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: 2-7 mm wide, c. 0.2-0.4 mm thick, discrete to adjacent to slightly overlapping, nearly completely adnate to the substrate, or the margins slightly raised, round to lobed upper surface: pale to dark brown, dull upper cortex: c. 30-60 µm thick; epinecral layer: (almost) lacking or up to 50 µm thick medulla: white, usually with numerous spherical cells ( c. 9-14 µm in diam.); algal layer: about 70-120 µm high lower cortex: hardly discernible or of densely aggregated roundish-angular cells (10-16 µm in diam.) lower surface: usually pale, but also blackening, attached with a dense rhizoidal weft; rhizohyphae: hyaline, 4.5-6.5 µm thick Perithecia: broadly pyriform, up to more than 0.5 mm wide and usually bulging the lower side of squamules; exciple: hyaline or yellowish, 25-30 µm thick; periphyses: 30-40 x 3-4 µm asci: cylindrical, 70-90 x 10-15 µm, 8-spored ascospores: uniseriate, ellipsoid, 12-16 x 5.5-7.5 µm Pycnidia: frequent, laminal, immersed conidia: oblong-ellipsoid, 2.5-4 x 1.3-2 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: soil, moss, debris, mostly over ± calciferous ground in open situations from low to high elevations World distribution: cosmopolitan, North and South America, Europe, Asia, Africa, Australasia Sonoran distribution: common in Arizona, southern California, Sonora, Baja California, and Baja California Sur. Notes: Together with Placidium lacinulatum, P. squamulosum is the most common species in the Sonoran region It is very variable species with a broad ecological amplitude. The lack of rhizines easily separates it from P. lacinulatum, otherwise these two species are almost identical in habit and anatomy with equally great variability in both species. Unusually large specimens with a thick, almost prosoplectenchymatous medullary tissue may be difficult to separate from Placidium chilense, but the latter species has a distinct lower cortex of large cells and narrowly oblong to shortly cylindrical (vs. ellipsoid-oblong) conidia. Placidium pilosellum differs further in having marginal pycnidia.variable species with a broad ecological amplitude. The lack of rhizines easily separates it from P. lacinulatum, otherwise these two species a