268 LIFE BY THE SEASHORE. 



species. In the mussel notice first that the two valves of 

 the shell are of the same size (equivalve); this is very 

 characteristic of Bivalves, and, together with the fact that 

 each valve is inequilateral, is a convenient means of dis- 

 tinguishing them from Brachiopods, or lamp-shells, which 



have inequivalve but 

 equilateral shells. In 

 other words, the shell 

 of a bivalve is divided 

 into two equal parts 

 when the valves are 

 separated, but not 

 when the valves are 

 divided by a median 



pi - n --^r^. ia eut 1 i ne \. The J she11 rf * 



Brachiopod, on the 



other hand, consisting as it does of two valves of unequal 

 size, is divided into two equal parts only by a line which 

 bisects the two closed valves. 



Again, the two valves of the shell of the mussel are 

 united at the hinge by an internal cartilage called the 

 ligament, so placed as to cause the shell to gape, except 

 when it is forcibly closed by muscles. The hinge is over- 

 hung by two projections, or beaks, which form the oldest 

 part of the shell. In the inside of the shell are to be seen 

 markings indicating the places where the muscles of the 

 shell have been attached. In the mussel these are two in 

 number, and are placed at the side of the shell opposite to 

 that where the foot is protruded. In addition to these 

 markings, there is an uninterrupted line near the margin of 

 the shell which marks the line of attachment of the mantle 

 to the shell. If the mussel shell be compared with that of 

 Tapes, it will be seen that in the latter case this line of 

 attachment does not follow uninterruptedly the margin of 

 the shell, but is at one spot inflected to form a deep, 

 rounded bay, called the pallial sinus. This sinus marks the 

 attachment of the muscles which move the siphons, and its 

 presence in a shell enables one to conclude at once that the 

 living animal possessed siphons. 



The shell of Tapes differs in several other respects from 

 that of Mytilus. Thus the hinge, instead of being smooth, 



