42 Troxell — Hyrachyus and its Subgroups. 



The new specimen of H. bairdianus, apotype, Cat. No. 

 11081, Y. P. M., discovered in the Bridger beds near 

 Fort Bridger, Wyoming, by 0. Harger in 1871, consists 

 of a fairly complete skull and jaws, with all the dentition 

 except I 2 . 3 and F 1 . The series of lower teeth measure 

 precisely the same as do those of the type of H. agrarius 

 Leidy; the specimen therefore serves as a unit of meas- 

 ure for this group so ill denned by the types, and in addi- 

 tion gives us an idea of the character of the anterior teeth. 



Hyrachyus affinis Group. 



Hyrachyus affinis (Marsh) 1871. 



Hyrachyus implicatus Cope 1873. 



Hyrachyus intermedins Osborn, Scott and Speir 1878. 



Hyrachyus crassidens Osborn, Scott and Speir 1878. 



Hyrachyus paradoxus Osborn, Scott and Speir 1878. 



Hyrachyus affinis gracilis, subsp. nov. 



f Hyrachyus modestus (Leidy) 1870. 



This group of species, here associated about H. affinis 

 (Marsh) as a center, represents the smallest of the 

 hyrachyids. Seven names are listed, only four of which 

 are thought to be of value in our classification of the 

 early rhinoceroses ; H. crassidens, H. paradoxus and 

 ?H. modestus are based on imperfect specimens, inade- 

 quate as types. The three others are here considered 

 as subspecies. H. affinis intermedius Osborn, Scott and 

 Speir, founded on the upper molars, is slightly smaller 

 than //. affinis affinis, especially in the antero-posterior 

 dimension, due probably to wear; it is noted for the 

 cingulum which nearly encircles the tooth, the obscure 

 crista, the cross ridges arched forward, and the position 

 of the postero-external lobe far to the rear. H. affinis 

 implicatus Cope has for cotypes three specimens, the 

 first of which varies in measurements from H. affinis 

 affinis in having a shorter last molar but otherwise being 

 larger; it has been referred by Cope (1884, p. 675) and 

 others to H. agrestis Leidy, while the second cotype is 

 presumed to stand for a species under the same name. 

 It is the opinion of the writer that the species must stand 

 on the merits of its first type, and that, in order to make 

 the second specimen a valid type, it must be renamed. 



