OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 181 



The cipiiotheres are especially characteristic of the European Oligocene, 

 although a few traces of the family appear as early as the Ludian (Upper 

 Eocene). The caenotheres and surviving dichobunes of Europe, in spite of 

 certain resemblances in their tooth structure, are very remote from one 

 another; in fact, the cajnotheres form a somewhat isolated group (see p. 548). 

 These peculiarly European forms reached their culmination in the Middle 

 Oligocene, declined before the Oligocene ended, and seem to have l^ecome 

 entirely extinct soon after the appearance of the invaders of the Miocene.^ 



Fui. 76. — Entelodonts, giant pigs of Europe and America. A Middle Oligocene stage. 

 (The position of the ears in this restoration is erroneous ; they are placed too high. See 

 Fig. 83.) After original by Charles R. Knight in the American Museum of Natural History. 



In America the majority of the Oligocene mammals have been discovered 

 in the single geographic region of Dakota and in a continuous series of dep- 

 ositions not exceeding 600 feet in thickness, which are known as the White 

 River Group. Similar forms have been found in Wyoming, Montana, 

 Colorado, and British Columbia. In Europe we find an analogous fauna 

 in beds of very different geologic origin, and interspersed with rich records of 

 plant life which are practically wanting in America. 



This is one of the most conspicuous instances of the advantages of 

 correlation. Were it not for the convincing evidence to the contrary af- 

 forded by the Old World, we should be inclined to regard the American 

 Oligocene as a period marked by fcnv geographic changes, but by certain 



' Stehlin, H. G., Die Saugoticre dcs schwcizcrischen Eocajns, 1903-H)()6, pp. 075, 687, 690. 



