Description: Thallus foliose-umbilicate, monophyllous, rather large, (10-)13-30(-45) mm wide, rather thin (0.24-0.42 mm thick when dry), attached by a central holdfast. Upper surface brown to dark brown, bulging; lower surface more or less concolorous with upper surface, distinctly reticulate. Upper cortex hyaline, except the brown outermost part, with an epinecral layer of compressed hyphae; medulla white; lower cortex colourless except the brown outermost part. Perithecia black, the ostiole immersed or slightly protruding, without involucrellum, the wall colourless except in the brown uppermost part. Hamathecium or periphyses and periphysoids, interascal filaments absent; hymenial jelly hemiamyloid. Asci 8-spored, clavate, the wall thickened above, not amyloid. Ascospores simple to rarely 1(-4)-septate, (14-)16-21(-26) x (5.5-)6- 8(-10.5) µm. Pycnidia rather common, black, sometimes immersed in warts. Conidia bacilliform. Photobiont chlorococcoid. Spot tests: cortex and and medulla K-, C-, KC-, P-, UV-; medulla I-. Chemistry: no lichen substances.
Note: a widespread, cool-temperate to arctic-alpine, circumpolar species found on periodically submerged siliceous rocks, in seepage tracks or along small streams, often completely inundated during summer; it also occurs in melt-water seepages below snow-beds and along lakeshores.