Page:American Seashells (1954).djvu/463

shells are smooth and round. The hollow is covered over by a sheath of peri- ostracum. Hinge of adult with large triangular tooth in left valve which fits snugly between tM^o smaller ones in the right valve. Found in shallow water under stones. External sculpture of scaled ribs and concentric ridges. Color light-brown. Shell thick, translucent glaze inside. Fimtily CORBICULIDAE Genus Folymesoda Rafinesque 1820 Folymesoda caroliniana Bosc Carolina Marsh Clam Plate 3obb Virginia to north half of Florida and Texas. I to I % inches in length, about as high, subtriangular in outline, rather obese and with a strong shell. Exterior of smoothish shell is covered with a very fuzzy or minutely scaled periostracum which is mostly glossy-brown and rather thin. Interior white, rarely stained with purple. Each hinge with 3 small, almost vertical, equally sized teeth below the beaks and each hinge with I anterior and posterior lateral. Ligament external, long, narrow and dark-brown. Common at the mouths of rivers where the influence of the tides is felt. Genus Pseudocyrena Bourguignat 1854 Pseiidocyrena fioridana Conrad Florida Marsh Clam Plate 3oy Key West to northern Florida and to Texas. I inch in length, quite similar to Folymesoda caroliniana, but more vari- able in shape (ovalish to elongate), without the fuzzy periostracum, with its beaks never eroded away and with 2 long, slender anterior and posterior laterals. Exterior with irregular growth lines, dull dirty-white, commonly flushed with purple or pink. Interior white with a wide margin of deep purple or entirely purple. Brackish warm water in mud. Common. Family ARCTICIDAE Genus Arctica Schumacher 1817 (Cyprina Lamarck) Arctica islandica Linne Ocean Quahog Plate 32f Newfoundland to oflF Cape Hatteras, North Carolina. 3 to 5 inches in length, almost circular in outline, rather strong, porcel- laneous, but commonly chalky. Exterior covered with a brown to black, rather thick periostracum. The posterior laterals and the absence of a pallial