RODENT QUADRUPEDS. 69 



Mice. 4. Beavers. 5. Porcupines. 6. Guinea Pigs. 7. 

 Chinchillas. 8. Hares. This order contains about three 

 hundred species, and is the most generally distributed of 

 all the orders of terrestrial Mammals. Its species are 

 found in all quarters of the world, a few of them even in 

 Australia. The furs of some of them are very valuable, 

 as the Beavers, the Chinchillas, and the Gray Squirrels. 

 114. The grand peculiarity of this order is in their 

 gnawing teeth. These are in front, two in each jaw, and 

 they are peculiarly constructed. The front covering of 

 the tooth is enamel, and its rear portion, that is, the body 

 of the tooth, is ivory, which is by no means as hard as 

 enamel. Observe the effect of this arrangement. As 

 the upper and lower teeth are brought together in gnaw- 

 ing, the enamel does not wear away as fast as the ivory, 

 because it is harder. The thin enamel, therefore, always 

 presents a sharp chiseling edge above the level of the 

 ivory. No other class of animals has this peculiarity. 

 These teeth are used for different purposes, as, for exam- 

 ple, by Squirrels in opening the shells of nuts, and by 

 Rats in making holes in wood. The teeth of other Mam- 

 malia have a limit to their growth, but not so with these 

 front teeth of the Rodents. These grow continually, but 

 are kept always of the same length by the wear of the 

 gnawing operation. If, therefore, one of them be lost, 

 the one opposite will attain a great length. In Fig. 53 

 you see the lower jaw of a rabbit in 

 which the two teeth are very long 

 because the upper teeth were lost. 

 A Rodent in such a plight is essen- 

 tially disabled, and may die of starv- 



Fig. 53. Overgrown Teeth ation. 



of Rabbit. n5 rp he Qther teeth in the RQ _ 



dents are situated far back, as seen in Fig. 54 (p. 70). 

 These back teeth are of different kinds in the different 

 families, according to the nature of their food. Thus in 

 the Squirrels, which live on nuts, these teeth are rounded, 



