On shell-interior of P. depressa, whitish projecting points of ribs have short, unglazed, chalky, pure-white central line (1), but reduced or lacking where projecting points of ribs eroded (2) [This applied to all in large sample examined but unsure if universal on P. depressa and exclusive of P. vulgata and P. ulyssiponensis.]

OTHER SPECIES ALBUMS
marinvert.senckenberg.science/image-browse/