The largest and finest pearls, also the greatest number, are found usually in distorted shells. This has given rise to the idea that they are a symptom of disease in the fish, but having in mind the functions of the three zones of the creature's mantle by which they supply separately material for the epidermis, middle shell and lining, one may conceive that if, by some extraordinary cause, the secretions of one of these is largely withdrawn from the natural channel, the losing part of the shell would warp the normal growth of the others to its own dwarfage. When the nacre grows to a pearl, contrary to the intent of nature, instead of a lining for the shell endeavoring to keep pace with the growing oyster, the full-growing exterior is distorted in accommodating itself to the undersized lining. In view of the fact that an oyster sometimes contains a large number of pearls (one shell in New Caledonia contained 256) the diversion of nacre sufficient to cover them, or to produce one large pearl, might reasonably be expected to result in a considerable distortion of the shell. It may also be that the displacement of the mantle, caused by the wrapping of itself about the growing pearl, interferes with the even deposit of shell material about the edges of the shell and so distorts it. Because deformed shells are more fruitful of pearls some have advocated the practice of throwing perfectly-formed shells back into the sea unopened, but, inasmuch as the mother-of-pearl of the shells often exceeds in value the pearls found in them, this is not likely to happen. Few fisheries could be made to pay if they were fished for the pearls alone. In many of them the shells yield 90 per cent. of the total value and are in fact the sole incentive for the investment of the necessary capital. Luckily for the world's supply of pearls, however, the disturbers of the mollusk which cause these gems by their intrusions appear to be more abundant in waters where the shell is valueless, the banks about Ceylon especially being infested with the cestodes which are commonly the nuclei of Indian pearls. It is interesting also to learn that Mr. James Hornell (inspector of the pearl banks) finds these worms in another stage in the file-fish, which frequents the banks to prey upon the oysters, and confidently expects to find them in the adult stage in the shark, which in turn devours the file-fish. It is the opinion of Jameson of London and others, that the parasite which causes the formation of pearls in the mussels of Europe is frequently the larva of distomum somaterœ, from the eider-duck and scoter, and that the larva first inhabits Tapes, or the cockle, before getting into the mussel.