SKELETON 



95 



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

 amphibians, but that of many theriomorphs is strikingly like that of the stego- 

 cephalans. 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. 



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

 amniotes in being tropibasic (see fig. 70) except in snakes and amphisbaenans. 

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

 persisting in Sphenodon (fig. 98) and the lizards than elsewhere. All four 

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



Fig. 98. — Chondrocranium of Sphenodon, stage "R," after Howes and Swinnerton. 

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

 process; h, hyoid; mk, Meckel s cartilage; nc, nasal capsule; oc, otic capsule; pi, pterygoid; 

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



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- 

 rate b firmly united to the squamosal and sometimes to other bones (monimo- 

 stylic). The pterygoids extend forward to the palatines. In the squamata and 

 the ichthyosaurs pterygoids and palatines are widely separated in the middle 

 line, but elsewhere they are closely approximate, the pterygoids even meeting 

 the basisphenoid. In all except chelonians, some dinosaurs and the typhlopida 

 an ectopterygoid (os transversum) extends from pterygoid to maxilla, while in 



