Thallus: squamulose or subfoliose squamules: rather large (up to 7 mm wide and 0.3 - 0.5 mm thick) and almost leafy, densely aggregated to slightly overlapping, lobate; margins: variously wavy, free from the substrate and upturned or downturned upper surface: medium to dark brown, dull upper cortex: 40-60 µm thick, overlain by a thin epinecral layer medulla: white, thick, composed of intricately interwoven filamentous hyphae; algal layer: about 100-130 µm thick lower cortex: distinct, subparaplectenchymatous, composed of roundish cells (8-13 µm in diam.), up to 50 µm thick lower surface: pale brown, naked where free from the substrate, central parts with a rhizohyphal weft; rhizohyphae: hyaline, c. 5 µm in diam., lacking in a broad marginal zone; also rhizinate; rhizines: few, whitish or pale brownish, thin (50-100 µm in diam.), slightly branched Perithecia: broadly pyriform, up to 0.4 mm broad, with colorless exciple asci: subcylindrical to narrowly clavate, 50-65 x 14-18 µm, 8-spored ascospores: primarily uniseriate, soon (sub)biseriate, ellipsoid, 11-14 x 6-7.5 µm Pycnidia: laminal, immersed conidia: bacilliform, 4-5 x 1-1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: directly on (volcanic) rock or over cyanophilic lichens thereon or on thin earth cover over rock World and Sonoran distribution: known only from Baja California Sur. Notes: Placidium pseudorufescens is the only rhizinate species of the genus that is found growing directly on rock with rhizines fixed in minute fissures. It is morphologically similar to P. rufescens, from which it differs in being rhizinate and having laminally immersed pycnidia with bacilliform conidia, and in having much smaller ascospores. The common P. lacinulatum is ecologically different in that it very rarely grows on thin earth covering (crumbling) rock, but usually it occurs on fine-grained soil. Furthermore, it differs in having a medulla rich in spherical cells, and usually thicker, more conspicuous rhizines.