Thallus: crustose, chinky-areolate stipe: subparaplectenchymatous with filamentous hyphae in lower parts areoles: in the center angular, 1-3 mm broad, flat or ± convex, 0.25-0.55 mm thick; the marginal areoles: effigurate, radiating, up to 4 mm long and 2 mm broad upper surface: brown, smooth, without pruina upper cortex: 20-30 µm thick, of cells c. 4-7 µ in diam., overlain by a compact smooth epinecral layer up to 25 µm high medulla: subparaplectenchymatous; cells: c. 7-12 µm in diam. in the upper part but somewhat smaller in the lower part and more densely packed; algal layer c. 70-100 µm thick, algal cells 5-10 in diam. lower cortex: not clearly delimited, of more densely compacted medullary globose cells 5-8 µm in diam. lower surface: brown, lighter along margins, attached to the substrate by 1 to several stipe-like holdfasts Perithecia: subglobose, up to 0.4 mm broad; exciple: colorless or slightly brownish asci: clavate, 8-spored ascospores: simple, oblong-ellipsoid, 15-18 x (4-) 5-6 (-7) µm Pycnidia: laminal, immersed, up to 300 µm wide conidia: shortly bacilliform, 4-5 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on (volcanic) rocks on exposed sites. World and Sonoran distribution: Baja California Sur, Sinaloa and Sonora. Notes: Placopyrenium insuetum is the only species of the genus with a brown, epruinose upper surface.