THE FRESH-WATER MUSSEL. 



117 



lime. When any thing is made hard by lime, it is said 

 to be calcified — calcium is the chemical name for lime, 

 so the last soft ring is said to be uncalcified, that is, 

 not filled and hardened by lime. Now all these lines 

 are called lines of growth (Fig. 107), because they show 

 the age of the anodon, just as the rings of wood in the 

 trunk of a tree show the age of the tree. When the 

 young mussel was away up under the little roof, close 

 to the knob, or umbo, its straps, or adductor muscles, 

 were fastened up there too, but, as it moved down and 

 became larger, it had to move its muscles with it, so 

 that it kept taking up and putting down the fastenings 

 of the muscles whenever it made a move forward, and 

 each fastening left a mark on the inside of the shell. 

 So now you know how the triangular marking (Fig. 108) 

 is made inside from the top to the bottom of the shell. 

 You can make the same shaped marking in the sand, if 

 you place your heels together and then step forward, 

 placing your heels farther apart at each step. Where 

 you start represents the umbo ; where you stop repre- 

 sents the bottom of the shell. So the little mussel 

 steps forward, leaving the print of his straps as he goes. 



Some of the shells of the mussel family grow to an 

 immense size, weighing as many as six hundred pounds. 

 When you go to Paris you can see a pair of clam-shells 

 weighing five hundred pounds, which are used to hold 



