The second or _Metachirine_ group includes three species found all over the tropical parts of the New World. They are of medium size, with short close fur, very long, scaly, and naked tails, and less developed ridges on their skulls than in the type species. As a rule there is no pouch adapted to carry the young, which commonly ride on their mother’s back, holding on by winding their prehensile tails round hers. The _Philanderine_ group is closely allied to the preceding, but is readily distinguished by the woolly hair, and the brown streak down the middle of the face. The Woolly Opossum (_D. lanigera_), which is represented in the accompanying woodcut (Fig. 38) carrying its young in the fashion mentioned above, is one of the two species of this group. In the fourth or _Micoureine_ group the numerous species are all smaller than in the preceding groups, and have short and close hair, and no dark streak down the face. The best known species is the Murine Opossum (_D. murina_), little larger than a House-Mouse, and of a bright red colour, which is found as far north as central Mexico, and extends thence right down to the south of Brazil. The last or _Peramyne_ group contains several extremely shrew-like species, of very small size, with short, hairy, and usually non-prehensile tails, not half the length of the trunk, and with wholly unridged skulls. The most striking member of the group is the Three-striped Opossum (_D. americana_), from Brazil, which is of a reddish-gray colour, with three clearly-defined deep-black bands down its back, very much as in some of the striped mice of Africa. [Illustration: FIG. 38.—The Woolly Opossum (_Didelphys lanigera_).] The numerous fossil species of Opossum found in the Upper Eocene and Lower Miocene of Europe are of especial interest from a distributional point of view, since they indicate how the Opossums of America may have been connected with the Australian Marsupials. These forms were originally referred to _Didelphys_, but have been subsequently described as _Peratherium_ and _Amphiperatherium_. The characters of the molar teeth on which these genera are based do not appear to be sufficiently important to justify their separation from _Didelphys_. Allied forms occur in the Tertiaries of North America, which were originally described under the name of _Herpetotherium_, but have been subsequently referred to _Peratherium_. Remains of many of the existing species of Opossum are found in a fossil condition in the Pleistocene cave-deposits of Brazil. _Family_ DASYURIDÆ