Some can glide, and some are adapted for semiaquatic life. These animals are remarkably uniform in structural characters. Canines and anterior premolars are lacking, leaving a space between the incisors and the cheek teeth. The number of teeth does not exceed 22 except in the genus Heliophobius, which has 28 teeth. The incisor teeth of rodents grow throughout life.
Growth is at the base, and the tooth is a segment of a true circle.
Thus, the tooth is continuously being pushed out of the end of the jaw, compensating for the portion worn away by cutting hard materials. The outer surface of the tooth is harder than the inner surface, much like a chisel, so that it is to some extent self-sharpening. There is no nerve in the tooth except at the growing base. The grinding, or cheek, teeth consist mostly of dentine. Enamel forms the outer layer, and in most rodents it also forms loops or folds in the body of the teeth.