in cross-section. Ossification terminates first in the
ventrolateral corners, followed by the lateral and dorsolateral areas.
The bony support of the latter area is furnished by the lamina
perpendicularis (_lam. perp._) of the frontoparietal (_fpar._, Fig.
14). Cartilage appears in the ventral part of the sphenethmoid in
posteromost sections; at the posterior levels of the orbit the
sphenethmoid is entirely cartilaginous.


Orbital, Otic, and Occipital Regions

_Orbital region._--The sclera (_scl._, Fig. 14) of the eye is
cartilaginous. The optic foramen (_opt. f._) is large and lies in
connective tissue at the posterior limits of the orbit and
sphenethmoid. At the posterior levels of the foramen the dorsolateral
cranial roof cartilages, taeniatectí marginales (_t. t. mar._)
converge medially to form the posterior margin of the frontoparietal
fontanelle and the tectum synoticum (_tect. syn._) of the occipital
region. At the posterior levels of the orbit the bursa angularis oris
(_b. ang. o._, Fig. 14) is present adjacent to the maxillary.

_Nerve foramina of otic and occipital regions._--The trochlear foramen
lies within the bony margins of the optic foramen. The trochlear nerve
is located posterodorsal to the optic tract, and separated from the
latter by connective tissue. The oculomotor foramen (_ocul. f._) lies
in connective tissue posterior and ventral to the optic foramen (Fig.
15). Anteriorly, dorsally, and ventrally the foramen has a bony margin
formed by the prootic (_pro._); posteriorly, only a thin layer of
connective tissue separates the oculomotor from the large prootic
foramen (_pro. f._). The latter is bordered by bone dorsally and by
cartilage ventrally (Fig. 16). Posteriorly, bone separates the prootic
foramen from the anterior acoustic foramen (_ant. acus._ _f._),
through which the ramus acusticus anterior and medius pass (Fig. 17).
An extremely narrow bridge of cartilage separates the anterior
acoustic foramen from the larger posterior acoustic foramen (_post.
acus. f._). The latter has a bony posterior margin and is widely
separated from the bony jugular foramen (_jug. f._) posteriorly
(Fig. 19).

 [Illustration: FIGS. 14-15. Transverse sections through skull: 14)
 at level of optic foramen; 15) at level of oculomotor foramen.
 Abbreviations: _angspl._ angulosplenial; _ant. r. pter._, anterior
 ramus of pterygoid; _ant. sq._, anterior arm of squamosal; _b. ang.
 o._, bursa angularis oris; _fpar._, frontoparietal; _fpar. fon._,
 frontoparietal fontanelle; _l. perp._, lamina perpendicularis of
 frontoparietale; _max._, maxillary; _Mc. c._, Meckel's cartilage;
 _ocul. f._, oculomotor foramen; _opt. f._, optic foramen; _pasph._,
 parasphenoid; _psdbas. proc._, pseudobasal process; _pter. proc._,
 pterygoid process; _scl._, sclera; _t. t. mar._, taenia tecti
 marginalis; _tymp. r._, tympanic ring.]

_Pterygoid._--The anterior terminus of the pterygoid (_pter._, Fig. 1)
appears at approximately the mid-length of the orbit as a small
arcuate bone closely applied to the posterior maxillary process.
Farther posteriorly the maxillary decreases in size, and the pterygoid
and posterior maxillary process diverge medially from it. Posterior to
this point of divergence, the posterior maxillary process is known as
the pterygoid process (_pter. proc._). The anterior terminus of the
quadratojugal (_qj._) lies medial to the maxillary at the level of the
oculomotor foramen (Fig. 14).

