THE SKULL OF REPTILES II 



canal, with the internal opening in front of the external. The in- 

 ternal nares, primitively (Figs. 6, 21, 47) divided by the prevomers 

 and surrounded by the premaxillae, maxillae and palatines, may 

 sometimes (Figs. 49, 71c) lie between the prevomers and palatines. 



The parietal or pineal foramen, very large in certain shell-eating 

 cotylosaurs (Fig. 22), had become inconstant even in that order. 

 It is present, so far as known, in all the Theromorpha (Figs. 33-42), 

 and in the Therapsida (Figs. 43-45), with the exception of a few 

 forms; in the Proganosauria, Ichthyosauria (Fig. 50), Sauropterygia 

 (Fig. 48), the Diaptosauria (Figs. 60-62), and most lizards (Figs. 

 55 A, 56), but is absent in some true lizards, the chameleons, and all 

 snakes (Fig. 59). It has been reported in certain doubtful Pseudo- 

 suchia and more or less doubtfully in a few phytosaurs and dinosaurs, 

 but with these possible exceptions appears to be absent in all the 

 Archosauria (Figs. 65 b, 66 b, 68, etc.) as also the Chelonia (Figs. 

 30, 31, 32). Usually located between the parietals anteriorly (Figs. 

 22, T^T,, 43, 44, 45, 46, etc.), it may occur between the frontals pos- 

 teriorly (Fig. 55). In the Anomodontia and Gorgonopsia (Fig. 43) 

 there is a separate bone, the preparietal, a small unpaired element of 

 doubtful homologies, absent in other reptiles, in front of or surround- 

 ing the foramen. 



The orbits, directed upward sometimes in aquatic animals (Fig. 

 32), but usually laterally, were primitively (Figs. 2, 3, 22, 23, 33, 43, 

 44, 65) surrounded by the prefrontal, postfrontal, postorbital, jugal, 

 and lacrimal. The frontal usually forms a part of the upper margin, 

 the maxillae sometimes below (Figs. 30, 48, 49, 55, 56, 59). In 

 snakes (Fig. 59), only the prefrontal and postorbital may be left. 

 Sometimes the postorbital bar is incomplete in lizards (Fig. 56), 

 snakes (Fig. 59), and therapsids (Fig. 45 d). The prefrontal is ex- 

 cluded in some dinosaurs, the postorbital in Araeoscelis (Fig. 52) 

 and Hyperodapedon (Fig. 62 d), leaving not a single element invari- 

 ably associated with the orbit. Antorhital or preorhital vacuities are 

 very characteristic of the Archosauria, occurring in all phytosaurs 

 (Figs. 66 b, 67 a) and true pseudosuchians (Fig. 65 b, d), most 

 Saurischia (Fig. 70 a, b) and Pterosauria (Fig. 71) and some Croco- 

 dilia. Usually there is but one, but there may be two or even three 

 on each side in certain Theropoda (Fig. 70 a). 



