TEETH AND SPINES. 43 



teeth are held in place by an elastic ligament, 

 which permits of their being bent backwards 

 into the mouth in swallowing food, but prevents 

 any escape thereof from the mouth most effectu- 

 ally. More frequently the teeth are immovably 

 fixed to the jaw^ as in the eel, haddock and 

 mackerel. Only rarely are the teeth implanted 

 in sockets, as in the file-fish {BaUstes). 



Everybody must be familiar with the deadly- 

 looking weapon like a double-edged saw displayed 

 in the windows of the natural history and 

 curiosity dealers. These have been taken from 

 a kind of shark known as the saw-fish, an 

 inhabitant of tropical or sub-tropical seas. 



The teeth forming this saw are set in sockets, 

 but are not replaced by a regular succession of 

 new teeth ; instead they grow continuously, 

 probably as long as the fish lives. This saw is 

 formed by an outgrowth from the head, and is a 

 very powerful and terrible weapon. By its aid 

 other fish are attacked and ripped open, and 

 pieces of jagged flesh, or protruding viscera, are 

 then nipped off and seized by the comparatively 

 feeble teeth borne by the mouth. 



The growth and succession of the teeth has 

 already been hinted at ; we must now briefly 

 survey the main fact concerning these processes. 



Of all the teeth - bearing animals except the 

 mammalia, we may say with tolerable certainty 

 that the teeth which we see at any given time 

 have not been long in use, and will soon be 

 replaced by others. That is to say, provision 

 is made for a constant supply of new teeth 

 to replace those in use, and this succes- 



