I have handled a few hundred Pecten plica/striatus over the years, and in my
opinion there is no reliable way of separating the two consistently into two
groups, based on the number and development of ribs.  Any place you draw the
line there are intergrades between the individuals on either side of the
line.
Pecten is a masculine noun.  Therefore adjectives modifying it should have
the masculine (-us) suffix.  Placing a prefix (like decado-) before the word
Pecten doesn't change its gender.  Therefore Pecten striatus is correct,
striatus being an adjective (meaning finely grooved or lined) modifying
Pecten.  Pecten plica is also correct however, because plica is not an
adjective.  It is a feminine noun, meaning a fold or wrinkle.  When a noun
is used as the specific name following a genus name, each noun retains its
own gender ending.  Therefore plica retains the feminine (-a) ending.
Paul M.