376 



THE SONORAN REGION. 



[CHAP. 



hemispheres contain intermediate types such as Hyotherium and 

 Chcerohyus which are apparently the ancestral stock of both 

 families. 



FIG. 79. FRONT VIEW OF RIGHT HIND FOOT OF Agriochcerus. 



Another very peculiar type of North American Tertiary ungu- 

 lates is represented by Protoceras, from the upper division of the 

 White River Oligocene, which forms a family (Protoceratidce} by 

 itself. In these creatures the feet approximate to the ruminant 

 type; but the skull, as shown in the accompanying illustration, 



FIG. 80. SKULL OF Protoceras, WITHOUT THE LOWER JAW. 



has at least two pairs of large bony processes, probably covered in 

 life with horns, and a pair of large upper tusks, in both of which 

 respects it exhibits a curious parallelism with the perissodactyle 



