TRANSACTIONS OF WAGNER 

 46 



UINTA SELENODONTS 



metatarsus is much less elongate than in the latter, but in the extreme reduction 

 of the lateral digits, which are long, filiform splints, the pes is more advanced 

 in differentiation than the manus. The phalanges are more slender than they 

 afterwards became, while the unguals are rather shorter and more curved. 



Hardly an instance has yet been discovered in which the relationship 

 between two genera of successive geological epochs is more clearly that of 

 ancestor and descendant than in the case of Protylopus and Poebrotherium, yet 

 when we inquire concerning the forerunners of the Uinta type, the answer 

 must remain doubtful. I see no reason, as yet, to modify my former conclu- 

 sion ('91$, p. 46) that the Bridger genus Homacodon should be regarded as 

 the probable ancestor of Protylopus, but the gap between the two genera is so 

 wide that the inference as to their connection must remain largely conjectural. 

 We need to recover the connecting links, which will doubtless be found in the 

 Washakie and in the overlying transitional beds, before a definite conclusion 

 can be reached. The starting-point of the tylopodan line is, probably, as 

 Cope long ago suggested, the Wasatch Trigonolestes (Pautolestes), a form 

 whose exceedingly primitive dentition might be either oreodont or lemuroid. 

 It seems highly probable that this genus will prove to be ancestral to all of 

 the indigenous North American selenodonts. 



If the conclusion that Poebrotherium and Protylopus are the real, if remote, 

 ancestors of the modern Tylopoda be well founded, certain inferences of far- 

 reaching significance for the philosophy of evolution will necessarily follow. 



(1.) Schlosser's dictum ('87, p. 42) that a closed dentition without diaste- 

 mata indicates the end of a phyletic series is proved not to be tenable, at least 

 for all cases. Protylopus is without diastemata. In the smaller species of Poe- 

 brotherium they are very inconspicuous, but they become longer and longer in 

 the later species of that genus and in Gomphotherium, not to mention the suc- 

 ceeding genera. This growth of the diastemata is, it should be observed, 

 not due to any loss of teeth, for the John Day genus retains the full number, 

 but to an elongation of the jaws. The case of the oreodonts is also in point : 

 the Uinta representative of this family (Protoreodo/i) has no diastema, and yet 

 the group flourished abundantly and became highly diversified throughout 

 the whole of Oliogocene and Miocene times. Schlosser's principle, though 

 doubtless true of the cases to which he applied it, is thus seen not to be 

 of general application. 



(2.) In discussing the modes in which evolution acts it has often been 

 disputed whether development is always by a series of direct and unswerv- 



