Thallus: indistinct, up to 0.15 mm thick, continuous and sparsely fissured to verruculose with convex "areoles" 0.2-0.3 mm wide surface: grayish white to pale tawny, dull anatomy: subparaplectenchymatous, composed of cells 5-9 µm in diam. with small interspersed substrate particles, with irregularly distributed algal cells 7-13 µm in diam. Perithecia: half immersed to almost superficial, hemispherical emergent with their bases immersed in the thallus which is slightly raised in their vicinity; exciple: 0.2-0.3 mm wide, subglobose or somewhat deplanated, brown; involucrellum: appressed, covering at least two thirds of the exciple, mostly extending to exciple base level, lateral up to 100 µm thick ; periphyses 20-30 µm long, thin asci: clavate, 70-80 x 15-25 µm, 8-spored ascospores: hyaline, simple, ellipsoid , (18-)20-25 x 8-12 µm Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on sandstone in more or less sheltered places from lower to montane level, also found on volcanic rock in a seasonal streambed World distribution: mainly eastern North America Sonoran distribution: few collections from Arizona and southern California. Notes: The difference against the European Verrucaria gudbrandsdalensis is not clear-cut; the perithecia of V. calkinsiana are somewhat smaller and have thicker involucrella. Another European species, Verrucaria cincta, has the same type of involucrellum as V. calkinsiana but smaller spores (17-21 x 7-8 µm). Verrucaria amylacea has smaller spores and thinner involucrella. In Verrucaria muralis the involucrellum is also thinner and rarely reaches more than half-way down the exciple.