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are represented by one species only, the well-known Virginian opossum (Fig. 29), which is found with slight modifications from the Southern States of North America
Fig. 2'X— The Virginian Opossum.
southwards over the greater part of South America. This animal would, perhaps, judging merely by the present distribution of life, be considered to have intruded into the Nearctic Region from South America, where alone members of this family still survive; but, on examining its past history, we ascertain that the genus Didelphys was formerly found both in Europe and in North America during Eocene and Miocene times, so that it is possible that the Virginian
opossum may be a survivor rather than an intruder in North America.
The next order, the Edentates, is represented in this Region only by a single species of armadillo (Tatusia novem-cincta), which almost certainly came up from the south, and is only met with just inside the southern borders.
Fig. 31.—The Rocky-moustain Goat.
Among the Ungulates, the most remarkable form is the prong-buck (Antilocapra), which forms a distinct family of that order, and is entirely confined to this Region (Fig. 30, p. 156). It is allied in some respects to the Antelopes of the Old World, but it is unique among all the hollow-horned ruminants from the fact that it sheds its horns every year.
Two other genera, belonging to the family Bovidse, are confined to the Nearctic Region; these are the Rocky Mountain goat (Haploceros), found only in the Rocky
Fio. 32.—The Musk Ox.
Mountains (Fig. 31, p. 157); and the Musk Ox (Ovibos), which ranges over the barren grounds at the extreme north of the continent, and spreads into Greenland (see Fig. 32). The latter, however, was also found in the northern parts of the Old World until a comparatively recent epoch, geologically speaking.
The Bisons (Fig. 33) are still common to the Nearctic and Palrearctic Region, though now nearly extinct in both hemispheres.
Rodents are very numerous in the Nearctic Region. According to the tables here used, which have been compiled from Flower and Lydekker's text-book of
mammals, out of a total number of twenty-eight genera, thirteen are endemic. One of these, Raplodon, a small animal resembling the Prairie-dog in its habits, and found only west of the Rocky Mountains, forms a distinct family.
The Carnivora are also well represented, especially the genera of Cats, Dogs, Bears, and Weasels, all of which,