DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 97 



they are slight, but dense; there is a small septo-maxillary (fig. G, 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, 2^0-8.) is not so attenuated in front, 

 nor do the wings turn backwards as in the last kind ; the tract between the wings is 

 st)-ongly 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 la.st, but they are broader, and with 

 smaller notches m their margin. To the discrepancies between the last and the type, 

 we must add (l) tlie 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. Pleurodema Bihronii. — 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 

 magnimi (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 ti'act of cartilage running over 

 the crest of the posterior canal (jj.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 ti-act (fig. 1) ; this latter is a large wedge-shaped tract (t.cr.) 

 which runs to, and forms the hiiid margin of, the great single fontanelle {fo.). 



This is bounded by the ex-occipitals behind and by the prootics in front [j^r.o., e.o.), 

 and the latter nm forwards just as far as the cartilage above, up to the optic fenestra 

 (II.); they therefore take in the whole " alisphenoidal " wall as weU as the "prootic" 

 region. The "tegmen tympani" {t.ty.) is extended far outwards, and this parotic 

 pi-ojection is largely cartilaginous; beyond the horizontal canals [h.s.c.) 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 {vh.), with its fenestra and 

 cartilaginous plug or stapes {st.). 



MDCCt'LXXXI. o 



