Indeed, despite the fact that they present much diversity of habit - some being arboreal, as the squirrels, many of which are provided with expansions of skin or parachutes on which they glide from tree to tree; some cursorial, as the hares; others jumpers, as the jerboas; others fossorial, as the mole-rats; and others aquatic, as the beavers and waterrats - no important structural modifications are correlated with such diversity of habit.
In the Jaculidae the metatarsals are greatly elongated, and in some of the species, as jerboas, they are welded together.
In general habits and appearance these animals recall large jerboas, from which group they are, however, distinguished by the four pairs of rooted cheek-teeth, the premolars being as large as the molars, and the latter having one outer and one inner enamel-fold.
- The next Myoxoidea, or dormice, mouse group, have the three sections of the order, namely, the Dipodoidea, or jerboas, and Myoidea, or the following characteristics in common.
In the typical jerboas, Jaculus (or Dipus), ranging from North Africa to Persia, Russia and Central Asia, there are only three hind toes, the incisors are grooved, and the premolars are generally wanting.
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.
As regards the true jerboas, there is a curious resemblance in the structure of their hind-legs to that obtaining among birds.
It may also be noticed that in mammals and birds which hop on two legs, such as jerboas, kangaroos, thrushes and finches, the proportionate length of the thigh-bone or femur to the tibia and foot (metatarsus and toes) is constant, being 2 to 5; in animals, on the other hand, such as hares, horses and frogs, which use all four feet, the corresponding lengths are 4 to 7.
In addition to their long hind and short fore limbs, jerboas are mostly characterized by their silky coats - of a fawn colour to harmonize with their desert surroundings - their large eyes, and long tails and ears.
They have elongated hind-limbs and long hairy tails; and progress by leaps, in the same manner as jerboas, from which they differ in having five hind-toes.
Among jumping animals it may serve as a balance, as in the case of jerboas and kangaroos, while in the latter it is also used as a support when resting; among many hoofed mammals it is used as a fly-whisk; and in whales and dolphins, as well as in the African Potamogale and the North American musquash, it plays an important part in swimming.
In jerboas, for example, a bunch of twelve or thirteen hairs springs from the same point, while in the polar bear a single stout hair and several slender ones arise together, and in the marmosets three equal-sized hairs form regular groups.