Description: Thallus crustose, episubstratic, rimose-areolate, pale yellow-green to whitish yellow, forming patches 1-6 cm in diam., delimited by a dark prothallus. Areoles contiguous, 0.2-0.8 mm wide, round to slightly angular, flat to usually slightly convex, smooth. Cortex without distinct structure, c. 40 µm thick; medulla white, I+ deep blue. Apothecia frequent, lecideine, arising between the areoles, round, 0.3-0,7 mm across, with a black, convex, epruinose disc, and a very thin, soon excluded proper margin. Proper exciple brownish red, K+ intensifying red; epithecium reddish brown, K- or K+ reddish violet; hymenium colourless, 70-90 µm high; paraphysoids strongly conglutinate, richly branched and anastomosing, clavate at apices; hypothecium brown, 100-200 µm high, K-. Asci 8-spored, clavate, fissitunicate, with a well-developed tholus, lacking an ocular chamber, Rhizocarpon-type. Ascospores 1-3-septate, rarely submuriform and with a single transverse septum, soon becoming dark green to brown, ellipsoid, 11-23 x 6-14 µm, halonate. Photobiont chlorococcoid. Spot tests: medulla K-, C-, KC-, P- or P+ yellow. Chemistry: rhizocarpic and psoromic acids, sometimes with traces of gyrophoric acid.
Note: on siliceous rocks in underhangs protected from rain in cold sites with frequent fog, with optimum near or above treeline.