208 Troxell — DiceratJierium and the Diceratheres. 



ent horn rugosities one is led to believe that its ancestor 

 was Ccenopiis tridactylus and therefore closely related 

 to the true Diceratherium of Marsh. 



This genus may include the f oUomng species : 



Metaccenopus niohrarensis (Peterson) 1906. 

 MetaccEYiopus petersoni (Loomis) 1908. Inadequate. 

 Metaccenopus egregius (Cook) 1908. Genoholotype. 

 Metaccenopus gregorii (Peterson) 1920. Of doubtful 

 validity. 



M. petersoni Loomis, whether or not it is specifically 

 distinct from M. niohrarensis or M. egregius^ is of impor- 

 tance as showing that the Agate Spring quarries have 

 a variety of rhinoceroses, and not Menoceras coohi alone. 



M. gregorii Peterson is specifically indistinguishable 

 from M. egregius ; it is unfortunately based on a fairly 

 complete skull but with characterless teeth; however, 

 it is of interest in so far as it shows the spread of Meta- 

 ccenopus into the region of South Dakota. 



SUMMARY. 



In restudying the American horned rhinoceroses of 

 Oligocene-Miocene time, one is impressed with the need 

 of a systematic grouping which, first, will distinguish 

 those of the Great Basin of Oregon, Diceratherium 

 Marsh, from those of the Great Plains ; second, will differ- 

 entiate that well known group of animals from Nebraska 

 and Wyoming, here designated Menoceras coohi (Peter- 

 son) gen. nov. from Metaccenopus Cook; and third, 

 will separate all from Aceratherium Kaup of the Old 

 World. 



In the present paper, all the known species of dicer- 

 atheres are classified, the more important ones are 

 redescribed, and two new species are proposed from 

 the abundant material collected by Professor Marsh. 

 From this study the conclusion is reached that in the 

 light of our present knowledge it can not be reasoned that 

 a hornless rhinoceros is of an aceratherine species, for 

 the adult male may have been well armed. 



