Description: Thallus crustose, fully endolithic, usually covering large surfaces, grey, dark grey-green or blue-green, sometimes dark lead-grey, often glossy, uncracked. Upper cortex (lithocortex) densely prosoplectenchymatous, of conglutinated hyphae intermingled with microcrystals, with a green pigment; algal layer discontinuous, photobiont cells in clusters; medulla prosoplectenchymatous, sometimes with oil cells (macrospheroids) in the lower part. Perithecia completely immersed in the rock and leaving pits when they fall off. Involucrellum black, flat to slightly convex, much narrower than the body of the perithecium (0.15-0.22 μm diam.), spreading sideways but scarcely downwards, in surface view usually with 3-6 fine cracks radiating from ostiole; excipulum 0.2-0.3 mm diam., colourless to pale brown; hymenium colourless, K/I+ blue; hamathecium of periphyses in the ostiolar canal, and short periphysoids in the upper half of the perithecial cavity. Asci 8-spored, two-layered, fissitunicate, with a thin tholus reacting I-, Verrucaria-type. Ascospores 1-celled, hyaline, ellipsoid, often poorly developed, 14-21(-25) x 6-8(-12) μm. Photobiont chlorococcoid. Spot tests: all negative. Chemistry: no lichen substances.
Note: a mild-temperate lichen found on compact limestone and dolomite in sheltered situations (e.g. in forests), with optimum in submediterranean areas; in the Mediterranean belt it is confined to humid-shaded situations, reaching near treeline in the mountains of the South; forms from southern Italy with a dark grey thallus are common, and deserve further study.