332 MAMMALIA. 



three digits of the pes are nearly imiform in size, and there are no rudi- 

 ments of digits nos. i and v. One of the best known species is Titano- 

 therium (Brontops*) robustum (fig. 190), between four and five metres in 

 length and about two-and-a-half metres high. Its dental formula is 



i. 2, c. 1, pm. 4, m. 3 



-; , -T- . The generic name Brontothenwn has been given to 



i J- 3 c. Xj pm. 4. m. o 



those forms with two incisors above and below, with the upper canine close 

 to the first premolar, and with this premolar lost in the mandible. Various 

 remains occur in the Lower Miocene, not only of Dakota, but also of 

 Nebraska, Wyoming, Colorado, and the region of the Cypress Hills, District 

 of Assiniboia, Canada. 



Palaeosyops, from the Lower and Middle (Wind River and Bridger) 

 Eocene of the United States, does not bear any horn-like prominences on 

 the skull, and its premolars are all simpler than the molars (fig. 189). 

 The facial region is somewhat more elongated than in Titanotherium; the 

 frontal bears a large postorbital process ; and the parietal region rises into 

 a sagittal crest. The cerebral hemispheres of the brain are also proved to 

 have been less convoluted than in Titanotherium. The skeleton is almost 

 completely known, and much resembles that of the latter genus, though it 

 is of more slender proportions. The typical species Palceosyops paludosus, 

 from the Bridger Eocene of Wyoming, is an animal about two metres in 

 length and one metre in height. 



The rhinoceroses, or family Rhinocerotidae, begin to be 

 recognizable towards the close of the Eocene,- bojbh in Europe 

 and North America ; and the Miocene genera in b&th countries 

 differ very little in their skeleton from the surviving rhinoce- 

 roses of the Old World at the present day. At ^-st they are 

 entirely hornless, and some of them retain a fudimentary 

 fourth digit (no. v) on the manus. The incisors 'and canines 

 are also a more persistent feature in the earfrer than in the 

 later forms, and the premolars are sometimes simpler than the 

 molars. In North America a normal horn does not appear to 

 have ever been acquired, and the race disappeared before the 

 close of the Pliocene without attaining a much highe^ 1 degree 

 of specialization than that exhibited by the Miocene Acera- 

 therium of Europe. In the Old World, however, the gradual 

 development of the horn can be clearly traced ; the ancestry of 

 the surviving rhinoceroses of Asia and Africa is revealed to a 

 considerable extent by fossils both from Europe and Asia ; and 

 one northern rhinoceros (Elasmotherium), discovered in the 

 Pleistocene deposits of Siberia and some parts of European 



