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. [Illustration: 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. [Illustration: ALACTAGA.]