The premolars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series.
Sewellels are medium-sized terrestrial rodents, with no postorbital process to the skull, which is depressed in form, and rootless cheek-teeth, among which the premolars number I, the first in the upper jaw being very small.
The second section, Castoroidea, of the present group includes only the family Castoridae, represented by the beavers, which are large aquatic rodents characterized by their massive skulls, devoid of post-orbital processes, with the angle of the lower jaw rounded, the molars rootless or semi-rooted, with re-entering enamel-folds, and one pair of premolars above and below.
The dentition includes one pair of premolars above and below, and rooted or rootless molars with but few enamel folds.
The humerus lacks a foramen at the lower end; and the molar teeth, as explained and illustrated in the article Vole, consist of two longitudinal rows of triangular alternating vertical prisms, and may be either rootless or rooted.