RODENTIA 153 



teeth continue to grow throughout life. They are strongly 

 curved, especially those of the upper jaw, and are so arranged 

 in opposition as to wear away their cutting surfaces one against 

 the other. If one happens to be broken, great disturbances 

 result. The corresponding tooth in the opposite jaw, having 

 no means of wearing away its cutting edge and finding its 

 growth unchecked, continues to push its head forwards in a 

 circle or spiral, until it may eventually kill the unfortunate 

 animal, either by penetration or starvation resulting from 

 inability to feed. A good figure of such a case occurring in 

 a rabbit was published by Tegetmeier in the Field, 8th March 

 1890, 360. 



The two cavities of the mouth are formed by a folding in- 

 wards of the hairy skin of the face behind the incisors. Thus 

 these teeth when engaged in gnawing through an obstacle do 

 not fill the mouth with inedible or objectionable material, as 

 happens when a dog or other animal with a simple mouth uses 

 its teeth to remove roots or soil. 



Classification :— The surviving British members of the order 

 as here arranged fall into four families : — Leporidce, containing 

 the hares and rabbits ; Muscardinidce, the dormice ; Murida, 

 the mice, rats, and their allies ; Sciuridce, the squirrels ; and a 

 fifth, the Castoridce, containing the beavers, of which a British 

 representative only became extinct in early historical times. 

 The above families are usually grouped in two main divisions 

 or sub-orders. Of these the Duplicidentata, including only, 

 amongst living British rodents, the Leporidce, have always two 

 pairs of upper incisors. The second division, that of the Simpli- 

 cidentata, includes all British rodents except the Leporidce, 

 and its members are easily recognised by their single pair of 

 these teeth. No rodent possesses more than a single pair 

 of mandibular incisors. 



An older arrangement whereby the order was divided into 

 groups, each of higher rank than a family, such as Lagomorpha 

 or leporines, Myomorpha or murines, and Sciuromorpha or 

 squirrel-like rodents, has been superseded as being in many 

 ways unnatural and unsatisfactory. Under it, for instance, the 

 dormice must necessarily be placed either with the mice or the 

 squirrels, in either case an unnatural alliance. 



VOL. II. L 



