Thallus: areolate, uneven, 0.25-0.35 mm thick, up to several centimeters across, without a black basal layer, with a distinct margin, without an apparent prothallus areoles: ±round-angular to rounded, flat to slightly convex, wrinkled-verrucose, 0.2-0.5 mm wide, somewhat dispersed to contiguous, ±secondarily diffracting along wrinkles, with pale to brown side-walls surface: brown, rugose, dull anatomy: upper cortex: poorly defined, uneven, composed of 1-3 layers of cells 4-6 µm in diam., with a brown uppermost cell layer, overlain by a hyaline, 10-30 µm thick epinecral layer composed of flattened dead cells; algal layer: (sub)paraplectenchymatous, 80-150 µm thick, partly sprinkled with clouds of small crystals, with 5-8 µm in diam. algal cells; alga-free medulla: colorless to pale brown, having a looser texture, with filamentous parts, discontinuous, penetrated by substrate particles and crystals and thus grading into the substrate Perithecia: fully immersed in areoles or with upper portion emergent, tips bare; exciple: pale to dark brown, broadly pyriform with a short conical neck, 0.30-0.45 mm wide, 20-25 µm thick; involucrellum: flanking the exciple, reaching halfway or deeper, 70-100 µm thick, at the base sharply delimited or somewhat lacerate or with inner part brownish transparent; periphyses 40-50 µm long, thin, slightly branched-anastomosing asci: clavate, 90-110 x 27-33 µm, 8-spored ascospores: hyaline, simple, ellipsoid to broadly ellipsoid, 24-35 x 13-17 µm Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on limestone and calciferous sandstone World distribution: Europe and southwestern North America Sonoran distribution: Arizona. Notes: The thallus of the Sonoran sample is less well-developed (smaller, heavily diffracted areoles) with more protruding perithecia than in European samples, which may be environmentally or substrate-induced. Verrucaria murorum is often treated as synonymous with V. macrostoma, but this latter species has larger, smooth, non-wrinkled areoles, a less developed involucrellum, and narrower spores.