Anadara brasiliana Lamarck, 1819
The well-inflated (highly-rounded) Incongruous Ark is unusual in that the bivalve's basal ventral margin, or edge, curves gently instead of forming a straight line. This curve can be seen by holding the complete bivalve, both halves joined, with the beaks facing away from yourself. You can see a slight curve on the bottom edge.
The two-inch white shell can be found on beaches from North Carolina to Brazil. The exterior is scored with broad, beaded ribs. The beak is centered with the smaller hinge teeth closest to it, and the larger hinge teeth out farthest on the hinge line.
Though this is a commonly-found shell, one should not ignore its beauty.
(The photo shows two white Incongruous Ark halves and one discolored valve. Note especially the beading on the darker shell.)