The Collaborative International Dictionary of English v.0.48:

Mactra \Mac"tra\, n. [NL., fr. Gr. ? kneading trough, fr. ? to
   knead.] (Zool.)
   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.
   [1913 Webster]