BIVALVE MOLLUSCA. 351 
Rhine mussel, a large species, the nacre of which is employed for 
ornamental purposes, Unzo dittoralis (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. Linnzeus, 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 (Linnzus, ) 
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 unsuccesstul. 
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, 
