squamules: up to 3 mm wide, rounded to elongate, adnate to partly ascending, adjacent to imbricate, weakly concave to weakly convex upper surface: olivaceous to dark brown, dull, epruinose, smooth to sparingly fissured margin: concolorous with upper side, straight or down-turned, lobed upper cortex: up to 150 µm thick (including up to 80 µm thick epinecral layer), composed of thin-walled hyphae with round lumina, not containing crystals of lichen substances or calcium oxalate medulla: containing lichen substances but no calcium oxalate lower cortex: poorly developed, of mainly periclinally oriented hyphae, not containing calcium oxalate lower surface: medium brown, K- Apothecia: up to 1.5 mm diam., laminal, immarginate even when young, medium brown, epruinose ascospores: ellipsoid, 9.5-13.5 x 5-7 µm Pycnidia: unknown Spot tests: upper cortex and medulla K-, C-, KC-, P- Secondary metabolites: two unknown compounds (A5:B5-6:C6 and A5:B4:C6, both orange brown after charring). Substrate and ecology: on soil and rock in open habitats, in Californian coastal scrub and Madrean evergreen woodland, from about sea-level up to 2000 m alt. World and Sonoran distribution: rare in California, Baja California and Chihuahua. Notes: The species is most easily recognized by its unique chemistry. The small squamules and brown apothecia make it most likely to be confused with P. pacifica and P. pruinosa. Those species differ in being more or less pruinose. Furthermore, the former has more flat and marginate apothecia (at least when young), and the latter has squamules with a characteristic up-turned, epruinose margin.
The identity of Psora mexicana de Lesd., described from the state of Puebla, Mexico, is still unsettled because the original material is missing. A specimen identified as that species by de Lesdain and examined by Timdal (1986) resembles P. brunneocarpa morphologically, but does not contain lichen substances.