MAMMALIA ORDER V.RODENT1A. 



Fig. 60. A JERBOA 

 (Dipus). 



from the other members of the family in the equality of the length of the 

 fore and hind-limbs ; the upper jaw having a single pair of premolars, while 

 there are none in the lower. The North American jumping-mice (Zapus) 

 are distinguished by the elongation of the hind-limbs, and the presence of a 

 pair of premolars in both jaws ; the hind-feet being furnished with five toes, 

 and the metatarsal bones separate from one another. The common species 

 ranges from Hudson Bay to Colorado. Agreeing with the last in their elongated 

 hind-limbs, the next four genera differ in having only 

 three functional hind-toes, and by the union of the 

 metatarsal bones of the hind-foot. Of these four 

 genera Dlpus has only three hind- toes, while Eucho- 

 retes, Alactaga, and Platycercomys have five, of which 

 the first and fifth do not touch the ground. The first 

 three of these have a long, cylindrical and tufted tail ; 

 Dipus being distinguished from the other two by 

 its grooved incisors, and the frequent absence of 

 premolars. Euchoretes is represented only by the 

 Yarkand jerboa, characterised by its elongated muzzle 

 and large ears ; while Platycercomy^ of which there 

 are several species, ranging from Siberia to Nubia, 

 differs by its flattened and lancet-shaped tail, and the 

 invariable absence of premolars, the incisors being 

 smooth. The true jerboas (Dipus), of which there are 

 several species, extend from Algeria through Egypt and North Arabia to 

 Eastern Persia and Central Asia. Alactaga is 

 best known by the Kirghiz jerboa (A. decumana) 

 of the Kirghiz steppes. All the jerboas are 

 essentially desert-haunting animals, living in 

 the open, and progressing by long leaps. The 

 last and largest member of the family is the 

 so-called Cape jumping-hare (Pedetes caffer\ 

 ranging from the Cape to Angola and Mozam- 

 bique, and differing from all the other genera 

 in having rootless molars. The hind-limbs 

 are elongated, the metatarsal bones separate, 

 and the hind-feet furnished with four toes ; 

 while the ears are long and narrow, and the 

 very long tail is clothed with elongated hair. 



With the exception of the hares and picas, 

 which form a group by themselves, the whole of 

 the remaining members of the order are in- 

 cluded in a section known as the Hystrico- 

 morpha, which embraces six families. As a whole, this section is charac- 

 terised by the stoutness of the zygomatic arch of the skull, in which the 

 jugil bone forms the whole of the central portion without 

 any support from a process of the maxilla, while the angle 

 of the lower jaw arises from the outer side of the bony 

 sheath of the socket of the incisor. The collar-bones may 

 be either complete or imperfect ; in the lower part of 

 the hind-leg the fibula is distinct from the tibia ; and there is almost 

 always a single pair of premolar teeth in each jaw. The members of 

 this section are particularly characteristic of Central and South America 



^_ 



Fig. 61, CAPE JUMPING- 

 HARE (Pedetes coffer). 



Octodont Tribe. 



Family Octo- 



dontidce. 



