THE SKULL OF REPTILES 1 3 



(Fig. 21 b), and Theromorpha (Fig. 45 g), and is generally present 

 in later reptiles though absent or vestigial in the Crocodilia. 



The cranial region thus exposed by these various openings has 

 been exposed to a greater or less degree in most Chelonia (Figs. 30- 

 32) in a different way: by the emargination of the roof bones from 

 behind or from behind and below, until, in some forms like the 

 terepenes, the whole temporal roof is lost. 



Posterior palatine or suborbital openings occur in most reptiles 

 (Figs. 55, 63, 66, 69, 72) since the Theromorpha, but are absent in 

 some turtles. They are situated between the palatines and maxillae 

 posteriorly, and are usually also bounded in part by the ectoptery- 

 goids. They do not occur in the Cotylosauria (Figs. 6, 21 a, 24, 

 29) or Theromorpha (Figs. 40 c, 42 c), though present in many 

 Therapsida. 



The Skull Elements 



The primitive relations of the skull elements may be discussed seri- 

 atim, with their chief modifications in later reptiles. 



Premaxillae (px). Primitively short (Figs. 2, 3, 4, 19, 22, 7,3, 43)- 

 articulating with maxillae, nasals and prevomers, the posterior 

 process forming a partial division between the nasal chambers. 

 They form the anterior boundary of the external and internal nares. 

 Four or five teeth in each. 



Elongate in the strictly aquatic reptiles (Figs. 46, 47) and in the 

 Pterosauria. In the plesiosaurs (Figs. 46, 47), pterosaurs (Fig. 72 a), 

 some lizards (Fig. 56), and thalattosaurs (Fig. 61), a median pro- 

 longation separates the nasals, articulating directly with the frontals, 

 in the first group (Fig. 46) sometimes directly with the parietals, 

 separating the frontals. Edentulous in the chameleon lizards (Fig. 

 55), they take no part in the boundary of the nares. They are also 

 edentulous in the turtles (Figs. 30-32), anomodonts (Fig. 44 c), 

 some dromasaurians, the later pterodactyls (Fig. 72), most orni- 

 thischians (Fig. 700,0), the chameleon lizards, and many snakes. 

 Teeth, when present, are in a single row and rarely exceed five or six 

 in number in each, though there are as many as twenty- three in 

 some phytosaurs (Fig. 66) and even more in ichthyosaurs, where the 

 dentigerous border is greatly elongated. The dentigerous part is 

 short in the long-faced plesiosaurs (Fig. 48 c). They are often fused 

 (Fig. 72), and sometimes united with the nasals (Fig. 54 c). 



