There is even the same ankylosis of the neck vertebrae. We find, moreover, the same association of long-legged and shorter-legged forms that characterises the Heteromyidae.
Fig. 239.—Bones of right pes of Jerboa, Dipus aegyptius. × ¾. a, Astragalus; c, calcaneum; c2, middle cuneiform; c3, outer cuneiform; cb, cuboid; n, navicular; I-IV, first to fourth toes. (From Flower's Osteology.)
The typical genus Dipus is a smallish quadruped with long naked ears and a long tail. The ten species are all Palaearctic in range. The fore-limbs are short and five fingered, and the short pollex has no claw; the hind-limbs are excessively long and only three-toed. The bony structure of these limbs is remarkable. The three metatarsals are elongated almost like those of a bird, and are ankylosed together. The digits have long phalanges which alone reach the ground as the animal hops. It is a curious fact, and one not so easily identifiable with the way of life, that the neck vertebrae of this genus are ankylosed together with the exception of the atlas, which is free; the arrangement is precisely like that of the Sperm Whale. The last vertebra is, however, sometimes free. The Jerboas not only leap but they burrow, and their strong incisors are said to be used in burrowing through stony ground. They are eaten by the Arabs, and are, or have been, called Daman Israel, i.e. Lamb of Israel. In D. hirtipes the body and tail measure respectively 4½ and 7 inches. The hind-feet have a tuft of long hairs below. Mr. W. L. Sclater's newly-founded genus Euchoreutes[1] is somewhat more primitive in its characters than is Dipus. The general form is the same, with long ears and a long tail. But there are five toes to the hind-limb, the two lateral ones though nailed being much shorter than the middle three. It has a "long pig-like snout," and the tail is cylindrical as in most other Jerboas, with a tuft of longer hairs at the end. The incisor teeth, grooved in Dipus, are here smooth, as in Alactaga. The species was probably obtained "in the sandy plains round the city of Yarkand."
- ↑ Proc. Zool. Soc. 1890, p. 610.
