This cooking is a convenient method of opening the shells, but unquestionably it injures the quality of many pearls. In some instances when the shells open, the pearls fall out and descend to the heated iron bottom, where they are quickly injured. The surface of one exposed too long to the heat shows numerous minute cracks, which increase in number and size when subjected to changes of temperature. Some choice gems have in this manner been rendered almost valueless. If a jacket boiler, or one with a double bottom, were used, there would be less danger of injuring the pearls; or a similar result could be accomplished by placing a wire screen a few inches above the bottom. Several fishermen have endeavored to devise mechanical methods for removing the pearls and thus avoid the painstaking search among the flesh tissues now necessary; but these contrivances have not proved satisfactory, and have not been employed except experimentally. In the Mississippi and its tributaries, where the fishery is very extensive, after the pearls have been secured, the shells are sold to button manufacturers and to exporters at prices ranging from $4 to $40 per ton, according to species, quality, and market conditions. This provides a fairly remunerative income to the fishermen even if no pearls whatever are found. But in the small tributaries and where the mollusks are less numerous, the shells are of little value owing to the expense of bringing them together and conveying them to market. Not every mollusk contains a pearl, and the village belle, intent on her evening toilet, need not buy a bushel of clams with the pleasant anticipation of finding a sufficient number of gems for a necklace. Small and irregular pearls are not at all uncommon, but choice ones are decidedly scarce, and each one represents the destruction of tens of thousands of mollusks. Quantities of irregular and imperfect nodules known as slugs are collected, which sell for only a few dollars per ounce. In some sections of the Mississippi, the slugs are so very numerous that their aggregate value exceeds that of the choice pearls. In the Mississippi, the percentage of pearls found in a definite quantity of mollusks is less than in the tributary streams, yet the much greater quantity of shells collected raises the total yield to a very considerable amount. Pearling is subordinate and incidental to gathering the shells for manufacture. In that length of the river from St. Paul to St. Louis, a fair average yield to the fishermen is about fourteen dollars’ worth of pearls and slugs to each ton of shells. Of course, this is not the individual experience, for a single Unio may contain a gem worth $5000, and on the other hand several tons of shells may yield only a few cents’ worth of baroques. The market for the shells places the Mississippi fishing upon an industrial basis, and guarantees a substantial income to every fisherman even when no pearls whatever are found.