382 RODENTIA empties into the gulf of California ; the aver- age rainfall in this vast area is only 15 inches. N. and W. of the Colorado drainage is the great interior basin, between the Wahsatch mountains and the Sierra Nevada, with no known outlet, the great rivers sinking; here the average annual rainfall is only 12 inches. To the north is the great drainage of the Co- lumbia, the branches of which rise in the main chain of the Rocky mountains far to the east, in Idaho; the average annual rainfall is only 18 inches. There are several smaller areas of drainage on the Pacific coast. The limited rainfall in all these regions shows that success- ful agriculture is only possible with the aid of irrigation. E. of the Sierra Nevada the rains are not frequent, the snows are very light, and the amount not great, so that the supply of water from the melting of the snows is not extensive. The difference between high and low water mark is very great. For a short time in May and June the streams are high and large, but they soon dwindle greatly, and even disappear altogether. So little snow falls on the E. ranges that the streams which flow into the plains from the E. slope will not sup- ply water to irrigate more than one fourth of the agricultural area. RODET1A (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 mous- taches; the opening of the mouth is small, but Rodent Skull and Incisor. a. Enamel. 6. Dentine or soft tooth substance. 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 di- vers ; the thumb is never opposable, when pres- ent; 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 -Jc}, grow from persistent pulps, and are covered with a coat of enamel only in the front ; the leporidce (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 me- dium, 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, den- tine, and enamel, and either form roots (thus limiting the duration of growth) or grow from persistent pulps like the incisors. The sum- mits of their crowns are generally traversed by parallel transverse ridges, placed in oppo- sition to the antero-posterior movement of the lower jaw, thereby greatly facilitating masti- cation. The order comprises such animals as the capybara, beaver, porcupine, squirrel, mar- mot, dormouse, rat, hamster, lemming, jerboa, hare, rabbit, muskrat, Guinea pig, agouti, and chinchilla. Waterhouse divides them into the five families of leporidce, hystricidce, murida, tciuridcB, and saccomyidce, all represented in North America, and the last peculiar to it. Rodents form nearly one third of all mam- mals, 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 spe- cies are found within the limits of the United States ; the pouched rats are entirely Ameri- can ; of the rat family, the field mice are best represented in North America ; of the porcu- pine family, more than seven eighths are South American, the capybara, the largest living ro- dent, being among them, itself greatly sur- passed in size by the extinct castoroidet Ohio- ensis of North America ; while many species of hares are found in North America, only one is met with in South America. This or- der 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
