Shell small, subtrapezoidal, inflated, inequilateral, right valve smaller, more strongly sculptured, clasped by the left; ventral margin sinuate, crenulate within; hinge straight, median third vertically striated, broader at the ends; anterior and posterior with few, irregular, horizontal, or oblique, striated teeth; radial ribs crossed by growth striae. Distribution: Antarctic, Australia, New Zealand. Type species: Arca pectunculoides Scacchi.

Bathyarca Kobelt, 1891. Type species (SD Verrill and Bush, 1898): Arca pectunculoides Scacchi, 1834. Pliocene, Mediterranean. Shell small, inflated, ovate to elongate. Periostracum thin, adherent, sometimes hirsute. Umbones anterior to subcentral, prominent. Cardinal area short, narrow. Sculpture feeble, with radial and commarginal striae. Hinge line nearly straight; teeth in two series; distal teeth may be horizontal. Inner fold of mantle extensible, acting as siphons. Ctenidia large. Labial palps small. The reduced dentition and its horizontal orientation distally would appear to relate this genus to the extinct cyrtodontids, but the shell microstructure and the anatomy place it with the arcids. The genus appears to be a comparatively recent adaptation to the deep-water habitat and is known from the late Cretaceous. Members of the genus represent a significant biomass in many soft abyssal sediments, and some species are very widely distributed in several ocean basins. The name is derived from the Greek bathys, deep, and the Latin arca, an ark shell, and is feminine. Arnaud (1978) argued that Arca grenophila Risso, 1826, is the oldest name for the type species of this genus, but Salas (1996) has shown otherwise. We acknowledge the advice of Anders Warén on this genus.