278 Transactions of the South African Philosophical Society. 



and has no serrations. It would thus seem that Chelyoposaurus 

 bears a similar relation to Prodicynodon that Opisthoctenodon does 

 to Pristerodon. 



Affinities of the Endothiodonts. 



It will be observed that while the various Endothiodonts have 

 many characters in common, they can be divided into two groups, 

 the members of which are much more closely related to each other 

 than to the members of the other group. Thus it is manifest that 

 Esoterodon is closely related to Endothiodon and Cryptocynodon, 

 and that Pristerodon is a near ally of Opisthoctenodon. Prodi- 

 cynodon, though less specialised than the other genera, comes nearer 

 to Pristerodon than to Cryptocynodon. 



We may imagine the common ancestor of the Endothiodonts to 

 have been a form somewhat intermediate between Cryptocynodon 

 and Prodicynodon, with well-developed canines and with a single 

 row of molars serrated in front and behind. By one line of descent 

 there arose Endothiodon, the intermediate stages being Crytocyno- 

 don and Esoterodon. A second line gave rise to Pristerodon and 

 Opisthoctenodon, where the molars are only serrate behind. In the 

 third line the molars became degenerate as in Prodicynodon, and 

 finally lost as in Dicynodon and Oudenodon. Though there is great 

 probability that Dicynodon is descended from Prodicynodon, or a 

 closely allied form, there is considerable doubt as to whether 

 Oudenodon is descended from Dicynodon by the loss of the tusk. 

 The tuskless Endothiodonts, such as Opisthoctenodon, resemble 

 Oudenodon much more closely in general characters than does 

 Dicynodon, but it may be that the loss of the tusk in Opistho- 

 ctenodon and Oudenodon has brought about a similar development 

 in the two genera. In the meantime we may regard Oudenodon as 

 bearing the same relations to Dicynodon that Opisthoctenodon does 

 to Pristerodon. 



The discovery of the small Endothiodonts has pretty well bridged 

 over the gap between Endothiodon and Oudenodon, and the ques- 

 tion might well arise, are we justified in putting Endothiodon in a 

 different family from Oudenodon ? Had Dicynodon been a terminal 

 type it would have been convenient to have put it in the same family 

 as the Endothiodonts, but no sooner would it appear were the molars 

 lost than a large number of molar-less forms arose — Lystrosaurus, 

 Kistecephalus, Gordonia, Geikia, and probably one or two others. 

 It therefore seems advisable to place those forms which apparently 

 have sprung from Dicynodon or a closely allied form in one family — 



