MOLAR TEETH OF THE JERBOA.
If we regard the American Jumping Mouse as constituting a peculiar section of the family, ZAPODINÆ, a second group, DIPODINÆ, is formed by the TRUE JERBOAS, which make up the greater part of the family. These either possess only three molars, or a very small additional tooth exists in front of each series in the upper jaw. The molars are rooted, and diminish in size backwards in each series. The cervical vertebræ are anchylosed; the fore feet have the thumbs rudimentary, but sometimes furnished with a small nail; the hind feet have only three toes fully developed, and the metatarsals are united into a single bone of great length; the soles are furnished with elastic balls; and the tail is very long, well-clothed with hair, and tufted at the end.
JERBOA.
Of these pretty little creatures, which are in some respects singularly bird-like, about twenty species have been recorded, and these occupy the whole of the Old World area of the family, except South Africa. The JERBOA (Dipus ægyptius) may serve as an example of this section of the family. This is a most lively and active little creature, which inhabits the deserts of north-eastern Africa as far south as Nubia, and extends its range into Arabia and south-western Asia. On these arid plains, so scantily clothed with a few grasses and dry shrubs that it is difficult to conceive how any animal can find a living on them, the Jerboa lives, often in numerous societies, and in company with the few birds and lizards which enliven the wilderness. These animals dwell in subterranean abodes consisting of many branched galleries, which they dig out in the hard soil not far from the surface. The Arabs assert that these habitations are produced by the joint labour of the whole society. They retreat into their burrows at the least alarm. The females are said to produce from two to four young at a birth in a nest made in the deeper part of the burrow, and lined with hair pulled from the under surface of her own body. When going along quietly, the Jerboa walks and runs by alternate steps of the hind feet, but when there is occasion for rapid motion it springs from both feet at the same time, covering so much ground at each leap, and touching the ground so momentarily between them, that its motion is more like that of a bird skimming close to the surface of the ground than that of a four-footed beast.
The Jerboa is about six inches long, with a tail about eight inches in length exclusive of the tuft with which its tip is adorned. Its upper surface is of a greyish sand-colour, like that of many other desert animals; the lower surface is white; and the tail pale-yellowish above and white beneath, with the tuft white, with an arrow-shaped black mark on its upper surface.
ALACTAGA.
Several other species of Jerboas are known, some from the deserts of North Africa, others from the steppes of Central Asia. The latter region harbours some forms, which differ from the preceding, among other characters, by having five toes in the hind feet, whereas the true Jerboas have only three, but of the five toes only three are sufficiently developed to take part in the animal’s progression. The best known of them is the ALACTAGA (Alactaga jaculus), a rather larger species than the Jerboa, and with a still longer tail, reddish-yellow with a greyish tinge above, white beneath and on the hind legs. Its range extends from the Crimea and the steppes of the Don across Central Asia to the borders of China. It walks upon all-fours, and when advancing quickly springs along after the fashion of the Jerboa. Its food consists of all sorts of vegetable substances, but it is especially fond of the bulbs of plants, and does not refuse occasionally to eat insects, or even the eggs and young of the birds which inhabit the steppes with it. The Alactagas live in very complicated burrows, with many passages and branches, and they are said always to make one passage from the central chamber of their residence, which terminates close to the surface of the earth at some distance, but is only opened in case of danger, when the inhabitants escape through it, the position of its intended aperture being previously unrecognisable. In cold weather they sleep in their nests. The female produces from five or six to eight young, in a nest lined with her own hair. Species of Alactaga occur not only in Central Asia, but also in Arabia and North Africa.
