Source : Webster's Revised Unabridged Dictionary (1913)

Mactra \Mac"tra\, n. [NL., fr. Gr. ? kneading trough, fr. ? to
   knead.] (Zo["o]l.)
   Any marine bivalve shell of the genus {Mactra}, and allied
   genera. Many species are known. Some of them are used as
   food, as {Mactra stultorum}, of Europe. See {Surf clam},
   under {Surf}.