Thallus: , continuous to irregularly rimose, rarely areolate, thin to ±thick or hardly apparent, with an indistinct margin and usually without a visible prothallus areoles: subangular to irregular, 0.3-0.6 mm wide surface: gray, grayish white or pale brown, dull, epruinose, smooth or (sub)farinose anatomy: upper cortex indistinct, algal layer: (sub) paraplectenchymatous, ±irregular, 70-150 µm thick; algal cells: 7-12 µm in diam., dispersed or clustered to densely packed; medullary hyphae: having a looser texture, partly filamentous, with interspersed (or obscured by) substrate grains and crystals, colorless to patchily pale brown, grading into the decomposed rock surface Perithecia: usually semi-immersed or almost sessile, rarely 3/4 immersed (in thick thalli), black, dome-shaped or hemispherical, emergent portion naked or with a narrow thalline covering at the base, ostioles inconspicuous to excavate; exciple: sub-globose, 0.25-0.35 mm wide, colorless to brownish, dark at the apex, rarely brown throughout in old perithecia, 15-25 µm thick; involucrellum: extending down to middle part of the perithecium, appressed to the exciple or slightly spreading, c. 50-70 µm thick, rather even in thickness or slightly thinning downwards; periphyses 25-35 µm long, sparsely branched and anastomosing asci: clavate, 65-75 x 16-22 µm, 8-spored ascospores: hyaline, simple, narrowly to broadly ellipsoid, 17-25 x 8-12 µm Pycnidia: not seen Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: semi-endolithic to epilithic, usually on or in soft or weathered calcareous rocks, walls and mortar, also on hard limestone and siliceous rocks, in a wide range of habitats World distribution: Eurasia, North Africa, North America, Japan, Taiwan, Australia, and New Zealand Sonoran distribution: Arizona, southern California, and Baja California. Notes: Verrucaria muralis is characterized by a semiendolithic to epilithic, pale thallus, medium-sized and partly immersed perithecia with involucrellum enclosing the upper third or half of the exciple, and medium-sized spores. Verrucaria calkinsiana has smaller perithecia with darker exciples and thicker, deeper reaching involucrella. Verrucaria amylacea is distinguished by narrower spores and involucrella extending down to the exciple-base level. Verrucaria mimicrans has larger perithecia and broader spores.