122 The American Naturalist. [March, 
and simpler than those of Leptomeryx, so that these two 
forms must have been derived from an ancestor which com- 
bined the simplicity of both forms. For this we must again 
recur to Dorcatherium, and I therefore insert this genus at 
the base of the following diagram. With its entirely pris- 
matic molars Hypisodus has one element of superiority, but 
the number of its superior premolars is unknown. 
Prodremotherium Bachitherium 
Two species of this family are very abundant in the 
Ca^nozoic beds of North America. These are the Leptomeryx 
evansi Leidy of the White River series, and the Hypertragu- 
lus calcaratus Cope of the same, and of the John Day Miocene 
series. Either species was of the size of a spaniel, and had 
delicately formed limbs. The H. calcaratus had large eyes, 
and a compressed muzzle. Larger species are found in 
Canadian beds. The least species of the family belongs also 
to the White River Beds. This is the Hypisodus minimus 
Cope, whose size does not exceed that of a gray-squirrel. 
Like the Leptomeryx, it does not extend upwards into the 
John Day beds. 
The remaining families of the Booidea agree in possessing 
the following characters. 
The second and generally the third superior premolar 
teeth possess an internal crest as well as the fourth fig. t6). 
The inferior premolar teeth have oblique transverse crests. 
The keel of the distal extremity of the metapodial bones ex- 
tends to the front of the condyle (fig. ii). The lateral met- 
apodials are represented by their extremities only, the middle 
