This is a Marsh Periwinkle, Littorina irrorata.
5/8-1 1/2" (1.6-3.8cm) high. These were found on the high tide line on Ocean Isle Beach, NC. Broadly ovate, think, sharply pointed except when eroded; Whorls smooth, slightly convex. Columellar area and inside of outer lip whitish, the latter with a dark margin and dark brown deep within. Habitat: on rocks intertidally. In Europe, this very common and abundant shell is still gathered and eaten by many people. Information from "The Audubon Society Field Guide to North American Seashells pages 401-2.
The Florida Melampus, Detracia floridana is found at the high tide line. (5/8 ") Small, cone-shaped shell with a short, blunt spire. Long, narrow aperture with front end expanded. Color: Light to dark brown, occasionally with three to four darker-colored spiral bands. Also called a salt-marsh snail, this family has a primitive lung in place of gills and breathes air. The snail eats decaying plant matter but is believed to get its nutrients from the bacteria that live on decaying matter. Information found in"Seashells of North Carolina - NC Sea Grant College Program Porter and Houser". 
