Sally Thomas: I opt for a member of the bittersweet family... most likely the Atlantic bittersweet.
Sharon: The closest bittersweet in the NC Sea Grant handbook that sounds similar to the Atlantic Bittersweet, is Giant Bittersweet:
Bivalves, Ark-Shaped bittersweets (Glycymerididae)
Giant Bittersweet Glycymeris Americana (DeFrance)
Description: (4 inches) Round, somewhat flat shell. Indistinct broad radial ribs sculptured with radiating scratches. Central beak. Long, curved hinge with 19 to 24 teeth. Scalloped margin. No pallial sinus. Velvety periostracum.
Color: Grayish tan exterior, mottled with yellowish brown. Dark brown periostracum.
Habitat: Lives offshore. Dense concentrations at 75-foot depths off Cape Fear. Commonly found on beaches near and south of Cape Fear. Occasionally netted as incidental catch by offshore fishing boats.
Range: North Carolina to Florida.
Notes: It has a bitter taste, as its name implies, so it cannot be considered for commercial harvest. It has a muscular foot.
Source: Seashells of North Carolina, North Carolina Sea Grant College Program