8o Pearls. 



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 Unio, nevertheless presents 

 less difference to the ordinary observer than really 

 exists between many members of the genus Unio 

 itself. There are about one hundred species of the 

 Anodon distributed over North America, Europe and 

 Siberia ; while the genus Unio comprises upwards 

 of 400 species. 



The shell of the Unio is a more or -less elon- 

 gated oval, generally of a dark brown or almost 

 black exterior, while its inside is slightly nacreous. 

 Though very unlike the Aviciila in outward appear- 

 ance, yet in anatomical structure the two molluscs 

 are very similar, being surrounded by a correspon- 

 ding mantle, which secretes not only the shell but 

 the Pearls contained in it. These however are 



