104 THEORY OF THE EARtH. 



alternate perpendicular layers of hard enamel and 

 softer bone. Teeth of this structure necessarily 

 require horizontal motions, to enable them to tritu- 

 rate or grind down the herbaceous food j and, ac- 

 cordingly, the condyles of the jaw could not be 

 formed into such confined joints as in the carnivo- 

 rous animals, but must have a flattened form, cor- 

 respondent to sockets in the temporal bones, which 

 also are more or less flat for their reception. The 

 hollows likewise of the temporal bones., having 

 sinaller muscles to contain, are narrower, and not 

 so deep, &c. All these circumstances are deduci- 

 ble from each other, according to their greater or 

 less generality, and in such manner that some are 

 essentially and exclusively appropriated to hoofed 

 quadrupeds, while other circumstances, though 

 equally necessary to that description of animals, 

 are not exclusively so, but may be found in animals 

 of other descriptions, where other conditions per- 

 mit or require their existence. 



When w r e proceed to consider the different or- 

 ders or subdivisions of the class of hoofed ani- 

 mals, and examine the modifications to which the 

 general conditions are liable, or rather the particu- 

 lar conditions which are conjoined, according to 

 the respective characters of the several subdivi- 

 sions, the reasons upon which these particular con- 

 ditions or rules of conformation are founded become 

 less evident. We can easily conceive, in general, the 

 necessity of a more complicated system of diges- 



