Some conchologists recognize a large number of species of _Margaritiferæ_, while other authorities consider many of these as local variations of the same species. There is much difference in the size, color, and markings of the shells in different localities, owing to varying geographical and physical conditions. The distinction of species and the nomenclature herein adopted are those of Dr. H. L. Jameson, who has recently revised and rearranged the collection of shells belonging to this family in the British Museum of Natural History,[80] and to whom we are indebted for descriptive notes relative to several of the species. The greatest pearl-producer in the family of pearl-oysters is the _Margaritifera vulgaris_ of the Gulf of Manaar and the Persian Gulf, and to a much less extent of the Red Sea. It occurs in various other inshore waters of the Indian Ocean, and about the Malay Archipelago and the coast of Australia and New Guinea, although it is not the principal pearl-oyster of those waters. An interesting account of its immigration into the Mediterranean Sea through the Suez Canal was given by Vassel in 1896.[81] This species is quite small, averaging two and a half inches in diameter in Ceylon waters, and somewhat more in the Persian Gulf, whence large quantities of the shell are exported under the name of “Lingah shell.” The Ceylon variety has the nacreous lining almost uniformly white over the entire surface, only the lip having a slightly pinkish ground color. The exterior is marked by seven or eight reddish brown radial bands on a pale yellow ground. In addition to its greater size, the Persian variety is darker, and the lip of the shell has a reddish tinge. For centuries the _Margaritifera vulgaris_ has sustained the great pearl fisheries of Ceylon, India, and Persia, and at present yields the bulk of pearls on the market, especially the seed-pearls and also those of medium size. It produces relatively few large ones, rarely exceeding twelve grains in weight. These pearls are commonly silvery white, and for their size command the highest prices, because of their beautiful form and superior luster. Excepting the Venezuelan species, this is the only pearl-oyster which at present supports extensive fisheries exclusively for pearls; in the fisheries for all other species the value of the shells furnishes considerable revenue, and in some localities this represents several times as much as the income from the pearls.