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.
Fig. 238.—Jerboa. Dipus hirtipes. × ⅓. Eastern Europe.
Fam. 7. Dipodidae.—This family consists of small, plain-living, and leaping or arboreal creatures, commonly known as 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
