Life habit: lichenicolous Thallus: crustose, immersed in the host thallus or areolate; prothallus: absent or poorly developed areoles: up to 1 mm in diam., round to angular, weakly convex to bullate surface: pale to grayish yellow, smooth or scabrose, epruinose medulla: white, KI+ faintly violet Apothecia: round or angular, up to 0.8 mm in diam. disc: black, plane or weakly convex, epruinose margin: disappearing epihymenium: brown, K-, containing crystals dissolving in K hymenium: hyaline, 80-120 µm tall; paraphyses: tips single; hypothecium: brown asci: clavate, 8-spored ascospores: hyaline or pale green-brown initially, becoming dark green to brown, submuriform, 18-26.5 x 10-14.5 µm Spot tests: medulla K+ yellow or red, C-, P+ orange Secondary metabolites: rhizocarpic acid, stictic acid (major), and norstictic acid (minor). Substrate and ecology: on Dimelaena oreina over non-calciferous rock in open sites at 2250-2380 m (two localities thus far known) World and Sonoran distribution: Arizona and southern California. Notes: Rhizocarpon dimelaenae differs from R. viridiatrum in the choice of host, in forming smaller and less continuous thalli, and in having larger, more strongly convex and often scabrose areoles.