The Collaborative International Dictionary
Boat shell
Boat shell \Boat" shell`\ (Zo["o]l.)
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.
Wiktionary
boat shell
n. 1 (context zoology English) A marine gastropod of the genus (taxlink Crepidula genus noshow=1). The species are numerous. 2 (context zoology English) A marine univalve shell of the genus (taxlink Cymbium genus noshow=1), syn. (taxlink Cymba genus noshow=1).