What is "littorina littorea"

Periwinkle \Per"i*win`kle\, n. [From AS. pinewincla a shellfish, in which pine- is fr. L. pina, pinna, a kind of mussel, akin to Gr. ?. Cf. Winkle.] (Zo["o]l.) Any small marine gastropod shell of the genus Littorina. The common European species ( Littorina littorea), in Europe extensively used as food, has recently become naturalized abundantly on the American coast. See Littorina.

Note: In America the name is often applied to several large univalves, as Fulgur carica, and Fulgur canaliculata.