ARCTICIDAE 381 



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 



