While this strange method of reproduction is the most prominent peculiarity of the mar supials, they differ from other orders of mam mals in several anatomical features, such as the simplicity of the brain, in which the cerebellum is completely exposed, and the tendency to sepa ration of bones of the skeleton, usually solidly anchylosed in other mammals.
Since early Tertiary times marsupials have been confined to South America and the Aus tralasian region, with the single exception of the few North American opossum (q.v.). This family (Didelphyidce) is restricted to the western hemisphere and alone remains of the large number of Tertiary forms once prevalent in South America, save a single Patagonian diprotodont, the opossum-rat (q.v.). The home of the group, then, is Australia, Tasmania and the Papuan group, where about 125 species are known; and the race seems to have survived in that insular region owing to the absence of destructive enemies, for most of them are almost defenseless vegetable feeders. They have, however, developed into a great variety of forms under the influence of varying condi tions and long competitions, and present a most curious parallel to the diversities observable among the higher and more widely diffused mammalia. Some have large size, go in herds
and occupy grassy plains; others are smaller, more agile and confined to mountainous dis tricts. Others are still smaller, burrow and feed upon roots or resemble little terrestrial rodents in appearance and habits; while many forms dwell altogether in trees and often simulate squirrels of various kinds. In another direction have been evolved a variety of predatory mar supials, whose needs have developed bodies, teeth and powers resembling those of wolves or bears, and which are wholly flesh-eaters. There is, in fact, hardly a group of mammals which does not find a counterpart among the marsupials,— even the moles and shrews.
The classification of the order divides it into 10 or 12 well-defined families, some of which are wholly extinct, as follows: