BIVALVE MOLLUSC A. 351 



Rhine mussel, a large species, the nacre of which is employed for 

 ornamental purposes. Unio littoralis (Cuvier), is represented in Fig. 

 150, and Unio pictorum, Fig, 151. The flesh of the river mussels 

 is leathery, of an insipid taste, and scarcely eatable : the finest 

 species are found in the great American lakes and rivers. 



Fresh-water mussels, as we have seen, produce pearls of moderate 

 value. Linnaeus, who was aware of the origin of the Pintadine pearls, 

 and of pearls in general, was also aware of the possibility of producing 

 them artificially from various molluscs. He suggested bringing 



Fig. 151. Unio pictorum (Linnaeus). 



together a number of mussels, piercing holes in their, shells with 

 an auger in order to produce a wound, and afterwards leave them 

 for five or six years, to give the pearl time to form. The Swedish 

 Government consented to try the experiment, and long did so in 

 secret pearls were produced, but they were of no value, and the 

 enterprise was abandoned as unsuccessful. 



Scottish pearls were much celebrated in the middle ages ; and 

 between the years 1761 and 1784 pearls to the value of ; 10,000 

 were sent to London from the rivers Tay and Isla ; " and the trade 

 carried on in the corresponding years in the present century," says 

 Mr. Bertram, " is far more than double that amount." The pearl, 



