220 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



favors condylarth rather than aniblypod affinities for the periptychids, 

 and the Hmbs might perhaps support either view equally well and cer- 

 tainly do not oppose condylarth affinities. The recognition of the 

 affinities of Coriphagus, as discussed under that genus, adds to the 

 evidence for the opinion supported here. 



It has been generally recognized that the teeth of the periptychids 

 could not give rise to those of pantolambdids or coryphodonts. They 

 are in fact aberrant and developing along a line, or series of fines, of 

 their own. It has sometimes been recognized, and can readily be 

 shown, that their greatest resemblance is with the Condylarthra and 

 that they could all be immediately derived from types well known in 

 that order. Indeed, they intergrade with certain mioclaenids to such 

 a point that the families are difficult to distinguish on this basis. The 

 pantolambdid-coryphodont dentitions, on the other hand, are widely 

 different. Union of periptychids and pantolambdids has, then, rested 

 entirely on fimb, and especially on foot structure. Indeed, without 

 slighting the fact that other resemblances occur, it has depended more 

 on the astragalus than on any other point. If this arrangement is a 

 natural one, it seems necessarily to imply that the "taligrade" astraga- 

 lus and fimb structure arose in a stock with extremely primitive teeth 

 and that the widely divergent periptychid and pantolambdid denti- 

 tions developed later. Such a thesis seems a priori rather improbable, 

 but certainly it is not impossible. The apparently, but I think falsely, 

 analogous case of the divergence of, say, suid and camelid dentitions 

 after the artiodactyl foot structure arose suffices to demonstrate that 

 such a history is conceivable. 



The analogy is probably false and the thesis indefensible because 

 in the case of the artiodactyls the teeth, followed back in time, dis- 

 tinctly converge and are rather plainly derivable from a common type 

 possessed by animals that already had all the essential artiodactyl Hmb 

 characters. This is not true of the periptychids and pantolambdids. 

 Even within the fimits of the Periptychidae there are forms with hardly 

 any suggestion of the "taligrade" foot, but wdth teeth much too dis- 

 tinctly periptychid to give rise to the pantolambdids. A common 

 ancestor, if it existed at all, can hardly have had taligrade feet but 

 must alm.ost certainly have been a condylarth and a very primitive 

 condylarth. A review (table 50) of typical astragali of the groups 

 conceived will make the situation clearer. 



These genera are all of about the same age. The Hyopsodontidae 

 are represented by isolated Gidley Quarry specimens surely of this 

 group but not exactly determinable, as there are several hyopsodontids 

 of about this size in the quarry. All five genera have numerous other 

 characters of the astragalus in which they are closely similar. These 

 are nondifferentiated primitive protoungulate characters, most of 

 them disappearing in more advanced forms. 



