Porpidia cinereoatra (Ach.) Hertel & Knoph

    Thallus thick, cracked to verrucose and areolate, areoles to 0.5-2 mm across, flat to convex, dull white, dull cream, pale greyish, distinctly delimited by a black, more or less important prothallus, evidencing colonies of this species. Apothecia 0.5-1.5 mm diam., isolated or in small clusters, most often immersed in the thallus, disc black, flat or convex, covered with a brey-blue pruine, at least when young, exciple black excluded when mature. Ascospores simple, colourless, ellipsoid, 12-18 x 6-10 µm. Photobiont: Chlorococcoid. Chemical spot tests negative. Not especially maritime, but rather fequent on exposed acid rocks. Not to be confused with Porpidia macrocarpa of which the apothecia are larger and not covered with grey-bluish pruine, and with  Porpidia platycarpoides that displays a more greyish and thinner thallus with large sessile and not immersed apothecia. N.B. Species also very close to Porpidia irrigua whose apothecia are sessile. If it occurs on calcareous or very little acid rocks see Porpidia speirea which is I +.