E. L. Troxell — Horned Eocene Ungulates. 33 



Colonoceras agrestis 

 Strong ribs on metacone of 



molars. 

 M 3 rounded, ant. and post. 



sides not parallel. 

 M 3 heavy projecting post, cin- 



gulum. 

 Transverse valley of molars 



not blocked. 

 Peculiar metastvie on molars. 



Hyrachyus affinis 



Ribs faint or absent. 



M 3 subquadrate, ant. and post. 



sides straight. 

 Short cingulum, not extended 



backward. 

 Cingula across valleys. 



Metastvie inconspicuous. 



11082 TYPE 

 Y. P. M 



Fig. 3. — Colonoceras agrestis Marsh. X%- 



As compared with the more complete skull of H. affinis 

 gracilis nobis, Cat. No. 11170, Y. P. M., one sees these 

 differences : C. agrestis has more slender condyles, 

 broader frontals, especially between the orbits, and 

 finally has the incipient horn rugosities as the generic 

 name implies. 



Except for the presence of horns, none of these features 

 separates C. agrestis widely from H. affinis, and only 

 in a most rigid splitting of groups can the two be distin- 

 guished generically. It has been suggested that the 

 horns of the early rhinoceroses indicate sex distinctions, 

 and this may be true in the present case. 



M et airy r achy 'lis bicornutus, gen. et sp. nov. 



(Figs. 4, 5.) 

 Holotype, Cat. Xo. 10258, Y. P. M. Eocene (Bridger), Millersville, near 

 Fort Bridger, Wyoming. 



One of the rarer skulls of early rhinoceroides in the 

 collection is this specimen found by R. E. Son in 1873. 



Am. Jour. Sci. — Fifth Series, Vol. IV. No. 19.— July, 1922. 

 3 



