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as already stated, peculiar to the Neotropical Region, where they seem to take the place of the absent Insectivores. The Opossums are best arranged in two genera only, Didelphys with about twenty-five species, and the very distinct Water Opossum (Chironectes) with a single species only. The Tree Opossums (Didelphys) of which Mr. Thomas recognizes five sub-genera, are distributed all over the Neotropical Region and a single species, Didelphys virginiana, is a well-known inhabitant of the southern United States, and is therefore unquestionably a Nearctic mammal. Whether, however, it is certainly indigenous there, a remnant of the Marsupial-life of the tertiaries, or whether it may not have extended its range northwards from Central America in more recent times it is hard to say. Certain it is that the Virginian Opossum is very closely allied to its South American representative Didelphys azarm, and by some authors is considered merely as a sub-species. Besides the Virginian Opossum five others of the same genus are registered by Alston as occurring within the boundaries of the Central American Sub-region. The remaining species of Didelphys are distributed all over South America down to Paraguay and La Plata, and one species at least occurs in Chili to the west of the Andes.
The Water Opossum (Chironectes) has likewise a wide distribution—from Guatemala to Southern Brazil, but it is absolutely restricted to the Neotropical Region.
Section V.—Summary And Deductions
Table of the genera of the Order Marsupialia, showing the distribution of the species.
1. The Order of Marsupials consists of about 172 species divided into thirty-nine genera and eight families.
2. Six of these families (with thirty-six genera and 144 species) belong to the Australian Region; the remaining two families (with three genera and twenty-eight species) are Neotropical.
3. Two species of Phalanger have passed the northern boundaries of the Australian Region into Celebes which is in the Oriental Region (see Chapter V., p. 146).
4. In a similar way a single form of Opossum (Didelphys) is found in the southern part of the Nearctic Region.
5. With these exceptions no Marsupials are found in the Palsearctic, Nearctic, Oriental, and Ethiopian Regions.
Section VI.—Distribution Of Monotremes
The geograpical limits of the fourteenth and last order of mammals may be very easily pointed out, the Monotremes (which are correctly regarded as a sub-class, Prototheria) being absolutely restricted to the Australian Region and found even here only in certain parts of it. The Monotremes in these days are a failing group, consisting only of about five species, which belong to three genera and two families—the Echidnidm or Echidnas, and the Ornithorhynchidm or Duck-bills. Echidna, the first-known genus of the former family, is somewhat widely distributed from New Guinea over the whole of Australia and Tasmania. The Papuan, Australian, and Tasmanian forms of Echidna, which are closely related, are classed by some naturalists as species, and by others as sub-species. It is preferable to place them in the former category, as no intermediate forms are certainly known. But in Papua, besides typical Echidna, the family is also represented by its larger brother, Proechidna, with a single well-known species (Proechidna bruijni) and perhaps another species not yet certainly distinguished.
The second family of Monotremes embraces only the excessively anomalous aquatic and fossorial form commonly known as the Duck-bill (Ornithorhynchus anatinus). The Duck-bill occurs in Northern Queensland, and spreads southwards thence into New South Wales, Victoria, and South Australia. It is likewise found in Tasmania, but so far as we know has never yet been obtained in any part of Western Australia.
Table of the Order Monotremata, showing the distribution of the species.
1. The Order of Monotremes contains five species referable to three genera and two families.
2. It is entirely restricted to the Australian Region.