Thallus: crustose, often very thin but up to 0.5 mm thick, areolate or with scattered appressed convex angular or lobe-like fragments; prothallus: sometimes visible, pale brown areoles: mostly sharply angular and 0.3-2(-2.5) mm wide, plane, the edges often slightly curling upwards upper surface: yellowish brown to brownish gray, not discolored when wet, sometimes covered with cyanobacteria, edges sometimes partly paler than the center, smooth or slightly granular-verrucose with 0.05 to 0.2 mm wide granules, epruinose upper cortex: not always well developed, paraplectenchymatous with isodiametric hyphal cells, usually 10-15 µm thick, overlain by a distinct epinecral layer up to 20 µm thick medulla: not developed; algal layer: not clearly separated, algae scattered in small or large groups Apothecia: broadly sessile, often numerous and covering the areoles, 1-10(-14) per areole, often deformed, up to 0.8 mm in diam. disc: pale yellowish brown to brownish black, paler when moist, with occasional dark spots, plane to slightly convex when young to convex-hemispherical, epruinose margin: thalline, thin (under 0.1 mm wide), +level with disc, becoming excluded in strongly convex apothecia, entire to flexuous or crenulate amphithecium: consisting of an algal rich, inner zone and a cortical zone of paraplectenchymatous, isodiametric hyphal cells, with an epinecral outermost layer parathecium: poorly developed, narrow, pale brown, encrusted toward the outer edge and c. 50 µm wide epihymenium: pale brown-orange to dark brown; epipsamma sometimes present, pale yellow with a pink substance around the paraphyses hymenium: hyaline below, 50-60 µm tall; paraphyses: simple, with cells 9-12 x 1.5-2 µm wide below, mostly not or slightly swollen towards apices, a few strongly swollen and up to 3-6 µm wide and strongly coherent through epipsamma in a gelatinous matrix; hypothecium: hyaline, with intricate hyphae asci: narrowly clavate, Bacidia-type, 65-70 x 12 µm, 8-spored ascospores: hyaline, (0-)1-septate, not constricted, narrowly to somewhat broadly ellipsoid, straight, 9-12(-14) x 4-4.5(-5) µm, thin-walled Pycnidia: rare, immersed 80-120 µm in diam, hyaline and pale to reddish brown pigmented around ostiole; conidiogenous cells: elongate, branched at base, 6-8 x 1.5-2 µm conidia: filiform, strongly curved, 16-22 x 0.8 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on various rocks, ranging from natural calcareous ones to artificial substrates such as old walls, on mortar brick etc., rarely on bone or over moss; very nitrophilous; found mostly in sheltered and shaded habitats World distribution: widely distributed throughout Europe and very common in some places, northern Africa, known also from southwestern USA. Sonoran distribution: rarely found, only on the Channel Islands of southern California and the coastal mainland of Santa Barbara Co. Notes: Lecania rabenhorstii is characterized by its plane to slightly uneven upper areole surface, its numerous convex to convex-hemispherical apothecia (up to 14 per areole) with a thin amphithecium, that often becomes excluded. It closely resembles L. inundata in anatomy, but that species has more bullate-papillate areoles, and its apothecia are usually grouped, less numerous with a more conspicuous margin, and have flatter to sometimes moderately convex discs. Lecania rabenhorstii is confusable with some morphs of L. turicensis but that species has a more glaucous whitish gray thallus without the more brown tinge found in L. rabenhorstii, and also the latter never has pruinose apothecia. Lecania polycycla externally resembles L. rabenhorstii, but the ascospores of L. polycycla are constricted and its hymenium is rather different. In the field, it is sometimes overlooked for a diminutive Rinodina species, especially R. gennarii. The latter species occurs also in the study area in Arizona.