Marsupials, lit. 'ponched animals' (Marsupialia, Didelphia, or Metatheria), a sub-class of mammals, the members of which, with the exception of the American opossums, are now restricted to the Australian and Austro-Malayan regions. They are in many ways simpler than the higher mammals, notably in the structure of the brain and in the absence of a close connection between the unborn young and the womb of the mother. The young are born very helpless, after a short gestation, and are usually stowed away in an external pouch or marsupium, where they are fed from the enclosed teats. From the wide occurrence of fragmentary marsupial remains in Triassic and Jurassic strata both in the Old and the New World, it seems that the pouch-bearers have been once widely distributed. Before the stronger mammals which rose up after them they have, however, succumbed, except in the case of the above-mentioned refugees in neo-tropical forests, and those saved by the insulation of the Australasian regions before any higher mammals gained a foothold. In the retreat thus afforded the marsupials have developed along numerous lines, as it were prophesying the carnivores, insectivores, rodents, and herbivores among the placental mammalia. Thus, apart from the carnivorous and insectivorous American opossums (Didelphyidae), of which one is strictly North American and the rest neo-tropical, there are five Australasian families: the Dasyuridae or 'native cats,' carnivorous and insectivorous marsupials as large as wolves and as small as mice, of which very pronounced types are the Tasmanian 'tiger' (Thylacinus) and the native ant-eater (Myrmecobius); the rodent-like Peramelidae or bandicoots; the herbivorous kangaroos and kangaroo-rats (Macropodidae); the very varied family of arboreal Phalangers (Phalangistidae), including the flying opossums (Petaurista, &c.), the native sloth or koala, the honey-sucking Tarsipes, and other curious forms; and finally the Phascolomyidae or wombats, rodent-like root-eating forms about the size of badgers.
See MAMMALS; also the well-known works of Owen, Huxley, and others on Vertebrates; the relevant parts of Cassell's and the Standard Natural History; Chisholm's trans. of Vogt and Specht's Mammals (1887); Waterhouse, Natural History of Mammalia, i. (1846); and Gould's Mammals of Australia (3 vols. 1845-63).