76 RODENTIA. 



lines, which renders their surface like that of a mill-stone. Finally, 

 the lower jaw of these animals, in place of being articulated with 

 the cranium by a transverse condyle, (like that in the carnaria,) 

 is joined to it by a longitudinal condyle, which only permits 

 motion forwards and backwards : on this account, these animals 

 cannot use their teeth either for tearing flesh or even cutting the 

 substances upon which they feed, and they are forced, therefore, 

 to file them as it were, in order to reduce them by continual 

 labour to very delicate particles, and it is from this circumstance, 

 they have obtained their name of gnawers or rodeutia. 



16. In conformity to this mode of organization, the gnawers 

 must necessarily be designed to feed chiefly upon vegetable sub- 

 stances ; some of them are omnivorous (rats, (or example ;) but, 

 for the most part, they live upon fruits, herbs, barks or roots. 



17. Most of these animals are of small stature, and in general, 

 their hind paws are much longer than the fore, so that they rather 

 leap than walk. The hare affords us an example of this arrange- 

 ment, which, in some other gnawers, (the Jerboas,) is carried so 

 far that the animal only uses his hind paws to leap with, and to 

 rest upon. (Plate 4.fg. 11.) 



18. As regards intelligence, the Rodentia are, in general, 

 less favoured by nature than the quadrumana and carnaria, and 

 it is remarked that their brain is less developed and presents 

 scarcely any convolutions. Nevertheless, we find in this order 

 those mammalia whose instinctive faculties are most admirable, 

 as we shall see when we treat of the castors, and squirrels. 



19. This order is composed of several small tribes, the most 

 important of which, with their distinctive characters, are indicated 

 in the following table: in which it will be perceived, they are 

 all arranged in two principal sections, according as they possess 

 a perfect, or an imperfect clavicle : the first section has been 

 called Rodentia with clavicles ; and the second, Rodentia with 

 imperfect clavicles : 



16. What is the general food of the Rodentia ? Does their organization 

 indicate the kind of food on vvhicli they live .' 



17. What is the general mode of progression amongst Roden'.ia? How is 

 their mode of progression accounted for ? 



18. What decree of intelligence is pos?e-scd by the Rodenlia compared 

 with other animals ? What is the comparative clcvclopenienl of tlieir brain ? 



19. (Answered in the table.) How is the order Rocieniia divided? What 

 is the composition of the teeth of Squirrels and Rats? Have they any 

 clavicles ? What numb* r of toes have Castors 1 What tribes of this order 

 have no clavicle, or only a very short one? What tribes of tliis order have 

 molar teeth without roots? What tribes of this order have molar teeth with 

 roots ? In what tribt?s do the molar teeth cease growing when completely 

 formed ? In what tribes do the molar teeth continue to glow through life ? 



