The Collaborative International Dictionary of English v.0.48:

Trochus \Tro"chus\, n.; pl. Trochi. [NL., fr. Gr. ? a wheel.]
   (Zool.)
   Any one of numerous species of marine univalve shells
   belonging to Trochus and many allied genera of the family
   Trochidae. Some of the species are called also topshells.
   [1913 Webster]