Description: Thallus crustose, episubstratic, pale grey to almost white, smooth to usually warted, forming patches to 5 cm diam, sometimes delimited by a blackish prothallus. Apothecia common, biatorine/zeorine, sessile and constricted at base, (0.5-)1-2(-2.5) mm across, with a rust red, usually flat disc and a concolorous, persistent, often flexuose proper margin. Proper exciple prosoplectenchymatous, non-amyloid, K+ red, C+ purple in section; epithecium orange-brown, granular, K+ red, C-; hymenium colourless, (60-)70-100(-130) µm high, not inspersed with oil droplets, amyloid; paraphyses simple or sparingly branched in upper part, 1.5-2 μm thick, the apical cells 2.5-3 μm wide; hypothecium colourless, not amyloid. Asci 8-spored, cylindrical-clavate, Teloschistes-type. Ascospores 1-septate, polarilocular, hyaline ellipsoid, 12-18 x 6-10 μm, the septum 4-6(-8) μm thick. Pycnidia red in apical parts. Conidia bacilliform, rarely narrowly ellipsoid, 3-5 x 1-1.5 μm. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-; apothecia K+ red; pycnidia and apothecial margin K+ red, C+ purple in section. Chemistry: non-chlorinated non-anthraquinones in apothecial disc; chlorinated anthraquinones in exciple and pycnidial wall; thallus without or rarely with traces of anthraquinones; Cinereorufa-green in prothallus.
Note: a mild-temperate species, with optimum on oaks in the submediterranean belt, widespread throughout the country, but absent from heavily disturbed areas.