DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 
97 
they are slight, but dense; there is a small septo-maxillary (fig. 6, s.mx.). The post- 
orbital process of the squamosal (sq.) is straight, the supra-temporal tract small, the 
descending part normal. The parasphenoid (fig. 7, pa.s.) is not so attenuated in front, 
nor do the wings turn backwards as in the last kind ; the tract between the wings is 
strongly ribbed in a cruciform manner, and the whole hinder part is largely anchylosed 
to the ossified endocranium. 
The vomers (fig. 7, v.) are similar to those of the last, but they are broader, and with 
smaller notches in their margin. To the discrepancies between the last and the type, 
we must add (1) the highly osseous state of the skull, (2) the ossified ethmo-palatines 
and quadrates, (3) the anchylosed parasphenoid, (4) the fibrous supra-stapedial, and (5) 
the additional centres of the basal plate. 
Fourth genus. Pleurodema. 
22. Pleuroclemct Bibronii. —Adult female ; 1\ inch long. Chili. 
The skull of this species is very evenly semi-oval, and its greatest breadth is to its 
length as 8 to 7 ; it, therefore, is one of the short skulls (Plate 18, figs. 1, 2). 
At first sight its main difference from the “norma” seems to consist in its open 
fontanelle (fo.) and the absence of the two secondary fontanelles, which open, like 
sky-lights, in the hind skull of more typical forms. 
More attentive observation, however, brings out several aberrant characters. 
The occipital condyles, the basi- and superoccipital tracts of cartilage, and the foramen 
magnum (figs. 1 and 2, oc.c.,f.m .) are all normal. The occipito-auditory region is more 
extended, laterally, and more ossified than in the type. 
Moreover, there is here a character which I have already described in another 
aberrant Neotropical Frog, namely, Pseudis ; this is a tract of cartilage running over 
the crest of the posterior canal ( p.s.c ., e.o.), and which, in Pseudis , is due to segmenta¬ 
tion of the originally continuous bony matter answering to both prootic and ex-occipital 
—that is an unmistakably archaic character. The basioccipital cartilage (fig. 2) is only 
half the width of the upper tract (fig. 1); this latter is a large wedge-shaped tract ( t.cr .) 
which runs to, and forms the hind margin of, the great single fontanelle (fo.). 
This is bounded by the ex-occipitals behind and by the prootics in front (pr.o., e.o.), 
and the latter run forwards just as far as the cartilage above, up to the optic fenestra 
(II.); they therefore take in the whole “ alisphenoidal” wall as well as the “prootic” 
region. The “tegmen tympani ” (t.ty.) is extended far outwards, and this parotic 
projection is largely cartilaginous; beyond the horizontal canals (h.s.e.) the auditory 
region loses one-third of its breadth, and this is bevelled off from the hind margin, 
mainly. In front the cartilage is nearly one-third the breadth of the whole auditory 
region; behind, it is nearly one-half, that is above. But below (fig. 2) the bony matter 
reaches to the edge of the sub-convex vestibular region (vb.), with its fenestra and 
cartilaginous plug or stapes (st.). 
mdccclxxxi. o 
