Page:The New International Encyclopædia 1st ed. v. 13.djvu/129

* MARSUPIAL FROG. 107 MARSUPIALIA. infolding of the skin, which extend forward over the back; but in one species the opeAiig is longitudinal. The eggs are few in number and of large size, with much food-j'olk, for in most species the embryos remain in the pouch until they are fully matured. How the eggs get into the poui'li is ncjt known, hut (iadow thinks it most likely that tlic.y are placed there bj' the help of the male immediately after fertilization. Five or six species of these small, briglitly colored frogs have been described from the tropical for- ests of Venezuela and the Upper Amazon. Con- sult Gadow, Amphibia and Reptiles (London, 1901). MARSUPIA'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 coexten- sive with the subclass Jletatheria (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 Eiitheria. The mammary glands are provided with long teats, and are usually in- closed in a marsupiuni or pouch, which serves to protect the helpless young. This pouch, how- ever, may be very imperfect or even wholly want- ing, the young being protected only by the hair of the mother's abdomen. The young when born ai'e very minute and undeveloped. That of a big kangaroo is no larger than a man's little linger. They are not merely imperfect fcetuses, but 'actual larvie.' inasmuch as they are pro- vided with a special sucking mouth, in adapta- tion to their needs, which is later replaced by a true mouth. 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- rary 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. _The anus and urino-genital opening are surrounded by a common sphincter muscle. It was formerly supposed that no allantoic 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, insectivorous, and omnivorous forms are all well known. In distribution, one family, the Didelphyidae (opossums), is peculiar to the American continent, where it is spread I'rom New York State to Patagonia: only one of the 24 species, however, occurs north of Mexico. All the other marsupials (except one) are cnntincd to the Australasian region, where they conipletel.v domi- nate all other mammals, and form the most char- VoL, XIII,— 8. acteristic feature of the fauna. Their survival and prosperity iu Australia is doubtless due to the entire absence there of destructive carnivores, except the dingo, of doubtful antiquity; and they have become diversilied within their limited 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 liow they came there, so remote from Australia. Geological researches show that during the Mczozoic Age marsupials inhabited Europe and North America, but none of that period have been found in Australian rocks. These oldest ancestors of the race appear to have been mainly of the polyprotodont 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 Soutli America and Australia. At any rate, before the Tertiary Age was finished pouched marsu])ials disappeared from the Northern Hemisphere and survived only in Australasia and South America. The hypothesis of a former land connection be- tween Australia and Patagonia is no longer re- garded as tenable ; ' but it is interesting to know that a diprotodont (see Opossum-Rat) 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 (monotremes) and higher Eu- theria, and in a sense this is true, but the former belief that it re])resents a stage of development from the Prototheria to the monodelpliic mam- mals is not now accepted. The di-stinetions 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 gi'eat 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 eutherian mammal, since the time when the stock had acquired diphydonty and the allantoic placenta. See the article Mam- malia. Cla.ssification'. Rather less than 150 species are known, but they exhibit a most extraordinary variety of size, form, and color. The classification of the marsupials is based primaril.y upon the dentition, although the characters of the feet have been given mucli weight recently. There are two principal groups, the Poh/pro/odontia, which have numerous small, subequal incisor teeth, and the Uiprodnntin. which have not more than six incisors in each jaw and usually liave only two in the lower jaw. The former includes the opossums, Tasmanian wolf and 'devil,' the dasyures, bandicoots, and the like, while in the latter are the wombats, phalangers, koala, and kangaroos. Descriptive articles will be found under each of these terms and the related words. BiRLiooRAPHY. In addition to standard works and books descriptive of Australia, consult the great folio volumes, with magnificent colored plates, of .J. Gould, entitled Monograph of the M acropodidce (London, 1841), and ilummals of