THE SKELETON IN REPTILES. THE SKULL. 277 



by a secondary or false roof of membrane bones, which is best 

 seen in the Ichthyosauria and some of the Chelonia. In other 

 reptiles the false roof is more or less broken up by vacuities 

 exposing the true cranial walls. The ethmoidal region is the 

 only one in which much of the primordial cartilaginous cranium 

 remains. The lateral parts of 'the sphenoidal region are also 

 as a rule not well ossified. 



In some reptiles, such as most Lacertilia and Chelonia, the 

 orbits are separated only by the imperfect interorbital septum, 

 while in others, such as the Ophidia, Crocodilia and Amphis- 

 baenidae, the cranial cavity extends forwards between the 

 orbits. 



In the occipital region all four bones are ossified. The 

 great majority of reptiles have a single convex occipital 

 coiidyle, but some of the Theromorpha such as Cynognatlius 

 have two distinct condyles as in mammals. Sometimes, as 

 in Chelonia, Ophidia and Lacertilia, the exoccipitals, as well 

 as the basi-occipital, take part in the formation of the single 

 condyle ; sometimes, as in Crocodiles, it is formed by the basi- 

 occipital alone, as in birds. The relations of the bones to the 

 foramen magnum vary considerably, in Chelonia the basi- 

 occipital generally takes no part in bounding it, and in the 

 Theromorpha, Crocodilia, and Ophidia, the supra-occipital is 

 excluded. The parietals are paired in Geckos and Chelonia 

 alone among living forms, and in the extinct Ichthyosauria 

 and some Theromorpha ; in all other reptiles they are united. 



The frontals are paired in Ichthyosauria (fig. 32, 5), 

 Chelonia, Ophidia, Sphenodon (fig. 52, B, 4) and some extinct 

 crocodiles, such as Belodon. They are completely fused in 

 living Crocodilia and some Lacertilia and Dinosauria. In 

 the gigantic Polyonax they are drawn out into a pair of 

 enormous horns, and the parietals and squamosals are greatly 

 expanded behind. 



An interparietal foramen occurs in the Theromorpha, the 

 Ichthyosauria (fig. 32, 10), Sphenodon, the Sauropterygia and 



