DAKOTA AND NEBRASKA. 359 



In comparing the two lists representing the North American tertiary mammals, 

 mainly from the States of Dakota and Nebraska, with the third list representing the 

 quaternary mammals of the same continent, a remarkable dissimilarity is observed, 

 and there is also noticed a greater resemblance of the former with the tertiary and 

 quaternary mammals of the old world. 



Of thirty-two genera of miocene terrestrial mammals, chiefly from the Mauvaises 

 Terres of Dakota, not one occurs in the quaternary formations of North America ; 

 and of twenty-one genera of jjliocene terrestrial mammals, chiefly from the Niobrara 

 Eiver of Nebraska, only eight are common to the quaternary formations of North 

 America, and of these eight, three are absent in the existing fauna of the continent. 



The eight genera alluded to as common to the pliocene tertiary and the quaternary 

 formations are Cauls, Cervus, Dicotyles, Mastodon, Elei^has, Equus, Hipparion, and 

 Castor. 



It is uncertain how far the species of Canis attributed to the Niobrara pliocene 

 formation are peculiar to it. Part of the fossils may be quaternary, or perhaps even 

 recent remains. Of Cervus, part of the specimens referred to it may be of a recent 

 species, while the antler viewed as pertaining to the same may represent a peculiar 

 genus subsequently extinguished. The only remains indicative of Dicotyles was an 

 ujjper canine tooth, which may really have belonged to a quaternary or perhaps a 

 recent species. The remains of the pliocene Mastodon pertain to the subgenus Tetra- 

 lo2)hodon, while those of the quaternary period belong to the subgenus Trilophodon. 

 The remains of Elephas probably indicate a species distinct from the quaternary E. 

 americanus, though it is not positively ascertained. The remains of Equus appear to 

 be different from those of the later E. fraternus. The genus Hipparmi is clearly 

 common to both the pliocene and quaternary periods, but the species are different. 

 P)-oto1iippus, one of the solipedal genera of the Niobrara pliocene, appears also to have 

 existed during the quaternary period in Chili, S. A. A small species of Castor, of 

 the Niobrara pliocene, is represented by the larger quaternary and still existing 

 Beaver. 



The quaternary fauna of both American continents was especially distinguished by 

 the presence of those wonderful creatures, the giant Sloths, no trace of which has 

 been detected in the tertiary formations of North America. This appears the more 

 remarkable from the circumstance that remains of several edentate genera have been 

 discovered in the miocene formations of Europe. 



The presence, in the quaternary fauna of North America, of the great Sloths, to- 

 gether with other ordinal and generic forms, which likewise existed, and in part still 

 continue to exist, in South America, leads to the impression that the North American 

 continent during the quaternary period was peopled by the extension of life from the 

 south. The greater similitude of the miocene and pliocene faunie, which we have 



