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- 
7A tially disabled, and may die of starv- 
Fig. 53.—Overgrown Teeth ation. 
of Bohit. 115. The other teeth in the Ro- 
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, 
