MARSUPIALIA, the marsupial mammals, a group ranked as an order, yet embracing the whole of the superior group Metatheria or Di delphia, as it has been variously named. The latest investigations, however, tend to invalidate the distinctions upon which these groups were formerly sharply separated from the higher mammals and to cause the marsupials to be re garded only as an order of Eutheria (q.v.), now distinguished chiefly by their extremely local distribution and degenerate non-placental type of reproduction. Their origin was extremely ancient and its sources are not known; but the group appears to have risen to Mesozoic times among the earliest of mammalian forms and to have begun, even before the advent of the Tertiary period, a course of special modifica tion and degeneration, especially in the line which has survived to the present. The former belief that the marsupials stood in the direct line of ancestry of mammals generally, which were thus considered as modified and diversi fied offshoots from the stock, is no longer held; on the contrary, the marsupials are regarded as a branch from some very early generalized stock, if not a group of independent origin. It is noteworthy, according to Woodward ((Verte brate Palaeontology,' 1898), that the earliest known complete mammalian skeletons, which i pass upward by insensible gradations into un doubted Eutheria, are scarcely distinguishable from the skeletons of the more generalized existing marsupials (for example, Thylacinus). In the later Mesozoic Age the marsupials were apparently scattered over all the laud-area of that time, as their remains have been discovered in many parts of both hemispheres; but even previous to the Eocene epoch they had disap peared entirely from north of the equator. From the first they are divisible into the two branches or suborders of Polyprotodontia and Diprotodontia. The former, characterized by numerous small incisor teeth, includes a major ity of the most ancient forms and such modern groups as the opossums and desyures ; while the latter, characterized by only abo six upper incisors and two, much enlarged, lower incisors, contain, besides some ancient forms, the major ity of modern representations of the order, as the kangaroos, phalangers, wombats, etc.
The marsupials take their name from the ventral pouch of skin, covering the mammary glands, in which the young are nourished or protected in most families, and which is indica tive of the peculiar method of reproduction characteristic of the order, and for the support of which two bones (the epipubic bones), not present in higher mammals, project forward from the pelvis. The internal organs of re production are double, the two oviducts not uniting into a single uterus or vagina, although the separation of the two parts is often im perfect; hence the term Didelphia (q.v.). The testes of the male arc suspended in a scrotum in front of the penis, the glans of which is often double. As a rule no allantoic placenta is present, but there is reason to suppose that the primitive marsupials were placental, and rudiments of this structure persist in the exist ing Australian bandicoots —a fact which in validates the former prime distinction made between the marsupials and higher Eutheria.
The young are dropped from the mother's womb as minute, undeveloped foetuses, those of the largest kangaroos being not half as large as mice when born. These larva (for they are that) are then taken by the lips of the mother and placed, one by one, within her ventral pouch, where each is attached to one of her teats, where it clings by means of its temporary sucking-mouth and is nourished by the milk which oozes or is pressed down its throat. They remain there a length of time varying with the size of the species, until they have grown to an advanced stage of development, when they gradually emerge; but for a long time afterward return to the mother's pouch for refreshment, rest or safety when alarmed. The pouch varies in its capacity and complete ness, in some families being quite absent, so that the young are shielded only by the long hair upon the mother's belly.