PEARL FISHERIES 10 I 



internal nacreous laminae of the shell of this species. The most 

 important fisheries are those of the Aru Islands, the Soo-loo 

 Archipelago, the Persian Gulf, the Eed Sea, Queensland, and the 

 Pearl Islands in tlie Bay of Panama. The shell also occirrs in 

 several of the groups of the South Pacific — the Paumotu, 

 Gambler and Navigator Islands, Tahiti being the centre of the 

 trade — and also on the coasts of Lower California.^ 



Pearls are the result of a disease in the animal of this species 

 of Avicula and probably in all other species within which they 

 occur. When the Avicula is large, well formed, and with 

 ample space for individual development, pearls scarcely occur 

 at all, but when the shells are crowded together, and become 

 humped and distorted, as well as affording cover for all kinds of 

 marine worms and parasitic creatm'es, then pearls are sure to be 

 found. Pearls of inferior value and size are also produced by 

 Placuna placenta, many species of Pinna, the great Tridacna, 

 the common Ostrea cdulis, and several other marine bivalves. 

 They are not uncommon in Unio and Anodonta, and the common 

 Margaritana margaritifera of our rapid streams is still said to 

 be collected, in some parts of Wales, for the purpose of ex- 

 tracting its small ' seed-pearls.' Pink pearls are obtained from 

 the giant conch-shell of the West Indies {Stroiiibus gigas), as 

 well as from certain TurhincUa. 



In Canton, many houses are illuminated almost entirely by 

 skylights and windows made of shells, probably the semi- 

 transparent valves of Placuna i^laccnta. In China lime is 

 commonly made of ground cockle-shells, and, when mixed with 

 oil, forms an excellent putty, used for cementing coffins, and in 

 forming a surface for the frescoes with which the gables of 

 temples and private houses are adorned. Those who suffer from 

 cutaneous diseases, and convalescents from small-pox, are washed 

 in Canton with the water in which cockles have been boiled.- 



A recent issue of the Peking Gazette contains a report from 

 tlie outgoing Viceroy of Fukhien, stating that he had 1 landed over 

 the insignia of office to his successor, including inter alia the 

 couch -shell bestowed by the Throne. A conch -shell with a 

 whorl turning to the rigid, i.e. a sinistral specimen, is supposed 

 when blown to have the effect of stilling the waves, and hence is 



^ See in particular, P. L. Simiuouds, The Comtncrcial Products of the Sea. 

 - H. rrieiid, Field Cluh, iv. 1893, p. 100. 



