The least specialized genus is Zapus, containing the jumping-mice of North America, with one outlying Siberian species, in which the five metatarsals are free, as are also the cervical vertebrae, the small upper premolar being retained.
The jerboa group (Dipodidae, or Jaculidae) is also mainly an Old World type, although its aberrant representatives the jumping-mice (Zapus) have effected an entrance into Arctic North America.
JUMPING - MOUSE, the name of a North American mouselike rodent, Zapus hudsonius, belonging to the family Jaculidae (Dipodidae), and the other members of the same genus.
Moreover, the Szechuen jumping-mouse differs from the typical Zapus by the closer enamel-folds of the molars, the shorter ears, and the white tail-tip, and is therefore made the type of another sub-genus.
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