Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
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: ellipsoid, 10.5-13 x 5-6.5 µm Pycnidia: laminal, immersed conidia: bacilliform, 5-6 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ red, C-, KC-, P+ orange Secondary metabolite: norstictic acid. Substrate and ecology: on soil in open habitats, in Vizcaíno subdivision of Sonoran Desert and in Californian coastal scrub, up to 500 m World and Sonoran distribution: rare in southern California and Baja California. Notes: 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 de Lesd. 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.