_Otic region._--The anterior end of the otic capsule (_ot. cap._) is
present at the anterior level of the oculomotor foramen. The anterior
terminus of the pseudobasal process (_psdbas. proc._) lies within the
medial portion of the pterygoid at the posterior border of the
oculomotor foramen (Fig. 15). The pseudobasal process abruptly
increases in size. At the level of the prootic foramen (Fig. 16) the
medial branch of the pterygoid diverges from the posterior ramus and
is closely applied to the medial surface of the pseudobasal process.
The otic process extends along the medial surface of the squamosal
from the dorsolateral edge of the pseudobasal process, and then
expands medially to meet the bony edge of the otic capsule and form
the crista parotica. Posterior to the formation of the crista
parotica, the ventral part of the otic process splits. The medial part
forms the ventrolateral ledge of the otic capsule (_vl. l. ot. c._,
Fig. 20a-f), whereas the lateral part moves ventrad in association
with the ventral arm of the squamosal and fuses with the pterygoid
process posteriorly.

Posterior to the bony closure of the prootic foramen, the ventromedial
part of the pseudobasal process joins the prootic and forms the
ventrolateral edge of the otic capsule. The posterior terminus of the
medial branch of the pterygoid lies ventral to the lateral part of the
otic capsule. The posterior branch of the otic process merges with the
pterygoid process ventrally.

 [Illustration: FIGS. 16-17. Transverse sections through otic
 region: 16) at level of prootic foramen; 17) at level of anterior
 acoustic foramen. Abbreviations: _angspl._, angulosplenial; _ant.
 acus. f._, anterior acoustic foramen; _cr. par._, crista parotica;
 _fpar._, frontoparietal; _max._, maxillary; _Mc. c._, Meckel's
 cartilage; _ot. cap._, otic capsule; _pasph._, parasphenoid;
 _pro._, prootic; _psdbas. proc._, pseudobasal process; _pter._,
 pterygoid; _pter. proc._, pterygoid process; _qj._, quadratojugal;
 _sq._, squamosal; _tect. syn._, tectum synoticum; _tymp. r._,
 tympanic ring.]

 [Illustration: FIGS. 18-19. Transverse sections through otic capsule:
 18) at level of posterior acoustic foramen; 19) at level of jugular
 foramen. Abbreviations: _angspl._, angulosplenial; _corn. prin._,
 cornu principalis; _cr. par._, crista parotica; _exocc._, exoccipital;
 _fpar._, frontoparietal; _jug. f._, jugular foramen; _max._, maxillary;
 _Mc. c._, Meckel's cartilage; _ot. cap._, otic capsule; _p. ext. pl._,
 pars externa plectri; _p. int. pl._, pars interna plectri; _p. med.
 pl._, pars media plectri; _pasph._, parasphenoid; _post. acus. f._,
 posterior acoustic foramen; _postlat. cr. par._, posterolateral edge
 of crista parotica; _pro._, prootic; _pter._, pterygoid; _pter. proc._,
 pterygoid process; _quad. proc._, quadrate process; _qj._
 quadratojugal; _sq._, squamosal; _tect. syn._, tectum synoticum;
 _tymp. r._, tympanic ring.]

At the level of the anterior acoustic foramen the cornu principalis of
the hyale (_corn. prin._) appears as a lateral ledge at the
ventrolateral corner of the otic capsule (Fig. 20a-b). The cornu
principalis diverges from the ledge at the level of the abbreviated
bridge between the anterior and posterior acoustic foramina. In
posterior sections the cornu lies medial to the squamosal-pterygoid
process-pterygoid complex (Fig. 18). The posterior terminus of the
cornu lies at a level with that of the posterior acoustic foramen.

The pars externa plectri (_p. ext. pl._, Fig. 20a-b) is cartilaginous
and first appears dorsal to the ventral arm of the squamosal in
association with the tympanic membrane. The pars externa plectri
expands dorsomedially and is fused briefly to the crista parotica by
the pars ascendens plectri (_p. asc. pl._, Fig. 20b). The pars interna
plectri (_p. int. pl._, Fig. 20b-f) is cartilaginous and appears
medial to the pars media plectri and the ventrolateral ledge of the
otic capsule at the level of the anterior acoustic foramen. The pars
media plectri (_p. med. pl._, Fig. 20b-f), a cartilage and bone
element, appears proximally at the dorsolateral edge of the otic
capsule and distally, ventral to the squamosal at a level between the
anterior and posterior acoustic foramina. At the level of the
posterior acoustic foramina the pars media plectri is bony, greatly
expanded in size, and joined to the pars interna plectri medially.

