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-lmb 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 
Fic. 238.—Jerboa.  Dipus hirtipes. x4. 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 
