526 Mollusca. 



Persian Gulf and at Ceylon, where they form an impor- 

 tant article of commerce. 



The Chinese form pearls by casting into the shell of a 

 certain kind of muscle artificial beads, which at the end 

 of a year become covered with a pearly crust, in such a 

 manner that they cannot be distinguished from the 

 natural pearl.* 



THE COMMON OYSTER, (Ostrea edulia,) 



HAS long been in favour with man for its delicacy as an 

 article of food ; the Lucrine lake used to be as much in 

 renown among the Romans for the choicest kind of 

 Oysters, as Cancalle Bay with the French, and the Col- 

 chester beds with us. The two shells of the Oyster are 

 generally unequal in size ; the hinge is without teeth, 

 but furnished with a somewhat oval cavity, and gene- 

 rally with lateral transverse grooves. Oysters sometimes 

 grow to a very large size ; in the East Indies they are 

 said sometimes to measure nearly two feet in diameter. 



The principal breeding season of oysters is in the 

 months of April and May, when they cast their young, 

 which are enveloped in slime, and in this state called 

 spats by the fishermen, upon rocks, stones, shells, or any 

 other hard substance that happens to be near the place 

 where they lie; and to these the spats immediately 

 adhere. Till they obtain their film or crust, they are 

 somewhat like the end of a candle, but of a greenish 

 hue. The substances to which they adhere, of whatever 



* For a very interesting article on this subject, see Beckmann's 

 " History of Inventions," vol. i. p. 259. (Bohiis Standard Library.} 



