ORDER RODENTIA. 97 



jaw they are in the maxillary, and not in the intermaxillary 

 bone. This provision for renovation is confined to the in- 

 cisive teeth. 



In some species,, as the hares and the pikas, the two- 

 large upper incisors have two other smaller, and placed 

 behind them ;. the use of which to the animal is not appa- 

 rent. 



The lower incisors are sometimes moveable indepen- 

 dently of each other, by the nonconsolidation of the sym- 

 physis of the lower jaw. 



The molars undergo, as to their detrition, the same laws 

 as in other animals ; those that feed on vegetables alone, 

 have their sutures deeply divided by small ridges, so that 

 the upper surface of the teeth is flat, and traversed by 

 lines or ridges of enamel, very various in their directions 

 in the different genera. The species which feed on 

 softer substances are less fitted for trituration by having a 

 smoother surface. 



In the Herbivorous Rodentia these teeth increase for a 

 very longtime, from their base, in proportion to their wear 

 on their crowns ; they do not become divided into roots till 

 very late, and in some species it is not certain whether this 

 process ever takes place ; in others, again, these teeth cease 

 to grow when the animal attains maturity, and the roots 

 are formed very early. 



The cheek-teeth vary in number, and still more in the 

 shapes or directions of the enamel ridges in the different 

 genera or divisions of the order ; these will be noticed in 

 their places as generic characters ; to which we have added 

 copies of the figures of these teeth given by the Baron in 

 his Ossemens Fossiles, which will display their peculiari- 

 ties much more effectually than verbal description. 



We know very little of the laws and the periods of the 

 succession of teeth of the Rodentia. Like other viviparous 

 quadrupeds, many of them are known to have deciduous', 



