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These animals have been regarded as ancestral Rodents, to which the tooth characters just mentioned clearly show likenesses. The earliest known form is Esthonyx. This genus shows such primitive characters, compared with its later representatives, as the existence of all three pairs of incisors in the upper jaw, but only two in the lower jaw. The enlarged incisors of both jaws do not seem to have grown from persistent pulps.

Anchippodus, a later form, still preserves the upper pair of first incisors in a vestigial form; the strong second incisors grew from persistent pulps. The most recent genus, Tillotherium, shows the characteristics of the group at their height. The strong Rodent-like, chisel-shaped incisors, which are reinforced by a small additional pair in the upper jaws only, are persistent. The grinding teeth are of the tritubercular pattern; there are three of each kind in the upper jaw, but in the lower jaw only two premolars on each side. This is at any rate the case with some, while others have three. The canine, though present in both jaws, is insignificant. As in many ancient types, there is an entepicondylar foramen in the humerus. The feet were five-toed, and bore sharp, laterally-compressed claws. The skull has been compared in general aspect to that of a Bear.