THE SKELETON IN REPTILES. THE SKULL. 281 



much. In many reptiles, such as the Squamata and Ichthyo- 

 sauria, the palate is not complete, both palatines and pterygoids 

 being widely separated in the middle line. In others, such as 

 the Crocodilia, Sauropterygia, and Chelonia, there is a more 

 or less complete bony palate. In many Chelonia this is 

 chiefly formed of the vomer, palatines, and pterygoids, the 

 posterior nares being mainly bounded by the palatines. In 

 living Crocodilia, however, outgrowths are formed from the 

 pterygoids and palatines which arch round and meet one 

 another vent rally, forming a secondary palate (fig. 43, A), 

 which completely shuts off the true sphenoidal floor of the 

 skull, and causes the posterior nares which are bounded by 

 the pterygoids to open very far back. Though this feature 

 is common to all postsecondary crocodiles, it is interesting to 

 notice that it is not found in the earlier forms, but that its 

 gradual evolution can be traced. In the Triassic Belodon, for 

 instance, the posterior nares open far forwards, and are not 

 surrounded by either the palatines or pterygoids. In the 

 Jurassic crocodile, Teleosaurus, the posterior nares lie further 

 back, being surrounded by the palatines, but the pterygoids 

 do not meet them. Finally, in the Tertiary forms the arrange- 

 ments are as in living crocodiles. 



A short secondary hard palate is found also in the Therio- 

 dontia. The palatines of Ichthyosaurus are noticeable for their 

 transverse position, which recalls that in the Frog. 



The various fossae or vacuities in the false roof of the 

 skull are important, and their relations may best be understood 

 by a description of their mode of occurrence in Sphenodon, a 

 form in which they are very completely developed. 



In Sphenodon, then, on the dorsal surface of the skull, are 

 the large supratemporal fossae (fig. 52, 20). Their inner 

 margins are separated from one another by the parietal walls 

 of the cranium, while externally each is bounded by a bony 

 arch, the supratemporal arcade, formed of the postfrontal, 

 post-orbital, and squamosal. Posteriorly the boundary is 



