The middle metacarpals and metatarsals generally confluent, the outer ones (second and fifth) slender but complete, i.e.
The Dipodinae, on the other hand, are leaping rodents, with the metatarsals elongated, a small upper premolar present or absent, and the crowns of the molars tall.
In all the more typical members of the family the three middle metatarsals of the long hind-legs are fused into a cannon-bone; and in the true jerboas of the genus Jaculus the two lateral toes, with their supporting metatarsals, are lost, although they are present in the alactagas (Alactaga), in which, however, as in certain allied genera, only the three middle toes are functional.
They are small rat-like rodents, with one pair of upper premolars, which are mere pins, as is the last molar, and the two pairs of limbs of normal length, with the metatarsals separate; the infra-orbital opening in the skull being triangular and widest below, while the incisive foramina in the palate are elongated.
In the Jaculidae the metatarsals are greatly elongated, and in some of the species, as jerboas, they are welded together.
In the forelimbs the bones corresponding to the third and fourth metacarpals of the pig's foot are fused into a cannon-bone; and a similar condition obtains in the case of the corresponding metatarsals in the hind-limbs.
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 hind-limb is typically avine, with intertarsal joint, distally reduced fibula, and the three elongated metatarsals which show already considerable anchylosis; reduction of the toes to four, with 2, 3, 4 and 5 phalanges; the hallux is separate, and as usual in recent birds posterior in position.
Third and fourth metacarpals and metatarsals confluent into cannon-bones (fig.
The three middle metatarsals become fused together into a cannon bone; the upper part of the third middle metatarsal projects behind and forms the so-called hypotarsus, which in various ways, characteristic of the different groups of birds (with one or more sulci, grooved or perforated), acts as guiding pulley to the tendons of the flexor muscles of the toes.