SKELETON. 87 



floor of the cranium is completed by a -shaped parasphenoid, which extends to 

 the premaxillaries in the aglossa, elsewhere only to the sphenethmoid. 



In the lower jaw there is a mento-Meckelian in front, followed by dentary and 

 angulare, Meckel's cartilage persisting through life. The larval branchial and 

 hyoid arches are typical, there being four gill arches. With the loss of gills the 

 posterior arches disappear, and the broad hyoid plate of the adult has four processes 

 which are new formations. 



REPTILES. The skull of existing reptiles is very different from that of amphib- 

 ians, but that of many theriomorphs is strikingly like that of the stegocephalans. 

 The principal differences alluded to in the first sentence have arisen by reduction 

 and disappearance of bones appearing in the more primitive types, but aside from 

 these there is little except the parasphenoid to separate the two groups. 



FIG. 89. Chondrocranium of Sphenodon, stage 'R,' after Howes and Swinnerton. 

 ep, epipterygoid; es, ethmosphenoidal plate; ex, extranasal cartilage; exp, extranasal process; 

 h, hyoid; ntk, Meckel's cartilage; nc, nasal capsule; oc, otic capsule; pt, pterygoid; q, quad- 

 rate; sb, subnasal process; 1-5, exits of nerves. 



The chondrocranium is known in but a few forms and these agree with other 

 amniotes in being tropibasic, except in snakes and amphisbaenans (see fig. 62). 

 In the adults cartilage largely disappears, except in the ethmoid region, more 

 persisting in Sphenodon (fig. 89) and the lizards than elsewhere. All four occipitalia 

 are ossified, but some may not participate in framing the foramen magnum, the 

 basioccipital being excluded in many chelonians, the supraoccipital in snakes, 

 crocodiles and theriomorphs. There is but a single occipital condyle (except in a 

 few theriomorphs), which is borne on the basioccipital as in the crocodiles, or on 

 this and the exoccipitals as in chelonians and squamata. Basi- and presphenoids 

 are present, orbito- and alisphenoids are but slightly ossified and the ethmoid region 

 is largely cartilaginous. Pro-, epi- and opisthotics are present, the epiotic fusing 

 with the supraoccipital, while the opisthotic in all recent forms except the turtles 

 unites with the exoccipital in the adult. 



In all except the squamata, in which it is movable (streptostylic), the quad- 



