42 E. L. Troxell — Ccenopus, the Ancestral Rhinoceros. 



Likewise the word Ace rath erium, heretofore consid- 

 ered to stand for a genus incorporating all aceratheres or 

 hornless rhinoceroses, loses its etymological significance, 

 and by exact definition, based in great measure on the 

 teeth and parts of the skull other than the nasals, comes to 

 be the name of a group which may be and probably is 

 limited to the Old World. 



Classificatiox of Species. 



Trigonias oshorni Lucas 1900. Genoholotype. 

 Ccenopus mitis (Cope) 1874. Genoholotype. 

 Ccenopus piunilis (Cope) 1886. Synouym of C. mitis. 

 Ccenopus {Leptaceratherium) trigonodus (Osborn and Wort- 

 man) 1894. Subgenoholotype. Leads to C. platycephalus, 

 Ccenopus trigonodus alius, subsp. nov. Figs. 1, 2. 

 Ccenopus copei (Osborn) 1898. Leads to C. tridactylics. 

 CceJiopus occidentalis (Leidy) 1851. Inadequate. 

 Ccenopus tridactylus (Osborn) 1893. 



Ccenopus dakotensis Peterson 1920. Synonym of C. tridactylus, 

 Ccenopus tridactylus proavitus (Hatcher) 1894 . 

 Ccenopus tridactylus metalophus, subsp. noY. Fig. 4. 

 Ccenopus tridactylus avus, subsp. nov. Fig. 5. 

 Ccenopus platycephahis (Osborn and Wortman) 1894. 

 Ccenopus platycephalus nanolophus, subsp. nov. Fig, 3. 

 Ccenopus simplicidens Cope 1891. Of doubtful validity. 



Discussion of Oexera and Species. 



Trigonias Lucas. 



The earliest species of the true rhinoceros is made the 

 genoholotype, Trigonias oshorni Lucas. As summarized 

 by Hatcher (1901), the upper teeth are unreduced, Ci 

 alone absent, simple superior premolars, P^ large, P^ 

 with non-parallel lophs, four digits on the manus. Its 

 geological age is that of the lower Titanotherium beds 

 (lowest Oligocene). The ancestry of the genus is obscure, 

 but it undoubtedly gave rise to Ccenopus Cope. 



Ccenopus Cope. 



Ccenopus is a genus which includes all of the White 

 River (Middle and Upper Oligocene) species; it is based 

 upon the holotv^e of C. mitis (Cope), a fragmentary lower 

 jaw quite indeterminate specifically. The species is usu- 

 ally represented by the parat^^e maxillary with molars 

 and premolars (Xo. 6325, A. M. X. H.). 



