The only other division of the family which is represented in North America is that of the bisons, of which the fast vanishing remnant of a single species[11] (_Bison bison_) is all that is left of what was once an extensive and varied assemblage. The bisons differ from the true oxen in the form and structure of the skull, in the shoulder-hump, which is produced by the very long spines of the dorsal vertebræ and in consequence of which the back slopes downward from the shoulders to the croup. They differ further in the character of the hair, which is short and woolly on the body and hind quarters, very long and shaggy on the head and neck. In the Pleistocene of North America there were at least seven recognizable species of bisons, which ranged over the continent from Alaska to Florida, though it is not probable that they were all contemporary. One of the earliest and by far the largest of these was the gigantic _B. †latifrons_, a specimen of which in the American Museum of Natural History measures six feet across the horns in a straight line; this was a Mississippi Valley species and extended from Ohio to the Gulf of Mexico and westward to Kansas and Texas. Another gigantic species (_B. †crassicornis_) lived in Alaska in association with a second and smaller species (_B. †occidentalis_) which ranged as far south as Kansas. _B. †occidentalis_, though smaller than the preceding species, was larger than the existing one and was remarkable for the great size of the hump. The bisons were migrants from the Old World and are the only members of the great ox-tribe that ever reached America. At present the Old World has but a single species of _Bison_ (_B. bonasus_), which has been saved from extermination only by the most rigid protection. Neither sheep nor bison extended their range to South America; both are and have been essentially northern groups and seem to have been unable to cross the tropics. * * * * * From the foregoing account, confused as it unavoidably is, one thing at least stands out clearly, that North America played a very insignificant rôle in the evolution of the Pecora, and has only two peculiar groups, the Prong Buck and the American types of deer, and of these, the probable American ancestry does not extend back of the lower Miocene and perhaps not so far. Even in the Old World the story, so far as it has been deciphered, is by no means clear and consistent, which is no doubt due to the fact that the regions from which Tertiary mammals have been obtained are so small in comparison with those that have yielded nothing. Certain broad outlines of the history may, nevertheless, be discerned.