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DescriptionLife habit: lichenized, not lichenicolous; Thallus: crustose, areolate, contiguous; prothallus: black, mainly marginal, often indistinct; areoles: round to angular, weakly convex, up to 1.4(-2) mm in diam.; surface: bright yellow, smooth, dull, epruinose; medulla: white, KI+ deep blue; Apothecia: round or angular, up to 0.7 mm in diam.; disc: concave to plane, epruinose; exciple: pale brown; epihymenium: brown, K- or K+ red, not containing crystals; hymenium: hyaline or green, 150-220 µm tall; paraphyses: hyaline, clavate; hypothecium: pale brown, K-; asci: clavate, 8-spored; ascospores: hyaline or pale green-brown initially, becoming dark green-brown, muriform, ellipsoid, 32-60 x 15-22 µm; Spot tests: medulla K-, C-, KC-, P+ yellow; Secondary metabolites: rhizocarpic and psoromic acids.; Substrate and ecology: on non-calciferous rock in open sites in coniferous regions, at 1475-3030 m, apparently rare; World distribution: Africa, Asia, Europe, and North America; Sonoran distribution: Arizona and southern California.; Notes: In the present delimitation, Rhizocarpon macrosporum differs from R. geographicum only in the size of the spores. Intermediates do occur, but are rare. The present delimitation conforms more or less with Runemark's (1956a) R. macrosporum and R. sphaerosporum.