98 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



temporal region. In the lower cotylosaurs the cranial roof is without fossae 

 (Broom doubts the infratemporal fossa of Frocohphon). In other theromorphs 

 quadratojugal and supratemporal are lacking, the squamosal meeting the 

 parietal. Placodus has only the supratemporal fossa, but in the majority the 

 upper arcade has disappeared, leaving a large temporal vacuity, much as in 

 mammals. 



Little is known of the lower jaw. The bones are sometimes discrete, some- 

 times extensively fused. The teeth are thecodont, and in the theriodonts are 

 differentiated into incisors, canines and molars, but in the anomodonts teeth 

 are absent, or at most there is a pair of large incisors in the upper jaw. 



mx 



Fig. ioi. — Dorsal and ventral views of the skull of turtle, Trionyx. bo, basioc- 

 cipital; bs, basisphenoid; eo, es, exoccipital; /, frontal; j, zygomatic (jugal) ; m, mx, maxil- 

 lary; «, nasal; opis, opisthotic; p, (behind naris) prefrontal, (others) parietal; pal. 

 palatine; pmx, premaxillary; pno, postorbital; pt, pterygoid; q, quadrate; s, supraoccipital 

 p, vomer. 



In the Plesiosaurs (fig. loo) and their allies the ^kuII is about a twelfth of 

 the total length. There is a parietal foramen between the parietals, which have 

 a process for articulation with the squamosal, the supratemporal being absent. 

 The large prefrontals intervene between the frontals and the orbits; lacrimals 

 and nsually nasals are absent. The large temporal fossa is bounded externally 

 by the zygomatic which extends back to the quadrate. The choanae are in front 

 of the palatines; an os transversum is present and there is frequently a para- 

 sphenoid in the interpterygoid vacuity. All have a subtemporal vacuity and 

 there is another in the plesiosaurs in the angle between palatine and transversum. 

 The usual bones are frequently distinct in the lower jaw-. 



In the Chelonians the cranial cavity extends forward between the eyes and 

 the mesethmoidal cartilage largely persists in the adult. Although the bones 



