Page:American Seashells (1954).djvu/423

Rh 1% to 2 inches in length, oblong. Characterized by its squarish, up- turned posterior end; coarse, dull, flaky periostracum; large circular chon- drophore; and the coarse, oblique rib running from beak to posterior ventral margin. A4oderately common from shallow to deep water. Found in fish stomachs. Faintly MALLETIIDAE Shell not pearly inside, oval, compressed, gaping at both ends; ligament external, elongated, resting on nymphs; numerous teeth; no resilium. A linear depression extends from the umbonal cavity to the anterior muscle scar. Worldwide, usually deep water. Includes several genera and subgenera in- cluding Tindaria Bellardi 1875, Neilonella Dall 1881, Malletia Desmoulins 1832 and Frotoniiciila Cotton 1930. Genus Tindaria Bellardi 1875 Shell small, resembling a tiny Venus clam; fat; beaks facing slightly for- ward; ligament minute, external; hinge smooth, continuous just below beaks. Generally deep water and rare. Tifidaria brunnea Dall Brown Tindaria Bering Sea, Alaska, to Tillamook, Oregon. /4 inch in length, fat, moderately pointed at posterior end. Very fine concentric scratches. Exterior dark olive-brown. Interior glossy cream. Has been dredged abundantly in a few places in deep water. There are 8 other rare species on the West Coast of America. Order FILIBRANCHIA Suborder TAXODONTA Superfamily ARCACEA Key to Families a. Shell elliptical, hinge straight Arcidae aa. Shell circular or lopsidedly circular, hinge curved: b. Ligament partly sunk into shell Limopsidae bb. Ligament external Glycymeridae Family ARCIDAE The ark shells have undergone intensive study in the last few years, and the nomenclature is still not settled. It is obvious, though, that not all of the