1000 The Perissodactyla. 
and the Rhinocerontide. Czenopus embraces, as yet, only 
American species, of moderate and small size; the C. occidentalis 
Leidy had four toes in front and three behind, representing the 
type which is the phylogenetic ancestor of the lowest and only — 
four-fingered genus of Rhinocerontide, Aceratherium, A cusp 
is sometimes present within the inner extremity of the incurved, 
_ transverse crest of the premolars in Canopus. Fusion of this 
with the transverse crest has produced the double transverse 
crests of the premolars of the Rhinocerontide. Dihoplus must 
be regarded as a collateral outgrowth from Canopus. It is rep — 
resented by one European species, the D. schleiermacheri, whith — 
is of larger size than either of the known species of Canopus. 
ize, from 
Fic. 14. Cenopus occidentalis Leidy; cranium, one-fourth natural ray a 
below; from the White River Miocene of Nebraska. From Leidy, “ 
Fauna of Nebraska.” 
In the Hyracopontip& we have a direct descendant n 
the Lophiodontidæ, but presenting a modification quite different 
from the Cænopidæ. In that family the canines wers m. 
the simple premolars retained ; in the present family the gye 
_ are retained and the premolars become complex, as in the 
noceroses. Three, perhaps four, genera are known: 
I. Canines very distinct from incisors. 
Premolars 4: Amynodon Marsh. 
Premolars Fy Seok Metamynodon b and O- 
II. Canines small, similar to incisors. i 
Prem Hyracodon Leidy. 
These genera are all hornless. . species 
Amynodon is the oldest and ancestral genus; its ee in 
belong, one in the Diplacodon formation, and the pane 
Bridger. Desmatotherium (Scott) belongs here, but 
