484 THE JERBOA CHAP. 



Allied to this family, and indeed united with it by Tullberg, 

 but kept separate by Thomas, is the 



Fam. 6. Heteromyidae. The members of this family are also 

 American, but are not confined to the northern-central regions 

 of that continent, for the genus Heteromys extends into South 

 America. 



The genus Dipodomys, with twelve species, is of a Jerboa-like 

 form, as the following measurements of an example of D. merriami 

 will show. The length of the head and body was 85 mm.; of 

 the tail 127 mm.; the hind-foot is 32 mm. It has but four 

 toes. The hind-limb is longer than the front-limbs. 



In Perodipus the same form is exhibited. There are, however, 

 five toes, and the sole of the foot is hairy. The axis vertebra 

 and the two following vertebrae are fused together. 



Perognathus is a third genus. It has the same general slender 

 form, but the tail is not so long, being but little longer than the 

 body. The hind-limbs, too, are shorter. The teats of this and 

 of Perodipus are as in Geomys. The two remaining genera of the 

 family are Heteromys and Microdipodops. 



Fam. 7. Dipodidae. This family consists of small, plain- 

 living, and leaping or arboreal creatures, commonly known as 



FIG. 238. Jerboa. Dipus hirtipes. x J. Eastern Europe. 



Jerboas. The main anatomical characters of the family are the 

 following: There is a large infra-orbital foramen. The molars 

 are always reduced, the premolar being either absent in the 

 lower jaw alone or in both jaws. This family presents an 

 obvious likeness to Dipodomys (hence the name of the latter) and 

 to some other members of the American family Heteromyidae 



