Distinguished from all other species of the Pertusariales by its silver-grey thallus with isidia with dark pigmented caps and distinctive thalline chemistry (?protolichesterinic acid).
Type: Falkland Islands, West Falkland, Mt Adam, feldmark on summit ridge, 25 January 1968, Imshaug 41061 & Harris (MSC—holotype).
Thallus effuse, silver-grey, thin, usually growing over terricolous bryophytes (one collection spreading on to rock) and following the contours of the substrate; isidia usually present, often in groups of 4–5 that probably arise through branching of the ‘stalk’; originating as dark green-grey pigmented warts emerging from the thallus, finally 0.4 mm high and with a short, unpigmented ‘stalk’ 0.2 mm wide, and a wider ‘cap’, 0.3 mm across that is usually dark pigmented and in section has a dark olivaceous-brown cortex 25–30 µm thick (K+ brownish, H–, N + blue-green), occasionally this pigment is absent and then the cap is creamy white throughout; medulla I–. Photobiont trebouxioid; cells 10–15(–17) µm diam.
Apothecia rare, lecanorine, 0.5–0.8 mm diam.; disc concave, black with grey pruina; margin raised, c. 0.05 mm wide and persistent. In section: proper exciple poorly developed, hyaline. Hymenium 140–170 µm high; paraphyses simple, c. 1 µm wide; epihymenium 25–30 µm, patchily olivaceous (K+ brownish, H–, N+ greenish). Hypothecium hyaline, 25–30 µm high, composed of randomly arranged hyphae. Asci clavate; ascospores not observed (dissolved into ascoconidia) but notes on herbarium packets indicate they were hyaline, simple 1/ascus, (127–)151.00±18.20(–172) × (58–)66.167±5.63(75) µm, l/b ratio (1.867–)2.29±0.31(–2.57), (n = 6).
Conidiomata not observed; ascoconidia bacilliform 4–5 × 1–1.5, formed from old ascospores within the ascus.
Chemistry. All spot tests negative; two or three pale, creamy yellow-pink spots at Rf 6 in solvent C by TLC.
Etymology. Named for the silver-grey colour of the thallus
Distribution and Ecology. Known only from the Falkland Islands. Reported from boulder fields on mountain summits on both main islands where it overgrows bryophytes, rarely spreading onto siliceous rocks. Associated species: Coccotrema corallinum Messuti.