The Prong Buck, or Prong-horned Antelope (_Antilocapra americana_), occupies a very isolated position, so much so that a distinct family, the Antilocapridæ, has been created for its reception. It differs from all other Cavicornia in having a branched horn, though the bony core is simple, and in annually shedding and renewing the horny sheath; the horn is directly over the eye; there are no dew-claws and all traces of the bones of the lateral digits have completely disappeared. The grinding teeth are thoroughly hypsodont. The genus occurred in the older Pleistocene, where it was associated with the last of the †deer-antelope, or _†Merycodus_ series (_†Capromeryx_), and which, so far as it is known, would seem to connect the two families, though this is doubtful. A middle Miocene genus (_†Dromomeryx_ Fig. 128, p. 237) would be a more probable ancestor of the Prong Buck, if it were not for the long, unfilled gap of the upper Miocene and the whole Pliocene. _†Dromomeryx_ had erect horn-cores placed directly above the eyes as in the modern genus, but low-crowned grinding teeth; it was the most ancient American cavicorn yet known. It remains to be determined by future exploration, whether this middle Miocene genus was actually the ancestor of _Antilocapra_, or merely an anticipation of it. In the lower Pliocene have been found the remains, very incomplete, of several antelopes, which appear to have been immigrants from the Old World, but are too imperfectly known for any definitive reference. One resembles the flat-horned, or goat-horned, antelopes of the European Miocene and Pliocene. Others had spirally twisted horns like those of the Recent strepsicerine, or twisted-horn antelopes of Africa and Asia, but may, nevertheless, be referable to the Antilocapridæ. Antelopes even penetrated to South America, and three genera of them have been reported from the Pleistocene of the Brazilian caverns and the Argentine pampas, but they were less successful in establishing a foothold than were the deer, and form no part of the modern Neotropical fauna. 11. _Bovidæ. Sheep, Bisons, Oxen, etc._ A series of genera, of disputed systematic position, is represented to-day by the so-called Musk-Ox (_Ovibos moschatus_), which is now exclusively North American, but in the Pleistocene ranged over northern Asia and Europe as far west as Great Britain. The Musk-Ox, which is at present found only in the extreme north, is a heavy, short-legged animal, three and a half to four feet high, and six feet or more in length; the body is covered with a dense coat of woolly hair overlaid by a thatch of long, straight hair, which gives the animal a very shaggy appearance. The horns are broad at the base, especially so in old males, in which they meet in the middle line and cover much of the head as with a horny casque; they curve downward and then upward and forward, with the tips directed toward the front; in the females and young males the horns are very much smaller.