LOUP rOKK EPOCH. 
3G5 
the Eocene period is revealed by paleontology. The fauna of the Eocene 
was the richest in all respects; that of the White River epoch was less 
prolific, though much more so than the present period; the Loup Fork 
epoch exhibits a great reduction of both species and genera. But this 
diminution of numbers was accompanied by greater specialization of struct¬ 
ure, variety being gained while abundance was lessened. An exception 
must be made in the case of the Carnivora, for the saber-toothed Tigers have 
not yet been observed in its deposits. But the elimination of ElotJieriwn, 
most of the Oreodontidce, of Hyopotamus, and of Poebrotherium, and their 
replacement by Camels and Merychyus, is a case of the substitution of more 
for less specialized genera. The case is still stronger in the Perissodactyla, 
where the types with half Bunodont dentition {Anchitlierium, Titanotlierium, 
etc.) are dropped, and three-toed Horses, with the more complex Seleno- 
dont teeth, are introduced; and the Rhinocerotic forms with full incisors 
{Hyracodon) or premolars (Aceratherium) are represented by a genus 
(ApJielops), with both reduced by one degree, and in so far approaching the 
most specialized recent genera. We may then safely maintain that, in the 
lapse of Tertiary time in North America, decrease in the number of specific 
and generic forms of Mammalia was accompanied by increasing specializa¬ 
tion or perfection in those that remained. 
