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BOYS AND GIRLS IN BIOLOGY. 



and shining, and this white substance is very useful ; it 

 is called " mother-of-pearl," and is made into knife-han- 

 dles, buttons, and many kinds of ornaments. On the in- 

 side of the valve you can see two three-cornered, or trian- 

 gular marks, where the posterior and anterior adduc- 

 tor muscles are fastened (Fig. 108). These two marks 

 are joined by a curved line, which shows where the 

 muscles in the edge of the mantle are fastened (Fig. 

 108). From each adductor muscle you can trace two 

 faint lines, getting nearer and nearer together as they 

 go toward the shiny knob, or umbo (Fig. 108). These 

 lines show where the adductors, or straps, used to be at- 

 tached. You will understand how these marks are made 

 when you see how the shell grows. At first the shell is 

 just the size of the little mussel, but as he grows he needs 

 more covering, so he puts out another layer from the 

 outside of his outer skin, or mantle. This layer, like 

 the first, gets hard from lime which fills up its openings. 

 Its upper edge lies under the lower edge of the first 

 layer, like the shingles or slates on the roof of a house, 

 and this lapping makes the first ring which you see on 

 the outside (Fig. 107). By-and-by the house gets too 

 small again, and the third row of shingles is added to 

 the roof, and so on till you come to the last row, which 

 you find is soft. Now you know the reason why this is 

 soft ; it is the last layer, and is not yet filled with 



