STRUCTURE OF JERBOA 



485 



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. 



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, how- 

 ever, sometimes free. 



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. liirtipes the body and tail measure respectively 4^- and 7 

 inches. The hind-feet have a tuft of long hairs below. Mr. 

 W. L. Sclater's newly-founded genus JZuchoreutes 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." 



1 Proc. Zool. Soc. 1890, p. 610. 



Thp Tprboas not FIG. 239. Bones of right pes 

 of Jerboa, Dipus aegyptius. 

 x f . a, Astragalus ; c, 

 calcaneum ; c 2 , middle 

 cuneiform ; c 3 , outer 

 cuneiform ; cb, cuboid ; 

 n, navicular ; I-IV, first- 

 to fourth toes. (From 

 Flower's Osteology.) 



