

CHAPTER I. 

 RATS AND MICE. 



THE ORDER RODENTIA — THE FAMILY MURID/E — RATS AND MICE — THE BLACK RAT — THE BROWN- 

 RAT — THE MOUSE — THE HARVEST MOUSE — THE BARBARY MOUSE — THE HAMSTER — THE MUSK 

 RAT — THE WATER RAT — THE FIELD MOUSE — WILSON'S MEADOW MOUSE— LE CONTE'S MOUSE 

 — THE COTTON RAT — THE LEMMING. 



THE RoDENTiA or Gnawers (from the hatin rodcre " to gnaw "), 

 constitute a well-defined order characterized by the possession 

 of two long, curved, sharp-edged, rootless incisors in each jaw. 

 The order contains a very considerable number of subdivisions, namely, 

 sixteen families, some of which comprise over .two hundred species. It 

 may be inferred from this fact that the animals of the order differ very 

 widely in all other respects than that which has given them their name. 

 The gnawing teeth are necessarily placed in strong heavy jaws which 

 are large in proportion to the head, for they not only require a stout 

 support, but room for continual development. These teeth are worn 

 continually by the friction they undergo, and necessitate a provision for 

 their continual renewal ; the base passes deepl}^ into the jawbone, where 

 a pulpy core supplies the material for constant growth. As those of the 

 upper jaw meet those of the lower jaw at the tips, they are perpetually 

 worn away by their action upon each other, and upon the hard food 

 which they are formed for nibbling. The growth at the base and the 

 wearing away of the tips thus balance each other. If one of the incisors 

 is lost, the opposing one in the other jaw being no longer worn, grows 

 till it projects like the tusk of an elephant. These incisors are chisel- 

 shaped, and are provided in front with a layer of enamel hard as the 

 hardest steel, while behind they are composed of a much softer material 



