An order of bivalves in subclass Lamellibranchia whose hinge dentition is characterized by a series of numerous similar alternating teeth and sockets. Typical genera of so-called ark shells are Arca, Barbatia, and Anadara, all with heavy coarse or matted periostracum, and mostly with strong radial ribs on globose or purse-shaped shells. The filibranch gills are enlarged to form the four double lamellae of the filter-feeding organs as in all true lamellibranchs, and the labial palps form complex sorting surfaces.