Sub-Fam. 2. Hydromyinae.—The genus Hydromys,[[330]] of which there are several species, the best known being H. chrysogaster, is an exclusively Australian form, and is aquatic in habit. It is a foot or so in length, and has a fairly long tail. The fore- and hind-limbs are webbed, in correspondence with its habits. The Australian Water-Rat is black, with an admixture of golden-coloured hairs dorsally and golden colour below, with a lighter median stripe. The thumb is small, and the webbing of the hands is not so marked as is that of the feet. The molars are only two in each half of each jaw. The caecum is rather small, the measurements of the alimentary canal being: small intestine, 895 mm.; large intestine, 278 mm.; caecum, 70 mm. Allied to the last is Xeromys, a genus which is also Australian, but limited to Queensland. It has been established by Mr. Thomas,[[331]] who discovered that it has the same reduced formula as Hydromys. Xeromys, however, is not an aquatic animal, and has unwebbed feet.
In the Luzon highlands Mr. Whitehead has discovered, and Mr. Thomas quite recently described,[[332]] a number of peculiar Rodents. Of these the genera Chrotomys, Celaenomys, and Crunomys are allied to the Australian and New Guinea Hydromys.
Chrotomys whiteheadi is unusual among Muridae, in its coloration being marked by a pale stripe down the back. The creature is the size of the Black Rat (Mus rattus). It is terrestrial not aquatic in habit, in spite of its likeness to Hydromys. The molars, however, are 3/3.
Crunomys fallax is more like Hydromys. It has, however, three molars, as in the last genus. But the skull has the flattened form characteristic of Hydromys as opposed to Mus.
Like Batomys, Celaenomys silaceus is also somewhat intermediate between Hydromys and Mus. It is described as very Shrew-like in appearance, and has a very pointed muzzle. Its habits Mr. Whitehead is "quite unable even to guess at." Like Hydromys and Xeromys this Rodent has but two molars.
Sub-Fam. 3. Rhynchomyinae.—The genus Rhynchomys, containing but one species, Rh. soricioides (of Thomas), is also, as both its generic and specific names imply, a somewhat Shrew-like form in external aspect. The skull, too, is Insectivore-like in its
elongation, and the lower incisors are worn to needle-like points. The two molars are excessively minute, and thus the always large gap in the jaws is greatly exaggerated. It is suggested that this Rat is an insect-eater, but nothing positive is known.
Sub-Fam. 4. Gerbillinae.—The Gerbilles form another sub-family, Gerbillinae, of the Muridae, or a family, according to some. The best-known genus is Gerbillus, including the Gerbilles proper. These animals are Old World in range, belonging to the three regions of that part of the world. There are a large number of species in the genus, over thirty. They have a Jerboa-like form, with rather long hind-limbs and a long and hairy tail. But the hind- as well as the fore-feet are five-toed. The molar teeth have no trace of tubercles, but only transverse lamellae of enamel. The incisors are orange; they are white in Dipus. Gerbillus pyramidum is 90 mm. long, with a tail of 125 mm. The ears are long, 13 mm. The tail has longer hairs at the tip.
Fig. 236.—Gerbille. Gerbillus aegyptius. × ½.