Page:Natural History, Mollusca.djvu/251

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The members of this family effect a transition from those genera which have but a single muscle, to those which have two; for though their shells display two, or even more muscular impressions, one is always found to predominate greatly over the rest, which are but small.

The shell has its exterior portion commonly black, or of the colour of horn, and composed of prismatic cellular substance; the interior layers are composed of nacre, and are therefore brilliantly pearly. Their form is very irregular, the valves being unequal, and always developed more towards one side than the other. The hinge is without teeth, the ligament thin, simple, or notched.

The animals have a mantle which does not adhere to the shell, entirely open all round, prolonged into irregular lobes, without siphons; there is a small foot which is grooved, and capable of spinning a strong byssus.

The most important genus of the family contains the Wing-shells, one species of which is the Pearl-oyster, from the interior shell of which we obtain the substance known as mother-of-pearl, which is largely imported into this country, and manufactured into shirt-buttons, knife-handles, and various trinkets; it is also much used for ornamental inlaying. But that which renders this shell of much more value is the production of pearls, the beauty of which has always rendered them fit companions of the choicest precious stones, for the purpose of personal adornment.