UNIONHLE. 341 



pictorum (Fig. 152), employed in the arts to contain certain colours. 

 Those known as the river mussels are leathery, of an insipid taste, 

 and scarcely eatable : the finest species are found in the great American 

 rivers. 



Mussels, as we have seen, produce pearls of moderate value. Linnseus, 

 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 together a number ol' 

 mussels, piercing holes in their shells with an augur in order to pro- 

 duce a wound, and afterwards leave them for five or six years, to 



Fig. 152. Unio pictorum (Linna2us). 



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 un- 

 successful, r 



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, 

 according to Mr. Bertram, is found in a variety of the mussel, which is 

 characterised by the valves being united by a broad hinge, and having 

 a strong fibrous byssus, with which it attaches itself to other shells, to 



