boat shell

from The Century Dictionary.

• noun The common slipper-limpet or half-deck, Crepidula fornicata.
• noun The English name of the shells of the genus Cymbium or Cymba, belonging to the family Volutidæ. See cut under Cymbium.

from the GNU version of the Collaborative International Dictionary of English.

• A marine gastropod of the genus Crepidula. The species are numerous. It is so named from its form and interior deck.
• A marine univalve shell of the genus Cymba.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A marine gastropod of the genus Crepidula. The species are numerous. It is so named from its form and interior deck.
• noun zoology A marine univalve shell of the genus Cymbium, syn. Cymba.