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.
As regards the Duplicidentata, it appears that the families Ochotonidae and Leporidae had become differentiated as early as the Lower Miocene.
In general dental characters, especially the retention of three pairs of molars, this genus approximates to the Leporidae, although in the absence of post-orbital processes and the pattern of the molars it departs less widely from the modern Ochotonidae than does Prolagus.
Lastly, among the Duplicidentata, the picas (Ochotonidae or Lagomyidae) form a group confined to the colder or mountainous regions of the northern hemisphere.