Description: Thallus crustose, fully endosubstratic, white, pale grey or pale ochre-coloured, usually poorly evident and weakly differentiated, filled with crystals of calcium oxalates. Apothecia lecideine, 0.3-0.8 mm across, deeply immersed in pits in the rock, with a flat, dark red-brown to black (reddish brown when wet), rarely thinly white-pruinose disc, and a thin, black, usually persistent proper margin. Proper exciple usually thin and poorly developed, reddish brown in outer part, almost colourless within, of radially arranged hyphae; epithecium yellowish brown to brown; hymenium colourless, 55-150 µm high, I+ partly pale blue; paraphyses sparingly branched and anastomosing, septate but not moniliform, 2-4.5 µm thick at mid-level, scarcely or not swollen at apex; hypothecium colourless to pale brown, I-. Asci 8-spored, elongate-clavate, with a thin, outer amyloid layer and a thickened tholus penetrated by a pore, the sides of which stain I/KI+ deep blue, Porpidia-type. Ascospores 1-celled, hyaline, ellipsoid, 10-16(-19) x 6-9 µm, with a > 1 µm thick, gelatinous perispore when young, the wall of old spores with a warted ornamentation. Pycnidia black, laminal or marginal and arranged in rows, partially immersed. Conidia bacilliform 3-8 x 1-2 µm, with apical and lateral conidiogenesis. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a widespread, temperate to southern boreal-montane lichen found on a wide variety of calciferous rocks, especially limestone, with a wide altitudinal range.