Squamules: up to 4 mm wide, rounded to elongate, adnate or ascending near the margin, rarely imbricate, weakly concave to plane upper surface: medium brown to dark brown, dull, epruinose or faintly pruinose, smooth to sparingly fissured margin: concolorous with upper side, straight to weakly up-turned, entire or lobed upper cortex: up to 200 µm thick, composed of rather thin-walled hyphae with more or less round lumina, not containing crystals of lichen substances, sometimes containing calcium oxalate (mainly in the epinecral layer) medulla: not containing lichen substances or calcium oxalate lower cortex: of mainly periclinally oriented hyphae, not containing calcium oxalate lower surface: pale to medium brown, K- Apothecia: up to 1.5 mm diam., laminal, convex and immarginate even when young, (medium-) dark brown to black, epruinose ascospores: ellipsoid, 9-14 x 5-7 µm Pycnidia: laminal, immersed conidia: bacilliform, 6-9 x c. 1 µm Spot tests: upper cortex and medulla K-, C-, KC-, P- Secondary metabolites: none detected. Substrate and ecology: on soil and rock, mainly in deserts, up to 1830 m World distribution: western North America Sonoran distribution: locally common in Arizona, California, Baja California and Baja California Sur. Notes: The species is sometimes difficult to distinguish from P. globifera and P. tuckermanii. It differs from the former in its often smaller and more closely adnate squamules with a more smooth, dull, and paler brown upper surface, and in having more thin-walled hyphae with more rounded lumina in the upper cortex. The stainable layer of the upper cortex is roughly 10-15 cells thick in P. globifera and 15-25 cells thick in P. luridella. The apothecia of P. luridella are dark brown to pure black, rarely medium brown, and always epruinose; those of P. globifera are medium to dark brown, rarely black, and sometimes yellow pruinose. Psora tuckermanii differs morphologically in having generally larger, usually somewhat paler and more pruinose squamules and medium brown, sometimes white or yellow pruinose apothecia. Furthermore, it usually contains calcium oxalate in the medulla. Timdal ( p. 274) regarded some specimens from California and Baja California with dark brown to black apothecia and lacking calcium oxalate in the medulla as belonging to P. tuckermanii; these specimens are here regarded as belonging in P. luridella.