28 FIRST BOOK OF ZOOLOGY. 



sels, and the remains of their feasts may be found in piles of 

 mussel-shells all along the shores of cer.tain lakes. 



The shell is composed of two pieces, or valves, as they are 

 called. The two valves are often found united, and the 

 margin along which they are connected is called the hinge- 

 margin, because the shells hinge at this part, and will open 

 and shut as a door swings upon its hinges. 



Let the pupil now examine a perfect fresh-water mussel, 

 that is, a mussel in which the valves are united in this way, 

 and he will observe that they are connected by a brownish 

 substance, which is quite elastic when the shell is alive, but 

 becomes brittle when dried. The shells are held together as 

 the covers of a book are held together by the back. 



This substance is called the ligament, and the position of 

 this ligament will indicate the back, or dorsal region of the 

 animal. 



27. On the outside of the shell will be seen fine lines, 

 which run nearly parallel to the outside margin of the shell. 

 These lines are the lines of groiuth, and indicate the succes- 

 sive stages of growth, or increase of the shell, as in the lines 

 of growth in the snail-shell already studied, and, as in the 

 snails, the growth takes place at the margin of the shells. 



The pupil may trace these concentric lines back, as they 

 grow smaller and smaller, till they are found to start from 

 one point at the back of the shell, and this point is called 

 the leah or unibone. It represents the starting-point in the 

 growth of the shell. In fresh-water mussels, the umbones 

 are eaten away by some corrosive action of the water, and the 



