Thallus 6-8 cm diam. (to 20 (-40) cm diam. in Scandinavian material), loosely attached, forming rosettes or fragmented and irregularly spreading. Lobes to 15 mm (rarely to 30 mm) wide, ± rounded, the margins entire, undulate, rarely crenulate. Upper surface yellow-green when wet, straw-coloured when dry, smooth, glabrous, with prominent bluish green, convex, circular cephalodial warts. Medulla white. Lower surface corticate, pale brown-white at margins, with a conspicuous brownish black tomentum towards the centre. Soredia and isidia absent. Photobiont Coccomyxa (green alga) in the main part of the thallus. Cephalodia internal, lying adjacent to the upper cortex, containing Nostoc cyanobacteria. Anamorph: conidiomata pycnidia, rare, marginal, in dark brown semi-immersed warts. Conidiogenous cells 4-5 µm diam., irregular but usually ampulliform, proliferating percurrently. Conidia bacillar, 3-4 x 1-2 µm in size. Teleomorph: ascomata apothecia (not observed in British material), at the margins of the lower surface of elongate fertile lobes, which are recurved to expose the disc. Disc 15-25 µm long and 20-30 mm broad, elliptical to reniform, the disc rufous-brown. Margin ± entire with a raised thalline cuff. Asci cylindrical, tapering to a short stalk, IKI-, 8-spored. Ascospores 23-27 × 4-6 μm, fusiform-ellipsoidal, 3-septate, pale brown. Chemistry: thallus C–, K+ yellowish, KC+ yellow, Pd+ yellow; medulla UV+ (nephroarctin, phenarctin, zeorin, ± usnic acid).