Description: Thallus crustose, endosubstratic, or of inconspicuous granules around the apothecia, usually consisting of the apothecial margin only. Apothecia lecanorine, sessile and constricted at base, 0.3-1 mm across, with a slightly concave to finally slightly convex, yellowish grey, brown or almost black, sometimes slightly pruinose disc, and a white, more or less prominent, smooth, entire, thalline margin. Thalline exciple corticate, obscured by granules insoluble in K, but soluble in N; epithecium yellowish brown, with fine granules insoluble in K and in N (sometimes with an epipsamma insoluble in K but soluble in N); hymenium colourless or pale yellow; paraphyses slender, sparingly branched throughout, with few anastomoses, not expanded at apex, coherent in K; hypothecium colourless, often with some granules. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 8-12(-14) x (3-)4.5-6(-7) μm. Photobiont chlorococcoid. Spot tests: thallus (apothecial margin) K-, C-, KC-, P- or rarely (apothecial margin only) P+ orange. Chemistry: usually without lichen substances; some samples with 2.7-dichlorlichexanthone and pannarin in apothecial margin.
Note: most frequent in urban areas (e.g. on monuments, mortar walls, asbestos-cement) up to the montane belt; records from natural habitats and from upland areas may refer to other species, especially to M. semipallida.