boat shell

from The Collaborative International Dictionary of English v.0.48

Boat shell \Boat" shell`\ (Zool.)
   (a) A marine gastropod of the genus {Crepidula}. The species
       are numerous. It is so named from its form and interior
       deck.
   (b) A marine univalve shell of the genus {Cymba}.
       [1913 Webster]