Pearls are a calculus, or morbid concretion, formed in consequence of some external injury which the shell receives, particularly from the operations of certain minute worms which occasionally bore even quite through to the animal. The pearls are formed in the inside on these places. Hence it is easy to ascertain, by the inspection of the outside only, whether a shell is likely to contain pearls. If it be quite smooth, without cavity, perforation, or callosity, it may with certainty be pronounced to contain none. If, on the contrary, the shell be pierced or indented by worms, there will always be found either pearls or the embryos of pearls. It is possible, by artificial perforations of the shells, to cause the formation of these substances. The process which has been chiefly recommended is to drill a small hole through the shell, and to fill this hole with a piece of brass wire, rivetting it on the outside like the head of a nail; and the part of the wire which pierces the interior shining coat of the shell will, it is said, become covered with a pearl. As to the value of British pearls, some have been found of size so large as to be sold for 20l. each and upwards; and 80l. was once offered and refused for one of them. It is reported in Wales, that a pearl, from the river Conwy, which was presented to the queen of Charles the Second, was afterwards placed in the regal crown. 273. _The ORIENTAL PEARL MUSCLE_ (Mytilus margaritiferus) _to which we are indebted for nearly all the pearls of commerce, has a flattened and somewhat circular shell, about eight inches in diameter; the part near the hinge bent, or transverse, and imbricated (or covered like slates on a house) with several coats which are toothed at the edges._ _Some of the shells are externally of sea-green colour, others are chesnut, or reddish with white stripes or marks; and others whitish with green marks._ _These shells are found both in the American and Indian seas._