Rhizocarpon furfurosum H. Magn. & Poelt

Thallus rimose to continuous, sometimes partly areolate near the margin, up to 5 cm diam. or several thalli confluent; hypothallus poorly developed, black; areolae up to 1 mm diam., contiguous, brownish grey to dark grey, dull, mainly angular, densely covered with short, fragile isidia or granular soredia; medulla KI–. – Apothecia not common, up to 0.8 mm diam., black, epruinose, orbicular, persistently plane or becoming weakly convex, with a thick and more or less persistent margin and an often umbonate disc; excipulum dark brown in the rim, inner part paler brown, containing crystals partly dissolving in K, K+ yellow; hypothecium dark brown, K–; hymenium colourless or pale brown; epihymenium medium brown, containing crystals dissolving in K, K+ yellow; ascospores 8 per ascus, muriform, persistently colourless, 14–24 × 7–12 µm. – Conidiomata not seen.

Stictic acid; spot tests: medulla and exciple PD+ orange, K+ yellow, C–.

On siliceous rock rich in iron minerals.

Rhizocarpon furfurosum is the only non-yellow, isidiate or sorediate species of the genus.