INTRODUCTION 



an open one, families which had already become distinctly differentiated as 

 such in the Uinta epoch. 



The most interesting and striking result to which the study of the Uinta 

 selenodonts has led is the very unexpected conclusion that, zvitli tlie possible 

 exception of the orcodonts and agrioclioerids, all of the strictly indigenous North 

 American selenodonts are derivatives of the tylopodan stem. Paradoxical as 

 this conclusion may appear, I believe it to be fully justified by the evidence 

 which will be laid before the reader. The Tylopoda are thus seen to be a 

 very ancient and highly diversified group, comparable in this respect to the 

 Pecora, or true ruminants, which they so closely resemble in many features. 

 The Pecora are an Old World group, which underwent great expansion and 

 diversification in Eurasia, but did not reach this continent till late Miocene 

 times, and never attained the importance here that they have so long had in 

 the Eastern Hemisphere. Their place was to a very great extent taken in 

 America by the Tylopoda, which ran a course of development in many ways 

 parallel to that of the Pecora and Tragulina, but with a variety and diversity 

 of structure, habit, and appearance such as are not attained in either of the 

 latter groups. 



It is this very parallelism with the Pecora which has led most students of 

 the American selenodonts astray. We have constantly been endeavoring to 

 find relationships between these forms and the European ruminants or tragu- 

 lines, where no such relationships existed, but only analogies, parallelisms, or 

 convergences. The truth appears to be that the indigenous American seleno- 

 donts make up a natural assemblage of forms, which, with much diversity of 

 size and structure, are yet all quite closely related among themselves, and 

 only distantly with the European forms which more or less resemble them. 

 Just as the Pecora are typically Old World, both in origin and development, 

 so the Tylopoda are typically New World, and did not reach the Eastern 

 Hemisphere till the end of the Miocene or beginning of the Pliocene, and then 

 only in very limited numbers, Caiiielus and its immediate forerunners being 

 the only known Eurasian representatives of the group. 



It is an admirable example of the keen insight which characterized Riiti- 

 meyer that he had practically reached this conclusion at a time when the White 

 River fauna was very imperfectly known, and that of the Uinta not at all. In 

 a passage which has not attracted the attention it deserves he says of Lcpto- 

 meryx : "Die Merkmale des Schadels mit Einschluss namentlich des Unter- 

 kiefers, scheinen weit eher auf eine nahe Beziehung von Lcptomeryx zu den 



