140 



VERTEBRATE SKELETON 



development of several living reptiles, fusing later with either supra- 

 or exoccipital. When the supratemporal bone is lacking the squa- 

 mosal extends to the parietal. 



The floor of the cranium is shown in diagram in figure 148. 

 Beginning behind, in the median hne are basioccipital and basisphen- 

 oid, preformed in cartilage and ossified in the adult. Usually 

 (Ichthyosaurs, Squamata, Rhynchocephals, Dinosaurs) the basi- 

 sphenoid is continued forwards by a slender 'rostrum,' apparently 

 cartilage in origin and representing the presphenoid; it supports the 



interorbital septum. The bas- 

 ioccipital is flanked on either 

 side by exoccipital and opistho- 

 tic, the latter meeting the 

 quadrate at the postero-lateral 

 angle of the cranium. A ptery- 

 goid, dermal in origin, extends 

 forwards from the quadrate, 

 and in more specialized reptiles 

 comes into more or less intimate 

 relations with the basisphenoid, 

 usually by a basipterygoid 

 process of the latter. Still 

 farther forwards the pterygoid 

 series is continued by a (dermal) 

 palatine, and this, in turn, by a 

 vomer. In most living groups 

 an OS transversum extends from 

 pterygoid to maxilla. The only 

 pterygoid element of cartilage 

 origin is the epipterygoid 

 (columella cranii) occurring, among recent species, in most lizards 

 and in Sphenodon. It extends from the dermal pterygoid up to the 

 lower side of the parietal. It was probably present in some Thero- 

 morphs, Ichthyosaurs and Dinosaurs. 



The rostrum (part of which may be parasphenoid) is smallest in monimostylic 

 species. The basisphenoid is visible in its whole extent in Squamata, partially 

 in Chelonia; in Crocodilia it is largely covered by the pterygoids. Chelonia, 

 Plesiosaurs, Theromorphs and Pterosaurs have no transversum, a bone which 

 is not the same as the ectopterygoid of fishes as sometimes stated. 



Fig. 148. — Scheme of floor of reptilian 

 cranium (Biitschli, 'lo). bo, basioccipital; 

 bp, basipterygoid process; bs, basisphenoid 

 and rostrum; w, maxilla; pm, premaxilla; pa. 

 palatine; pt, pterygoid; 9, quadrate; qj, quad- 

 ratojugal; /, transversum; y, vomer; 2, 

 zygomatic. 



