Troxell — Hyrachyus and its Subgroups. 



47 



which makes the posterior end of the ectoloph seem 

 small in comparison. The crista, the fold of enamel 

 on the inner side of the paracone, is relatively small on 

 each molar. 



The first and second molars have forms qnite different 

 from the third, due mostly to their larger metacones and 

 the greater posterior extension of the ectolophs which 

 reach ont beyond the general border of the crown, in a 

 strong inwardly curved fold. Just behind the point of 

 union of the meta- and ectolophs is the distinct metacone, 

 on the outer side of which is a basal ridge or cingulum. 



Fig. 5. 



71157 TYPE 



v. p. m 



Pig. 5. — Holotype, Hyrachyus princeps Marsh. Molars and premolars of 

 a very large hyrachyid. M 3 is transposed from the right maxillary. y(2/2>. 



Marsh has mentioned the small size of M 1 ; its diame- 

 ters are 26 mm. transverse and 21 mm. fore and aft. 



On the premolars the anterior and posterior sides 

 converge inward. They are rounded on the inner side, 

 not angular as in the rhinoceroses ; they are squared on 

 the broad outer sides, where distinct grooves separate 

 the two main cones and the anterior and posterior styles. 



The deuterocone dominates the inner half of the tooth, 

 and the protoloph supersedes the metaloph, which is 

 small and thin and resembles that of certain species of 

 the early rhinoceroses. The inner slope of the deutero- 

 cone is not encircled by a cingulum. The parastyle of 

 the premolar is distinct but much smaller than in the 

 molar. 



On P 3 the protoloph does not actually join the ectoloph ; 

 both it and the anterior cingulum are somewhat broken 

 and interrupted. 



P 2 has an incomplete metaloph, as in Ccenopus nanolo- 

 plnis, which does not reach the deuterocone, and the 



