DENTITION. 



163 



284 



in order to replace those which have been 

 broken or worn down. In some fishes the teeth 

 advance in proportion as the jaw lengthens, and 

 as the fore teeth are worn away : in other cases 

 they rise from the substance of the jaw, which 

 presents on its surface an assemblage of teeth in 

 different stages of growth ; so that in this class 

 of animals the greatest variety occurs in the 

 mode of the succession of the teeth. 



The teeth of the Crocodile, which are sharp- 

 pointed hollow cones, composed of ivory and 

 enamel, are renewed by the new tooth (as is 

 shown at a, in Fig. 284), being formed in the 

 cavity of the one (b) which it 

 is to replace, and not being 

 inclosed in any separate cavity 

 of the jaw bone (c). As this 

 new tooth increases in size, it 

 presses against the base of 

 the old one, and entering its 

 cavity, acquires the same co- 

 nical form ; so that, when the 

 latter is shed, it is already in 

 its place, and fit for immediate use. This suc- 

 cession of teeth takes place several times during 

 the life of the animal ; so that they are sharp 

 and perfect at all ages. 



The fangs of serpents are furnished, like the 

 stings of nettles, with a receptacle at their base 

 for a poisonous liquor, which is squeezed out by 



