I50 



VERTEBRATE SKELETON 



directed downwards and backwards, has no columellar notch and is covered 

 externally for most of its length by the squamosal. 



The basisphenoid is small with (usually) a small parasphenoid in front of it, 

 lying in a pterygoid vacuity. The pterygoids are large, meet in front, and extend 

 forwards in the middle line to the vomers, separating the palatines; behind, they 

 articulate with the basipterygoid process of the basisphenoid. A transversum 

 with the pterygoid and palatine, bounds a suborbital vacuity in some species, 

 while a large subtemporal vacuity lies between pterygoid and zygomatic. The 

 choanae are just in front of the palatines and between vomers and maxillae. 

 The thecodont teeth are confined to the margins of the jaws in Plesiosaurs. 



Fig. 159. — Skull of Plesiosaurus tnacrocephalus (Andrews), aw, angulare; ar, 

 articulare; bo, basioccipital; bs, basisphenoid; ch, choana; d, dentale; fr, frontal; iptv, 

 interpterygoid vacuity; j, zygomatic; 7nx, maxilla; orb, orbit; pa, parietal; pal, palatine; 

 pas, parasphenoid; po + pof, postorbital and postfrontal; ptnx, prema.xilla; prf, pre- 

 frontal; pt, pterygoid; q, quadrate; sang, surangulare; sf, supratemporal fossa; sov, 

 suborbital vacuity; sq, squamosal; tr, transversum. 



The separate bones of the lower jaw are indistinguishable as a rule, but in 

 Polycotylus dentale, angulare, surangulare, goniale and articulare are recognized. 

 ICHTHYOSAURIA have skulls (fig. 160), evidently adapted for an aquatic 

 life, the snout being very long and formed almost wholly of premaxillae. The 

 single temporal fossa, bounded laterally by supratemporal and postfrontal, has the 

 squamosal entering but slightly or not at all into the arcade. Sclerotic bones 

 occur in the large orbits which are close to the nares, only the lacrimal inter- 

 vening. Frontals and parietals are short, the parietal foramen lying between 

 the four bones. The short maxilla forms a small part of the upper jaw, but 

 enters the border of the orbit. Pre- and postfrontal meet above the orbit, 

 excluding the frontal from its margin, while the posterior and inferior borders 

 are formed by postorbital and zygomatic. 



