The following description of the skull, hyoid, sternum, and prepollex
is based on a male specimen of _P. spinipollex_ (KU 68632) that has
been cleared and stained. The broad, flat skull (Fig. 3) has a large
frontoparietal fontanelle. The ethmoid is large and has a flange
laterally. The nasals are of moderate size and in broad contact with
the ethmoid, but are separated from one another medially. The anterior
half of the maxillary bears a thin, high flange. The anterior process
of the squamosal is short and widely separated from the maxillary. The
quadratojugal is a small spine-shaped element projecting anteriorly
from the ventral base of the quadrate; the quadratojugal does not
articulate with the maxillary.
[Illustration: FIG. 3. Dorsal aspect of skull of _Ptychohyla
spinipollex_ (KU 68632). Arrow indicates reduced
quadratojugal.]
The posteromedian part of the hyoid plate is calcified; from this
plate the long bony, posterior cornua (thyrohyales) extend
posterolaterally.
The omosternum is calcified, widest anteriorly, and has a convex
anterior edge. The calcified xiphisternum is roughly bell-shaped
having short lateral processes anteriorly and a deep notch
posteriorly.
The swollen thumb is supported by a dorsoventrally flattened spine
that does not extrude through the skin.
VARIATION.--In general, the skull varies little. Usually the
quadratojugal is present only as a short element attached to the
quadrate, but in one specimen of _P. spinipollex_ the quadratojugal
articulates with the maxillary and forms a complete
quadratojugal-maxillary arch on each side of the skull. One specimen
of _P. leonhardschultzei_ has a complete arch on one side and an
incomplete arch on the other.
Only _P. spinipollex_ has lateral processes anteriorly on the
xiphisternum; in the other species the xiphisternum is deeply
bell-shaped.
_Ptychohyla schmidtorum_ and _P. ignicolor_ have slightly longer
premaxillaries than the other species. The longer premaxillary is
reflected in the larger number of teeth on the bone--9 to 11 (average
10) in four specimens of _P. schmidtorum_ and 10 teeth in one _P.
ignicolor_, as compared with 6 to 10 (average 7.9) in seven specimens
of the other species. The number of maxillary teeth in the various
species are: _P. euthysanota euthysanota_, 43; _P. euthysanota
macrotympanum_, 38; _P. leonhardschultzei_, 38 and 40; _P.
spinipollex_, 34 and 40; _P. schmidtorum schmidtorum_, 37 and 43; _P.
schmidtorum chamulae_, 40 and 41; _P. ignicolor_, 43. The teeth on the
premaxillary and anterior part of the maxillary are long, pointed, and
terminally curved backwards. Posteriorly on the maxillary the teeth
become progressively shorter and blunter.
Variation in number of vomerine teeth is shown in Table 1.
[Illustration: FIG. 4. Tadpoles of the _Ptychohyla euthysanota_ group:
(A) _P. euthysanota euthysanota_ (KU 60042),
(B) _P. euthysanota macrotympanum_ (KU 60049),
(C) _P. leonhardschultzei_ (KU 68556), and
(D) _P. spinipollex_ (KU 60053).]