Thanks to all who answered my request. (The website Wolfgang indicated is
really very useful). From what I have seen, I believe that the Atlantic
forms (M. cinerea) should be regarded as a separate species, unless there
were intermediate specimens in some overlap zone.
What do the nomenclature rules state when the original author (the great
Linné himself in this case) names two taxa (here: "Cardium" stultorum and
corallinum), but subsequent authors lump them together? Which name has
priority?

Greetings from Germany
Michael