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ARCIDAE 341
1% to 2 inches in length, oblong. Characterized by its squarish, upturned
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