Marine Pearls. 



77 



between the Pearl itself and the shell of the 

 mollusc producing it. The only shell however of 

 true commercial value is what is known as mother- 

 of-pearl shell, the substance of which, termed " nacre," 

 is familiar to every one, and has been transformed 

 into so many articles of ornament and utility from 

 a period almost as remote as the introduction of 

 the Pearl itself. A description of the shell forms 

 the subject of a separate chapter. 



Little need be said about the other marine 

 shells, which subordinately yield Pearls. From the 

 Melcagrina Califoriiica Pearls are obtained in the 

 Gulf of California and the Bay of Panama. Pearls 

 are occasionally found in the curious Malleus or 

 "Hammer Oyster;" in the Pinna, a wedge-shaped 

 shell usually moored by a byssus or tuft of fibres ; 

 in the delicate Placuna or "Window shell;" and 

 even in the common edible oyster. But the last is 

 rather a zoological curiosity than an object of 

 commercial value or artistic interest. 



It is perhaps worth mentioning that large 

 white Pearls are occasionally found in the great 

 clam shell, Tridacua gigas. These are alv/ays sym- 

 metrical and of some beauty, having a faint but 

 pleasing sheen when looked at side-ways, but are 

 of little or no value. Tlie giant clam is probably the 



