THE ANIMAL KINGDOM. 67 



enamel in most other animals covers the whole crown of 

 the tooth, it is only placed in the front of those of rodents, 

 with the ivory at the back, so that, when the animal works 

 at its trade, the softer part wears away, leaving the front 

 or harder portion standing sharply up; and further, as 

 these hard edges get worn away a little by working 

 against each other, the teeth continue growing, so that 

 they are always kept of the proper length. The back 

 teeth are flat, with ridges of enamel, which in those 



Fig. 24. SKULL OF A RODENT, 



rodents who live on vegetable matter ^are rounded, whilst 

 in the others they are sharp. The Rodents, too, have the 

 'power of moving their jaws backward and forward, so 

 that the teeth work on each other like rasps or files. 



The Rodents may be conveniently divided into those 

 which possess collar-bones, and those which are without 

 them ; and as the collar-bone acts as a sort of buttress to 

 support the shoulder against the top of the breast-bone, you 

 will not be surprised to find amongst the first such animals 

 as use their front legs, not only for walking, but as arms 

 in the hard work they have to do, such as the Beaver> 

 the Squirrel, and the Rat, and amongst the latter the 

 Porcupine, Hare, and Guinea-pig. 



3 



