OPOSSUM. The animals to which this name is applied are the only non-Australian members of the Marsupials or pouched animals, being found throughout the greater part of the continent of America, from the United States to Patagonia, the number of species being largest in the more tropical parts (see MAMMALIA - Marsupialia, vol. xv. p. 380). They form the family Didelphyida,, distinguished from the other Marsupial families by their equally developed hind-toes, their nailless but fully opposable hallux, and by their dentition, its formula being cl..2mt4 total 50, a number only exceeded among heterodont 3.Iammalia by the Australian illyrnzecobius fasciatus. The peculiarity in the mode of succession of these teeth has been explained in the article referred to, where also (p. 378) a figure of the teeth may be seen. 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 their great toes so fully opposable to the other digits as to constitute a functionally perfect posterior pal r of "hands." These opposable great toes are without nail or claw, but their tips are expanded into broad flat pads, which are no doubt of the greatest use to such a climbing animal as an opossum. On the anterior limbs all the five digits are provided with long sharp claws, and the pollex or thumb is but little opposable. Their numerous teeth are covered with minute sharply-pointed cusps, with which to crush the insects on which they feed, for the opossums seem to take in South America the place in the economy of nature filled in other countries by the true Inseetivora, the hedgehogs, moles, and shrews.

The family consists of two well-recognized genera only, viz., Didelphys, containing all the members of the family, with the exception of the Yapock, a curious animal which forms by itself the second genus, Chironectes, and is distinguished from all other opossums by its webbed feet, nori-tuberculated soles, and peculiar coloration. Its ground colour is light grey, with four or five sharply-contrasted brown bands passing across its head and back, giving it a very peculiar mottled appearance. It is almost wholly aquatic in its habits, living on small fish, crustaceans, and other water animals ; its range extends from Guatemala to southern Brazil.

The other genus of opossum; Didelphys, is an extremely heterogeneous one, and has been split up into several groups, some of which perhaps ought also to be recognized as genera. The first of these consists of three or four large dull-coloured long- and coarse-haired species, with perfect marsupial pouches, large leafy ears, and greatly-developed muscular ridges on their skulls. The best known of these, and indeed of all the family, is the Virginian Opossum, Didelphys virginiana, an animal spread over all temperate North America ; it is extremely common, being even found living in the towns, where it acts as a scavenger by night, retiring for shelter by day upon the roofs of the houses 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 is from fourteen to seventeen days. Another very similar species is found in central and tropical South America, and is known as the Crab-eating Opossum (D. cancrivora) The second group, or sub-genus, named ilfetachirus, 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 pouches 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. The accompanying woodcut represents Lord Derby's Opossum (D. derbiwia) carrying its young in this manner.

OPOSSUM The third group is Hicoureus (Gryntwomys of Burmeister), differing only from Megachirus by the comparatively 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 house-mouse, of a bright-red colour, which is found as far north as central Mexicq, and extends thence right down to the south of Brazil. The last sub-genus contains three or four wonderfully shrew-like species, of very small size, with short, hairy, and 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. tristriata), from Brazil, which is of a reddish-grey colour, with three clearly-defined deep-black bands down its back, very much as in seine of the striped mice of Africa. This sub-genus has been named Ilenentrus or " half-tail " by Geoffroy Saint-Hilaire (Microdelpkgs of Btmneister), and should perhaps be allowed full generic rank.

The numerous fossil remains referable to species of the Didelphyiclse are of special interest as showing some of the connecting links in geographical distribution between the opossums and the Australian Marsupials, now so widely and absolutely separated. They consist of the bones of a considerable number of species from the Eocene and Early Miocene deposits of central France, one or two Eocene species having also been found in southern England These ancient opossums have been separated generically from Didelphys on account of certain differences in the relative sizes of the lower premolars, but as nearly the whole of the species have been formed on lower jaws only, of which some hundreds have been found, it is impossible to judge how far these differences are correlated with other dental or osteological characters. In the opinion of Dr Filhol, who has devoted considerable attention to the subject, the fossils themselves represent two genera, Peratkerium, containing the greater part of the species, about twenty in number, and Anzphiperatlteriunt, with three species only. All are comparatively small animals, few of them exceeding the size of a rat.

Besides these interesting European fossils, a certain number of Didelphian bones have been found in the caves of Brazil, but these are either closely allied to or identical with the species now living in the same region. (O. T.)