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NATURAL HISTORY. 



6KULL OF THE TAGUAX, A FLYING SQUIRREL. 



may be looked upon as " a feeble folk," their numerous species render them a most important 

 section of the mammalian fauna of nearly all countries, and this importance is greatly increased,, 

 practically, by the immense number of individuals by which each species is usually represented. 



The Rodentia, or gnawing mammals GLIRES, as Linnaeus and some modern zoologists call 



them notwithstanding the great number of the species, 

 and the immense variety of forms which they display, con- 

 stitute, 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. 



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,* 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. 



The great incisors, which are characteristic of the Rodents, exhibit the following peculiarities : 

 They possess no roots, but spring from a permanent pulp, so that they continue growing during; 

 the whole life of the animal ; and their form, and 

 that of the cavity which constitutes their socket, is 

 always that of a segment of a circle,t in consequence 

 of which, they always protrude from the front of the 

 jaws in the same direction, and meet at the same angle. 

 By this means, as the teeth are worn away at their 

 summits by iise in gnawing, a fresh supply of tooth is 

 continually being pushed forward to take the place of 

 the portion thus removed, and, in fact, so intimately 

 are the two functions of use and growth correlated in 

 the teeth of these animals, that if by chance one of 

 the incisors should get broken, or the natural opposition 

 of these teeth should be disturbed in consequence of 

 injury to the jaw, the teeth, thus deprived of their 

 natural check, continue growing, and, following the 

 curve of their sockets, gradually form circular tusks, which must always be greatly in the way of the 

 animal when feeding, and sometimes, by actually penetrating again into the mouth, cause its death by 

 absolute starvation. The teeth themselves are composed of dentine, coated along the front surface with 

 a layer of hard enamel, which substance is wanting on the other surfaces of the teeth, except in the 

 Hares, Rabbits, and other forms with additional rudimentary incisors in the upper jaw, in which, as 

 further evidence of their relationship to the other Mammalia, the whole surface of the incisors is 

 encased in enamel, although this coat is excessively thin except on the front or outer face. The purpose 



* In the young there are four of these small additional teeth, but the outer pair disappear after a short time. 

 . f The upper teeth always constitute a larger segment of a smaller circle than the lower ones. 



DENTITION OF THE HAKE. 



