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The Collaborative International Dictionary
topshells

Trochus \Tro"chus\, n.; pl. Trochi. [NL., fr. Gr. ? a wheel.] (Zo["o]l.) Any one of numerous species of marine univalve shells belonging to Trochus and many allied genera of the family Trochid[ae]. Some of the species are called also topshells.

Wiktionary
topshells

n. (plural of topshell English)