“Arca, Gray, l.c. 155 = Byssoarca, Swainson. Shell oblong, subquadrate, gaping beneath; hinge-margin rather produced at each end; umbo subanterior, curved; front cardinal tooth moderate, divided into small equal crests like the hinder one; the scar of the hinder pedal muscle very large, oblong, triangular, one the hinder half of the dorsal surface of the cavity of the shell. Periostraca paleaceous.
a. Teeth small. Arca Noae, A. pacifica, A. truncate, A. navicularis, A. angulata – if these presumed species are more than local varieties.
b. Teeth larger. A. zebra.
II. The hinge-line broad, more or less curved; the teeth wider at the distal ends; the crest of the inner portion small, transverse, of the distal portion larger and more or less oblique or longitudinal; the umbones more or less separated by an elongate area, marked with angular concentric cartilage-grooves, or small, with slight cross-lines.
A. Shell radiately striated or nearly smooth; lower margin entire or subdentate, sometimes gaping; cardinal teeth subequal; the umbonal area marked with angular concentric cartilage-grooves.”
(Gray, 1857: 368)
