1887] The Perissodactyla. ` 995 
ring in the Wasatch (or Suessonian) epoch. A common ancestor 
gave origin to Systemodon and Hyracotherium. It resembled 
the former in the absence of diastemata, and the latter in the 
three-quarters natural size; ~ 
a 
Fis. 5. Skull of Z7 s j c 
eh of Hyracotherium venticolum Cope, Fig. a from above; 4 from 
p River Eocene of Wyoming. Original. 
Steater distinctness of the cusps of its molars. The succession 
Me genera may be represented somewhat as follows : 
Es eee Tapiride. Equide. 
Hyrachyus, = Lophiodon. Protapirus. 
Heptodon. Pliolophus. 
Helaletes. Hyracotherium. 
Systemodon. 
