Thallus areolate to rimose, up to 5 cm diam.; prothallus often poorly developed, black, usually faintly pruinose; areolae up to 0.5 mm diam., medium grey, dull, contiguous, angular, plane to weakly convex or often verrucose; medulla KI–. – Apothecia up to 1 mm diam., black, epruinose, often immersed among the areolae, orbicular to angular, remaining more or less plane, usually with an indistinct margin; excipulum narrow, brownish or partly greenish black in the rim, inner part sometimes paler brown, K–; hypothecium dark brown, K–; hymenium colourless; epihymenium greenish to reddish brown, K– or K+ faintly red; no crystals or granules in the apothecia; ascospores 8 per ascus, muriform, persistently colourless or becoming faintly brown with age, 25–32 × 11–16 µm. – Conidiomata not seen.
No lichen substances.
On siliceous rock in the spray and inundation zones by streams and lakes. Sparingly collected.
The species may be confused with R. reductum, but differs mainly in having partly immersed, more angular apothecia with an indistinct margin and somewhat less septate ascospores.