The fresh-water Pearl-mussel is sometimes also called Margaritana margaritifera ; the sub-genus Mar-garitana having been established by a German naturalist named Schumacher.
The
Pearl-mussel is found inhabiting lakes and rivers in cool temperate
zones, in almost all parts of the world. It occurs in mountain streams,
not only in Great Britain and Ireland, but in all Northern Europe, in
Bavaria and Saxony, and in the United States, and Canada. These Pearls
are also occasionally produced by the Anodon, which although not included in the genus Unto, nevertheless presents less difference to the ordinary observer than really exists between many members of the genus Unto itself. There are about one hundred species οι the Anodon distributed over North America, Europe and Siberia ; while the genus Unto comprises upwards of 400 species.
The shell of the Unio is
a more or less elongated oval, generally of a dark brown or almost
black exterior, while its inside is slightly nacreous. Though very
unlike the Avicula in outward appearance, yet in anatomical
structure the two molluscs are very similar, being surrounded by a
corresponding mantle, which secretes not only the shell but the Pearls
contained in it. These however are
