The Rodentia, or gnawing mammals--GLIRES, as Linnæus and some modern zoologists call them--notwithstanding the great number of the species and the immense variety of forms which they display, constitute, perhaps, the most definitely circumscribed order of the Mammalia. In most other groups of the same value, we find that some types exhibit divergent characters, which render it difficult to frame a general description of the order which shall include them; or else some species present a marked tendency towards some other order; but in the case of the Rodents, we never have any difficulty, a cursory inspection of the dentition is always sufficient to decide whether a quadruped belongs to the Rodentia or not; and in spite of an almost infinite variety of form, the structure of the rest of the organism is most clearly in accordance with the evidence derived from the teeth. [Illustration: SKULL OF THE TAGUAN, A FLYING SQUIRREL.] The teeth are only of two kinds--incisors and grinders (_see_ the above figure of the skull of the Taguan)--and the number of efficient teeth of the former kind is never more than two in each jaw. Almost throughout the order, indeed, there are actually, even from the first, only two incisors present; but in the Hares and Rabbits, and some allied forms, there are in the upper jaw, in addition to the working teeth, a pair of rudimentary incisors,[48] placed immediately behind the large ones, but quite incapable of taking any part in the business of gnawing, for which the latter are so admirably fitted. Their presence is, however, of interest, as indicating the direction in which an alliance with other forms of Mammalia more abundantly supplied with teeth is to be sought. [Illustration: DENTITION OF THE HARE.]