arc a

[Lat. arc a = a chest.] A genus of Conchiferous Molluscs, the typical one of the family Arcadæ. The shell is strongly ribbed, or cancellated, hinge straight, with very numerous transverse teeth. They are universally distributed, but are commonest in warm seas. They inhabit the zone from low water to 230 fathoms. In 1875, Tate estimaten the known recent species at 140, and the fo il ones at 400, the latter commencing with the Lower Silurian rocks. Of the recent species, A. Noœ, A. tetragon a, A. lactea, A. raridentata, and A. barbata occur in Britain. The fo il species are found in the United States, Europe, and Southern India.