Thallus: determinate, areolate to subsquamulose, often in extensive patches, up to 0.4 mm thick, with isidia-like protuberances, mostly sterile areoles: contiguous, plane but margins often subascending, initially with (sub)acute margins and polygonal, later becoming irregular in outline, 0.5-1 mm wide, developing brown isidioid outgrowths along margins surface: pale to dark brown, smooth or uneven to sulcate-verrucose, dull to slightly glossy isidia: developing along thallus fissures, becoming confluent to cover larger part of the thallus, originating as globose outgrowths 25-75 µm in diam. and then becoming somewhat elongate and slightly coralloid, paraplectenchymatous, containing algal cells anatomy: upper cortex: paraplectenchymatous, composed of cells 4-6 µm in diam., colorless to pale brown, up to 20 µm thick but may be reduced to a single layer of cells, overlain by an amorphous, hyaline, epinecral layer 5-30 µm thick composed of collapsed flattened cells; algal layer: subparaplectenchymatous, up to 200 µm thick, with algal cells 5-10 µm in diam., cells of paraplectenchyma 5-7 µm in diam.; medulla: colorless to pale brown (especially around perithecia), partly with substrate crystals, with filamentous hyphae in the lowermost zone grading into the decomposed surface of the rock Perithecia: rare, immersed in areoles with apical part more or less emergent, the projecting part narrowly covered by the thallus, apex bare, black, convex; exciple: 0.3-0.35 mm wide, broadly pyriform, with a short conical neck, colorless to brown, c. 25 µm thick; involucrellum: contiguous with the exciple, reaching half-way or deeper, broadening to 75-100 µm thick toward the base, basal part dissolving, with mineral enclosures, merging with brownish parts of the medulla.; periphyses: those in the neck shorter and simple, lower ones 35-50 µm long and slightly ramified-anastomosing asci: clavate, 75-95 x 20-30 µm, 8-spored ascospores: hyaline, simple, ellipsoid, 20-27 x 11-13 µm Pycnidia: inconspicuous, immersed, small (up to 100 µm wide) conidia: oblong to subcylindrical, c. 4 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on ±calcareous rocks World distribution: widespread in Europe and North America Sonoran distribution: Arizona (Maricopa and Yavapai Co.), southern California (Santa Rosa Island, San Bernardino Mts., San Diego Co.), and Mexico (Sonora). Notes: Verrucaria furfuracea is the isidioid counterpart of V. macrostoma and deserves to be treated as a distinct species according to the species pair concept.