Opegrapha atra Pers.
Thallus crustose, thin, smooth, effuse, poorly delimited or rarely by a greyish 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, 1-2 x 0.2-0.3 mm long and 0.1-0.15 mm tall, superficial and little immersed, discs a slit in the middle of lirellae, slit seldom opened. Ascospores more or less clavate, 3-septate, 14-20 x 3-5 µm. Pycnidia immersed, black. Thallus C-, K- or seldom K+/- pale yellow, Pd-, hymenium I+ blue. Common, slightly thermophile, on smooth bark of trees.*