The Collaborative International Dictionary
Ark shell
Ark shell \Ark" shell`\ (Zo["o]l.) A marine bivalve shell belonging to the genus Arca and its allies.
Wiktionary
ark shell
n. (context malacology English) Any of the family Arcidae of saltwater clams or marine bivalve molluscs.
WordNet
ark shell
n. marine bivalve mollusk having a heavy toothed shell with a deep boat-like inner surface