RODENTIA. 1 Gy 
ORDER V. 
——_ 
RODENTIA. 
We have just seen, in the Phalangers, canini so very small, that we 
cannot consider them as such. The nutriment of these animals, accord- 
ingly, is chiefly derived from the vegetable kingdom. Their intestines 
are long, and their cecum ample; and the kanguroos, which have no ca- 
nini whatever, subsist upon vegetables only. The Phascolomys might 
stand first in that series of animals of which we are about to speak, and 
which have a system of mastication still less complete. 
Two large incisors in each jaw, separated from the molars by an empty 
space, cannot seize a living prey nor tear flesh; they cannot even cut the 
food, but they serve to file, and, by continued labour, to reduce it into se- 
parate molecules—in a word, to gnaw it; hence the term Rodentia, or 
Gnawers, which is applied to animals of this order. It is thus that they 
successfully attack the hardest substances, frequently feeding on wood and 
the bark of trees. The more easily to accomplish this object, the incisors 
have no thick enamel except in front, so that their posterior edges wear- 
ing away faster than the anterior, they are always naturally sloped. Their 
prismatic form causes them to grow from the root as fast as they wear 
away at the edge; and this tendency to increase in length is so powerful, 
that if one of them be lost or broken, its antagonist in the other jaw, hav- 
ing nothing to oppose or comminute, becomes developed to a most mon- 
strous extent. The lower jaw is articulated by a longitudinal condyle, in 
such a way as to allow of no horizontal motion except from back to front, 
and vice versa, as is requisite for the action of gnawing. ‘The molars also 
have flat crowns, whose enamelled eminences are always transverse, so as 
to be in opposition to the horizontal motion of the jaw, and to serve the 
better in trituration. 
The genera in which these eminences are simple lines, and the crown 
is very flat, are more exclusively frugiverous; those in which the emi- 
nences of the teeth are divided into blunt tubercles are omnivorous; while 
the small number of such as have no points more readily attack other ani- 
mals, and approximate somewhat to the Carnaria. 
The form of the body in the Rodentia is generally such, that the hinder 
parts of it exceed those of the front; so that they rather leap than walk. 
In some of them this disproportion is even as excessive as it is in the 
Kanguroos. : 
The intestines of the Rodentia are very long; their stomach simple, or 
