Hi Tom,
Yes, Pecten (Decadopecten) plica (Linne.) and P. striatus (Schumacher) are
impossible to separate on the basis of rib counts, for exactly the reason you
mentioned - the abundance of intergrade forms.  And there are few if any other
reliable criteria for separating them.  For that reason, many authors now
consider them synonymous, under the Linnaean name P. plica.  In the Guidebook
to Pecten Shells, Rombouts, in his description of P. plica, says "D. striatus
(Schumacher, 1817) is very much the same if not synonymous with D. plica.  It
is claimed that D. striatus has less radial folds, but there are
intermediates, so eventually, no doubt, they will all be called D. plica."
Paul M.