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.
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 umbone. 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.
Fig. 7.—a Fresh-Water Mussel.
l, Ligament; u, Umbone; f. 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 uni-valve shells, because they have but one valve or piece).
