When a growing pearl is very near to the nacreous lining of the shell, the pressure between the two hard substances results in a rupture of the pearl-forming sac and the epithelial layer of the shell, and the pearl comes in actual contact with the nacre. The pearl gradually becomes attached to the shell, and the under portion is prevented from growing further; the upper or exposed surface receives other layers, resulting in the formation of a _bouton_. As the shell around the pearl continues to grow, it gradually closes about, and almost wholly conceals the pearl. Since it is constantly wasting away on the exterior surface as it grows on the interior, it follows that in time the shell passes the pearl quite through to the outside, where it rapidly decays. Thus the oyster virtually forces the annoying intruder directly through the wall of its house instead of by way of the open door, and magically closes the breach with its marvelous masonry. These embedded pearls are generally faulty and of diminished luster, but in the aggregate, large quantities of imperfect ones, and especially half and quarter pearls, are secured in this manner. Sometimes—particularly in the Australian fisheries—large pearls are thus found, weighing twenty, forty, sixty, and even eighty grains; and when the faulty outside layers of nacre are removed, a subjacent surface of fine luster may possibly be revealed. In bivalves, these adherent pearls are commonly in the deep or lower valve, except in those unusual cases where the mollusks have been lying in a reverse position. At the fisheries, the surfaces of the shells are carefully inspected for evidence of pearly nodules, and these are broken open in search for encysted objects. Cutters of mother-of-pearl occasionally find embedded pearls of this kind which have escaped the vigilant eyes of the fishermen. We read of an instance in an important paper treating of the jeweling trade of Birmingham: “A few years since [the paper was written in 1866] a small lot of shells was brought to Birmingham, which either from ignorance or mistake had not been cleared of the pearls at the fishery. A considerable number were found and sold, and one especially was sold by the man who had bought the shell for working into buttons, for £40. The purchaser, we believe, resold the same for a profit of £160; and we have heard that it was afterward held in Paris for sale at £800.” A choice gem which was found in New York, in October, 1905, in an Australian shell, sold finally for $1200.