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 creatures, 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 edulis_, 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 extracting its small ‘seed-pearls.’ Pink pearls are obtained from the giant conch-shell of the West Indies (_Strombus gigas_), as well as from certain _Turbinella_. In Canton, many houses are illuminated almost entirely by skylights and windows made of shells, probably the semitransparent valves of _Placuna placenta_. 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.[211] A recent issue of the Peking Gazette contains a report from the outgoing Viceroy of Fukhien, stating that he had handed over the insignia of office to his successor, including _inter alia_ the conch-shell bestowed by the Throne. A conch-shell with a whorl turning to the _right_, _i.e._ a sinistral specimen, is supposed when blown to have the effect of stilling the waves, and hence is bestowed by the Emperor upon high officers whose duties oblige them to take voyages by sea. The Viceroy of Fukhien probably possesses one of these shells in virtue of his jurisdiction over Formosa, to which island periodical visits are supposed to be made.[212]