338 
VERTEBRATA. 
BEAVERS. 
OKDER 7. EODENTIA. 
The Rodentia or Gnawing animals, are all of small size, many of them the most diminutive of 
their class; but the species are exceedingly numerous, and usually very prolific, so that no mam- 
malia are more generally or abundantly distributed. Their most conspicuous character is to be 
found in their dentition, which is very peculiar. The teeth are of two sorts, incisors and molars, 
the canine teeth being entirely deficient. The incisors are two in number in each jaw; their 
bases pass far into the jaw, where usually, beneath the molar teeth, there is a permanent pulp, by 
the action of Avhich the incisors are kept constantly growing during the life of the animal, so as 
to supply the continual wear going on at the extremities, where the tipper and lower teeth come 
in contact. The substance of the body of these teeth is moderately soft, but their anterior sur- 
face is covered with a layer of very hard enamel, secreted by a membrane coating the anterior 
Avail of the socket. The thin layer of hard enamel which coats the front of the tooth resists abra- 
sion much better than the dentine of which the body of the tooth is composed; the latter conse- 
quently wears away most rapidly, and thus the enamel always constitutes a shai"p projecting edge 
like that of a chisel, of which the dentine forms the beveled portion. 
The object of this arrangement is very apparent. These animals feed to a great extent upon 
hard substances, or substances inclosed in hard coverings, such as nuts, &c. ; and in order to get 
at their food they require both sharp and strong teeth; the requisite sharpness is furnished by 
the thin plate of enamel; but as this by itself would break away directly when applied to its or- 
dinary purposes, it is strengthened by a thick layer of dentine, which, although it furnishes the 
necessary support, wears away so readily as never to interfere with the efficiency of the cutting 
edge. 
Behind the incisors there is a large gap, beyond Avhich the molai's are situated. These vary 
in number from two to six, and are usually destitute of true roots; in fact, like the incisors, they 
generally continue growing throughout the animal's existence. They are often composed of 
a simple prism of dentine coated with enamel; the latter is sometimes folded so as to produce 
transverse ridges at the surface of the tooth, which is usually worn flat, and in some cases exhib 
its small isolated spots of enamel in the body of the dentine. These teeth are evidently adapted 
