MAMMALS. 35 



delicate sense of touch has its seat not only in the 

 flying-membrane, but also in the skin of the ears, 

 •which are often very large, and in the membranous 

 flaps which, in a few genera (the " leaf -nosed " bats), 

 occur on the nose and lips. As is well known, bats 

 sleep in the day ; and they also hibernate in chimneys, 

 hollow trees, ruins, and other similar places of conceal- 

 ment. 



They principally devour night-flying moths, and 

 spiders ; and, since they use a great quantity of 

 nourishment, are of great service, since the cater- 

 pillars of many of the species they destroy are very 

 injurious to agriculture or forestry. About nine 

 species live in Britain, but there is no use in enume- 

 rating them here. 



o 



Order: Rodentia (Gnawing Mammals). 



Two long incisors (Fig. 19), the crowns of which 

 are continually being worn down, while a correspond- 

 ing growth takes place at the root-end. These incisors 

 are used for gnawing, in which process the lower jaw 



F[G. 19.— Skull of Squirrel. 



is rapidly moved backwards and forwards. Gnawing 

 wears down these teeth less in front than behind, 

 owing to the presence of a thick layer of enamel in 

 the former position. Their crowns, therefore, maintain 

 a chisel-edge. That the incisors never stop growing 



