A single mollusk may contain several small pearls,—more than one hundred have been found,—but in such cases usually none has commercial value. Ordinarily only one is found in the examination of very many shells. Of these objects it may be truthfully said that “many are found, but few are chosen,” few that are of first quality or are worthy of a fine necklace. In many instances, several pounds of cheap pearls would be gladly exchanged for a choice gem weighing an equal number of grains. On the Atlantic seaboard of America, the Anodontas, or “mussels,” as they are known locally, are more numerous than the Unios. They prefer the still waters of the ponds and lakes, rather than the swift currents of the streams. The shell is much thinner than that of the Unios, and it is usually not so brilliant in color and iridescence; consequently the pearly concretions obtained from them are less lustrous. The rivers of Europe, and of Asia also, contain numbers of pearl-bearing mussels. In many localities the yield of pearls has at times attracted attention and produced much profit, though probably never equaling the present extent of the Mississippi River finds. The principal pearl-bearer of Europe is the _Unio margaritifera_, the shell of which has been of some local importance in the manufacture of pearl buttons. In Great Britain it is known as the pearl-mussel; in France as the _moule_ or _huître perlière_; in Germany as _perlenmuschel_; in Belgium as _paarl mossel de rivieren_; in Denmark as _perle-skiael_; in Sweden as _perlmussla_; in Russia as _schemtschuschuaja rakavina_, and in Finland as _simpsuckan cuosi_. The _Unio margaritifera_ likewise exists in Siberia, and possibly elsewhere in Asia. Other species of Unio exist there and in Mongolia, Manchuria, etc., as, for instance, _U. mongolicus_, _U. dahuricus_, etc. A leading species in eastern China, the _Dipsas plicatus_, has long been extensively employed in the artificial production of pearly objects or culture pearls.[83] Unio pearls show less uniformity of tints than those derived from the pearl-oysters. They present an extended series of shades, corresponding to those on the interior of the shells, from almost perfect white through various tints of cream, pink, yellow, bright red, blue, green, russet, and brown. The metallic shades are numerous, especially the steels and the coppers. Most of the members of the _Mytilidæ_ family, which includes the marine mussels, are of slight luster; and the pearly concretions found in them are of the grade known as “druggists’ pearls,” so-called because, formerly, they were used in a powdered form in astringent and other medicines. However, some of these mussels on the European coast yield pearls that are fairly lustrous. The white and the pink are most numerous, but purple, red, bronze, and yellow are by no means uncommon.