RODENTIA, or GLIR,ES, an extensive order of Mammalia, represented by such forms as mice, beavers, porcupines, squirrels, rabbits, lemmings, etc. Of the characteristics tooth structures are the most important. The canines are always absent and the incisors large and chisel-like and, with the exception of the Le poridce, which have four in the upper jaw, never more than a single pair in each jaw. These teeth consist each of a front layer of hard enamel and a posterior part of softer dentine, which ensures a persistent sharp edge, since the more rapid wearing of the softer dentine leaves the harder enamel of the front surface as a chisel-like edge. These teeth continue to grow from persistent pulps throughout life. The incisors are long and curved and each forms a segment' of a circle. Between the in cisors and • the molar teeth a wide ex ists. The molars are few and their crowns may exhibit a variously •laminated or tuberculate pattern. This structure of the molars has rela tion to the motion of the jaws in gnawing, the jaws being so mticulated by narrow condoles fitting in longitudinal grooves that they slide backward and forward instead of moving ver tically, as in most other animals. The trans verse ridges of the molar teeth act, therefore, in Won to this sliding motion of the jaws. opposition to in some cases •as in the beavers) also grow from persistent pulps and possess un divided fangs. There is always a succession of milk and permanent dentitions. The chief chewing muscle is the masseter, which is greatly developed, while the temporal muscle is small. The.usual number of toes is five on both feet, but the pollen may be rudimentary or absent, and in. the hind feet the number may be reduced to four, as in the hareS, or to three, as in the agouti, jerboa, etc. Generally they are all clawed, but sometimes, as in the capybara, have hoof-like terminations. The scapula is narrow and remarkable for the long aeration process, and the clavicles are generally well developed,' but may be imperfect or absent, as in the hares and guinea-pig. The femur generally possesses a third trochanter, and the tibia and fibula are distinct or, as in the rats and hares, united: The beaver has the hind feet webbed, to adapt it for its semi-aquatic life. Some rodents have
the hind legs of extreme length, as the jerboas. Usually a more or less complete hairy screen extends across the mouth at the narrow region of the palate behind the incisor teeth, thus pre venting the entrance into the hinder portion of the mouth of the chips or dust that result from gnawing operations. Many rodents arc pro vided with cheek-pouches for carrying food; usually these open within the mouth, but in the Geolnyider they open externally.
The stomach, generally a simple structure, may in some forms, as the lemmings, become quite complex, and its anterior portion, as in beavers, may be provided with glandular ap pendages. This same portion in the dormouse is glandular and dilated so as to resemble the proventriculus or digestive stomach of birds. A large sacculated exeunt generally exists, and the intestine is usually very long. A gall-bladder is sometimes wanting. The surfaces of the cerebral hemispheres are smooth or destitute of convolutions and, when viewed from above, the cerebellum is seen to he in great part uncovered by the cerebrum. The corpus callosum is well developed. The penis is usually retractile and contains a bone, and the testes normally remain in the abdomen, but descend periodically into the groin at the breeding seasons. The uterus is frequently completely divided into two cor nua or horns, each of which opens separately into the vagina. In others the two cornua unite to form a single uterus or womb. The placenta is of the deciduate type and of discoid form. Rodents are extremely prolific. Most species of rodents have characteristic and penetrating odors which arise from scent glands variously located in the vicinity of the anus. The skin, generally covered with hair, is spiny in the porcupines (q.v.), some genera of which group possess prehensile tails. Many of these ani mals furnish furs of value in commerce.