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DescriptionLife habit: lichenized, not lichenicolous; Thallus: crustose, areolate, verrucose; prothallus: black among the areoles; areoles: rounded, flat to more often convex, 0.3-0.8 mm in diam.; surface: usually gray, sometimes gray-brown, somewhat pruinose; cortex: thin, 5-15 µm thick, overlain with an epinecral layer 5-15 µm thick; medulla: white or yellow due to rhizocarpic acid; Apothecia: black, adnate or among the areoles, 0.4-0.6(-1) mm in diam.; disc: black, flat or becoming convex, dull, epruinose; margin: at first +thick, then mostly absent; epihymenium: brown to brownish black, 15-25 µm thick, K+ red; hymenium: hyaline, 100-140 µm tall; paraphyses: coherent, scarcely branched; hypothecium: brown; asci: clavate, 2-spored; ascospores: pale green initially, becoming olive-brown to brown, muriform, ellipsoid, (28-)35-60(-67) x (13-)1828(-35) µm, halonate; Pycnidia: frequent, black, almost globose, on the prothallus; conidia: hyaline, filiform, 8-12 x 0.3-0.6 µm; Spot tests: medulla K+ weak yellow or occasionally K+ red, rarely K-, C-, KC-, P+ yellow to red, P-; Secondary metabolites: rhizocarpic, stictic and +norstictic acids.; Substrate and ecology: in cold and wet regions; World distribution: throughout western and northern North America and Greenland, Europe and South America; Sonoran distribution: Arizona and southern California.; Note: A chemotype with yellow, K-, P- medulla usually has dark brown, small, plane, angular areoles and is restricted to +calciferous rocks in the lowlands may represent a distinct taxon.