White-tailed, Sonora, and grey mule-deer (Odocoileus) are found in the south-western counties; and there are a few antelope (Antilocapra Americana) in the west.
No true antelopes are American, the prongbuck (Antilocapra), which is commonly called "antelope" in the United States, representing a distinct group; while, as already mentioned, the Rocky Mountain or white goat stands on the borderland between antelopes and goats.
On the latter view Merycodus, the prongbuck (Antilocapra) and the antelopes must be regarded as representing three branches from an original common stock, divergent as regards the structure of their cranial appendages, but parallel in other respects.
If, therefore, Antilocapra deserves to be separated as a family from the Bovidae, the same can scarcely be refused for Merycodus.
By many modern writers the American prongbuck, pronghorn or " antelope," alone forming the genus Antilocapra, is regarded as representing merely a sub-family of the Bovidae, to which latter group the animal is structurally akin.