In the first place all these parts of the mantle which supply the material for the epidermis, the middle shell, and the lining, are enclosed within the shell and in touch with the lining yet each receives the exudations of that part of the mantle which supplies the material suitable for it, the mantle invariably pushing the coarser excretions outwardly to the shell's exterior. Again, whatever the quality of the skin of the pearl may be, it is never of conchiolin like the outer epidermis and though sometimes similar to the plates, of which the conchiolin is the exposed fringe, it always contains sufficient nacre to render the surface smooth. The fact that the skins of a pearl do sometimes correspond with the different parts of the shell, and that the same skin on the surface is occasionally partly nacreous and unnacreous, in connection with the variation of quality which exists in the internal composition of the skin, favors an idea that the mixed and variable quantity of nacre in the skins may be caused by the abnormal position of the mantle wrapped about the growing pearl which would thereby come more or less under the influence of the calcite and conchiolin zones distorted from their normal extension and action. It has also been suggested that the oyster deposits the nacreous layer in a fluid state and then rests until the deposit hardens, when the process is repeated. To a certain extent this may be true though apparently it could not be a yearly process as pearls found in the small varieties of the avicula which mature in four to six years and die out in seven years, often contain a greater number of layers than the years of the mollusk's life, and no pearl is ever found with a soft exterior, though it seems possible that pearls with a dead white chalky exterior are taken from the oyster at a period when the crystallization of the outer skin has not been perfected, or that they have escaped some action, chemical or of the animal, necessary for the formation of the lustrous waves of nacre. Mr. Ludwig Stross, who has had much experience at the pearl fisheries, says that he has frequently found pearls of fair size in shells of the Lingah type which could not be over twelve to fifteen months old. Some of these pearls weighed fully three grains. As there are many apparent skins in a pearl of that size, the divisions could not mark either years, seasons, or breeding periods. In some experiments made by Mr. Stross, he found that borings made to the interior of a living mollusk's shell were closed by a film of hard nacre in two days.