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.