360 ON THE EXTINCT MAMMALIA OF 



investigated in the present work, with the cotemporaneous faunae of the old world, 

 suggests the probability that the North American continent was peopled during the 

 tertiary period from the west. Perhaps this latter extension occurred from a conti- 

 nent, whose area now forms the bottom of the great Pacific Ocean, and whose tertiary 

 fauna is now represented east and west by the fossil remains of tertiary age in 

 America on the one hand, and Asia, with its peninsula Europe, on the other. 



In comparing the miocene and pliocene faunae with each other, as represented 

 mainly by the remains from the Mauvaises Terres and the Niobrara River, we observe 

 the remarkable fiict that in upwards of fifty genera belonging to the two faunae to- 

 gether, scarcely a genus is common to both. In view of the consecutive order and 

 close approximation in position of the two formations and faunae, such an exclusive- 

 ness would hardly have been suspected. The circumstance may in some measure 

 appear exaggerated, from the fact that certain genera which I have considered as dis- 

 tinct would by other naturalists be viewed as the same. Thus, for instance, the 

 pliocene Merychyus may be regarded as identical generically with the miocene Oreo- 

 don, but after all these are the only ones which could be looked upon as the same, 

 unless perhaps Rhinoceros is included. In this case, however, the miocene Rhinoceros 

 occidentalis appears to have been an Aceratherium, while that of the pliocene forma- 

 tion was probably a true or horned Rhinoceros. 



Of all other known faunae, extinct and recent, those of Dakota and Nebraska under 

 consideration appear to approximate most in their relationship with the tertiary faunae 

 of Europe. 



Of the carnivora of the former localities, comprising eight genera and fifteen 

 species, five of the genera, or more than one-half, are found in the European tertiaries, 

 as, for instance, Ganis, Amphicyon, Hycenodon, Pseudcelurus, and Drepanodon. The 

 feline Dinictis of the Dakota miocene has not elsewhere been discovered. The re- 

 maining two carnivorous genera are too imperfectly known for comparison. 



It is truly wonderful that of the numerous ruminantia, comprising fourteen genera 

 and nearly double that number of species, none excepting the genus Gervus belongs 

 to any other known fauna, extinct or recent. Even in the case of the excepted genus, 

 it is probable that part of the remains attributed to it may belong to a peculiar sub- 

 genus, while others may be of a recent species. 



When we com.pare the family relationships of the North American tertiary and 

 quaternary ruminants, we find remarkable differences. A peculiar family, the Oreo- 

 dontid(E, is represented both in the miocene and pliocene ; in the former by three 

 genera and many species, in the latter by a single genus. This family has nowhere 

 else been discovered, neither in the American quaternary, nor the foreign tertiary 

 equivalents. 



