Glycymeris americana Defrance, 1829
The American Bittersweet varies in size depending upon whether it lives in cooler or warmer water. In the southern section of its range the bivalve may reach a length of almost 5 inches (it is sometimes called the Giant American Bittersweet), whereas in the more northern reaches of its range it is only about ½-inch long. Usually a beachcomber will find only one valve of this mollusk, rather than both halves of the shell.
The circular, somewhat compressed shell is a dull, creamy white, mottled with yellowish-brown. It has low, rounded radial ribs. The beak points straight down. The hinge is slightly curved with a row of hinge teeth. The teeth are faint or there are none at all directly below the beak. The posterior muscle scar usually has a built-up shelly ridge, and there is no pallial sinus. The inner margins of the shell are slightly crenulate (serrated-looking). The valves are solid and definitely would not be thought of as fragile or thin.
The American Bittersweet Clam lives, burrowed in gravel or sand, in moderately shallow water from Virginia to Texas and Brazil. There are approximately 150 species of Bittersweets worldwide.