Thallus: squamulose squamules: flat to almost bullate, rounded to slightly lobed, pedunculate, up to 1.5 mm wide and 0.6 mm thick, contiguous, secondarily divided by cracks, forming an areolate appearing thallus upper surface: brown, smooth, matt upper cortex: 20-40 µm thick medulla: white, subparaplectenchymatous with globose cells (8-11 µm in diam.); algal layer: c. 80-110 µm thick; algal cells: 6-12 µm in diam. lower cortex: not discernible, composed of more densely packed globose-angular cells (8-11 µm in diam.), the lowermost ones brown lower surface: brownish; rhizohyphae: hyaline, confined to the central peduncle Perithecia: subglobose, up to 0.45 mm wide; exciple: colorless except for the upper third; periphyses: 20-30 µm long asci: subcylindrical to narrowly clavate, 8-spored ascospores: uniseriate at early stages but soon biseriate, narrowly ellipsoid, 15-19 x 5-7 µm Pycnidia: laminal, immersed conidia: cylindrical, 4-6 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on (volcanic) rocks in dry, open situation. World and Sonoran distribution: known only from Baja California Sur. Notes: Placidium lesdainii is morphologically indistinct from P. acarosporoides, from which it differs in its cellular medullary tissue and narrow ellipsoid ascospores (15-19 x 5-7 µm versus 13-17 x 8-11 µm). The hyphal texture of the medulla is somewhat variable in P. acarosporoides and well developed material necessary to reveal the difference, but the shape of the spores is a constant diagnostic feature. Placidium lesdainii was possibly already described by de Lesdain (as one out of several Endopyrenium species) but the types were not available, and from the descriptions none of his species could be assigned with certainty to this taxon.