206 Troxell — Diceratherium and the Diceratheres. 



The Diceeatheees of the Geeat Plains. 

 Menoceras, gen. nov. 



Scarcely a more conspicuous or better known species 

 of extinct rhinoceroses is mentioned in our literature 

 than that which includes those specimens found and 

 named by Peterson (1906) from the famous Agate Spring 

 quarry on the ranch of Mr. Harold Cook at Agate, Neb. 

 With the approval of Mr. Peterson, this species, Dicera- 

 therium cooM, is here made the type of a new genus, 

 Menoceras (lU-eVo?, strength, Kcpa?, horn) and the following 

 forms may be classed under this head, most of which 

 Peterson considers invalid as species : 



Diceratherium coohi Peterson 1906. Genoholotype. 

 Diceratherium arrikarense Barbour 1906. 

 Diceratherium schiffi Loomis 1908. 

 Diceratherium stigeri Loomis 1908. 

 Diceratherium aherrans Loomis 1908. 

 Diceratherium loomisi Cook 1912. 



After a comparison with the genoholotype of Dicera- 

 therium, D. armatum, mth which it has always been asso- 

 ciated, it is very clear that Menoceras is probably the 

 farthest removed from Diceratherium of all the Miocene- 

 Oligocene rhinoceroses. This great difference was recog- 

 nized by Peterson (1920), who calls attention to the 

 features of Menoceras coohi here briefly enumerated : (1) 

 form of the horn cores {fi.g. 2), (2) form of the muzzle and 

 anterior nares, (3) expanded zygomatic arch Avith rugose 

 angles, (4) complication of the cheek teeth and union of 

 the crochet with the ectoloph. Other distinctive features, 

 widely differentiating it from Diceratherium, are the 

 smaller size, great geographical separation, unu-sual 

 deepening of the pits and sinuses on both molars and 

 premolars, almost complete absence of cingula, an extra 

 transverse loph on the second deciduous premolar, and 

 the closing of the external auditory meatus below. 



It has been suggested by Peterson (1920) that M. coohi 

 is derived from Ccenopus mitis, a lower Oligocene species. 

 C. dakotensis Peterson, also based on a lower jaw, but 

 from the Protoceras beds, is considered by its author to 

 be a connecting link because of the short symphysis and 

 diastema, the curving of the lower border of the ramus^ 



