Squamules: up to 5 mm wide, elongate, adnate and dispersed to adjacent or slightly imbricate, plane to weakly convex; upper surface: castaneous brown, shiny, epruinose, smooth or sparingly fissured; margin: concolorous with upper side, more or less straight, entire, crenulate or lobed; upper cortex: up to 100 μm thick, composed of rather thin-walled hyphae with round lumina, sometimes containing some crystals of lichen substances but no calcium oxalate; medulla: containing crystals of lichen substances; lower cortex: 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, black, shiny, epruinose; ascospores: 10.5-13 ´ 5-6.5 μm; Pycnidia: laminal, immersed; conidia: bacilliform, 5-6 ´ 1 μm; Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ red, C-, KC-, P+ orange; Secondary metabolites: norstictic acid.
The species resembles P. californica morphologically, but differs in forming somewhat smaller squamules without the often down-turned margin, in having an upper cortex composed of more thin-walled hyphae with more rounded lumina, in having pure black, more shiny apothecia, and in containing norstictic acid in the medulla. The Mediterranean-Macaronesian species P. gresinonis differs in forming smaller, more rounded and concave, olivaceous brown squamules with a slightly raised, often paler or grayish margin, and in having marginal to submarginal, smaller apothecia.
California and Baja California del Norte.
On soil in open habitats, in Vizcaino subdivision of Sonoran Desert and in Californian coastal scrub, up to 500 m.
Psora californica, Psora gresinonis.