MAMMALIA. 
108 
cessfully attack the hardest substances, frequently feeding on wood and the bark of trees. 
The better to accomplish this object, these incisors have enamel only in front, so that ' 
their posterior edges wearing away faster than the anterior, they are always naturally sloped 
[or chisel-like] . Their prismatic form causes them to grow from the root as fast as they wear 
away at the tip [their formative pulps being persistent] ; and this tendency to increase in 
length is so powerful, that if either of them be lost or broken, its antagonist in the other jaw, 
having nothing to oppose or comminute, becomes developed to a monstrous extent.* The 
inferior 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, the enamelled eminences of which are always transversal, so ■ | 
as to be in opposition to the horizontal movement of the jaw, and better to assist in ^ 
trituration. 
The genera in which these eminences are simple lines, and the crown is very flat, are more ^ 
exclusively frugivorous ; those in which the eminences of the teeth are divided into blunt 
tubercles are omnivorous ; while the small number of such as have no points more readily ■ 
attack other animals, 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 [with the exception of a large South American group, including 
the Guinea-pig and its allies,] they rather leap than run. In some of them, this disproportion 
is even as excessive as in the Kangaroos. 
The intestines of the Rodentia are very long; their stomach simple, or but little divided 
and their coecum often very voluminous, even more so than the stomach. In the subgenus 
Myoxus, however, this intestine is wanting. . J ; 
Throughout the present group, the brain is almost smooth and without furrows : the orbits ^ 
are never separated from the temporal fossae t> which have but little depth : the eyes are ; 
directed sideways : the zygomatic arches, thin and curved below, announce the feebleness of ■ 
the jaws ; and the fore-arms have almost lost the power of rotation, their two bones being • 
often united : in a word, the inferiority of these animals is perceptible in most of the details 
of their organization. Those genera, however, which have stronger clavicles, display a certain 
degree of address, and employ their fore-feet together to hold up food to the mouth : some of 
them even climb trees with facility. ; 
[We have seen that in the true Lemurs the middle superior incisors are separated by a wide i 
interval, which in the Colugos {Galeopithecus) is still more extended: in Propithecus of ! 
Mr. Bennett, on the contrary, the front pair are brought nearly contiguous, having more of | 
the Monkey character than in other Strepsirrhini. The lower canines also, which are directed ; 
horizontally forward throughout that group, and approximated so as to leave little room for 
the intervening incisors, which are accordingly extremely narrow or compressed, are even ; 
more approximated in the Propithecus, so that one pair of the incisors is necessarily sacri- i 
ficed; and hence the diminution of the interspace between the upper incisors. Now in ! 
this we may discern a slight approach to the rodent character of Cheiromys, in the loss of one ‘ 
pair of incisors. In the latter genus, the whole of the incisors disappear, the canines of both || 
[ jaws occupying their site : precisely as in the true Rodentia, wherein also the incisors and not j|| 
the canines or tusks are almost without exception obliterated, as is beautifully shown in the || 
instance of the Hare, where true incisors exist posterior to the upper gnawing teeth : it will m 
be observed that in all Rodentia the currently reputed incisors pass through the inter- ^ 
maxiliaries ; while the constant limitation of their number to two in each jaw, and the inva- 
riable absence of any trace of other teeth in the ordinary position of canines, assist in con- ' 
firming the opinion here decidedly entertained respecting the nature of what have been desig- 
nated incisive teeth in these animals. It may be added that the Marsupiata do not, therefore, as 
« We have seen one of these upper teeth thus prolonged, and I t They are so in Cheiromys, ranged by the author in this order. — 
gradually curling round, so as to destroy the eye of a Rat, — Ed. | Ed. 
