MAR'SUPIA'LIA. The marsupials form one of the great subdivisions of the class Mammalia, and are of special interest because of their an cestral history and relationships, and their re markable geographical distribution. Although ranked as an order, Marsupialia is coxten sive with the subclass Metatheria (q.v.). Its principal characters are as follows: the brain is small, with the surface-folding absent or very simple, the corpus callosum rudimentary, and the cerebellum completely exposed. Epipubic bones are present in both sexes, and there are other important skeletal characters, prominently a tendency to the separation of bones ankylosed in the higher Eutheria. The mammary glands are with long teats, and are usually in closed in a or pouch, which serves to protect the helpless young. This pouch, how ever, may be very imperfect or even wholly want ing. the young tieing protected only by the hair of the mother's abdomen. The young when born are very minute and undeveloped. That of a big kangaroo is no larger than a man's little linger. They are not merely imperfect foetuses, but 'actual lame,' inasmuch as they are pro vided with a special sucking mouth, in adapta tion to their needs, which is later replaced by a true month. The young when born are transferred by the lips of the mother to the pouch, where they are placed upon a teat to which the tempo any sucking mouth clings; and, as they are un able to suck, the milk is injected into them by the action of special muscles of the mammary gland. (See GLAND.) The organs of reproduc tion are peculiar to the group, which is often called `Didelphia' in reference to their character. The oviducts never unite to form a uterus and the vagina is always double, at least in part ; the testes hang suspended in front of the penis and the glans of the latter is often bifurcate. anus and urino-genital opening are snrrounded by a common sphincter niusele. It was formerly supposed that no allantoie placenta was present in the group, but it is now known to exist in some bandicoots (Perameles). The egg is minute, as in other Eutheria, but incompletely divides at first.
In dentition and habits as great a variety exists among the marsupials as in all the,rest of the mammals together. for carnivorous, herbiv orous, insectivormia, and omnivorous forms are all well known. In distribution. one family, the Didelphyidie (opossums). is peculiar to the American continent, where it is spread iron] New York State to Patagonia: only one of the 24 species, however, occurs north of Mexieo. All the
other marsupials (except one) are confined to the Australasian region, where they completely domi nate all other mammals, and form the most char acteristie feature of the fauna. Their survival and prosperity in Australia is doubtless due ,to the entire absence there of destructive carnivores, except the dingo, of doubtful antiquity; and they have become diversified within their lindted cir cumstances in the same way as have the larger company of mammals all over the world, to en able them to utilize all possible advantages. The fact of marsupials existing in America, and especially in the Neotropical region. has excited much speculation as to how they eame there, so remote from Australia. Geological researches show that during the Mezozoie Age marsupials inhabited Europe and North America, lint none of that period have been found in Australian rocks. These oldest ancestors of the race appear to have been mainly of the polyprotodent type, little differentiated from the diprotodonts, how ever; and either this differentiation occurred very long ago (in Jurassic or Cretaceous times) or the latter is a condition which has arisen, as Beddard suggests, independently in both South America and Australia. At any rate, before the Tertiary Age was finished pouched marsupials disappeared from the Northern Hemisphere and survived only in Australasia and South America. The hypothesis of a former land connection be tween Australia and Patag,onia is no longer re garded as tenable;' but it is interesting to know that a. diprotodont (see exists in Patagonia.
The relationships of marsupials have become much better understood than formerly. The name Metatheria was originally given with the idea that this group was intermediate between the Prototheria (monotreines) and higher En theria, and in a sense this is true, but the former belief that it. represents a stage of development from the Prototheria to the monodelphie mam mals is not now accepted. The distinctions be tween the marsupials and the Monotremata are fundamental, and there is no evidence of the derivation of the two branches from any common source. On the contrary. as Beddard concludes in a learned review of the subject, the great spe cialization of the structure of the marsupials (including evidence of degeneration), and their age, point to the fact that they are the descend ants of an early form of entherian mammal. since the time when the stock had acquired diphydonty and the allantoie placenta. See the article Alk MALTA.