144 



THE WONDERS OF GEOLOGY. 



Lect. II. 



consequence of their exquisite adaptation for these opera- 

 tions. In the skull of the Squirrel (Lign. 22, fig. 2) the 

 front teeth are of enormous size, as compared with the 



Lign. 22. — Skull and teeth of rodentia, or gnawers. 



Fig. 1. Inner side of the left lower jaw of a Florida rat, {Arvieola Floridava,) of the 

 natural size. 



2. Skull of the Squirrel. 



3. Molar teeth of the upper jaw of a Florida rat, magnified ; seen ohliquely. 



molar, and they lock together in such manner as to render 

 a grinding movement impossible ; a corresponding adjust- 

 ment, therefore, takes place, — the lower jaw is so constructed 

 as to work in the skull neither in a transverse nor in a rota- 

 tory direction, but lengthwise, like the action of a carpenter 

 using his plane, the teeth moving backwards and forwards ; 

 as may be observed in the rabbit while eating its food. 

 The enamel of the molar teeth, as for example in those of 

 the Rat, (see Lign 22, fig. 3) is placed vertically and trans- 

 verse to the jaw, so as to form an admirable grinding 

 surface. But this is not the only variation of structure 

 observable in these teeth. The incisors being implements 

 of constant use, are renewed by continual growth, and there 

 is a special provision for their support in a bent socket. 



