Page:American Seashells (1954).djvu/569

Rh I inch in length, thin but not too fragile, opalescent-white with a brownish border of periostracum. Interior opalescent. Valve semi-circular in outline. Right valve almost flat and with a single, fairly large tooth which juts laterally. Left valve moderately convex. The posterior dorsal margin is almost straight, the posterior end somewhat drawn out into a rostrum. Moderately common from lo to 75 fathoms. Pandora bilirata Conrad (Alaska to Lower California. Dredged) is half as large, not rostrate posteriorly, and with 2 strong radial ribs on the posterior dorsal margin of the left valve. Pafidora grantilata Dall (Santa Barbara south to La Paz, Mexico) is much like bilirata, but half its size, more elongate, and the 2 radial ribs are granulated. Family THRACIIDAE Genus Thracia Blainville 1824 Shells up to 4 inches in size, thin, chalky in texture, beaks so close that the right one becomes punctured by the left beak; ligament external; shell commonly moderately rostrate at the posterior end; right valve fatter than the left. Thracia conradi Couthouy Conrad's Thracia Plate 2 8y Nova Scotia to Long Island Sound, New York. 3 to 4 inches in length, about % as high. Valves obese and chalky-white. Hinge without teeth, only thickened considerably behind the beak and be- low the large, wide, external ligament. Right beak always punctured by the beak in the left valve. Pallial sinus U-shaped but not very deep. Posterior end of valves shghtly rostrate and with a weak radial ridge. Rarely washed ashore. Not uncommon offshore and down to 150 fathoms. Thracia trapezoides Conrad Common Pacific Thracia Alaska to Redondo Beach, California. 2 inches in length and not quite so high; thin, chalky, and with the posterior end broadly rostrate. The beak of the right valve has a hole punc- tured in it by the beak of the left valve. The posterior rostrated part of the valve is set off by a broad, radial, depressed furrow which is bordered by a low, rounded, radial ridge. Color drab, grayish white. Commonly dredged off the west coast. Compare with T. curta Conrad.