MOLLUSC A. 



CLAMS. 



37 



The common clam is in many respects a more de- 

 sirable type to teach this lesson with than the oyster, but 

 it is not so widely distributed, nor so easily obtained 

 in all localities. 



There are several species of edible clams quite dis- 

 tinct in aspect and characteristics. 



The common soft-shelled clam, Mya arenaria (Fig. 

 2 1 ) , is the one best known in New England, north of 

 Cape Cod, and most freely offered for sale in the 

 markets. 



In comparing this with the oyster, lay the specimens 

 on one side, and the outlines bearing the same rela- 

 tionship to those of the oyster, as in the diagram (Fig. 

 i6), where the outline of the clam is drawn above that 

 of an oyster, so as to exhibit the contrast of form. It 

 will then be found, as the analysis goes on, that they 

 have been placed with all their organs in a similar 

 position ; but this is a matter the scholars ought to find 

 out for themselves by the examination of the animal. 



By an examination of the shell, similar facts to those 

 detailed in treating of the shell of the oyster can be 

 ascertained step by step. 



The lines of growth leading back to the beaks and 

 encircling them, the hinge, and so on. 



The horny covering is particularly noticeable, clothing 

 the outer edges of the shell, and clinging closely to 

 the exposed fleshy parts, though worn away nearer tlie 

 beaks by the constant attrition of the sand or sandy 

 mud in which the clam burrows. 



