Squamules: up to 4 mm diam., when young isodiametrical and adnate with ascending margin, later becoming elongated and ascending, thin. Upper side: medium brown, dull, epruinose or faintly white pruinose, smooth or with a few shallow fissures; margin: white, entire or deeply lobate; underside: pale brown to white. Upper cortex: 40-80 μm thick, composed of rather thick-walled hyphae with round to angular lumina, norstictic acid present or absent in the stainable layer, calcium oxalate usually present in the epinecral layer. Medulla: thin, partly almost absent, containing calcium oxalate and norstictic acid. Lower cortex: well developed, not always sharply delimited from medulla, composed of mainly anticlinally oriented hyphae densely covered with calcium oxalate and with rather shortly cylindrical lumina. Apothecia: up to 1.3 mm diam., attached laminally to the squamules, simple or more rarely compound, soon becoming more or less hemispherical, immarginate, brownish black to black, dull, epruinose. Ascospores: 11-13 x 5-7 μm. Pycnidia: not seen. Thallus chemistry: norstictic acid and zeorin.
The species may be confused mainly with P. nipponica and P. vallesiaca. The former differs in having larger, thicker, and less pruinose squamules containing gyrophoric acid, lecanoric acid, and rarely anthraquinones. The latter differs in having more adnate, thicker squamules with a poorly developed lower cortex, and lacking zeorin. Psora tenuifolia is, with the exception of the occassional traces in P. californica, the only species in the genus known to contain a triterpenoid (zeorin).
Arctic and northern boreal regions of Asia and North America.
The species is terricolous.
Psora nipponica.
