Description: Thallus crustose, episubstratic, continuous to rimose-areolate, thin to moderately thick, pale white to greenish grey, dull, smooth, epruinose, sometimes delimited by a dark prothallus. Apothecia lecideine, 0.5-1.5 mm across, with a black, flat to rarely convex, epruinose disc, and a thick, persistent, raised proper margin. Medulla non-amyloid, I-. Proper exciple of radial hyphae, darker brown in outer part, extending from the deep reddish brown hypothecium; epithecium brown; hymenium colourless, (70-)90-120 µm high, strongly inspersed with oil droplets; paraphyses simple to moderately branched, apically swollen, with a brown pigment cap; hypothecium brown. Asci 8-spored, clavate, Bacidia-type. Ascospores 1-septate, but sometimes with two additional false septa, soon brown, narrowly ellipsoid with pointed ends, sometimes slightly curved, (12-)17-25(-30) x (5.5-)7-10(-13) µm, the wall thin (but thickened at septum) and not ornamented. Pycnidia rare, black, globose, unilocular. Conidia bacilliform, straight. Photobiont chlorococcoid. Spot tests: K+ yellow then brownish, C-, KC-, P-, UV-. Chemistry: atranorin, plus traces of fulgidin and fulgoicin.
Note: a holarctic, humid subtropical to southern boreal-montane lichen found on smooth bark in rather humid woodlands, especially in open beech forests of the montane belt. Some forms with pluriseptate spores from coastal Tyrrhenian Italy perhaps deserve further study: they might prove to belong to a distinct taxon, corresponding to Lecidea parasema sensu De Not., which has a different chemistry (Giralt & al. 2000). The species is included in the Italian red list of epiphytic lichens as “Near-threatened” (Nascimbene & al. 2013c).