Rodentia (Lat., from rodere, to gnaw), an order of mammals characterized by the chisel shape of the incisors, adapted for gnawing the hard vegetable substances upon which they principally feed, such as the wood and bark of trees, hard-shelled nuts, and occasionally bony structures like ivory. Rodents are generally small, numerous in species, very prolific, and found in all parts of the globe. They are un-guiculated, and in most the hind parts of the body and limbs exceed the front in length, so that they leap rather than walk, in some (as the jerboa) the disproportion being so great that they resemble the kangaroos in their mode of progression; the skull is small and flat, with the jaws, especially the lower one, strong; the snout is usually provided with long moustaches; the opening of the mouth is small, but the cheeks often form large pouches in which they convey food to their burrows; the legs are short in most, for walking or climbing, in the flying squirrels provided with a membrane extending from the sides, which answers the purpose of a parachute; many, like the beaver and muskrat, are excellent swimmers and divers; the thumb is never opposable, when present; the skin is ordinarily covered with soft fur, but sometimes interspersed with bristles or spines; tail hairy, or naked and scaly.

It is by their dental character that the rodentia are most clearly defined as a natural order. The bow-shaped incisors, usually 1/1-1/1, grow from persistent pulps, and are covered with a coat of enamel only in the front; the leporidoe (hares and rabbits) possess an additional pair of incisors in the upper jaw, situated directly behind the first and larger pair. The superior hardness of the enamel, which much more effectually resists the action of gnawing than the comparatively soft dentine, enables the rodents to retain unimpaired a sharp, chisel-shaped edge to their cutting teeth, the wearing away of the crown being constantly replaced by growth below.- The loss or breaking of one of these incisors, leaving the growth of the opposing tooth unchecked by a resisting medium, results disastrously, frequently causing an interlocking of the jaws. There are no canines. The grinders vary in number from four to twelve in the upper jaw, and from four to ten in the lower. When the number in either jaw exceeds six, the supernumerary teeth immediately preceding the last triplet must be regarded as true premolars, supplanting a milk dentition; they are composed of cement, dentine, and enamel, and either form roots (thus limiting the duration of growth) or grow from persistent pulps like the incisors.

The summits of their crowns are generally traversed by parallel transverse ridges, placed in opposition to the antero-posterior movement of the lower jaw, thereby greatly facilitating mastication. - The order comprises such animals as the capybara, beaver, porcupine, squirrel, marmot, dormouse, rat, hamster, lemming, jerboa, hare, rabbit, muskrat, Guinea pig, agouti, and chinchilla. Waterhouse divides them into the five families of leporidoe, hystricidoe, muridoe, sciuridoe, and saccomyidoe, all represented in North America, and the last peculiar to it. Rodents form nearly one third of all mammals, and in North America one half of all the land mammals, this last containing about one fifth of all the described species; of the squirrels, nearly one third of all known species are found within the limits of the United States; the pouched rats are entirely American; of the rat family, the field mice are best represented in North America; of the porcupine family, more than seven eighths are South American, the capybara, the largest living rodent, being among them, itself greatly surpassed in size by the extinct castoroides Ohio-ensis of North America; while many species of hares are found in North America, only one is met with in South America. - This order is generally considered as displaying very little intelligence, though manifesting (as the beaver) some of the most remarkable instincts; but the rat certainly shows an adaptation of means to ends, under circumstances often the most unnatural and unexpected, which makes it hard to draw the line between animal instinct and intelligence.

There is in many an extraordinary development of the sexual appendages, some of which are very complex and peculiar to the order; the testes are generally larger than the kidneys, and in most are not contained in a scrotum, but beneath the skin of the perineum; the intromittent organ is variously directed, with an internal bone, and in some armed with a formidable apparatus of horns, spines, and serrations; the preputial glands are often largely developed, secreting in the beaver the drug casto-reum, once much used as an antispasmodic; the uterus is two-horned; the mammary glands vary from 4 in the Guinea pig to 12 or 14 in the agouti. Rodents have existed from the earliest tertiary epoch, presenting genera sometimes different from, and sometimes the same as the present.

Rodent Skull and Incisor.   a. Enamel. 5. Dentine or soft tooth substance.

Rodent Skull and Incisor. - a. Enamel. 5. Dentine or soft tooth substance.