Thallus: superficial, rimose to areolate, thin to moderately thick (0.1-0.5 mm) areoles: polygonal to ±round-angular, plane or slightly convex, 0.2-1 mm wide, sometimes secondarily divided into small verrucules surface: dark brown or greenish brown, rarer pale brownish gray to grayish white, smooth to rough anatomy: upper cortex: thin and hardly discernible or up to 20 µm thick and consisting of ±round-angular cells 4-7 µm in diam., overlain by an amorphous, hyaline, epinecral layer 10-30 µm thick; algal layer: comparatively thin (25-100 µm) and irregular, with algal cells 7-10 µm in diam. in dense clusters; alga-free medulla: looser, sordid to chalky white, containing many substrate grains and crystals, grading into the substrate Perithecia: at least ?-immersed, bottle-shaped, their bases ± deeply sunken into pits in the substrate, the emergent part convex to, usually, conical, black; exciple: bottle-shaped, 0.30-0.55 mm wide and 0.45-0.75 mm high, initially pale brown, but soon blackening, 25-35 µm thick; involucrellum: apical, covering the upper third to upper half of the exciple, very variable, contiguous with the exciple or slightly spreading, usually rather diffusely delimited or lacerate, 30-100 µm thick, inner parts often paler or irregularly pigmented; periphyses 40-60 µm long, thin, simple or slightly branched asci: clavate, 100-115 x 33-42 µm, 8-spored ascospores: hyaline, simple, broadly ellipsoid, 25-33(-35) x (12-)14-18(-20) µm Pycnidia: immersed, 40-80 µm wide conidia: bacilliform, 7-10 x 1 µm, straight to slightly bent Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on ±calciferous rocks, rather common on sandstone, occasionally on soil, mostly upland, but also on coastal sites World distribution: Eurasia, North Africa, and North America Sonoran distribution: Arizona (Coconino, and Yavapai Counties), and southern California (Channel Islands). Notes: Verrucaria murorum has spores of about the same size as in V. viridula, and the perithecia have a short conical neck, but the involucrellum reaches halfway or deeper down the exciple. Verrucaria macrostoma differs in that the perithecial apex is not produced into a neck, and in having narrower spores. In both species the exciples are pale to dark brown but not black, and the bases of the perithecia lie within the thallus tissue instead of being sunken in pits in the substrate.