108 COMPARATIVE ANATOMY 



portion (" lower labial cartilage " of larva) is bent inwards towards 

 the median line and unites with its fellow in a symphysis, forming 

 in the adult the mentomandibular (p. 84). 



There is a much greater reduction of the branchial skeleton at 

 the close of larval life than inUrodeles. In the larva, representatives 

 of the hyoid and of four branchial arches can be recognised, but 

 these are all united together and form a continuous structure (Fig. 

 79, A). The greater part of the broad basal parts of this 

 apparatus, as well as the four branchial arches, disappear during 

 metamorphosis. The hyoid cartilage of the adult (B, c) is 

 formed partly from the remains of the hyobranchial cartilages of 

 the larva and partly by new outgrowths from it. 



Reptiles. 



The skull in Reptiles is extremely complex and varied as 

 regards its bones and their relations. Although differing markedly 

 in many important respects from the cranial skeleton of Am- 

 phibians, the ground-form of the latter is distinctly recognisable, 

 especially in the primitive Hatteria and in Lizards. On the other 

 hand, numerous points of similarity are seen in the skull, as 

 well as many other parts, of Reptiles and Birds, which are, 

 therefore, included together under the term Sduropsida. 



In spite, however, of the similarity of plan of the amphibian 

 and reptilian skull, it must be borne in mind that no recent 

 Amphibian lies on the direct line of descent of the Reptiles, 

 though certain fossil Amphibians (Stegocephali) and Reptiles, as 

 well as the existing Hatteria, help to bridge over the space between 

 the two Classes. 



In order not to cause confusion by reference to the multi- 

 farious details which present themselves in dealing with the 

 reptilian skull, it will be as well to consider first its more 

 important characteristics, many of which are common to the 

 Amniota in general, before treating specially of the various Orders. 

 In this general description, the lacertilian skull will be chiefly 

 referred to as a typical form (Figs. 80 and 82). 



Apart from its naso-ethmoidal region, the chondrocranium 

 plays no important part in Reptiles subsequently to the embryonic 

 period, and it no longer forms such a complete structure as, e.g., 

 in Anura, but is considerably reduced and frequently largely 

 fenestrated (Fig. 80). This want of completeness, however, is 

 later partly compensated for by the investing bones, and as the 

 ossification is very considerable, a firm and solid skull results. 



The cranium includes three more vertebral elements than in 

 Amphibia (p. 97), so that the foramina for the three roots of the 

 compound hypoglossal nerve perforate the skull. In all Amniota 



