Description: Thallus crustose, very variable, from fully endosubstratic to episubstratic, continuous or rimose, rarely areolate and up to 2 mm thick, white to beige, without a distinct prothallus. Cortex up to 100 µm thick in the areolate forms, overlain by an epinecral layer, usually with crystals; medulla white, I-. Apothecia lecideine, black, sessile and strongly constricted at base, 1-3 mm across, with a flat to finally convex, epruinose disc, and a thin, glossy, raised, often undulate proper margin. Proper exciple bright green, bluish green or brownish in outer part, colourless and with dense crystals and/or oil droplets within; epithecium bright green-black to bluish green, 15-25 µm high; hymenium colourless, 65-110 µm high, densely inspersed with oil droplets, I+ blue; paraphyses easily made free in K, mostly simple, rarely sparingly branched and anastomosing, 1.5-2.5 µm thick at mid-level, the apical cells 3-5 µm wide; hypothecium colourless to (rarely) very pale yellowish brown, usually strongly filled with crystals, without oil droplets. Asci 8-spored, clavate, with an intensely I+ blue tholus penetrated by a weakly amyloid, broadly cylindrical axial mass, and a poorly developed ocular chamber, Lecidella-type. Ascospores 1-celled hyaline, broadly ellipsoid to ovoid, 10-17(-19) x 6-10 µm, rather thick-walled. Pycnidia black, globose. Conidia thread-like, curved, 11-27 x c. 1 µm. Photobiont chlorococcoid. Spot tests: thallus K- or rarely K+ yellow, C-, KC-, P- or rarely P+ pale yellow. Chemistry: thallus with variable amounts of atranorin, zeorin, and rarely lichexanthone.
Note: a circumpolar, cool-temperate to arctic-alpine, nitrophilous lichen, one of the most common calcicolous lichens of upland areas throughout the country, reaching the nival belt in the Alps, related with L. stigmatea. For the chemistry see Knoph & Leuckert (1997, 2000).