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siphon and behind into a conical excurrent siphon. Foot strap-shaped. This genus was formerly known as Modiolaria Beck 1838. Hinge finely dentate. Musculus niger Gray Black Musculus Plate 2 8g Arctic Seas to North Carolina. Alaska to Puget Sound. About 2 to 3 inches in length. Similar to M. discors, but much more compressed and with strongly developed axial, decussated ribs on the pos- terior and anterior thirds. Center section with microscopic concentric wavy threads and pimples. Often pinkish on the inside. Common. Musculus laevigatus Gray Smooth Musculus Plate 28f Arctic Ocean to Puget Sound, and North Atlantic. I to I % inches in length. Distinguished from discors by its larger size, and in having no pronounced radial riblet or depression separating the pos- terior third from the middle area. The periostracum is more often black in this species. The posterior area very often has numerous microscopic con- centric scratches which, to the naked eye, give this area a dull finish. Like discors, this species is much fatter than niger. Musculus lateralis Say Lateral Musculus Figure yjd North Carolina to Florida and the West Indies. % inch in length, oblong, fragile, with a center area on the valve with concentric growth lines only. Remainder of shell with radial ribs. Color light-brown with a strong blush of blue-green. Interior slightly iridescent. Common offshore. Musculus discors Linne Discord Musculus Plate 28e Atlantic: Arctic Seas to Long Island Sound. Pacific: Arctic Seas to Puget Sound. I inch in length, oblong, fairly fragile. Anterior and posterior thirds of outer shell with very weak radial ribs; center section smooth except for irregular growth lines. Periostracum shiny and either dark black-brown or light-brown. Interior bluish white with slight iridescence. Commonly dredged.