Professor Herdman and James Hornell, after three consecutive inspections of the oyster banks in the Gulf of Manaar in 1902-3, stated in a paper contributed to the British Association for the advancement of science, that after examining many hundreds of oysters and decalcifying a large number of pearls, they had come to the conclusion, that grains of sand and other inorganic particles formed the nuclei of pearls only under exceptional circumstances, as for instance, when the shell was injured by the breaking of the ears, which would enable sand to get into the interior. Pearls, or pearly excrescences on the interior of the shell, were due to the intrusion of leucodore, clione and other borers. Pearls found in the mussels, especially at the levator and pallial insertions, were formed around calcospherules, minute calcareous concretions produced in the tissues. But most of the fine pearls found free in the body of the Ceylon oyster, contained the remains of platyhelminthian parasites. These observations agree with the opinions formed, after careful study, by several eminent conchologists. The action of the mollusk results differently as the object to be covered is free within the folds of the creature's mantle or, rising above the surface of the nacreous lining, presses upon it. If free, the intruder is enveloped by the animal's exudations and the deposits become concentric instead of level, or nearly so, as in the construction of the shell. It is said that the foreign substance acts as an irritant, causing the fish to exude its secretions abnormally in order to protect itself, and thereby creating a diseased condition; but from the fact that the process continues after the intruder has been enveloped and rendered as non-irritant as the natural lining of the shell, it would appear that the introduction of a foreign element simply draws upon it the normal impulse of the fish to cover with nacre anything with which it comes in contact, and that the method of doing it is similar to the instinctive rolling action of the tongue when some insoluble globule is put in the mouth, for not only do free pearls grow spherically, but a nucleus fast to the shell is not covered simply but it grows to a pearl, round and domelike, as nearly spherical as its juncture with the shell will permit. Not only is the composition of a pearl identical with the lining of the shell where it is formed, but in a general way its appearance and characteristics are the same, except that free pearls are sometimes colored when the nacre of the shell is white. Button pearls, warts and baroques, grown fast to the shell, are usually like the surrounding nacre in every respect. Salt-water pearls are characterized by the soft velvety luster of the oriental mother-of-pearl, and fresh-waters, like the lining of the unio, have a somewhat thinner looking and more chalky texture.