RODENTIA, an order of placental mammals characterised by their peculiar front or incisor teeth, which are reduced to a single functional chisel-like pair in each jaw, specially adapted for gnaw ing, and growing throughout life. Rodents may be characterised as terrestrial, or in some cases arboreal or aquatic, placental mam mals of small or medium size, with a milk and a permanent series of teeth, plantigrade or partially plantigrade, and generally five toed, clawed (rarely nailed or semi-hoofed) feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw. See TEETH.
In all rodents the upper incisors resemble the lower ones in growing uninterruptedly from persistent pulps, and (except in the hare group, Duplicidentata) agree with them in number. The pre molars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series. The orbits are always open behind, never being surrounded by bone. All rodents are vegetable feeders.
Anatomy.—The skull is characterised by large premaxillae, completely separating the nasals from the maxillae; by the pres ence of zygomatic arches ; by the wide space between incisors and cheek-teeth; and (except in Duplicidentata) by the antero-pos teriorly elongated glenoid vacuity for the articulation of the lower jaw. The orbit is more or less completely continuous with the temporal fossa. The palate is narrow from before backwards, this
being specially the case in the hares, where it is reduced to a mere bridge between the premolars. Tympanic bullae are always pres ent and generally large; in the gerbils and jerboas there are sup plemental mastoid bullae which form great hemispherical bony swellings at the back of the skull, in these genera and the hares the auditory meatus being tubular. The lower jaw is characterised by its abruptly narrowed and rounded front part supporting the pair of large incisors, by the small coronoid process, and the great development of the lower hind, or angular, portion. The cheek teeth may be either rooted or rootless, and either cusped or formed of parallel plates. When there are more than three cheek-teeth, those which precede the last three have succeeded milk-teeth, and are premolars. See TEETH : Alammalia; Dental Formulae.
Classification.—Some diversity of view obtains among natural ists with regard to the classification of the order; the scheme here followed being the one adopted (with some modifications of no menclature) by Mr. Oldfield Thomas. All authorities agree in dividing rodents into two great sub-orders, the one, Duplicidentata, comprising only the hares, rabbits and picas, and the other, Simplicidentata, all the rest. In the latter there is only one pair of upper incisors, the enamel is on the front surface only.