xm 



PHYLUM OHORDATA 



337 



elements of the plastron of the Chelonia are probably of a similar 

 character. 



In the skull ossification is much more complete than in the 

 Amphibia, the primary chondrocranium persisting to a considerable 

 extent only in some Lizards and in Sphenodon ; and the number 

 of bones is much greater. The parasphenoid is reduced, and its 

 place is taken by the large basioccipital, basisphenoid, and pre- 

 sphenoid. 



A fairly typical Lacertilian skull has been described in the case 

 of Lacerta. Its principal characteristic features are the presence of 



B 



Four 



FlG. 990. Skull of C'ulubriiH' Snake (Tropidonotus natrix). A, from above ; B, from below 

 Ag. angular ; Art. articular ; Bp. basi-occipital ; Bs. basi-sphenoid ; Ch. internal nares ; 

 Coco, occipital conclyle ; Dt. dentary ; Eth. ethmoid ; F. frontal ; F'. post-orbital ; Fov. 

 fenestra ovalis ; J/. maxilla ; N. nasal ; 01. exoccipital ; Osp. supra-occipital ; P. parietal ; 

 Pe. periotic ; P. f. pre-frontal ; PI. palatine ; Pmx. premaxilla; Pt. pterygoid; Qu. 

 quadrate; SA. supra-angular; Sgu. squamosal: Ts. transverse; Vo. vomer ; //, optic 

 foramen. (From Wiedersheim's Comparative Anatomy.) 



an inter-orbital septum, the presence of the epipterygoid, and the 

 mobility of the quadrate. The last of these features it shares with 

 that of the Ophidia. The epipterygoid is not universal in the 

 Lacertilia, being absent in the Geckos, the Amphisbsenians, and the 

 Chamseleons. The quadrate is not always movable. The skull of 

 the Chamseleons has a remarkable helmet-like appearance owing to 

 the development of processes of the squamosal and occipital regions, 

 which unite above the posterior part of the cranial roof. The skull 

 of the Amphisbsenians differs from that of other Lacertilia and 

 approaches that of Snakes in the absence of an inter-orbital septum. 

 In the skull of the Ophidia (Fig. 999) orbitosphenoidal and 



