Thallus 3-4(-9) cm diam., forming compact rosettes or dispersed and fragmentary, pale grey-brown to deep brown-black depending on degree of exposure and hydration, usually closely appressed. Lobes 2-6 mm wide, fragile, thin, spreading, smooth, even or partly undulate, without sulcae or reticulations, soredia or isidia, but developing scattered ± appressed or ascending crenulate laminal folioles 1-2 mm wide, occasionally so numerous as to form cauliflower-like clusters partially obscuring the underlying lobe and thallus, especially on older parts of the thallus. Lobe margins entire, often ascending and crisped, frequently developing horizontal or ascending folioles which are scattered or form dense imbricate clusters. Medulla pale yellow to yellow-orange, K+ violet to purple-red, composed of interwoven hyphae ca 4 µm diam., ± thickly encrusted by acicular pale yellow crystals. Lower surface matt, smooth to ridged-undulate, without a tomentum, pale at the margins, becoming brown-black towards the centre. Photobiont cyanobacteria (Nostoc). Anamorph: conidiomata not observed. Teleomorph: ascomata apothecia, rare, similar to those of N. laevigatum but smaller, not observed in British material. Discs to 3 mm diam., brown-black, the margins entire, dorsal surface above the apothecia faintly honeycombed. Hymenium ca 70 um high, ± conglutinate, with a red-brown epithecium. Paraphyses unbranched. Hypothecium ca 45 um thick, pale straw to colourless, compact, small-celled. Ascospores 18-21(-22) x 5-6um., red-brown, 3-septate. Chemistry: medulla with the same anthraquinones as N. laevigatum, but a wider range of hopane triterpenoids: ± T2, T3, ± T4, ± T6.