Arca
a genus of conchiferous mollusks, the typical one of the family arcade. 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. Of the recent species. A. пoæ, A. tetragona, A. lactea, A. raridentata, and A. barbata occur in England. The fossil species are found in the United States, Europe, and southern India.
ARCACHON (ar-ka-shôn'), a bathing place which has grown up since 1854, on the S. side of the Bassin d'Arcachon, 34 miles S. W. of Bordeaux, France. Its main street stretches 2½ miles along the shore, with the pine forest immediately behind. Its numerous villas among the first are much frequented in winter by invalids afflicted with lung disease. Scientific oyster culture is practiced here on a large scale. Pop. about 12,000.