What is "venus mortoni"

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.