The Pecten shell became a symbol not only for the Santiago de Compostela
pilgrimages but also for the crusades to the Holy Land.  For the latter, it
is quite clear that we are dealing with Pecten jacobaeus (treated
frequently as a subspecies of Pecten maximus), because Pecten Maximus does
not occur in the Eastern Mediterranean.  But regarding the former, it seems
that smart and eager souvenir vendors in Santiago de Compostela actually
imported pectens from other European countries farther north.  Since
Santiago itself as well as all 'European countries farther north' are on
the Atlantic coast rather than the Mediterranean, the most likely candidate
to be the Santiago pilgrimage symbol is Pecten maximus, because Pecten
jacobaeus does not occur in the Atlantic.  Because pilgrimages and crusades
are so closely related, and also because the two species are so closely
related, all of this usually gets mixed up into one single shell that is
the symbol of all holy voyages and also the Shell Oil Company (and, by the
way, also as the origin of so many pecten images in British heraldry), but
we are probably really dealing with two species that both became symbols of
various kinds in Medieval art and history.
 
Monika