About Crepidula fornicata and its appearance in Europe.
 
Crepidula fornicata Linné, 1758 was imported in England together with
Amarican oysters about 1880. These date can be found in the following
literarure aacording to the references given in the book  I consulted. I
have not seen these references, but I'm pretty sure they will content some
interesting data . Crepidula fornicata was reproted before 1940 from the
Netherlands, so there was no need  for Crepidula fornicata to use the fleet
for the D-day campain to cross the Manche.
Robson, G.C., 1915: On the extension of the range of the Amarican Slipper
limpet (Crepidula fornicata) on the east coast of England, Ann. Mag. Nat.
Hist. (8) vlol 16 p 496-499.
Burton, E.St.J., 1930: The distribution of the Slipper limpet (Crepidula
fornicat L.) in Britain. - Proc. Bournemouth Nat. Sci. Soc. Vol. 22 p 56-58.
So Linné's description was based on an American specimen if these data are
correct, you will have to see the original description  to find something
about the place from which this specimen originates.
In Europe we have several American species: Mya arenaria, Petricola
pholadiformis, of course Crepidula fornicata, the last one is Ensis
americanus, which can be found in thousends on the beach. Even the large
Mactra form the American East coast was life reported from Terschelling (The
Netherlands) in one specimen but this species not found here anymore.
                                        Martin Cadée, The Netherlands.