_Distribution._—The existing species of Marsupials are, with the exception of one family (the _Didelphyidæ_), limited in geographical distribution to the Australasian region,[39] forming the chief mammalian fauna of Australia, New Guinea, and some of the adjacent islands. The _Didelphyidæ_ are almost purely Neotropical, one or two species ranging northwards into the Nearctic region. Fossil remains of members of this family have also been found in Europe and America in strata of the Eocene and early Miocene periods; and it is probable that at least many of the polyprotodont Mesozoic mammals noticed in Chapter IV. are referable to the Marsupialia. [Illustration: FIG. 36.—Front view of skull of Koala (_Phascolarctus cinereus_), showing diprotodont and herbivorous dentition (_Quart. Journ. Geol. Soc._ vol. xxiv. p. 313).] _Classification._—In dividing the Marsupials into minor groups, it may be observed that one of the most obvious distinctive characters among them is derived from the form and arrangement of the teeth. In certain species, as the Opossums, Dasyures, and Thylacine, the incisors are numerous, small, and subequal in size, and the canines large, as in the typical placental Carnivores (Fig. 35). To these the term “polyprotodont” is applied, and they are all more or less carnivorous in their habits. In others the central incisors are very prominent, and the lateral incisors and canines absent or subordinate in function (Fig. 36). These are called “diprotodont,” and they are all wholly or in great part vegetable feeders. In one group of these, the Wombats, there are but two incisors above and the same number below; but all the others, including the Kangaroos, Koalas, and Phalangers, have two functional incisors below and as many as six above, three on each side, but of these the first or central pair is the most fully developed. Some hesitation has frequently been expressed as to whether the Polyprotodont and Diprotodont types are entitled to constitute distinct primary groups, owing to the presence of syndactylism among the _Peramelidæ_ in the former, as well as in the latter; but if Mr. O. Thomas is right in regarding this feature as acquired independently in the two groups we may safely adopt such a division. Taking various combinations into consideration, the existing Marsupials readily group themselves into six very natural families, the leading characters of which may be summarised as follows:— _Order_ MARSUPIALIA. _A._ POLYPROTODONTIA.—Incisors numerous, small, subequal. Canines larger than the incisors. Molars with sharp cusps. α. Incisors ⁵⁄₄. Hind feet with the four outer toes subequal, distinct, and a well-developed opposable hallux. _Didelphyidæ._ β. Incisors ⁴⁄₃. Hind feet with four outer toes distinct. Hallux small or rudimentary, rarely opposable. _Dasyuridæ._