Cavolinia tridentata (Niebuhr, 1775) forma atlantica van der Spoel, 1974
Shell with flat dorsal side and moderately developed ribs, ventral side moderately vaulted. Light-brown hue over nearly entire shell. No keel along lateral sides. Shell margin between caudal and lateral spines concave, its upper stretch more or less straight and horizontal. Growth lines faint, transverse striation poorly developed. Tips of lateral spines directed slightly caudally. Caudal spine short and straight. Well-developed arch-shaped rib at base of dorsal lip. Shell length: 16 mm, width: 10.5 mm.