38 Troxell — Hyrachyus and its Subgroups. 



Art. V. — The Genus Hyrachyus and its Subgroups; by 

 Edward L. Troxell. (With Plate I.) 



[Contributions from the Othniel Charles Marsh Publication Fund, Reabody 

 Museum, Yale University, New Haven, Conn.] 



Contents. 

 Introduction. 



Definition of the genus. 

 Subdivisions of Hyrachyus. 

 Hyrachyus agrestis group. 

 H. agrestis Leidy. 

 H, oairdianus (Marsh). 

 Hyrachyus affinis group. 

 H. affinis affinis (Marsh). 

 H. implicatus Cope. 



H. intermedins, crassidens, and paradoxus Osborn, Scott and Speir. 

 H. affinis gracilis, subsp. nov. 

 % H. modestus (Leidy). 

 Hyrachyus princeps group. 

 H. princeps Marsh. 

 H. eximius Leidy. 



H. imperialis Osborn, -Scott and Speir. 

 Summary. 



Measurements. 

 Eeferences. 



Introduction. 



As early as the Eocene period in America we find 

 representatives of the great group of animals called the 

 rhinoceros. With them, and not far removed, we see 

 also the ancestors of the horse and of the tapir, and still 

 other closely related forms which were early blotted ont, 

 and whose line has been discarded from the material 

 of the great evolutionary structure. 



Within the great family Rhinocerotidae, there was 

 already a differentiation in the Middle Eocene which gave 

 rise to later subdivisions: (1) Amynodon, leading to the 

 Amynodontidre, extinct with the Middle Oligocene; (2) 

 Metahyrachyus nobis, a possible progenitor of the true 

 rhinoceros through Trigonias and Ccenopus; and (3) 

 Hyrachyus, giving rise to the Hyracodontidse, which 

 existed throughout the Oligocene. 



The present study deals with the last of these three 

 groups, the cursorial hyracodonts, and with the single 

 genus Hyrachyus Leidy. 



As to what constitutes Hyrachyus there need be little 

 doubt ; there are about one half dozen valid species which 



