MOLLUSCA. 185 



a secretion produced by the animal in perfect 

 health, with a view of strengthening certain por- 

 tions of its shell, either on account of a slight 

 fracture, or to close up apertures pierced in it by 

 marine worms, or, again, to furnish strong points 

 of adherence for certain muscles or ligaments of the 

 animal's body. Be this as it may, Linnaeus, in his 

 experiments on fresh- water mussels (Unio), dis- 

 covered a means of causing the mollusc to produce 

 pearls artificially, as we shall see presently. 



As to the geographical distribution of Avicula 

 margaritifera, which produces mother-of-pearl and 

 the real Oriental pearl, it is found in the Persian 

 Gulf, on the coasts of Arabia Felix, on the coasts of 

 Japan. It is at Cape Comorin, and in the Gulf of 

 Manaar, at the island of Ceylon, that the most 

 productive and celebrated pearl fisheries have been 

 established. Oriental pearls are likewise met with 

 in America, on the coasts of California, at Mada- 

 gascar, and at the island of Tahiti. 



The Gulf of California is about 700 miles long, 

 and from 40 to 120 miles in width. One of the 

 first shells discovered in its waters was a pearl 

 oyster, the Avicula fimbriata (Margariphora mazat- 

 lantia of others), to obtain which the Spaniards, in 

 the seventeenth century, employed from 600 to 800 

 divers ; the value of the pearls obtained amounted 

 annually to about 60,000 dollars. This traffic was 



