THE STUDY OF SHELLS. 147 



impossible to open them without the aid of a sharp instru- 

 ment. Bivalves are provided with powerful muscles 

 passing across from shell to shell, by means of which they 

 pull the two halves of the shell together. Oysters and 

 clams have only one such muscle, but most others have two. 

 Examine the inside of the saucer shell and find at each 

 end of the mantle line an elliptical scar (Fig. 48). This 

 marks the place of attachment of the shell-closing muscle. 



In some shells it will be seen 

 that the posterior end of the 

 mantle line is indented into a 

 kind of bay (Fig. 50). Com- 

 mon shells showing this are 

 the " fool's mussel" (Mactra 

 stultorum), the Venus shell 

 (Venus gallina), and the large 

 well-known sand mussel (Lu- 

 traria elliptica). This bending 

 of the mantle line is due to the 



presence Of a respiratory tube cle marks and indented inuntlb 



ill these animals. This tube 



projects from the hinder end 



of the shell and can be more or less retracted within it. 



It is used as a means of conveying water into and out 



of the shell. The presence of a " bay " on the mantle line, 



therefore, is an indication that its owner possessed a 



respiratory tube. 



Lastly, the interlocking teeth forming part of the hinge 

 joint of the shell should be noted. 



Pupils should learn the names of at least a dozen com- 

 mon shells. Examples which will well repay a close ex- 

 amination in detail are the clam or scallop (Pecteri), which 

 has only one adductor muscle mark; ttue oyster, whose 

 left valve is deep and whose right is flat and which also 

 has but one adductor mark (the posterior) ; the razor shell 

 (Soleri), on which all the usual markings may be found, 

 but altered in shape; the common mussel; the wedge shell 

 (Donax)\ etc. 



