Thallus 3-6(-25) cm diam., foliose, irregularly plate-like, dull black-brown, often densely white, ± scabrid-pruinose when dry, green-brown when wet, attached by a stout, ± central umbilicate stalk. Upper surface densely pustulate, with numerous conspicuous, crowded, convex, round to oval pustules, especially towards the centre of thallus, flattening towards the margins which become ± eroded and lacerated, rarely perforated. Isidia coralloid, occasionally absent, as black clumps and tufts, especially around holes, tears and cracks. Upper cortex composed of densely aggregated hyphal tissue with short cells and ± isodiametric lumina. Lower surface grey, brown or black, roughened, with broad, excavate depressions corresponding to pustules on the upper surface, without rhizines, the lower cortex composed of densely aggregated cartilaginous hyphal tissue. Photobiont chlorococcoid.
Anamorph: conidiomata pycnidia, 150-180 μm diam., immersed with a brown wall of angular isodiametric cells. Conidia 2.5-3 × ca 1 μm, bacilliform.
Teleomorph: ascomata apothecia, very rare, 1-3 mm diam., sessile or ± stalked. Thalline exciple absent. Margin (true exciple) persistent, smooth or partially isidiate. Disc black, flat, smooth, epithecium black-brown, undulating; hymenium ± 100 μm in depth; hypothecium to 160 μm thick, dark brown. Interascal tissue of simple or branched paraphyses with enlarged apical cells. Asci 90-100 × 4-5 μm, elongate-clavate, thick-walled, apical dome K/I+ blue, 1(-2) spored. Ascospores 28-70 × 18-34 μm, becoming brown, muriform.
Chemistry: Medulla C+ red, K–, KC+ red, Pd–, UV– (gyrophoric acid).
Assessed by Woods & Coppins (2012) as of Least Concern. Widespread but rather local in distribution.
Easily distinguished from species of Umbilicaria by its thallus with a strongly pruinose upper surface with conspicuous domed to hemispherical pustules and ± numerous coralloid clusters of black isidia.
W & N Britain (Dartmoor, Lake District, Welsh Marches, N Wales, Scotland), extending to E England (rare); in Ireland mainly in the SW, but also Antrim, Galway and Wicklow.
On nutrient-, and often mineral-rich, siliceous rocks, standing stones, birds’ perching rocks and boulders. Sometimes forming spectacular swards of thalli in nutrient-enriched seepage tracks on steep rock faces and dry stone walls