44: E. L. Troxell — Camopus, the Ancestral Rhinoceros. 



yet should take precedence over either of these because 

 their types are not adequate nor dependable. C. copei 

 holds a very important place in taxonomy, since to a 

 large extent it has usurped that of C. occidentalism which 

 is so frequently mentioned in our literature, and stands at 

 the very beginning of that branch of the genus Ccenopus 

 which gives rise to C. tridactylus and probably many of 

 the later genera of American rhinoceroses. Its geologi- 

 cal level is the base of the Oreodon beds (early Oligocene). 



Ccenopus tridactylus (Osborn) is a very important spe- 

 cies having a variety of characters which are strongly 

 emphasized in later forms. One is therefore convinced 

 that this is a pivotal point in the racial evolution, and from 

 it there arise two or more lines of descent. Throughout 

 this species the following are notable features: (1) the 

 development of the parallel lophs on the premolars, (2) 

 the greater complication of the enamel of the molars by 

 the presence of cristse and crochets, (3) the loss of all 

 trace of C^ and P, (4) the reduction to the tridactylous 

 manus, and (5) the first appearance in the males of the 

 thickened and rugose nasals for the support of horns. 



The development of horns here is in a very primitive 

 state, but a considerable advance is made in the next 

 stage, D. armatum, where the horn supports are much 

 more rugose and elevated, but still widely separated and 

 situated v^'ell behind the tips of the nasals. 



Other subspecies are chosen to illustrate more fully the 

 variety of forms in C. tridactylus and the trend of that 

 species toward the true JDiceratherium^ especially with 

 regard to the simple parallel cross lophs on the premolars. 

 Specimen No. 10254, Y. P. M., is taken as the holotype of 

 one new subspecies, and specimen No. 10251 of another ; 

 both are described on a later page. 



Description of New Subspecies. 

 C(Enopus trigonodus alius, subsp. nov. 



(Figs. 1 and 2.) 



Holotype, Cat. No. 12052, Y. P. M. Middle Oligocene (White Eiver beds), 

 Nebraska. 



The type material consists of the anterior portion of the 

 face and the larger part of the lower jaws, not including 

 the symphysis. The tooth series (fig. 2) is larger by the 

 length of M'^ than that of the holotype of C. trigonodus or 



I 



