Life habit: lichenized, not lichenicolous Thallus: crustose, areolate or rimose to continuous; prothallus: poorly to well developed areoles: round to angular, plane or weakly convex, up to 2 mm in diam. surface: bright yellow, smooth, epruinose medulla: white, KI+ deeply blue Apothecia: round or angular, up to 1 mm in diam. disc: black, plane, epruinose epihymenium: brown, K- or K+ red, not containing crystals hymenium: green or more rarely hyaline, 120-200 µm tall; hypothecium: brown asci: clavate, 8-spored ascospores: hyaline or pale green-brown initially, becoming dark green to brown, eumuriform, 24-38 x 10-20 µm, halonate Spot tests: medulla K- or K+ yellow or red, C- or C+ red, P- or P+ orange Secondary metabolites: rhizocarpic acid and often one or more of the metabolites barbatic, gyrophoric, norstictic, and/or stictic acids. Substrate and ecology: on non-calciferous rock in open sites in coniferous regions, at 1370-3535 m, uncommon World distribution: Europe, and North and South America Sonoran distribution: coniferous regions of Arizona, southern California, and Baja California. Notes: In the Sonoran material of Rhizocarpon riparium, there are apparently correlations between the various chemotypes and other examined characters, e.g. spore size, pigment in the epihymenium, or thallus morphology. Some acid deficient specimens with small spores and a hyaline hymenium may be regarded as acid deficient specimens of R. geographicum, but such a delimitation from acid deficient specimens of R. riparium would have to be drawn rather arbitrarily. This circumscription of R. riparium conforms more or less with Runemark's (1956a) circumscription of R. lindsayanum subsp. lindsayanum.