The operculum (_op._, Fig. 20d-h) is cartilaginous and lies medial to
the lateral edge of the otic capsule between the pars interna plectri
and pars media plectri. The anterior end of the operculum (Fig. 20d)
lies at a level corresponding to the posterior part of the posterior
acoustic foramen. Posteriorly the operculum increases in size, and the
pars interna plectri and pars media plectri are reduced (Fig. 20e-f).
At a level corresponding to the posterior border of the posterior
acoustic foramen the medial portion of the pars interna plectri
disappears and leaves a small lateral rod of cartilage surrounded on
all but the ventral side by the operculum (Fig. 20f). The operculum
expands medially to merge with the main part of the otic capsule
(Fig. 20g). The lateral edge of the operculum expands ventrally and
then dorsomedially to form a complete tube. Slightly more posteriorly
the cartilaginous lateral edge of the otic capsule, lateral to the
operculum, dissipates into connective tissue and finally disappears,
leaving the posterior end of the operculum as the most distal element
of the otic capsule (Fig. 20h).

 [Illustration: FIG. 20. Transverse sections through otic capsule:
 _a_) level of anterior ledge of otic capsule; _b_) anterior level
 of pars interna plectri and pars ascendens plectri; _c_) level of
 pars media plectri; _d-f_) successive levels of operculum and pars
 media plectri; _g-h_) posterior levels of operculum. Abbreviations:
 _corn. prin._, cornu principalis; _cr. par._, crista parotica;
 _op._, operculum; _p. asc. pl._, pars ascendens plectri; _p. ext.
 pl._, pars externa plectri; _p. int. pl._, pars interna plectri;
 _p. med. pl._, pars media plectri; _sq._, squamosal; _tymp. r._,
 tympanic ring; _vl. l. ot. c._, ventrolateral ledge of otic
 capsule.]

_Ossification in otic and occipital regions._--The otic region of the
cranium is largely unossified. At the level of the optic foramen
(Fig. 14) the floor of the neurocranium is cartilaginous but completely
underlaid by the bony parasphenoid. The taenia tecti marginales and
the tectum synoticum are covered dorsally and laterally by the
frontoparietals. Perichondral ossification representing the prootic
bone occurs at the margin of the optic foramen and somewhat
posteriorly over part of the floor of the neurocranium. Perichondral
and endochondral ossification occurs in the sides of the neurocranium
ventral to the lamina perpendicularis. This ossification expands
laterally until it meets the crista parotica dorsolaterally and forms
the dorsal part of the prootic bone. The anteroventral edge of the
otic capsule remains cartilaginous. Posteriorly, at the level of the
anterior acoustic foramen, endochondral ossification is meager and
restricted to the dorsomedial parts of the otic capsule, plus a small
amount in the neurocranial floor; perichondral ossification is
restricted to the peripheral areas showing endochondral ossification.
Posteriorly, endochondral ossification is restricted in the dorsal
part of the otic capsule but somewhat increased in the floor of the
capsule. The lateral part of the otic capsule posterior to the
terminus of the operculum and the ventromedial and dorsomedial parts
of the neurocranium remain unossified.


Articular Region

In the anterior sections (at the level of the oculomotor foramen) the
angulosplenial (_angspl._) is a moderate-sized bone (Fig. 15).
Meckel's cartilage (_Mc. c._) is present as a small ovoid cartilage
lying dorsolateral to the angulosplenial. Posteriorly, Meckel's
cartilage is dorsal to the angulosplenial. The cartilage increases in
size at the level of the posterior acoustic foramen, and the
angulosplenial decreases in size posteriorly. At the level of the
posterior border of the posterior acoustic foramen, the maxillary
terminates and is replaced by the quadratojugal. The quadratojugal,
ventral arm of the squamosal, pterygoid process, pterygoid, and
Meckel's cartilage converge. At the level of the jugular foramen
(_jug. f._) (Fig. 19) the quadratojugal is incorporated into the
squamosal-pterygoid process-pterygoid complex. The complex is narrowly
separated by connective tissue from Meckel's cartilage ventrally. The
quadrate process (_quad. proc._) is represented by the cartilage
bordered dorsally by the pterygoid process and the ventral arm of the
squamosal, and ventrally by Meckel's cartilage. At the posterior
terminus of the skull all bony elements of the articular region
terminate, except for a small terminal part of the angulosplenial
underlying Meckel's cartilage.



