NTRODUCTiON 



THE selenodont artiodactyls found in the various Tertiary horizons of 

 North America may be roughly grouped into two somewhat hetero- 

 geneous assemblages: (i.) The first group comprises those which are generi- 

 cally identical with, or are clearly related to, Old World forms, such as the 

 anthracotheres and the true ruminants (Pecora), and which reached this con- 

 tinent by migration. (3.) The second group includes the forms indigenous to 

 America, the successive stages of whose descent may be traced, more or less 

 completely, through several of the Tertiary formations. 



Within this second group are found such peculiar and characteristically 

 North American forms as the Orcodoniidce, the Agriochozridce, the Leptomery- 

 cidcB, Protoceras , and PoebrotJicriiun. These forms have greatly puzzled nearly 

 all the palaeontologists who have studied them, and the most diverse opinions 

 have been expressed with regard to the systematic position and genetic re- 

 lationships of the various genera. Only concerning the poebrotheres has 

 there been any general consensus of opinion ; they have been well-nigh 

 universally regarded as representing the main line of tylopodan descent, lead- 

 ing directly to the camels and llamas of the modern period. For the other 

 groups we have been accustomed to seek various European connections and 

 analogues : the anthracotheres, the tragulines, the giraffes, and the deer have 

 all been called upon to explain the position of these problematical American 

 forms, though few students of the subject have been able to reach conclusions 

 which were altogether satisfactory even to themselves. 



The principal difficulty in dealing with these peculiar American families 

 has hitherto lain in the extremely incomplete and fragmentary phylogenetic 

 series by which they have been represented in the collections. Owing to this 

 absence of well-defined phylogenetic series, it has been almost impossible to 

 unravel the complicated tangle of resemblances and differences between the 

 American and the European groups of selenodonts, and to determine which 



