The Spalacidae are burrowing types, allied apparently to the ancestral Jaculidae, and characterized by the second and third molars being equal in size, the presence of enamel-folds in all these teeth, and the superiority in size of the claws of the second, third and fourth front toes over the other two.
The characteristics of the Muridae are those of the Myoidea generally, as given above under the heading of the Spalacidae.
Ellobius, which many naturalists place in this group, has been mentioned among the Spalacidae.
The beavers (Castoridae) are restricted to the northern hemisphere, whereas the dormice (Gliridae) and the mole-rats (Spalacidae) are exclusively Old World forms, the latter only entering the north of Africa, in which continent the former are largely developed.