Opossum, an American Indian name properly belonging to the American marsupials (other than Caenolestes), but in Australia applied to the phalangers (see Phalanger). True opossums are found throughout the greater part of America from the United States to Patagonia, the number of species being largest in the more tropical parts (see Marsupialia). They form the family Didelphyidae, distinguished from other marsupial families by the equally developed hind-toes, the nailless but fully opposable first hind-toe, and by the dentition, of which the formula is i. -, c. i, p. 1, m. 1-; total 50. The peculiarity in the mode of succession of these teeth is explained in the article referred to. Opossums are small animals, varying from the size of a mouse to that of a large cat, with long noses, ears and tails, the latter being as a rule naked and prehensile, and with the first toe in the hind-foot so fully opposable to the other digits as to constitute a functionally perfect posterior "hand." These opposable first toes are without nail or claw, but their tips are expanded into broad flat pads, which are of great use to these climbing animals. On the anterior limbs all the five digits are provided with long sharp claws, and the first toe is but little opposable. The numerous cheek-teeth are crowned with minute sharply-pointed cusps, with which to crush the insects on which these creatures feed, for the opossums seem to take in South America the place in the economy of nature filled in other countries by hedgehogs, moles, shrews, &c. The true opossums are typically represented by Didelphys marsupialis, a species, with several local races, ranging over the greater part of North America (except the extreme north). It is of large size, and extremely common, being even found living in towns, where it acts as a scavenger by night, retiring for shelter by day upon the roofs or into the sewers. It produces in the spring from six to sixteen young ones, which are placed by the mother in her pouch immediately after birth, and remain there until able to take care of themselves; the period of gestation being from fourteen to seventeen days. A local race found in Central and tropical South America is known as the crab-eating opossum (D. marsupialis cancrivora). The second sub-genus, or genus, Metachirus contains a considerable number of 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 have less developed ridges on their skulls. They have, as a rule, no pouch in which to carry their young, and the latter therefore commonly ride on their mother's back, holding on by winding their prehensile tails round hers, as in the figure of the woolly opossum. The latter belongs to the sub-genus Philander, which is nearly allied to the last; its full title being Didelphy (Philander) lanigera. The philander (D. [P.] philander) is closely related.
The fourth sub-genus (or genus) is Marmosa (Micoureus, or Grymaeomys), differing from the two last by the smaller size of its members and by certain slight differences in the shape of their teeth. Its best-known species is the murine opossum (D. murina), no larger than a mouse, of a bright-red colour, found as far north as central Mexico, and extending thence to the south of Brazil. A second well-known species is D. cinerea, which ranges from Central America to western Brazil, Peru and Bolivia. Yet another group (Peramys) is represented by numerous shrew-like species, of very small size, with short, hairy and non-prehensile tails, not half the length of the trunk, and unridged skulls. The most striking member of the group The Woolly Opossum (Didelphys lanigera) and young.
is the Three-striped Opossum (D. americana) from Brazil, which is of a reddish grey colour, with three clearly-defined deep-black bands down its back, as in some of the striped mice of Africa. D. dimidiata, D. nudicaudata, D. domestics, D. unistriata and several other South American species belong to this group. Lastly we have the Chiloe Island opossum (D. gliroides), alone representing the sub-genus Dromiciops, which is most nearly allied to Marmosa, but differs from all other opossums by the short furry ears, thick hairy tail, doubly swollen auditory bulla, short canines and peculiarly formed and situated incisors.
Whatever difference of opinion there may be as to the right of the above-mentioned groups to generic separation from the typical Didelphys, there can be none as to the distinctness of the water-opossum (Chironectes minimus), which differs from all the other members of the family by its fully webbed feet, and the dark-brown transverse bands across the body (see Water Opossum).
See O. Thomas, Catalogue of Marsupialia and Monotremata (British Museum, 1888); "On Micoureus griseus, with the Description of a New Genus and Species of Didelphyidae," Ann. Mag. Nat. Hist. ser. 6, vol. xiv. p. 184, and later papers in the same and other serials. (R. L.*)
