The Tridacna gigas is the largest of the bivalve shells; it is very thick and close in texture, and is said to have been found as much as five hundred pounds in weight. In Catholic countries the shells have been sometimes used as the receptacles for the holy water in the churches, and formerly they were considered sufficiently valuable to form a present to a king; those in St. Sulpice, at Paris, were given to Francis the First by the Republic of Venice. They have been found in India, as it is related, of so extreme a size, that more than one hundred persons have made a meal on the flesh of a single Tridacna gigas, but this, no doubt, is an exaggeration. These shells adhere to the rocks by their short and strong byssus with so much tenacity, as to require the assistance of a mallet and chisels, in the task of separating them from the rock.

Tridacna gigas.

ORDER BIMUSCULOSA,
(Shells with Two Pairs of Muscles.)

This Order contains by far the greatest portion of the bivalve shells, all interesting to the naturalist, from the variety and beauty of their structure, but few possessing a claim to notice on account of their use as food for mankind, not but that many are equally wholesome with those belonging to the last order, but as they possess two or more pairs of muscles, they are much more capable of moving from place to place, and, consequently, are seldom found in any great quantities in one spot, and being found, as they generally are, in a considerable depth of water, they are not so easily obtained.

The Fresh-Water Mussel, ([Anodonta cygnæa].)

Although this shell, and several other species which nearly resemble it, are known by the trivial name of Mussel, the only resemblance between them consists in their outward appearance, the animals which inhabit the shells being very distinct, both in organization and in habits. The Mussels, as we have already said, have very little power of moving from place to place, while, on the other hand, the Anodonta is at times far from being a sluggish animal, and, for the purpose of shifting its position, it avails itself of a very strong and broad muscular foot. The shell of the Anodonta is sometimes found to contain pearls, a circumstance which frequently occurs in all shells which are lined with mother-of-pearl.

Some of the Scotch rivers have produced numerous specimens of pearl, very large and beautiful, and which used to bear an extremely high price.