435 
MOLLUSCOUS ANIMALS. 
I. BIVALVES, or those having two shells. 
THE PEARL OYSTER. (Avicula Margaritifera.) 
WHO that sees the beauty and delicacy of pearls would 
imagine that they were the production of disease. Such, 
however, is the case, as they are either formed in the body 
of the oyster-like animal which inhabits the shell ; or 
they rise from cracks in the shell itself, the delicate, sil- 
very, half-transparent lining of which forms the substance 
generally called Mother-of-pearl, or Nacre. These shells 
are found in the Persian Gulf, and at Ceylon, where they 
form an important article of commerce. 
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