THE PLIOCENE OF EUROPE, ASIA, AND NORTH AMERICA 351 



These hipparions, in America at least, include the extreme desert-living 

 types. 



The American group of hipparions, or Neohipparion, differ from the 

 hipparions of Europe and Asia in several characters: (1) the anterior pillar 

 of the upper grinders is relatively larger and elliptical in cross s(>ction; 

 (2) the enamel foldings are comparatively simple; (3) the limbs and feet, 

 so far as known, indicate a more slender construction of the long bones and 

 especially longer proportions of the metapodials. The skeleton of A'', whit- 

 neiji (Fig. 123) was discovered by Mr. H. F. Wells of the American Museum 

 expedition of 1902 in the Upper Miocene or Lower Pliocene deposits on 

 Little White River near Rosebud Agency, South Dakota; the little cluster 

 of animals was huddled together as if they had perished in a desert storm. 

 It consisted of the superbly complete skeleton of an adult mare associated 

 with incomplete skeletons of five other younger individuals undoul^tedly 

 of the same species. The age of this type {N. whitneyi) is quite probably 

 Lower Pliocene.^ 



The increasingly arid conditions of climate were probably accompanied 

 by an extension of the areas of the dry grassy plains and uplands over 

 which these quadrupeds roamed, the hard conditions of the soil hastening 

 the transformation from the tridactyl into the monodactyl condition. 



Multiple phyla of rhinoceroses. — We have evidence also of the existence 

 of four and possibly of five contemporary phyla of rhinoceroses.^ As among 

 the horses, surprisingly primitive persistent forms mingled with the most 

 highly specialized. The polyphyletic character is also attributable to the 

 intermingling of American and Eurasiatic strains. (1) The first phylum 

 found among the aceratheres began with Ccenopus persistens in the Middle 

 Miocene and is continued into the C. hrachyodus of the Upper Mio- 

 cene; the former species is slender, long-headed, with short-crowned teeth 

 and primitive feet; the skull proportions are little changed from the Oligo- 

 cene type; all these animals are small. (2) A second phylum includes a 

 number of large, long-headed, long-limbed aceratheres with brachyodont 

 teeth ; these animals are closest to the Miocene aceratheres of Europe ; they 

 include the A. ceratorhinus and A. montanus found by Douglass in the 

 Upper Miocene of Montana; the nasals are long and tapering and exhibit 

 in the males a diminutive terminal horn. (3) A third phylum apparently, 

 introduced by the Aphelops megalodus of the Middle Miocene, is mesati- 

 cephalic, with smooth nasals, with a high occiput inclined forward; it per- 

 haps runs into the long-limbed A. malacorhinus of the Lower Pliocene. (4) 

 Then comes a phylum of extremely broad-headed aceratheres, perhaps 



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

 Art. xiii, July 24, 1903, pp. 465-476. 



^ Osborn, H. F., New Miocene Rhinoceroses with Revision of Known Species. Bull. 

 Amer. Mus. Nat. Hist., Vol. XX, Art. xxvii, Sept. 24, 1904, pp. 807-326; also, 



Douglass, E., Rhinoceroses from the Oligocene and Miocene Deposits of North Dakota, 

 and Montana. Ann. Carnegie Mus., Vol. IV, nos. 2 and 3, 1908, pp. 256-266. 



