MOLLUSCA. 195 



secretion ; it is a secretion of calcareous matter in 

 a globular form under circumstances that are yet 

 imperfectly known, though we can place the animal 

 in a condition that will induce it to secrete pearls. 

 For instance, if a specimen of Unio margaritiferus 

 be taken, and one of the valves of its shell be pierced 

 with a sharp instrument, so as to drill a hole almost 

 through it, care being taken not to allow the in- 

 strument to penetrate completely through the shell, 

 it will be found that the animal secretes a pearl upon 

 that part of its shell. 



Linnaeus succeeded perfectly in causing the for- 

 mation of pearls in the shell of this same fresh- 

 water mussel. He found that when grains of sand 

 were placed between the shell and the body of the 

 mollusc a pearl was produced which enveloped the 

 grain of sand. This might have been expected, for 

 sections of Oriental pearls often exhibit very fine 

 concentric laminae, surrounding a grain of sand, 

 or some such extraneous matter. 



We have only one or two more Bivalves to 

 mention before closing this chapter. 



Bufibn speaks of a mussel found in the Medi- 

 terranean which the Sicilians and Italians turn to 

 account for making gloves and stockings. It is a 

 species of Pinna. This genus of mollusca belougs 

 to the same group as the pearl oyster (Avicula) - } 

 like other mussels, the Pinna secrete a long byssus, 

 by which they hold to the rocks. The species vary 



