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 Octodontidae, which are exclusively South American, differ from the preceding family by the tympanic bulla being filled with cellular bony tissue, and by the par-occipital process curving beneath it, while the cheek-teeth are almost or completely rootless and composed of parallel plates.
In the circumpolar Evotomys (represented in England by the red-backed field-mouse) and the nearly allied North American Phenacomys, the molars develop roots in old age; but in Microtus (which includes the water-rat, and is circumpolar) they are rootless throughout life, the genus being' one of the largest in the mammalian class (see Vole).
Of these Ctenodactylus and Pectinator are characterized by the union of the incus and malleus of the internal ear, the free fibula and the almost rootless cheek-teeth.
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.