X.] ITS FAUNA. 367 



in Central America. Both these genera are represented in the 

 Pliocene of the Sonoran area. In the same family the three 

 genera of kangaroo-rats known as Dipodomys, Perodipus, and 

 Microdipodops appear to be confined to the region ; and the same 

 is the case with the allied genus Hderomys, although Perognathus 

 passes northwards into the Transition zone. 



FlG. 74. UNDER SURFACE OF LEFT FORE-FOOT OF Geomys. 



In the Ungulata, the deer belonging to the peculiar American 

 genus Cariacus are very abundant in the Sonoran region (where 

 those of the typical genus Cervus are entirely wanting), although 

 they also range into the Canadian sub-region of the Holarctic, and 

 extend right through South America. Since, however, they are 

 wanting in the earlier Tertiary deposits of the latter area, as they 

 are at all epochs in the Old World, there can be little hesitation in 

 regarding them as essentially Sonoran types. Even more decidedly 

 is this the case with the prongbuck (Antilocapra), the sole type of 

 the family Antilocapridce, which is distinguished from the Bovidce. 

 by the horn-sheaths of the males being branched and periodically 

 shed from their bony supports. Although the prongbuck pene- 

 trates a considerable distance into the Canadian sub-region of the 

 Holarctic, its true home is the prairie-district of the Sonoran lying 



