资料来源 : Webster's Revised Unabridged Dictionary (1913)

Ark shell \Ark" shell`\ (Zo["o]l.)
   A marine bivalve shell belonging to the genus {Arca} and its
   allies.

资料来源 : WordNet®

ark shell
     n : marine bivalve mollusk having a heavy toothed shell with a
         deep boat-like inner surface