Thallus crustose, conspicuous, dark grey to grey-brown, in exposed situations sometimes almost black, areolate. Areoles flat to convex or bullate, (0.2-) 0.3-0.6 (-0.8) mm diam. Prothallus often well-developed, black. Photobiont Trebouxia.
Anamorph: conidiomata pycnidia, immersed in the thallus. Conidia 5.6-6.5 × ca 1 μm, cylindrical.
Teleomorph: ascomata apothecia, small, 0.3-1 mm diam., black, immersed at first but becoming sessile. Disc at first concave, then flat and finally convex. Thalline exciple absent. True exciple sometimes distinct at first but then often soon excluded, dark brown, thin. Epithecium dark greenish-blue, sometimes encrusted with violet granules. Hymenium 90-110 (-120) μm tall. Hypothecium brown, the upper part (subhymenium) hyaline. Interascal tissue composed of paraphyses that become lax in K, simple below but occasionally branched above, apical cells hardly swollen. Asci 55-75 × 10-12(-15) μm, cylindrical to cylindric-clavate, fairly thick-walled but not fissitunicate, hardly thickened apically, only the outermost gelatinous layer K/I+ faint blue, discharge by splitting of the apex, 8-spored. Ascospores ± biseriately arranged, (10-) 12-16 (-18) × 5-6 (-8) μm, ellipsoidal to broadly pyriform, aseptate, hyaline, quite thick-walled, smooth, without a perispore or gelatinous sheath.
Chemistry: cortex and medulla C± pink, K–, KC-, Pd–, UV– (gyrophoric acid, ± unidentified compounds).
Assessed by Woods & Coppins (2012) as of Least Concern.
Frequently mistaken for Rimularia gyrizans or R. mullensis, with which they often grow, but those species have richly branched paraphyses, Rimularia-type asci and Pd+ yellow or orange, C- thalli. Somewhat similar to Clauzadeana macula with which it is often found, or the rare Orphniospora moriopsis. A morph from Arran with ascospores 14-17 × 6.5-8 μm and norstictic as well as gyrophoric acid has been reported and merits further study to clarify its status.
Frequent throughout western & northern Britain, from the Channel Islands to the Shetland Isles; extending locally to Ireland. BLS map here.
On hard siliceous rocks, in sunny exposed situations and often associated with yellow Rhizocarpon species.