Arthonia atra (Pers.) A. Schreid. = Opegrapha atra Pers.

    Thallus crustose, thin, smooth, effuse, poorly delimited or rarely by a greyish prothallus (never a black prothallus!), to 2-4 cm across, potentially mosaic-forming, whitish, silver-white, pale white-greyish sometimes tinged yellow or olivaceous. Apothecia lirelliform, simple or forked, seldom stellate, in patches, usually numerous and possibly covering almost all the thallus, and mainly the center, can form, if arranged parallel, like "writing lines" or a network of dense meshes, 1-2 x 0.2-0.3 mm long and 0.1-0.15 mm tall, superficial, discs a narrow slit in the middle of lirellae, slit seldom opened, blackish. Ascospores more or less clavate, 3-septate, 14-20 x 3-5 µm according to the literature, 15-18 x 4-5 µm according to our measurements. Pycnidia immersed, black. Thallus C-, K- or seldom K+/- pale yellow, Pd-, hymenium I+ blue. Common, slightly thermophile, on smooth bark of trees.