SKULLS OF CYNODONT REPTILES. 917 



Peculiarities of the Mammalian Shull, apparently derived 

 from a Cynod^ont Ancestor. 



Most of the bones of the mammalian skull have their homo- 

 logues in the Labyrinthodont skull, but they are also to be found in 

 the skulls of most reptiles. There is, however, no close resemblance 

 between the mammalian and the batrachian bones, and in many 

 cases the differences in cranial structure are so great that the 

 gap between the mammal and any known batrachian must be 

 enormous. When we examine the Cotylosaurian skull, we find that 

 the resemblance to that of the mammal is still remote, but any 

 little resemblance there was in the Labyrinthodont is here 

 increased, while there are many mammal-like characters not seen 

 in the lower type. 



The Pelycosaurians of the Lower Permian are so much more 

 mammai-like than any of the lower forms that, notwithstanding 

 their remarkable specialisations, one cannot help feeling, as Cope 

 felt, that here were forms fairly near to the remote mammalian 

 ancestor. 



The Therocephalians and Anomodonts of the Middle Permian 

 times are in essentials still more mammal-like. For the first time 

 we get a dentition clearly divided into incisors, canines, and 

 molars ; for the first time we get a lower jaw with a dentary which 

 has a large coronoid process. We get a zygomatic arch formed on 

 the mammalian type, and we lose for the first time the quadrato- 

 jugal. We also get most marked mammalian chaiucters in the 

 postcranial skeleton. 



The Therocephalians survived into Upper Permian times, but 

 hitherto they have not been found in Triassic beds. In Upper 

 Triassic times their place was taken by the Cynodouts. Though 

 the gap between the Therocephalians at present known and the 

 Cynodonts is very considerable, the primitive Oynodont Bauria is 

 to some extent a connecting link. 



Almost all the characters in which the Cynodont skull diflfers 

 from the Therocephalian are characters which are met with in 

 Mammals. Of these the most noteworthy are : — 



1. Formation of a secondary palate. 



2. Vomer very large, extending forward as a support to the 



secondary palate. 



3. Great reduction or complete loss of prevomers {Bauria). 



4. Loss of the postfrontal bone. 



5. Great reduction or loss of the pineal foramen. 



6. Two occipital condyles. 



7. Reduction of the quadrate. 



8. A large alisphenoid bone instead of the homologous rod- 



like " epipterygoid " or columella cranii of the Thero- 

 cephalians and Anomodonts. 



9. Pterygoids not extending back to the quadrates, the 



posterior extension being replaced by the alisphenoids. 

 10. Reduction of the angular and surangular, and greater 

 development of the dentary. 

 Proc. Zool. Soc— 1911, ^o. LXIII. 63 



