TEETH OF ANIMALS. 265 



trived," for we have license for such language in 

 our author, that the tooth itself shall undergo a 

 revolution, not being simply elevated from the jaw, 

 but turning on an axis. 



There are other modes in which Nature coun- 

 teracts the wear and tear of the engine ; and the 

 provision which we have now to mention supplies 

 not only a substitution of more perfect teeth for 

 those that are injured, but teeth of a size as well 

 as form suited to the growing jaw. In the croco- 

 dile, for example, the teeth are conical and sharp ; 

 but if not worn, they must be torn away, and 

 there is a necessity for a succession. It is thus 

 provided : under the exposed tooth, there is an- 

 other one lodged, of the same shape ; and under 

 that, a second and a third. Each tooth, as it is 

 deeper in the jaw, is larger in its base, and longer 

 and stronger. So it happens, that when a tooth 

 is torn off, it is only the uncapping of a sharper 

 and a stronger tooth. 



The same end is attained differently in other 

 creatures. In the rays, such as the skate, and in 

 the shark, the succession of the teeth is still more 

 curiously managed. The jaws resemble a part of 

 a cylinder, studded with many rows of teeth. 

 The teeth of the outermost row being in use are 

 liable to be torn off or worn down ; when this oc- 

 curs, their places are supplied by a revolving of 

 the solid base on which the teeth are studded, and 

 the posterior ranges advance in succession. Here, 

 then, we not only find sharp and cutting teeth, 

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