Rhizocarpon infernulum  

Description:
Thallus rimose to cracked areolate, brown or grey to very pale grey; areoles 0.3-0.4 mm diam., flat. Photobiont chlorococcoid, cells (10-)12-15(-18) µm diam.
Apothecia black, lecideine, flat and innate to slightly convex, 0.3-0.4(-0.5) mm diam.; proper exciple (if apparent) thick (0.02-0.03 mm) but barely raised. Hymenium hyaline, I+ blue, 70-90 µm tall; epihymenium usually with aeruginose to blue-black pigment;( K–, N+ red). Paraphysoidsc. 2.5 µm thick scarcely swelling at apex (to 3.0 µm) with sharply delimited brown (Atra-brown) cap, branched and anastomosing. Asci c. 70 x 30 µm, Rhizocarpon-type. Ascospores hyaline, halonate, 1-septate, 8/ascus, 15-18 x 7-8.5 µm. Hypothecium brown (Arnoldiana-brown). Excipulum well-developed, of radiating hyphae, pale brown with dark brown, carbonaceous outer cells.
Conidiomata not seen.
Chemistry : C–, K– or K+ yellow, Pd– or Pd+ orange. No substances or stictic acid detected by TLC.

Distribution:
Circumboreal in the northern hemisphere but, in the southern hemisphere, know for certain only from the Falkland Islands.