96 MR. D. M. S. WATSON ON THE 



in all non-iaammal-like reptiles, but which has arisen wit])in 

 the group, as it does not occur in Varanosaur us . 



The relation of the Theriodontia to the Dromosauria is obscure, 

 because of our very slight knowledge of the detailed skull-struc- 

 ture of the membei'S of that group. In general build the 

 Dromosaur skull difters from that of the Theriodouts and also 

 of the Pelycosaurs in the extreme shortness of the face. In this 

 feature, and also in the depth of the squamosal below the root 

 of the zygoma, it recalls the Dicynodonts, differing from them, 

 however, in retaining a very short temporal fossa. The Dromo- 

 saur skiiU recalls that of Bolosaiorus, and also still more strongly 

 that of Pakeobathria, if my interpretation of Credner's figures of 

 that animal be correct. There are in the structure of the 

 post-cranial skeleton no characters which show significant resem- 

 blances to any other South African form, and it seems not 

 improbable that the group represents the end membei'S of an 

 Artinskian group which survived into Upper Permian times. 



Thus, in my opinion, the three orders Deinocephalia, Dicyno- 

 dontia, and Theriodontia may have arisen from a common stock 

 whose direct conservative descendants are the Gorgonopsids, 

 and the Dromosauria may I'epresent a more widely separated 

 stock of the Anomodontia. 



One other line of argument which I have not before considered 

 is concerned with the dentition. 



In a discussion of the Dimeirodon dentition on p. 70, I pointed 

 out that it is desirable in a carnivorous animal Avith an enlarged 

 canine towards, but not actually on, the front end of the lower 

 jaw to have enlarged premaxillary teeth before the diastema to 

 balance the large canine behind that gap. In such earlier 

 Gorgonopsids as G'orgonops and Scymnognathus, the incisors are 

 actually of great relative size, much bigger than the cheek-teeth, 

 though, of course, not rivalling the great canines. 



In Deinocephalia, both Tapinocephaloids (e. g., Deiiterosa'arus) 

 and Titanosuchids(e. g., Anteosaitrus) , the incisors may be of very 

 great size, and in the former group very curiously converted into 

 eflfective crushing-teeth. Both these animals have an enlarged 

 canine, followed in the one case by a single molar, in the other 

 by a row of eight very small teeth ; in Deinosaunts{Gliorhizodon) 

 there is a single enlarged canine followed by ten smaller cheek- 

 teeth, all larger thaii the single incisor pi-eserved. 



Thus in this feature the Deinocephalia present a definite 

 resemblance to the Gorgonopsids, one which is shared also by 

 Dimetrodon. 



In later Gorgonopsids, as I have shown above, the incisors 

 become relatively smaller, and the " molars," although not 

 increased in number above the original five, become relatively 

 larger. 



There is some evidence of an irregular replacement of all the 

 teeth of a Gorgonopsid. 



