What is "hard clam"

Quahog \Qua"hog\, Quahaug \Qua"haug\, n. [Abbrev. fr. Narragansett Indian poqua[^u]hock.] (Zo["o]l.) An American market clam ( Venus mercenaria). It is sold in large quantities, and is highly valued as food. Called also round clam, and hard clam.

Note: The name is also applied to other allied species, as Venus Mortoni of the Gulf of Mexico.

The hard clam ( Mercenaria mercenaria), also known as a quahog (or quahaug), round clam, or hard-shell (or hard-shelled) clam, is an edible marine bivalve mollusk that is native to the eastern shores of North America and Central America, from Prince Edward Island to the Yucatán Peninsula. It is one of many unrelated edible bivalves that in the United States are frequently referred to simply as clams, as in the expression " clam digging". Older literature sources may use the systematic name Venus mercenaria; this species is in the family Veneridae, the venus clams.

Confusingly, the "ocean quahog" is a different species, Arctica islandica, which, although superficially similar in shape, is in a different family of bivalves: it is rounder than the hard clam, usually has black periostracum, and there is no pallial sinus in the interior of the shell.