Thallus: adnate to loosely adnate, appressed to pulvinate or subpanniform, foliose, up to 10 (-14) cm diam., lobate lobes: short and rounded or flabellate to somewhat elongate or occasionally linear-elongate, ± discrete to imbricate, (0.5-) 1-3 (-4) mm broad, flat to weakly convex upper surface: pale olive-brown to yellowish-brown or reddish-brown, dark brown or blackening, sometimes paler at the periphery, smooth to weakly pitted on the lobe-ends, inward smooth to rather strongly fissured and/or rugose; dull throughout or rather shiny, especially near the periphery; pseudocyphellate, the pseudocyphellae laminal, whitish to dark or ± concolorous with the upper surface, very distinct to occasionally rather obscure or rarely almost lacking soredia: granular to weakly isidioid, but highly variable in number of soralia produced (totally esorediate material is not rare), pale to rather dark brown, in laminal and marginal, punctiform to capitate soralia that arise in part from the pseudocyphellae lower surface: dark brown to black, paler on the lobe ends; smooth to somewhat plicate or rugose, dull to slightly shiny; moderately rhizinate, the rhizines ± concolorous with the lower surface Apothecia: frequent, up to 6 mm diam., sessile to short stipitate, concave to flattening or irregular, the margin entire to weakly crenate, pseudocyphellate and often becoming sorediate asci: clavate, 8-spored ascospores: ellipsoid, 8.5-11 x 4.5-7 µm Pycnidia: common, immersed conidia: cylindrical to weakly fusiform or subbifusiform, 5-7 x c. 1 µm Spot tests: cortex K-, C-, KC-, P-, HNO3-; medulla K-, C+ rose or rose-red, KC+ rose-red, P- Secondary metabolites: gyrophoric acid (major), ovoic acid (minor), unknown WG-2 (minor) (Esslinger 1992). Substrate: rocks World distribution: North America, Europe, North Africa, Asia Sonoran distribution: in forested regions, above 1800 m. Notes: Unless the sometimes rather obscure pseudocyphellae are overlooked, this species should not be easily confused with any other. The C+ rose or rose-red medulla and laminal pseudocyphellae, usually on relatively narrow lobes, will distinguish it from all others, especially in the study area. When the pseudocyphellae were not recognized, the sparsely sorediate or esorediate specimens have been misidentified as unrelated M. glabroides. A more closely related species, M. disjuncta, also occurs in the study area and has a somewhat similar appearance, with obscure submarginal pseudocyphellae and darkened soralia, but is easily distinguished by the C- medulla.