46 Troxell — Hyrachyus and its Subgroups. 



first studied and named ; it is based on a fragment of the 

 mandibular ramus with two teeth, one partly broken 

 away and both well worn. The species shown by Leidy 

 on his plate IY, figures 19, 20, is clearly inadequate as 

 a type ; it is somewhat smaller than H. princeps. 



Hyrachyus imperialis Osborn, Scott and Speir (1878, 

 p. 50) was "established on the second and third molars 

 of each upper jaw, and three premolars, and one lower 

 molar, portions of the skull and vertebrae." The 

 measurements agree precisely with the type of H. prin- 

 ceps, and so far as one can judge the description also 

 tallies. II. imperialis is either a synonym or at most only 

 a subspecies of H. princeps. 



Hyrachyus princeps Marsh. 



(Fig. 5.) 



Holotype, Cat. No. 11157, Y. P. M. Eocene (Bridger), Fort Bridger, 

 Wyoming. 



In the first description of this species (1872, p. 125) 

 Marsh characterizes it as the "largest of the Tapiridae 

 yet found in this country. ' ' He further says : 



"The remains representing it indicate an animal nearly three 

 times the bnlk of Lophiodon [Hyrachyus'] Bairdianus Marsh, and 

 probably twice that of the individual named Hyrachyus eximius 

 by Dr. Leidy. The specimens on which the species is based con- 

 sist of a nearly complete series of upper teeth, and several lower 

 molars, .... remarkably well preserved. The last two upper 

 molars are unusually large in proportion to the rest of the series, 

 and have the antero-external lobe quite separate, and with its 

 apex incurved. ' ' 



The following additional features will serve to define 

 the type of H. princeps more specifically. On the pos- 

 terior side of M 3 there is no deep fossa ; the cingulum is 

 weak here and is almost lacking on the inner side of the 

 tooth, leaving the median valley practically unobstructed. 

 The antero-external lobe is heavy and broad, and though 

 distinct, is not far removed from the curved protoloph. 

 The anterior and posterior sides of the tooth are parallel, 

 squaring the tooth to a degree duplicated in H. affinis 

 affinis only, within the genus. The straight exterior 

 and posterior sides unite to form a prominent angle 



