SUBORDER C 



PERISSODACTYLA 



139 



simpler than the molars. Narial opening deeply excavated ; nasal bones 

 and premaxillaries long. Manus with complete, but much reduced fourth 

 digit. Oligocene ; Lower White River beds. T. osborni Lucas. 



(c) Leptaceratherium Osborn. Dental formula: o^—^-V" Premolars some- 



what complex. Oligocene ; Lower White River beds. L. trigonodum Osborn. 



{d) Ronzothermm Aymard. Skeleton not known. Premolars much simpler 

 than molars ; those in the lower jaw with incomplete ridges. Superior 

 molars without crista, crochet and antecrochet. Oligocene ; Europe. E. 

 velautmm Aymard, Ronzon ; R. gaudnji Rames. In the Phosphorites of 

 Quercy and in the Brown Coal of Bohemia and Piedmont, as well as in the 

 Bohnerz of Swabia, Ehinocerinae also occur which probably belong to this 

 genus. 



(e) Aceratherium Kaup {Caenopus Cope) (Figs. 177-181). Dolichocephalous. 

 Nasal bones small, projecting freely above the narial openings, hornless ; 



dLl pZ P3 p4 



ml 



vnZ 



m.3 



Aceratherium platycephalum Osborn. Oligocene (White River beds), Dakota. Left upper cheek teeth. 



i/a- (After Osborn.) 



frontal bones smooth above ; mastoid process (post-tympanic) independently 

 developed and separated from the postglenoid process by a furrow. Dental 



formula: ' ' • Upper incisors with low laterally compressed crowns 



elongated antero-posteriorly and with the wear oblique. Lower inner incisors 

 diminutive, deciduous, chisel-shaped; outer very large, procumbent, triangular, 

 with wear posterior. Premolars less complex than the molars. Manus often 

 still tetradactyl, pes tridactyl. To Aceratherium belong the earliest repre- 

 sentatives of Rhinoceros, from the Oligocene Phosphorites of Quercy — 

 Aceratherium filholi Osborn. In the Lower Miocene are found A. lemanense 

 Pomel ( = R. gannatense Duvernoy) ; in the Middle Miocene, A. platyodon 

 Mermier ; in the Upper Miocene, A. tetradactylum Lartet, and A. zernowi 

 Borissiak from Sebastopol ; and in the Lower Pliocene, A. incisivum Cuvier. 

 From the Lower Pliocene of Samos are known A. samium and schlosseri 

 Weber, the latter nearly allied to A. Uanfordi Lydekker, occurring in the 

 Siwalik and in China. A. persiae Pohlig has been described from Maragha. 

 In the Oligocene of North America (White River beds) are found several 

 species, A. occidentale Leidy ; A. tridactylum Osborn ; A. platycephcdum Osborn 

 and Wortman ; A. mite Cope. 



(/) Diceratherium Marsh. Dental formula : ' ' . ' „' • Dolichocephalous. 

 ^ 2.0.4-3.3. 



