Arthonia trifurcata (Hepp) Cl. Roux

    Thallus crustose, thin and sometimes evanescent because more or less endolithic, rough, whitish, pale grey-brownish, white-greyish, sometimes with purplish-blue tones. Lirellate apothecia, long and slim, fewly prominent and more or less burried, scattered on the thallus and not piled or stacked, simple or branched and sometimes stellate, black. Ascospores club-shaped, 3-septate, clavate, 16-24 x 5-7 µm. Photobiont: Trentepohlia. C-, K-, KC-, Pd-. On hard calcareous rocks, especially on overhangs and in locations protected from direct sun and rain. N.B. This taxon was recently individualized from the Arthonia (Opegrapha) calcarea group of which the status is still a matter of discussion and variable according to authors.*