Protoblastenia rupestris (Scop.) J. Steiner

    Thallus crustose, thin and superfical, more or less areolate, areoles polygonal, pale grey-yellowish, pale grey-brownish, grey-greenish. Apothecia numerous, rather evenly distributed over the thallus, 0.3-0.8 mm diam., sessile - and not in a well-, flat becoming markedly convex up to a wart-shaped stage, brown-reddish, bright orange, sometimes orange-greenish when shaded, exciple thin soon excluded. Ascospores simple, ellipsoid,  8-17 x 5-8 µm. Photobiont: Chlorococcoid. Thallus : C-, K-, KC-, Pd-; apothecia : K+ purple. Not especially maritime, on calcareous rocks or walls, common in the maritime zone notably on mortar or concrete, notably of blockhaus. N.B. Another form may be found at the basis of old humid and shaded walls, Protoblastenia rupestris forma with a leprose thallus, white-greenish and with more flattened apothecia. Not to be confused with some Caloplaca species with flat apothecia, distinct exciple, greyish thallus and polarilocular ascospores. See also Protoblastenia calva which is much more scarce on only found on hard limestone rocks, with a very thin evanescent thallus, whitish to greyish (due to cyanobacteria), and larger globose apothecia, often with a blackish ring towards the base.