Page:Popular Science Monthly Volume 50.djvu/58

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S we watch the little pools of water left among the rocks by the retreating tide the pearly luster or the violet or golden tint of some tiny shell catches our eye. How exquisite its form and coloring!

Shells have always, from the most ancient times, been greatly prized. Prehistoric men discovered in the burial caves of Auvergne have chaplets of shells which scientific men tell us they must have traveled long distances to gather. It is only of late years that their curious little occupants have been interviewed and some ideas obtained with regard to their characteristics and mode of living.

All shells with their inhabitants belong to the immense class known as Mollusca, or soft-footed animals. Shells are divided into two groups—univalve, those having but one valve, as the snail, whelks, cowries, etc.; and bivalve, as the oyster, clam, and mussel.

If we take the clam as a typical mollusk we shall see that each little line on the inside or outside of the shell reveals an interesting fact. On the outer surface of each valve are a number of concentric lines parallel to the edge and growing fainter toward the hinge part. These are called the lines of growth, and are made by the mantle. The clam's mantle is quite as useful to him as are our hands to us, and he uses it for similar purposes. The mantle surrounds the clam's body inside the shell, its edge