Page:Natural History, Mollusca.djvu/261

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249
PEARL SHELLS.

many of them may every day be seen at this work."


Genus Pinna.

The largest British bivalves belong to the genus Pinna, in which the shell is fibrous, and horny in texture, rather fragile and delicate; the valves are equal, triangular and exceedingly one-sided; the hinge is straight, long and toothless, the ligament marginal, and almost wholly internal.

The animal is triangular, with the mantle freely open and having fringed edges; the foot is small, conical, or tongue-like, and grooved; there is a small anterior muscle in the angle; the posterior one is by far the larger.

The genus contains but few species, which are, however, widely distributed, and most of them are of large size; they range from deep water to near the shore, living on sandy or muddy bottoms, with their beaks plunged deep into the ground, and the broad extremity gaping upwards.

In a cabinet of British shells the eye is sure to be attracted by the great fan-like horny valves of the Pinna (Pinna pectinata), which are not uncommonly seen twelve inches long, and seven broad at the gaping end. Young specimens are of a semi-transparent horny texture, partially covered with rows of hooked spinous scales; but these spines are almost always worn away in old examples. The interior is pearly in a slight degree, and loose pearls of dull colour are sometimes found within the shells. The silky byssus spun by these mollusks, which is capable of being woven into small articles of wearing apparel, has been already noticed.

The great Pinna is found all around the British