94 Transactions of the South African Philosophical Society. 



Only one Theriodont tarsus is known — that of Microgomphodon (13) 

 — and unfortunately it is not satisfactorily preserved. It agrees, 

 however, with the Dicynodont tarsus in having the tibiale and 

 fibulare of large size. According to Seeley the calcaneum (=fibulare) 

 " does not' develop a posterior heel process." As, however, only the 

 anterior or upper surface of the tarsus seems to be displayed, it is 

 difficult to see what evidence there is for this statement. In the 

 Dicynodont fibulare there is a distinct heel process, and it is highly 

 probable that one also exists in the Theriodonts. There is in the 

 tarsus of Microgomphodon a wide space between the tibiale and the 

 first tarsale, so that the condition of the Theriodont tarsus is pro- 

 bably very similar to that in the Dicynodont, there being evidently 

 either a cartilaginous centrale or a bony centrale which is lost from 

 the specimen. 



While the mammalian carpus has become very slightly specialised, 

 and is hence of little service in guiding us to the mammalian ancestor, 

 the specialisation of the tarsus is so peculiar that all claimants to the 

 honour of being the immediate forefathers of the mammals may be 

 dismissed if they do not show some approximation to a similar 

 specialisation. We may thus put on one side all modern types of 

 reptiles and all the Amphibia, none of which can have any claim to 

 be the immediate mammalian ancestor, and we have left the Cotylo- 

 saurians, some of the primitive Diaptosaurians, the Therocephalians, 

 and the Dicynodonts and Theriodonts. In these latter we find more 

 or less approximation to the mammalian type, but if we take into 

 consideration the extreme mammalian specialisation— the presence 

 of a large tibiale and fibulare with a centrale which is not in the 

 centre but comes between the tibiale and the first tarsale — then we 

 are driven to the conclusion that the mammalian ancestor must have 

 been a Dicynodont, a Theriodont, or a form belonging to a closely 

 allied order. From the examination of the skull we have good 

 reason to believe that the ancestor was a Theriodont, and the 

 evidence of the tarsus fully confirms that derived from the skull 

 and other parts of the skeleton, and the carpus, while it does not 

 add any very strong evidence, certainly does not afford any evidence 

 that is not also in harmony with this conclusion. 



Addendum (Oct. 20, 1904) : — Case has recently published a short paper " On the 

 Structure of the Fore Foot of Dimetrodon " (Journ. Geol., vol. xii., No. 4, May- 

 June, 1904), in which he figures an almost perfect carpus, and shows that it is 

 strikingly like the carpus of Procolophon. The only differences of importance are 

 that in Dimetrodon the radiale is large and fully ossified, and that there is a 

 distinct ossified fifth carpale, — R.B. 





