Thallus: crustose, rimose-areolate stipe: similar in anatomy to the medulla but with more longitudinally arranged hyphae areoles: 0.5-2 mm wide, flat or slightly convex, grayish, up to 1 mm thick, basally constricted, subangular in the center; the marginal areoles: roundish or somewhat incised or lobed upper surface: grayish upper cortex: 15-20 µm thick, composed of roundish-angular cells (4-6 µm diam., the uppermost ones brown and overlain by a 20-40 µm thick amorphous layer) medulla: subparaplectenchymatous, composed of densely intricated hyphae which are often divided into spherical cells (6-9 µm diam.); algal layer: c. 100-150 µm thick; algal cells: 7-12 µm in diam. lateral and lower cortices: the same type but without amorphous layer lower surface: attached by a stipe Perithecia: broadly pyriform to subglobose; exciple: 25-30 µm thick, colorless or with the outermost layer darkening; periphyses: 30-40 µm long and 2-3 µm thick asci: clavate, 60-70 x 20-25 µm, 8-spored ascospores: simple, ellipsoid, 14-18 x 6-9 µm Pycnidia: laminal conidia: bacilliform, 5.5-7.5 x 1 µm long Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: parasitic on Staurothele areolata at least when young, later becoming independent World distribution: North America Sonoran distribution: only known from Baja California Sur. Notes: It is very similar to some Verrucaria species, from which it differs in its pycnidial type. Placopyrenium zahlbruckneri is similar in habit, but has larger areoles, is not parasitic, and has shorter periphyses and conidia and larger spores.