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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
Virginia to north half of Florida and Texas.
Plate 3obb
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
Key West to northern Florida and to Texas.
Plate 3oy
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
Newfoundland to oflF Cape Hatteras, North Carolina.
Plate 32f
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