ARCIDAE 341 



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 



