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fication have recently undergone important changes, and continual discoveries of new species and new alliances are made by several busy naturalists who are engaged mainly on a study of the smaller mammals. Under these circumstances it is hardly necessary for our present purpose to mention more than the names of most of the twenty-one families which constitute this complicated group, but we shall endeavour to pick out, as we go through them, some of the most noticeable facts connected with the distribution of these mammals.
Adopting Mr. Thomas's recent classification of the genera of this group* (with a few slight deviations) as the best authority, we find the Anomaluridæ, à singular group of Flying-Squirrel-like Rodents, at the head of the Order. This family, with its three genera (Anomalurus, Idiurus, and Zenkerella), is purely Ethiopian, the eleven or twelve species which are referred to it occurring only in tropical Africa. Passing on to the next family, the Squirrels (Sciuridæ), we have an extensive group of about 240 species divided into eleven genera distributed nearly all over the earth's surface, with the exception of the Australian Region and Madagascar, where they are entirely deficient. The most numerous genus is that of the true Squirrels (Sciurus) which, subject to the exception just mentioned, is fairly distributed over the whole of the earth.
The Castoride, or Beavers, which come next, are represented in the present day only by the genus Castor, with two species, one of which occurs in the high latitudes of the Palearctic and the other in those of the Nearctic Region. These two species are closely allied and perhaps scarcely distinguishable.
1 “On the genera of Rodents,” P. Z. S. 1896, p. 1012. Cf. Palmer, “Science,” N. S., vi., p. 103 (1897).
The Haplodontiidæ, or Sewellels, allied to the Squirrels, contain only the single genus Haplodontia, the species of which are confined to the Nearctic Region.
In the fourth family of the Rodents we meet with more familiar objects. The Gliridæ, or Dormice, with six genera and about nineteen or twenty species, have a curious distribution, being found only in the Ethiopian, Oriental, and Palæarctic Regions. Each of these Regions, however, has its peculiar genera, true Glis and its allies belonging to the Palæarctic Region, whilst Graphiurus is strictly Ethiopian, and the two remaining genera are restricted to the Oriental Region.
We now come to the Mice, or Muride, which, as already indicated, are exceedingly numerous and all-pervading creatures. Mr. Thomas places the 730 species of this family in seventy-eight genera. Mice are most numerous, perhaps, in the tropics, but are also well represented in Arctic latitudes, and in the shape of Lemmings (Lemmus), extend far towards the Pole.
They are not abundant in Australia proper, being represented there chiefly by the peculiar genus Hydromys and a few species of Mus. In Madagascar the seven genera of Muridæ met with are likewise altogether restricted to that anomalous island.
The Mole- rats (Spalacide), which follow next in Mr. Thomas's series, are a small and peculiar group, the members of which imitate the subterranean life of the Moles. The typical genus Spalax, with eight species, is confined to the Palæarctic Region, whilst the Bamboo Rats (Rhizomys), represent the group in the Oriental Region and Tachyoryctes in the Ethiopian Region.
The Pocket-gophers (Geomyide), which are entirely restricted to the Nearctic Region, contain only two genera and nine species. Allied to them are the Heteromyidæ, a more numerous group of seventy or eighty species, entirely restricted to the New World, and, with the exception of a few stray species of Pocket-mice (Heteromys), to the Nearctic Region.
The tenth family of Rodents, the Bathyergidæ, belong entirely to the Ethiopian Region, over which they are thinly represented by fifteen or sixteen species. The Naked Sand-Rat of Southern Abyssinia and Somaliland (Heterocephalus glaber), is one of the most extraordinary-looking Mammals in the world, being almost entirely without hair and covered with a yellowish naked skin; it is subterranean in its habits.
The Dipodidæ, or Jerboas, which we now come to, are well known for the great length of the hind limbs and the kangaroo-like manner of their progression; they consist of six genera and about thirty-three species, all of which, except one (Zapus), are restricted to the Palearctic Region. The six species of Zapus, are spread over the Nearctic Region from the far North down to Mexico, where, however, they are restricted to the highlands.
Allied to the Jerboas is the Jumping-Hare (Pedetes caffer), which forms an allied family of itself, and is restricted to Southern and South-eastern Africa,
We now arrive at the series of Porcupiny Rodents, of which as many as seven families are usually recognized. These are mostly found in the Neotropical Region, and four of them indeed, the Chinchillidæ, Dasyproctidæ,
Dinomyidæ, and Caviidæ, containing what are usually called the Chinchillas, Agoutis, Giant-mice, and Cavies, are entirely restricted within its limits, while a fifth family, the Erethizontidæ, or Tree-Porcupines, has a single genus in the Nearctic Region.
The Octodontida, a large group of seventy or eighty species, divided into some twenty-two genera, are also mostly Neotropical, but four peculiar types, Ctenodactylus, Massoutiera, Pectinator, and Petromys forming a little group by themselves, are Ethiopian. The true Porcupines, Hystricidæ, of which three genera are known, are found in the Ethiopian, Oriental, and Palearctic Regions, typical Hystrix being the only one met with in Europe and Northern Asia.
Finally, at the close of the long series of Rodents, we have the two groups of Pikas and Hares, markedly differing from the nineteen previous families in their dentition, and therefore assigned to a separate Sub-order of Rodents as Duplicidentati. The Pikas (Ochotoma), of which some sixteen species are recognized, are restricted to the highlands of the Nearctic and Palearctic Regions. The Hares (Leporide) have a much wider distribution, having representatives in every part of the world's surface except in the Australian Region and Madagascar. Of Lepus proper some sixty species are now recognized, the greater number of which occur in the Palearctic and Nearctic Regions, whilst they are generally scarcer further south, though well represented in Africa.