THE MAMMALS 233 
of the body have the form of stiff, barbed spines (Fig. 132), 
readily dislodged so that the animal requires no other wea. 
pon of defense. The rabbits and hares are of smaller size, 
and the cottontails especially are widely distributed. West 
of the Mississippi the jack-rabbits are familiar, and are 
Fie. 182.—Porcupine (Hystrix cristata). One-tenth natural size.—After BREHM. 
famous for their great speed. Like the porcupines, they 
feed on leaves and grass, and are often very destructive. 
The mice, especially the field and white-footed mice, are 
abundant in woodland and meadow throughout the United 
States. The house-mouse (Mus musculus) is a native of 
Europe, as is the common rat (Jf decumanus), which was 
imported over a century ago. The wood-rat (Neotoma), 
however, is native, and may be found in many localities 
from east to west. The muskrat (fiber zibethicus), beaver 
(Castor canadensis), and woodchuck (Arctomys monaz) were 
also more or less plentiful formerly, but in many localities 
are well-nigh exterminated. The squirrels, on the other 
hand, continue to exist in large numbers. The prairie- 
