Life habit: lichenized, not lichenicolous Thallus: crustose, composed of scattered areoles or areolate to rimose; prothallus: black, poorly to well developed areoles: round to angular, plane to strongly convex, up to 1.4 mm in diam. surface: greenish yellow or bright yellow, epruinose, smooth medulla: KI+ deeply blue Apothecia: round or angular, up to 1 mm in diam. disc: black, plane or weakly convex, epruinose exciple: pale brown to brownish red epihymenium: brown or reddish brown, K+ red, not containing crystals hymenium: hyaline or more rarely green, 100-180 µm tall; paraphyses: clavate, with hyaline tips; hypothecium: dark brown, K- asci: clavate, 8-spored ascospores: hyaline to pale green-brown initially, becoming dark green to brown, submuriform or muriform, ellipsoid, 20-36 x 10-20 µm, halonate Spot tests: medulla K-, C- or rarely C+ red, P+ yellow Secondary metabolites: rhizocarpic and psoromic acids, and rarely gyrophoric acid. Substrate and ecology: on non-calciferous rock in open sites, mainly in coniferous regions, at 1000-3720 m alt., uncommon World distribution: cosmopolitan Sonoran distribution: Arizona, southern California, and Baja California, most frequently above 1500 Notes: The present circumscription is based on the frequent correlation of psoromic acid in the medulla with a hyaline hymenium and with average spore length being less than 34 µm. Specimens having psoromic acid and average spore length larger than 34 µm usually have a green hymenium, and are here referred to R. macrosporum. Specimens lacking psoromic acid usually have somewhat longer spores than R. geographicum and a green hymenium; these are here referred to R. riparium. There are no apparent macroscopic differences between the three species in the Sonoran material, and some specimens show intermediate combinations of the above features. This circumscription of R. geographicum conforms more or less with Runemark's (1956a) circumscription of R. tinei and R. lindsayanum ssp. kittilense, but Runemark placed more importance on the K-reaction of the epihymenium, and less importance on the secondary chemistry.