CONCHIFERA OR BIVALVES. 119 



The Avicula Margaritifera, or Pearl Oyster, 

 is found chiefly in the Persian Gulf, and at 

 Ceylon ; it is lined with 

 a pearly substance, 

 which is formed by the 

 animal in thin layers, 

 and is imported into 

 Europe in large quan- 

 tities, for the purpose of 

 making knife handles, 

 buttons, &c, under the name of mother-of-pearl. 

 If by any accident the inner surface of the 

 shell is injured, so as to cause a fracture in the 

 mother-of-pearl, the layers in that place be- 

 come for the future irregular, and a lump is 

 gradually formed, which from accidental cir- 

 cumstances is sometimes oval, sometimes glo- 

 bular, and sometimes pear-shaped. Pearls, as 

 these lumps are called, are occasionally found 

 loose in the shell, and in this case the pearly 

 substance has been deposited upon a loose 

 grain of sand, or diseased ovum or egg of the 

 animal. 



" Pearls are small nacrous balls, that become 

 formed and hardened within the body of the 

 I 4 



