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. mongoliens, 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.1
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 Mytilidœ 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.
A few pearls are also obtained from the sea-wings or wing-shells (Pinna), the
silkworms of the sea, found in the Red Sea, the Mediterranean Sea, the
Indian Ocean, the southern coast of America, and elsewhere. These
shells are narrow at the umbo, or hinge, long, and fan-shaped ; they
are generally brittle, and present a horn-like appearance. The
interior is commonly of a silvery reddish or orange-colored hue, and
this tint is imparted to the pearls. The most characteristic feature of
the Pinna is the thick rope of silky fibers, from four to ten,
1 See p. 288.
