Thallus to 5 cm diam, mosaic-forming, finely verrucose, milk-white to grey, 200-300 μm thick, without pruina or densely white pruinose, soralia 0.5-2.5 mm diam, irregular in shape, blue-grey or white, often becoming widely confluent, soredia 20-40 μm diam; Prothallus usually present, conspicuous, black, to 2 mm wide. Cortex absent but cortical gel present, 20-30 μm thick. Medulla cretaceous, milk white, densely filled with calcium oxalate crystals, 10-15 μm diam.
Anamorph: not known.
Teleomorph: ascomata unknown.
Chemistry: thallus C–, K± pale yellow, KC–, Pd+ yellow, UV± pale cream (psoromic, ± conpsoromic acids).
Very similar to Roccellographa circumscripta but ± entirely sorediate.
The sorediate Arthonia endlicheri, Dirina massiliensis forma sorediata and Llimonaea sorediata are all C+ red, Pd–.
In GB&I, common on W coasts of British Isles (N to Westerness) & Channel Islands. Also known from W Europe, Macaronesia.
Widespread under sheltered overhangs of acid coastal rock faces, often with Roccellographa circumscripta and Dirina massiliensis f. sorediata.