Squamules: up to 4 mm wide, elongate, imbricate, weakly concave to weakly convex upper surface: medium brown, dull or shiny, epruinose or partly pruinose, smooth or sparingly fissured margin: white, straight or up-turned, entire or lobed upper cortex: up to 90-140 µm thick, composed of thick- to thin-walled hyphae with angular to round lumina, containing crystals of calcium oxalate medulla: containing calcium oxalate lower cortex: of mainly periclinally oriented hyphae, not containing calcium oxalate lower surface: white to pale brown Apothecia: up to 1.5 mm diam., laminal-submarginal, plane and marginate when young, later becoming more convex and often immarginate, reddish brown, epruinose or yellow pruinose ascospores: ellipsoid, 8-12 x 5-7 µm Pycnidia: unknown Spot tests: upper cortex and medulla K-, C-, KC-, P- Secondary metabolites: none detected. Substrate and ecology: on calciferous rock in open habitats, at 790-1720 m World distribution: mediterranean Europe and North America Sonoran distribution: uncommon in Arizona, Chihuahua, Sinaloa and Sonora. Notes: Some specimens are difficult to distinguish from P. tuckermanii. The apothecia of Psora pseudorussellii are more reddish brown, less convex and often partly marginate (especially when young); the squamules are generally smaller and usually somewhat darker than those of P. tuckermanii. The two species are largely allopatric: P. pseudorussellii has a mainly eastern and southern and P. tuckermanii a mainly western distribution in North America (Timdal 1986).