96 Div. 1. YERTEBRATE ANIMALS— MAM^IALTA. Class 1. 



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 develo])ed to a monsti-ous 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 ccecum often very voluminous, even more so than the stomach. In the subgenus 

 Myoxvs, however, this intestine is wanting. 



Throughout the present group, the brain is almost smooth and without furrows : the orbits 

 are never separated from the temporal fossfe t> which have but little depth : the eyes are 

 directed sideways : the zygomatic arches, thin and curved below, annoimce 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 

 interval, which in the Colugos {Galeopiihecus) is still more extended : in Propitliecus of 

 Mr. Bennett, on the contrary, the front pair are brought nearly contiguous, having more of 

 the Monkey character than in other Strepsirrliini. 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 apjiroximated in the Propithecus, so that one jiair of the incisors is necessarily sacri- 

 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 

 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 

 be observed that in all Rodentia the currently reputed incisors pass throvgh the inter- 

 maxillarics ; while the constant hmitation 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 oi)inion 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 h»Te seen one of ihe»f iippi-r teeth Ihu. prolonifcd, mid | t Thn- nrc so in Clidrowi/i, rnnged hj the author in this order.— 

 •OxIiuIIt curling round, III Rs to de»ira)' the ere of a Rat. — Ln. I En. 



