Thallus crustose, 0.5-4 cm diam., sometimes coalescing to cover large areas, areolate. Prothallus distinct, black. Areoles arising discretely on the prothallus, 1-2 mm diam., bright yellow to green-yellow, matt, smooth, contiguous to somewhat dispersed on the prothallus, initially round and convex, soon curving around adjacent apothecia and becoming crescent-shaped, eventually entirely surrounding the apothecia and forming a pseudo-lecanorine margin. Medulla I+ blue. Soredia and isidia absent. Photobiont chlorococcoid.
Anamorph: no information available.
Teleomorph: ascomata apothecia, to 1 mm diam., black, not pruinose, ± round, flat to concave. Thalline exciple absent. True exciple persistent but often thin and indistinct, brown-black at the rim, the inner part hyaline, K–. Epithecium pale olive-brown, K– or intensifying green. Hymenium colourless to faintly blue-green. Hypothecium thin, brown-black, K–; no crystals or granules in the apothecium. Interascal tissue of cellular pseudoparaphyses, conglutinate, richly branched and anastomosed, the apical cell only slightly thickened. Asci clavate, fissitunicate, with a well-developed tholus that is K/I– in the lower part and K/I+ blue near the apex, lacking an ocular chamber, Rhizocarpon-type, 4- to 8-spored. Ascospores 34-57 × 15-24 μm, ellipsoidal to ovoid-ellipsoidal, muriform, with 15-38 cells in optical section, dark brown when mature, without an epispore, gelatinous sheath or appendages.
Chemistry: medulla K+ yellow, Pd+ orange (stictic acid, often in low concentration, and rhizocarpic acid). According to Roca-Valiente et al. (2016), Pd reactions are variable in this species, and unhelpful for species recognition.
Assessed by Woods & Coppins (2012) as of Least Concern.
The crescent-shaped areoles, pale olive-brown epithecium, usually pale blue-green hymenium and Pd+ orange, K+ yellow medulla (stictic acid) are diagnostic. Rhizocarpon ferax is known from a single record from Ireland. It is said to differ from R. lecanorinum in the more
abundantly septate ascospores and in the occurrence of psoromic, not stictic acid, as the main medullary substance. The material on which the Irish record is based has not been located and the presence of this species in the British Isles needs confirming.
In upland areas of SW and N England, Wales and Scotland, local. BLS map here.
On siliceous rocks, slate roofs and walls, sometimes associated with mine workings.