568 



THE POPULAR SCIENCE MONTHLY. 



The two valves are often found united, and the margin along which 

 they are connected is called the Junge-margin, because the shells 

 hinge at this part, and Avill open and shut as a door swings upon its 

 hinges. 



Let the pupil now examine a perfect fi-esh-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 elas- 

 tic 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. 



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 growth, and indicate the successive 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. 



We 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 beak or timbone. 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 early stages in the growth of the shell are usually destroyed. In 

 very young shells, however, the early stages can be plainly seen. 



Back or dorsal region. 

 u I 



ex. 



g 5' 



CD O 

 P. 



/ Veutra) region. 



Fig. 7. a Fresh-Water Mussel. 

 /, Ligament ; w, Umbone ; j. Foot ; ex., Excurrent Orifice ; in., Incurrent Orifice. 



The ligament is always behind the beak, or umbone, in fresh-water 

 mussels, and in nearly all bivalve shells (so called, because they have 

 two valves or pieces, while the snail-shells are sometimes called loii- 

 valve shells, because they have but one valve or piece). 



