4:0 Troxell — Hyrachyus and its Subgroups. 



tuting isolated cones, with median valleys open, meta- 

 cones receded, with parallel cross ridges leading from 

 points anterior to the two main onter cones respectively, 

 and finally with a strong posterior extension of the ecto- 

 loph on the last npper molar ; dental formula 3. 1. 4. 3. 



All the species of the genus are from the Bridger beds 

 of the Middle Eocene. 



The Subdivisions of Hyrachyus. 



Hyrachyus agrestis Group. 



Hyrachyus agrestis Leidy 1871, genoholotype. 

 Hyrachyus agrarius Leidy 1871, synonym. 

 Hyrachyus bairdianus (Marsh) 1871. 



This group, besides the two species of Leidy already 

 discussed, includes also H. bairdianus Marsh, probably 

 a subspecies under H. agrestis. 



Leidy 's types, inadequate for accurate specific deter- 

 mination, serve only to define the genus. Marsh's type, 

 although also far from being complete, offers some dis- 

 tinguishing characters and is here redescribed along with 

 a nearly complete skull and jaws in the Yale collection 

 (Cat. No. 11081, Y. P. M., apotype). 



Hyrachyus bairdianus (Marsh). 



(Figs. 1, 2.) 



Cotypes, Cat. Nos. 11035 and 11057, Y. P. M. Eocene (Bridger), near 

 Fort Bridger, Wyoming. 



The first type consists of a portion of the left maxil- 

 lary with the three molars (fig. 1), of which M 1 and M 2 

 are so worn and broken that one gets little character of 

 the species from their study. M 2 shows a strong internal 

 cingulum across the transverse valley. The third molar 

 is, however, well preserved, and shows certain important 

 specific features. The tooth is wide on the inner side, 

 narrow on the outer; thus the ectoloph is short, and its 

 posterior end, making up the metacone and style, does 

 not extend beyond the bulging posterior cingulum. 



The posterior fossette cuts deeply, forming a sharp 

 angle between the ectoloph and metaloph. A strong cin- 



