Of them, the genus "meleagrina" is the largest, has the heaviest shell, and furnishes the greatest quantity of the beautiful substance known as mother-of-pearl. The other extreme is the small, frail-shelled variety taken off the coast of Venezuela, called sometimes avicula squamulosa. Similar to this is the margaritifera vulgaris, or avicula fucata, of Ceylon. The pearl oyster of the Persian Gulf though similar is somewhat larger. Exact and uniform classification of the pearl-bearing mollusks of the sea does not exist, nor is it necessary in this connection, as the one distinctive feature which places them in the class under consideration is the possession of a nacreous lining to the shell, for no shell fish can produce a true pearl without it. The fresh-water pearl-bearing mollusk is a mussel, unio margaritifera, also found in many varieties, but all characterized alike by the nacreous lining of the shell. These creatures, living upon the earth where water always covers it, create in the building of their habitations a material of great beauty, and sometimes produce gems which princes covet. Of the most delicate nature, they build for themselves out of the water by which they are surrounded, houses strong and enduring, fitted for their protection from the rough chances of life, yet so furnished within that they suffer no inconvenience from the rugged strength which encloses them. Few things are coarser than the exterior of these domiciles, but nothing in nature is finer or more exquisitely beautiful than the substance with which they are lined. The avicula margaritifera is a habitant of the coral reefs and shoals about the islands and shores of the tropics; there are none living now in northern latitudes, though fossils of many species are found north of the present boundary of their habitations. An idea can be formed of the general shape and appearance of pearl-oyster shells by the neighboring illustrations of three varieties. These show the two extremes of the marine mollusk, the meleagrina of the South Sea and Australia, and the squamulosa of Venezuela. [Illustration: AUSTRALIAN PEARL-SHELL] In some of the small species, that of the Venezuelan Coast for instance, the outer shell is yellowish, with fan-like markings of dark reddish brown radiating from the boss or beak and growing darker as they near the lip. This shell is thin and frail. The nacreous lining is also thin but brilliantly iridescent and shows a series of fine lines and irregular fissure-like markings extending outward from the hinge and crossed by bands of color which curve with the outline of the lip edge of the shell.