Description: Thallus crustose, thinly episubstratic, smooth to winkled, pale grey to green-grey. Apothecia biatorine, <0.4 mm across, pale yellow to rarely almost white, thinly pruinose, with a convex disc and a soon excluded proper margin. Proper exciple thin, of radially arranged hyphae, almost colourless in section, K-, with small crystals soluble in K; epithecium colourless or yellowish, with small crystals soluble in K; hymenium colourless, I+ blue; paraphyses mostly simple, c. 1.3 µm thick at base, the apical cells more or less clavate, 2-3.5 µm wide; hypothecium colourless. Asci 8-spored, clavate, with a K/I+ blue apical dome penetrated by a narrow, K/I– apical cushion surrounded by a narrow, deeply K/I+ blue zone, the wall K/I- but surrounded by an I+ red-brown, K/I+ blue outer layer, the ocular chamber relatively small, Biatora-type. Ascospores 3-septate, hyaline, bacilliform, 9-18 x 2-3 μm. Pycnidia half-immersed in thallus, very pale yellow in upper part, colourless below, the wall inspersed with pale yellow crystals in upper part. Conidia bacilliform, narrowly ellipsoid, or narrowly drop-shaped, 4-7.5 x l.2-1.5 μm. Photobiont chlorococcoid. Spot tests: thallus K- or rarely K+ very pale yellow, C- or C+ pale yellow, KC-, P-. Chemistry: low amounts of usnic acid and zeorin in thallus, and large amounts of usnic acid in apothecia.
Note: on the acid bark of broad-leaved and especially coniferous trees, widespread in the Holarctic region especially in the boreal zone, with a few records from the Alps outside Italy, some of which not far from the Italian border; to be looked for in the Alps.