Description: Thallus crustose, areolate to verrucose, forming a compact crust. Areoles at first pale greenish yellow, then bright yellow-green, thick, 1-2 mm wide, flat to usually strongly convex, contiguous, with an epinecral layer. Medulla well-developed, white to yellowish, filled with oxalate crystals (insoluble in acetone). Apothecia lecideine, sessile, to 1.5 mm across, with a black, at first concave then soon flat, smooth disc and a thin, smooth, black proper margin. Exciple brown-black in outermost part, blue-green within; epithecium and hypothecium dark grey green, K-, N+ emerald green; hymenium colourless, inspersed with oil droplets, I-; paraphyses lax, slender, branched and anastomosing, not apically thickened, c. 1.5 µm thick. Asci 4-6(-8)-spored, bitunicate, clavate, I-, scarcely thickened at apex, with a small ocular chamber. Ascospores 3(-4)-septate, hyaline, fusiform, thin-walled, (14-)16-22(-25) x 3-3.5(-4) µm. Photobiont chlorococcoid. Spot tests: thallus and medulla K-, C-, KC-, P-, UV+ orange. Chemistry: rhizocarpic acid, epanorin, and sometimes traces of atranorin.
Note: an arctic-alpine, circumpolar species found in humid soil near and above treeline. It starts the life-cycle as a parasite of Baeomyces placophyllus, later becoming autotrophic, and is the most calcium-tolerant among the Arthrorhaphis-species, often occurring over calcareous schists and even marmor (Obermayer in litt.). Probably more widespread in the Alps.
Growth form: Crustose
Substrata: soil, terricolous mosses, and plant debris