BEAVERS AND THEIR WAYS. 269 
monstrous extent. Sometimes the width of the incisors is very 
great, exceeding the depth. This is noticeable in Rodents which 
burrow, and live almost entirely under ground, their powerful 
teeth being doubtless used to gnaw through the roots which 
would otherwise obstruct their subterranean progress. Those 
of the upper jaw are always shorter than those of the lower, 
and usually describe a little more than half a circle. The larger 
incisors of the lower jaw form a smaller segment of a larger 
circle. 
The lower jaw is capable of horizontal movement, from side to 
side, as well as forward and back, the lower incisors moving to 
right and left of the upper ones, thus enabling the Beaver to 
masticate its food by a transverse and diagonal, as well as 
forward and backward, movement of the molars on each other. 
The molars (four on each side in both jaws) have flattened 
crowns, the enamelled ridges of which are always set trans- 
versely, so as to be in opposition to the longitudinal movement of 
the jaw, the better to assist trituration. These flattened crowns 
sufficiently indicate that the food which they are intended to 
masticate is entirely vegetable. 
The general form of the animal is stout and heavy, especially 
in the hinder parts; the tail is of moderate length, broad, 
flattened and covered with a scaly skin; the feet are all five-toed, 
the fore pair considerably smaller than the hinder ones, but all 
well furnished with claws, and the hinder pair fully webbed to 
the extremities of the toes. 
The eyes are small, have the pupil vertical, and are furnished 
with a nictitating membrane. The ears are small and short, 
and their antitragus can be so applied to the head as almost 
entirely to close the auditory aperture; the nostrils are also so 
contrived as to be capable of being closed under water. The 
animal is thus admirably adapted in structure for the peculiar 
mode of life which it pursues. 
With the exception of the Capybara of South America 
(Hydrocherus capybara), which is about one-third bigger, the 
Beaver is now the largest of living Rodents. It averages in 
length, from tip of nose to end of tail, about 4% in. (the 
tail alone measuring 9$ to 10 in.), and 2 ft. 6 in. in girth, 
the weight varying from 80 to 50 and even 60 pounds. A 
Capybara shot by Darwin, at Monte Video, measured 8 ft. 2 in. 
