Pertusaria hymenea (Ach.) Schaer.

    Thallus crustose, rather thin, somewhat waxy, smooth becoming coarse, more or less tuberculate and cracked, without prothallus, greyish, grey-greenish when dry, greenish when moist, covered with numerous fertile warts, flat to slightly constricted towards the base, often distorted due to crowding, 1-2 mm diam.. Apothecia punctiform, 1-3 (4) per fertile wart, disc pinkish to concolourous to the thallus, turning to blackish during the course of maturation. Ascii 8-spored, 245-350 x 40-80 µm according to our measures. Ascospores simple, colourless, narrowly ellipsoid, walls 6-20 µm thick, 60-110 x 30-50 µm according to literature, 80-120 x 40-55 µm according to our measures. Cortex: C+ yellow, K-, KC+ orange, Pd-, UV+ pale orange. Common, not especially maritime, on bark of deciduous  trees. N.B. This species is highly variable according to its micro-habitat: in shaded areas, apothecia are widely opened with pale discs and potentially suggest a Lecanora species, whereas in well-lit and dryer areas apothecia are less opened with dark or black discs. N.B. Not to be confused with Thelotrema lepadinum of which the apothecia have a different structure.