298 THE AGE OF MAMMALS 



'Nebraska' is preoccupied, and by the U.S. Geological Survey this forma- 

 tion is known as the 'Ogallala' of Darton. It consists of widely scattered 

 river channel and flood plain deposits in South Dakota and Nebraska. To 

 the south in New Mexico are the Santa Fe Marls, determined as of this age 

 by Cope in 1884. In northwestern Texas are the Clarendon beds of 

 Gidley, a river channel deposit overlying the Panhandle, Middle Miocene, 

 at the edge of the Llano Estacado (Fig. 167). On the northern plains 

 of Montana are the Madison Valley beds of Douglass, 1,200 feet in 

 thickness. 



From Montana on the northwest to Texas on the southwest, to Nebraska 

 and Kansas in the central west, we find a very similar list of mammals, so 

 that the homotaxis of the American horizons 'Nebraska,' 'Ogallala/ 

 'Clarendon,' 'Santa Fe,' 'Madison Valley,' and numerous others un- 

 named is singularly well established. They may be said to belong to the 

 Hipparion Zone, a term equivalent to the Procamelus Zone of Cope, 

 the Cosoryx Zone of Scott (1894), the Protohippus Zone of Osborn 

 (1907). 



The climatic conditions and flora of the high mountain region at about 

 this time are well pictured in the description of the Florissant Lake of Colorado 

 given on p. 283. There is evidence of occasional dust burials by a wind 

 or sand storm in the discovery of remains of six hipparions on Little Wliite 

 River near the Rosebud Agency in South Dakota.^ This discovery in- 

 cluded the perfect skeleton of a female, the type of Neohipparion whit- 

 neyi, found closely crowded against those of .several younger horses of 

 the same species, the group having perished together either in a sand storm 

 or by a stroke of lightning; the association of the younger horses with the 

 mare is a natural one, indicating that this group had not been brought 

 together by stream action. In the original description of the superb type 

 specimen (Fig. 123) of this group Gidley pointed out that its limbs had the 

 delicate proportions of those of the Virginia deer. With its strongly 

 hypsodont and long-crowned teeth it represents a typical horse of the arid 

 country. 



The conspectus of this Upper Miocene fauna shows it as of prevailing 

 American type. 



As set forth in this summary there is a balance 

 Prevailing Mammals between the grazing and browsing types of Her- 

 Proboscidea bivora. Each great order of herbivores exhibits an 



Trilophodon increasingly sharp division between the more con- 



Rhinoceroses servative browsing types with short-crowned teeth, 



Teleoceras, Aphelops and the more progressive grazing types with long- 

 Tapirs croAvned teeth. Among the horses, the browsers 



Tapiravus (Hypohippus and Parahippus) are beginning to 



' J. W. Gidley, A New Three-toed Horse. Bull Amer. Mus. Nat. Hist., Vol. XIX, 

 July 24, 1903, p. 465. 



