44 THE STORY OF FISH LIFE. 



sion goes on throughout life. A most enviable 

 arrangement ! 



In the sharks, the young teeth will be found 

 adhering to the inner surface of the jaw, within 

 a cavity closed by membrane. Herein they lie, 

 closely packed, row upon row, with their j)oints 

 downwards in the lower, upwards in the upper- 

 jaw. From this position they slowly erect them- 

 selves, as they approach the edge of the jaw^ until 

 the last row of teeth are actually vertical. The 

 picture of a section through a jaw exposing one 

 of each of these several rows may help to make 

 this clear. In sharks only one row, in the 

 rays and skate several rows are in use at one 

 time. 



In the Greenland shark {Lcemargus) the teeth 

 interlock one with another, and on this account 

 one complete row is shed at a time. But in 

 other sharks where this interlocking arrange- 

 ment does not obtain, every alternate tooth 

 is shed and replaced at once. So that two 

 rows continue to form one single functional 

 row. 



With the higher fishes the succession of the 

 teeth presents yet other modifications. 



In socketed teeth the succession is vertical, 

 somewhat as in ourselves, the new tooth being 

 formed immediately below the functional tooth, 

 and taking its place when this falls out. In the 

 majority of bony fishes, where the teeth are 

 numerous and closely packed together, the suc- 

 cession is irregular. When the teeth are less 

 closely packed the succession is alternate — there 

 is a young tooth placed between any two adult 



