264 0. C. Marsh — Description of Tertiary Artiodactyles. 



The humerus is perforated above the lower condyle, and the 

 inner condylar margin is without the process characteristic of 

 Homacodon. The radius and ulna are separate, but the latter 

 bone is quite slender. The fibula is incomplete. The lower 

 part has coalesced entirely with the tibia, but the suture 

 remains distinct, except in very old individuals. In figures 9 

 and 10, the distal end of the tibia and the astragalus of this 

 species are represented twice natural size. 



The present species is only about half as large as Homacodon 

 vagans, and is thus one of the smallest Eocene Artiodactyles 

 known. The first specimens discovered were found by the 

 writer in 1870, near Fort Bridger, Wyoming. The type 

 specimen here described is from the same region. All are 

 from the Dinoceras beds of the middle Eocene. 



n. 



Helohyus plicodon, 1872.* 



12. 13. 



Figure 11. — Last upper molar of Helohyus plicodon, Marsh; (type) seen from 

 below. 



Figure 12. — Second and third upper molars of same species. 



Figure 13. — Three worn upper molars of same. 



Figure 14. — Lower teeth of same. 



Figure 15. — Last lower molar of Helohyus (Thinolherium) validus, Marsh ; top 

 view. 



Figure 16. — The same tooth of Helohyus (Eloihevlum) lentus, Marsh. 

 All the figures are natural size. 



The genus Helohyus, as at present known, includes several 

 species of suilline mammals, all much larger than those of 

 Homacodon, and most of them, at least, from a higher horizon 

 of the middle Eocene. The type specimen of the present 

 species, which is also the type of the genus, includes the last 

 upper molar represented natural size, in figure 11 above. This 

 is in fine preservation, and shows clearly the characteristic 

 features of the crown. Other remains found with the type, 

 and still others from the same locality or horizon, give more 

 characters of the genus, although no single skeleton is known 



*This Journal, vol. iv, p. 207, September, 1872. 