SUMMARY


Since no accounts comparable to the preceding for _Smilisca baudini_
are available for other hylid frogs, it is meaningless to attempt any
discussion dealing with character significance or variation within the
Hylidae. There is considerable literature treating bufonids,
leptodactylids, ranids, and various Old World genera (see Baldauf,
1955, for a review of these works). Likewise, a comparison at the
familial level based on the study of a single species seems inadequate
and premature. By way of summary and synoptic description a list of
cranial osteological characters of _Smilisca baudini_ is presented.
The items selected enable comparison with similar compilations by
other workers, and are based in part on my unpublished observations of
other hylids.

  1. Compared to hylids not having integumentary-cranial
     co-ossification, the dermal roofing bones of _Smilisca baudini_
     are extensive, and the skull is well-ossified internally. In
     contrast to most casque-headed hylids (those having
     integumentary-cranial co-ossification), the dermal roofing bones
     are much less extensive, the dermal sphenethmoid (see Trueb, 1966,
     p. 563) is absent, and internal ossification is less extensive.

  2. The solum nasi is not ossified; the septum nasi is ossified only
     posteriorly, and the olfactory eminence is supported by the
     cartilaginous solum nasi and the bony prevomer.

  3. The lingual process is absent. There is no palatal cartilage
     isolated between the premaxillaries.

  4. The anterior end of the cavum medium lies anterior to the cavum
     inferius.

  5. The septomaxillary is basically a U-shaped structure and has a
     dorsal, anteriorly curved, ramus on the lateral branch and a
     longitudinal loop of bone ventrally.

  6. A distinct pars nasalis is absent on the maxillary.

  7. A cartilaginous sclera is present.

  8. The taenia tecta marginalis and the tectum synoticum are the only
     roofing cartilages present.

  9. The external part of the plectral apparatus (columella) is
     directed anterolaterally. The pars ascendens plectri is fused
     with the crista parotica.

 10. The pseudobasal process is fused to the otic capsule.

 11. The cornu principalis of the hyale fuses with the pseudobasal
     process.

 12. Two acoustic foramina are present.

 13. The sphenethmoid and prootic are synchondrotically united.

 14. The frontoparietal is separate from the prootic and exoccipital.

 15. The prootic and exoccipital are fused.

 16. A bursa angularis oris is present.



LITERATURE CITED


 BALDAUF, R. J.
  1955. Contributions to the cranial morphology of _Bufo w. woodhousei_
        Girard. Texas Jour. Sci., 7(3):275-311.

  1958. A procedure for the staining and sectioning of the heads of
        adult anurans. Texas Jour. Sci., 10(4):448-451.

 DUELLMAN, W. E. AND L. TRUEB
  1966. Neotropical hylid frogs, genus Smilisca. Univ. Kansas Publ.,
        Mus. Nat. Hist., 17:281-375, pls. 1-12.

 TRUEB, L.
  1966. Morphology and development of the skull of the frog _Hyla
        septentrionalis_. Copeia, 3:562-573.



_Transmitted April 18, 1968._



       *       *       *       *       *

Transcriber's Notes

Except for several minor typographical corrections which were made
(missing periods, commas, etc.) that are not detailed here, the text
presented is that which appeared in the original printed version. To
enhance readability, the Figure captions and list in the SUMMARY
section were outdented on the first line.


Emphasis Notation

  _Text_ = Italicized





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