AMERICAN RUMINANT-LIKE MAMMALS. 245 



to both. In the long interval between the Wasatch and the 

 White River, North America had ample opportunity to develop 

 a peculiar mammalian fauna, and one which was composed of 

 types specially adapted to the conditions of life on this conti- 

 nent, and well fitted to resist the invasion of more or less similar 

 forms from the other continents, when easy intercommunica- 

 tion was reestablished. 



Any fauna may, for our present purpose, be conveniently 

 divided into two somewhat heterogeneous assemblies, one of 

 which is composed of immigrants from other regions, and the 

 other is indigenous. By the latter term are meant those forms 

 which have long been established in a continent and whose 

 ancestry may be traced through several geological horizons. 

 Using this convenient mode of discrimination, the ruminants 

 which from time to time have inhabited North America may be 

 distinguished as either indigenous or immigrant types. The 

 indigenous ruminants (it will be more accurate to call them selen- 

 odonts) predominated for a very long period of time and only 

 toward the end of the Miocene did Old World types of seleno- 

 donts obtain a permanent foothold here. It is a remarkable 

 fact that of these indigenous types which so long held sway in 

 this continent, not one is left here at the present time, and, 

 except the camels of the Old World and the llamas of South 

 America, they have become altogether extinct and have left no 

 descendants among recent mammals. 



The indigenous selenodonts of North America have long puz- 

 zled the students who have attempted to work out their system- 

 atic position and their relationships to the selenodonts of other 

 continents. Even the complete skeletons of various genera, 

 recovered from time to time, seemed to give little help in solv- 

 ing the problem. The great obstacle to progress has been the 

 absence of well-defined phylogenetic series, which would enable 

 the observer to trace out the history of the various families and 

 groups, step by step, through their numerous ramifications to 

 final extinction, or to their modern representatives. An isolated 

 genus, standing by itself, the predecessors and successors of 

 which are unknown, offers almost insuperable difficulties to the 

 determination of its proper taxonomic position. Such a genus 



