The lower jaw partly in section to show the lower incisor tooth.
Rodentia (Lat. 'gnawers'), an order of Mammals more rich in species than any of the others, including among its familiar representatives squirrels, marmots, beavers, rats and mice, lemmings, porcupines, guinea-pigs, hares and rabbits. Most are terrestrial, and many are burrowers, but a few are arboreal or even semi-aquatic. All are vegetarian, and gnaw their food. They are represented in all parts of the world.
Among the anatomical characteristics of Rodents may be noted the chisel-like edge of the incisor teeth, which wear away in front less rapidly than they do behind, where the enamel coating is thinner or absent; the reduction of the incisors to two above and two below, except in the hares and rabbits, in which there are four above; the fact that the incisors and sometimes the back teeth also are rootless, and continue growing from persistent pulps; the absence of canine teeth, and the presence of a large space between incisors and premolars; the condyle in which the lower jaw works is elongated from before backwards—an adaptation to the peculiar motion of the lower jaw characteristic of rodent gnawing; the cerebral hemispheres are smooth, and leave the cerebellum uncovered; the intestine, as in many herbivorous animals, has a large cæcum; the uterus is two-horned, the placenta discoidal and deciduate; the reproduction is in many cases very prolific.
Classification.—Sub-order Simplicidentata—with only one pair of upper incisors, having enamel only in front. This sub-order includes squirrels (Sciurus), flying squirrels (Pteromys and Sciuropterus), marmots (Arctomys), beavers (Castor), dormice (Myoxidæ), rats and mice, voles, lemmings, muskrats (Muridæ), pouched-rats (Geomyidæ), the capybara (Hydrochærus), porcupines (Hystricidæ), agoutis (Dasyprocta), guinea-pigs (Cavia). Sub-order Duplicidentata—with two pairs of incisors in the upper jaw, the second pair behind the first, the enamel extending round the teeth, but thinner posteriorly. This sub-order includes only the Picas or tailless hares (Lagomys) and the hares and rabbits (Lepus).
See Waterhouse, Natural History of the Mammalia, vol. ii. 'Rodentia' (1848); Flower and Lydekker, Mammals, Living and Extinct (Lond. 1891).