Description: Thallus crustose, endo- to episubstratic in non-sorediate parts, continuous to more or less distinctly areolate, forming regular or irregular patches up to 10 cm in diam., usually delimited by a dark, bluish to brown prothallus, the blue pigment N+ red. Areoles 0.1-0.3 mm wide, convex, green to pale brown, soon bursting apically into minute, irregularly punctiform, 0.1(-0.3) mm wide, mostly flat soralia; soredia farinose, up to 20-30 μm in diam, the external ones brownish, the internal ones pale greenish. Apothecia not always present, biatorine, up to 0.6 mm across, the disc flat to strongly convex, blackish but often faintly pruinose and appearing bluish grey, lead-grey or grey-black, rarely with a brownish hue, the proper margin soon excluded. Proper exciple more or less colourless, of radiating hyphae; epithecium pale olive-green to brown; hymenium colourless to bluish green; paraphyses strongly adglutinated, sparingly branched and anastomosing; hypothecium pale olive-brown. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 9-17 x 4-7.5 µm. Photobiont chlorococcoid. Spot tests: thallus and soralia K- (or soralia K+ very pale yellow), C-, KC- P-, UV+ white. Chemistry: sphaerophorin (major), isosphaeric acid (traces).
Note: on bark, on basal parts of (mainly) coniferous trees, more rarely on lignum, often associated with Parmeliopsis ambigua, with optimum in the subalpine belt; most frequent in the Alps.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)