342 



THE HARMONIES OF NATURE. 



Dentition of Seal. 



Dentition of a Rodent. 



teeth adapted neither for shearing on the one hand, nor on 

 the other for grinding their food, either of which actions 



would be unavailable in 

 their particular case are 

 numerous, and furnished 

 with several angular points 

 extremely well-fitted for 

 holding the slippery scaly 

 surface of fish, and equally 

 so for crushing them be- 

 fore they are swallowed. 



The peculiar food of the rodents, which generally consists of 

 hard vegetable substances, naturally required a very different 



dentition. Here the canine teeth, 

 which would have been worse than 

 useless, are suppressed ; while the 

 incisors, which play a very im- 

 portant part, are converted into 

 powerful chisels. Their confor- 

 mation is beautifully adapted to 

 the purpose they have to fulfil: 



they are required to have a sharp edge, in order to make 

 their way through tough vegetable substances, and they must 

 at the same time be very strong and firm ; this is effected by 

 the principal substance of the tooth being composed of very 

 tough ivory, with a plate of hard enamel in front only, which 

 latter, wearing away more slowly, is always left as a sharp pro- 

 jecting edge. The molar teeth, which are separated from the 

 incisors by a wide interval, are composed of alternate plates 

 of enamel and ivory, which, wearing unequally, stand up 

 in ridges, and give them a rasplike surface. The ridges are 

 always transverse, or in a direction from side to side of the head ; 

 and as the jaw has considerable facility of moving backwards 

 and forwards, it greatly increases the power of trituration. In 

 the frugivorous species of the order, however, the surface of the 

 molar teeth is raised into rounded tubercles as in the squirrel, 

 for instance ; whilst in those animals which have any carnivorous 

 tendency (as in the rat), they are raised into sharp points, thus 

 bearing some resemblance to tHose quadrupeds which are wholly 

 carnivorous. 



