TRANSACTIONS OF WAGNER 

 Il6 



UINTA SELENODONTS 



The conclusion that all the indigenous North American selenodonts are 

 members of the suborder Tylopoda, though by no means demonstrated, seems 

 to be naturally deducible from the evidence as we now have it. At all events, 

 this hypothesis explains better than any yet suggested the complicated tangle 

 of resemblances and differences between the various American groups, as well 

 as those between the American and the European families. In the European 

 Dicliohinida; and the American HoviacodontidcE the Pecora and the Tylopoda 

 nearly approximate to a common term, if they do not actually merge. Tiigo- 

 nolestes, or some similar form, may be the common ancestor of both. 



Of the numerous phylogenetic lines included within the tylopodan sub- 

 order, only one — that of the CaniclidcB proper — has persisted to the present 

 day. The current definition of the suborder Tylopoda has been drawn from 

 the peculiarities of the later members of this single family, and hence it is too 

 narrowly restricted to embrace the great group of related selenodont families 

 which flourished so abundantly in the Upper Eocene and Gligocene of North 

 America. Indeed, so diversified is this group, that it is exceedingly difficult 

 to frame a definition of the suborder that shall be diagnostic of it — on the one 

 hand, embracing all its members, and, on the other, clearly distinguishing it 

 from the Pecora and Tragulina. The difficulty arises not only from the lack 

 of obvious characteristics which are common to all members of the Tylopoda, 

 but also from the numerous features in which one or other member of the 

 suborder has developed in a course parallel with that of the Pecora or the 

 Tragulina. This difficulty of definition, however, is the inevitable result of 

 the discovery of long and clearly marked phylogenetic series. One by one 

 the characteristics of the terminal members of the series disappear as the line 

 is traced back, and are replaced by more generalized features, which are 

 repeated in many other groups. However obvious may be the relationship 

 between a number of highly diversified families, the difficulty of expressing 

 that relationship in a definition increases with the length and completeness of 

 the phylogeny, because of the structural differences between the earlier and 

 later members of the series. 



In the Tylopoda the peculiarities of the cervical vertebrae, for example, 

 are found only in the main line of descent, the Cainclidce, and seem to be cor- 

 related with the elongation of the neck. The reduction of the ungual pha- 

 langes to nodular form and the cancellous structure of the tympanic bullae 

 are also confined to the same line, and were evidently acquired within the 

 limits of the family. In Pocbrotlicruim and Pi'otylopus the phalanges are of 



