I have located a possible semi-positive identification, in Usbourne Spotter's SHELLS. Two such shells with the shape as seen in the photograph. Both are called "limpets".
Keyhole limpets "Unlike other limpets, most keyhole limpets have a small hole in the top of their shell. They have to stay underwater so that they don't dry out."
Classed among Gastropods
Top-Shaped - keyhole limpets (Fissurellidae)
Cayenne Keyhole Limpet Diodora cayenesis (Lamarck)
Description: (1 inch) Shell shaped like a small, low cone or a coolie hat. Many ribs radiating from the small, subcentral keyholelike opening on top of shell. Inside of keyhole opening outlined by a truncate callus with a deep pit on its concave edge.
Color: Exterior white and pinkish gray or brown. Interior white to gray.
Habitat: Lives in inlets and offshore waters attached to rocks or shells. Occasionally found on sound and ocean beaches.
Range: New Jersey to Brazil.
Notes: Also called a little keyhole limpet. A herbivore, it uses radula to scrape algae off of rocks. Its powerful foot creates strong suction to keep waves from washing it off the rocks. Water enters under the edge of the shell and exits through the "keyhole" near the peak. Its eggs are yellow and stick to rocks. Hatched young crawl away.
Source: Seashells of North Carolina, North Carolina Sea Grant College Program