What is "mactra"

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.

n. Any marine bivalve shell of the numerous known species of genus ''Mactra'' and allied genera, some of which are used as food, as (taxlink Mactra stultorum species noshow=1), of Europe.

Mactra is a genus of medium-sized marine bivalve mollusks or clams, commonly known as trough shells or duck clams. Mactra is the type genus within the family Mactridae.

The word "trough" in the common name refers to the fact that all Mactra shells have a large ligamental pit at the hinge line, which in life contains a large internal ligament. Most bivalves in other families have an external ligament instead.