Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: squamulose squamules: c. 2-6 mm wide, rather thin (0.25-0.4 mm thick), roundish or lobed, flat and fully attached or ± wavy and with margins free from the substrate (raised or downrolled), adjacent or overlapping upper surface: yellowish brown, tan or dark brown, often with an orangish tinge upper cortex: c. 40-60 µm; epinecral layer: 10-40 µm thick medulla: white, with many spherical cells (9-17 µm in diam.); algal layer: c. 80-120 µm thick lower cortex: composed of more densely aggregated spherical cells (9-16 µm diam.) if discernible at all lower surface: usually pale, sometimes darkening centrally, rarely black throughout; attached by a rhizohyphal weft; rhizohyphae: hyaline, 4.5-6 µm in diam. Perithecia: broadly pyriform, up to 0.5 mm wide, bulging the lower side of the squamules; exciple: colorless or pale yellowish brown; periphyses: 30-40 x 2.5-3.5 µm asci: cylindrical, 70-90 x 10-15 µm, 8-spored ascospores: uniseriate, ellipsoid, c. 12-17 x 5.5-7.5 µm Pycnidia: marginal, forming knob-like projections conidia: oblong-ellipsoid, 3-4 x c. 1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: soil, moss, debris, preferably in dry, mild climate World distribution: Europe (excluding the Alps and northernmost parts), North Africa, SW Asia, Australia, temperate North America Sonoran distribution: common in Arizona, southern California, Baja California, Baja California Sur, and Sinaloa. Notes: The hyphal outgrowths along the lobe margins and even on the upperside of young squamules described from European material are often lacking in extra-European material. Anatomically the species is identical with Placidium squamulosum, but easily separable by its marginal pycnidia. The distinction from P. andicola may be difficult (see under that species). Placidium rufescens also has the same pycnidial characters but thicker squamules with a well developed prosoplectenchymatous medulla, thicker rhizohyphae, and much larger spores.