Porpidia hydrophila (Fr.) Hertel & A J Schwab

    Thallus crustose, smooth to minutely cracked, pale grey, pale grey-bluish often tinged with pale green or pale orange, prothallus black, more or less distinct but conspicuous in case of mosaics. Apothecia more or less abundant, flat then convex or rounded, disc black, often pruinose very rough when old, exciple more or less crenulate, inconspicuous at the end, apothecia cross-section view reveals a blue-green epithecium. Pycnidia, black, usually numerous. Ascospores simple, ellipsoid, 17-23 x 6-9 µm. Photobiont: Chlorococcoid. Chemical spot tests negative. Rather rare, on dripping acid rocks: waterfalls, sources, walls and cliffs. Quite similar to Porpidia cinereoatrata though differing by its habitat and the blue-green cross-section of apothecia  and also to Porpidia macrocarpa of which apothecia are larger. N.B. Rather easy to distinguish when growing together with Ionaspis lacustris or Ionaspis ceracea  which is rather common or with  Ephebe lanata et Rhizocarpon lavatum (see pictures).