Description: Thallus crustose, endosubstratic or thinly episubstratic, white to pale grey, continuous to rimose, rarely minutely granular. Apothecia lecideine, (0.2-)0.5-0.8(-1) mm across, at first yellowish brown, then brown to brown-black, with a flat to finally convex, epruinose disc, and a usually darker, finally sometimes excluded proper margin. Proper exciple brownish in outer part, colourless or pale yellow within, 60-95 μm thick laterally, 100-125 μm thick at base, of radiating hyphae, usually with a single row of up to 6 μm thick, rounded cells along the outer edge, the pigmented parts K- or K+ yellow; epithecium yellow-brown brownish or olive-brown, K-, N-; hymenium colourless, 45-70(-90) μm high, I+ blue, K/I+ deep blue; paraphyses 0.8-1.5 μm thick at mid-level, simple or irregularly forked in upper part, the apical cells up to 2.5(-4) μm wide; hypothecium colourless or yellow-brown and K+ intensifying yellow in upper part, 20-35 μm high. Asci 8-spored, clavate, surrounded by a gelatinous I+ blue coat, with a well-developed I+ blue tholus, a I+ darker blue tube and a well-developed ocular chamber, Bacidia-type. Ascospores, 3-7-septate, hyaline, acicular. (32-)35-55(-70) x 1.2-2(-2.5) μm. Pycnidia rare, inconspicuous, immersed. Conidia filiform, curved, 9-17 x c. 0.7 μm. Photobiont chlorococcoid, the cells 5-12 μm in diam. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: thallus without lichen substances.
Note: a mild-temperate to humid subtropical species found on bark of broad-leaved trees (especially Acer, Fraxinus and Populus) in open deciduous woodlands near rivers, very rarely calcicolous or muscicolous.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)