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374 Ostrea permollis Sowerby Sponge Oyster Plate 28b North Carolina to Florida and the West Indies. Rarely over 3 inches in size. Lives embedded in sponges with only the margins of the valves showing. The surface of the valves has a soft, silky appearance. Beak twisted back into a strong spiral. Exterior light-orange to tan; interior white. Inner margins with numerous small, round denticles. Common. Another flat, but larger and light-shelled oyster, Pycnodonta hyotis Linne, is found in deep water attached to old wrecks off Florida and in the West Indies. It is immediately recognized by the peculiar structure of the shell which under a lens appears to be filled with numerous bubbles or empty cells, much like a bath sponge. It reaches a diameter of 3 or 4 inches, is generally circular in outline and may be colored whitish cream, brownish or even lavender. Ostrea thovmsi McLean is this species according to the French worker, Gilbert Ranson. Ostrea lurida Carpenter Native Pacific Oyster Plate igi Alaska to Lower California. 2 to 3 inches in length, of various shapes; generally rough with coarse concentric growth lines, but sometimes smoothish. Interior usually stained with various shades of olive-green, and sometimes with a slight metallic sheen. It occasionally has purplish brown to brown axial color bands on the exterior. This is the common intertidal native species of the Pacific Coast. A number of ecological forms have been described: expansa Carpenter, rufoides Cpr. and possibly conchaphila Cpr. Genus Crassostrea Sacco 1897 This genus includes the commercially important American Oyster, C. virginica Gmehn, which was formerly placed in the genus Ostrea. In Crassostrea, the left or attached valve is larger than the right. The inner margin is smooth. The eggs are small, produced in large numbers at one spawning (over 50 million), and are fertilized and develop in the open wa- ters outside of the parents. The muscle scar is usually colored. The prodisso- conch hinge is short, and the valves asymmetrical. The Japanese Oyster (C. gigas)^ introduced to west American shores, the Portuguese Oyster (C angu- lata Lamarck), and C. rhizophorae Guilding from Cuba also belong to this genus. Gryphaea Lamarck is a fossil genus which should not be associated with this genus.