io REPTILES 



disappeared as a functional element in the mammal. In the 

 true theriodonts the den tary element is much larger proportion- 

 ally than in ordinary reptiles ; and while in Lycosuchus the sur- 

 angular, angular, and articular elements of the hinder part of 

 the lower jaw are fairly well represented, in Cynognathus the 

 former has vanished, and the two latter are evidently on the 

 point of following its example. In the words of the author : — 



" The examination of the theriodont lower jaw and of its 

 mode of articulation show that the condition is already so 

 nearly mammalian that only a very slight modification, and 

 that very easily understood, is required to convert the therio- 

 dont lower jaw into that of the mammal." 



In certain of the earlier and most primitive types of 

 reptiles, such as Pariasaurus and Procolophon among the mam- 

 mal-like group, the upper surface of the skull is more or less 

 completely roofed in by superficial, or membrane, bones, so 

 that, as in the former of these, the only apertures in this roof 

 are those for the eyes, the nostrils, and the median parietal 

 (or interparietal) foramen. In this respect the skulls of these 

 primitive types resemble those of the primeval salaman- 

 ders, or labyrinthodonts (Stegocephalia). On the other hand, 

 more advanced types show a gradual opening-out of this 

 roof, as if portions had been cut away with a knife, till 

 finally the whole of the upper surface of the cranium proper is 

 exposed. In some cases a single temporal arch, as in the 

 Anomodontia, Chelonia, and Sauropterygia, remains to protect 

 and strengthen the lateral region of the skull, but more 

 generally, as in Crocodilia and Rhynchocephalia, there are 

 two such temporal arches, with a space between. The ac- 

 companying illustrations show the closed and open types of 

 skull. Among the more specialised orders, the Ichthyo- 

 pterygia retain to a large extent the original in-roofing of the 

 skull. It should be .added that in the labyrinthodonts the 

 upper surface of the skull displays a characteristic sculpture, 

 which is retained in Pariasaurus and possibly also in croco- 

 diles. 



With regard to the above-mentioned parietal foramen, 

 which occurs in many reptiles, and attains unusually large 

 dimensions in the tuatera and the ichthyosaurs, it should be 

 explained that this is a perforation in the forehead between 



