Cat. Lich. Univers. 8: 589 (1932).

Basionym: Placodium sorocarpum Vain., Ann. Bot. Soc. Zool.-Bot. Fenn. Vanamo 9: 320 (1929).

Thallus: crustose, endo- to episubstratal in non-sorediate parts, indeterminate, often forming small, inconspicuous patches between other lichens, sometimes spreading irregularly and discontinuously up to a dm or more across, areolate, sorediate. Prothallus indistinct or visible as a dark stain on light-coloured bark. Areolae greyish brown to brown or whitish green, discrete or becoming more or less continuous, flattened to weakly convex, irregularly rounded to somewhat elongate, up to 0.2 mm in diam. Soralia brownish due to pigmentation of the external soredia, greenish or, where the soredia have been shed, whitish, usually discrete, sometimes becoming contiguous but never confluent, mostly one, occasionally 2-3 per areola, distinctly delimited, circular in outline, either flush with the level of the thallus or, more usually, raised, sometimes doliiform, surrounded laterally by a corticate sheet which in the upper part forms a white circular rim bordering the soralia, up to 0.3(-0.5) mm in diam. Soredia fine; external soredia brown, up to 20(-30) µm in diam., surrounded by a continuous wall of more or less isodiametric (up to 70µm diam.) to somewhat elongate cortical cells; brown pigment K+ fuscous brown, N+ reddish brown. Medulla absent. Photobiont green, globose to broadly ellipsoid, up to 12(-15) µm in diam. Apothecia rare, speringly present in 3 (7%) of the specimens, up to 0.3(-0.5) mm in diam, rust red; margin indistinct; disc plane, rough, densely beset with K+ purple crystals (Sommerfeltia 14 1992: 120).

Photobiont: Green alga.

Chemistry: Roccellic acid (xxx 1992: 120).

Distribution type: : Italy, Norway, Sweden, Canada: British Columbia, Denmark: Greenland, USA: Colorado