674 1)11. U. UllOOM ON THE STIUJCTUUE of TllE SKULL 



Classification of the Thekapsida. 



Classification of fossil animals must always be more or less pro- 

 visional, and will have to be constantly altered as new foims are 

 discovered, and as new light is thrown on previously imperfectly 

 known forms. And yet we must always have a classilication of 

 some sort to work hy. 



Watson, in his recent paper — " The Bases of Classification of the 

 Theriodontia" (P.Z.S. 1921), discusses at considerable length the 

 inter-relationships of the various groups of mammal-like reptiles. 

 The Theriodontia he divides into the following groups :— Gorgon- 

 opsidio, Therocephalida?, Cynognatliidfe, and Hauromorpha. With 

 til is "rouping 1 am in entire agreement, but 1 would prefer to 

 re^ai-d the groups not as families, but as suborders. Seeley 

 placed Diademodon and Oomphofjnailnts in a distinct order — the 

 Comphodontia. Few would be willing to follow him thus far, 

 but one feels that though they are doubtless allied to Cyno- 

 (/natlms, the differences are sufficiently great to justif}- their 

 being put in at least a distinct family. In the Therocephalians 

 also we have quite a lot of genera that can be conveniently 

 o-muped together in families within the suborder. If we regard 

 the Therocephalians as a Family merely, we cannot classify the 

 larae number of genera unless we resort to Subfamilies. All 

 living Jiirds resemble each other more closely than do the known 

 Therocephalians. Yet the living birds are subdivided into 

 Superordei-s, Orders, and Suborders. If the Therocephalians are 

 only entitled to be regarded as a Family, then all living birds 

 ought to be placed in one Family, which would be rather in- 

 convenient. If, however, we regard the T'herocephalians as a 

 Suborder, it seems to me no harm will be done, and it will be 

 much more convenient than looking on the group only as a 

 Family. 



Watson considers that "the three orders Deinocephalia, 

 Dicynodontia, and Theriodontia may have arisen from a common 

 stock whose direct conservative descendants are the Gorgonoj)sid!<, 

 and the Dromosauria may represent a more widely separated 

 stock of the Anomodontia." I expressed a somewhat similar 

 opinion in a paper I sent to the Williston Memorial Volume 

 before Watson's paper appeared. We will prol)al)ly, however, 

 require many connecting-links before we will be quite sure of the 

 lines of evolution of the Therapsids, and I fear we will never get 

 those links in South Africa. In onr oldest Karroo beds that have 

 land reptiles we get already typical Therocephalians, typical 

 Gorgonopsians, typical Dicynodons, and Dinocephalians. If we 

 ever get Ecca beds with good reptiles, we may then get the 

 common ancestors. 



We do not, however, need to know the origins of of the groups 

 to get a useful classification, and the one which I would propose 

 is as follows : — 



