MOLLUSCA. 



257 



which the new layers of shell proceed at the edge of the mussel or 

 oyster. The finest kind are formed in the body of the animal, or 

 originate in the pearly part of the shell. It is by the size, roundness, 

 and brilliancy of pearls that their value is estimated. They are found 

 either in the mantle of the animal, or attached to the inner surface of 

 the shell. 



Pearls have been produced in an artificial manner; Apollonius, 

 the philosopher of Tyana, who is supposed to have died at an advanced 



' 



fig. 111. Artificial Pearls in the shell of My a margaritifera. 



age in the year 97, being the first to notice such a practice, on the 

 borders of the Red Sea. Sir Joseph Banks had specimens of some 

 Chinese mussels, in which were small pieces of iron covered with a 



s 



