See text Mactra is a large 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.

[1] 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.