MOLLUSCA. 191 



be possible to cause this mussel to manufacture 

 larger pearls. However, such as they are, the pearls 

 of M. edulis have been for many years an article of 

 commerce in England. 



There are two kinds of fresh-water mussel which 

 resemble each other very closely ; the first are found 

 in pools and other stagnant waters, and are known 

 in English as "Pond mussels" (Anodontes). The 

 other description inhabit running water, and are 

 seen in sparkling streams. These belong to the 

 genus Unio, and are those to which I am about 

 to draw attention. 



FIG. 23. Unio margaritiferus (Fresh-water pearl-mussel). 



Our readers are probably acquainted with the 

 "painter's mussel" (Unio pictorum). It is seen in 

 the shop-windows of vendors of pencils, colours, 

 and engravings, with its edges gilt. It is used by 

 miniature painters to hold colours, and that is all 

 I have to say of it. A much larger and by far more 

 interesting mollusc is the fresh- water pearl mussel 

 (Unio margaritiferus) (Fig. 23), a species which is 



