All authorities are agreed in dividing rodents into two great sections or sub-orders, the one, Duplicidentata, comprising only the hares, rabbits and picas, and the other, Simplicidentata, all the rest.
The remaining rodents, which include two families - the picas (Ochotonidae) and the hares and rabbits (Leporidae) - constitute a second sub-order, the Duplicidentata, differing from all the foregoing groups in possessing two pairs of incisors in the upper jaw (of which the second is small, and placed directly behind the large first pair), the enamel of which extends round to their postericr surfaces.
The Ochotonidae are represented at the present day only by the single genus Ochotona (Lagomys), which includes all the picas, or mouse-hares.
From the picas the hares and rabbits (Leporidae) are distinguished by the imperfect clavicles, the more or less elongated hind-limbs, short recurved tail (absent in one case) and generally long ears.
In the loss of the last upper molar, the Liu-Kiu rabbit approximates to the picas, as does the tailless rabbit in the abortion of its caudal appendage.
Lastly, among the Duplicidentata, the picas (Ochotonidae or Lagomyidae) form a group confined to the colder or mountainous regions of the northern hemisphere.