squamules: discrete, up to 5 mm wide and 0.25-0.35 mm thick, ± deeply lobate, fully appressed with only the very edges somewhat elevated upper surface: pale to medium brown, dull upper cortex: 40-60 µm thick, with a thin epinecral layer medulla: white, with many spherical cells (10-16 µm in diam.); algal layer: c. 50-80 µm high lower cortex: hardly discernible, composed of more densely aggregated spherical cells, the lowermost ones brownish lower surface: pale, brownish, with a rhizohyphal weft over all of the underside and ± projecting outwards; rhizohyphae: hyaline, 5-6 µm in diam. Perithecia: broadly pyriform, up to 0.4 mm wide, with colorless to pale brownish walls; periphyses: 30-35 x 2-3 µm asci: cylindrical, 70-80 x 11-15 µm, 8-spored ascospores: uniseriate, ellipsoid, 11-15 x 5-6.5 µm Pycnidia: laminal, immersed conidia: bacilliform, 5-7 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: soil and bark (Quercus douglasii in California) in warm, dry places World distribution: rare in Spain, Canary Islands (Tenerife) and SW North America Sonoran distribution: Arizona and southern California, only a few collections. Notes: The corticolous specimens agree in all important respects with terricolous material. All Californian samples were found on bark of oak, growing side by side with Catapyrenium psoromoides. Placidium fingens comes close to P. squamulosum, from which it differs mainly in its longer, bacilliform conidia. Those of the latter species are oblong-ellipsoid and shorter. Placidium pilosellum has marginal pycnidia with short, oblong-ellipsoid conidia. The primarily bark-inhabiting P. tuckermanii is much different (see under that species).