Thallus: crustose, distinctly areolate, dispersed areoles often becoming subsquamulose and aggregated in irregular patches, or areoles closely aggregated and the thallus typically forming rosettes with marginal lobes, thick, ±continuous or becoming dispersed; prothallus: absent surface: usually ivory, beige to deep brown or gray, rarely dark olive, dull or rarely shiny, smooth to deeply fissured, with fine or coarse pruina, rarely epruinose, phenocorticate, esorediate medulla: white, with moderate to large amounts of calcium oxalate (H2SO4+ needle shaped crystals) Apothecia: lecideine; (0.3-)0.4-0.7(-0.8) mm in diam., sessile margin: black, thin to thick, usually persistent, rarely excluded with age disc: black, usually epruinose or rarely with white pruina, plane, often becoming strongly convex with age proper exciple: dispersa-type, inner excipular hyphae distinct, not reduced, pigmented, prosoplectenchymatous (textura oblita), extending from the deep reddish brown hypothecium (leptoclinoidesbrown, textura intricata), outer excipular hyphae short-celled, cells angular, distinctly swollen (textura angularis) and ±carbonized with various amounts of a brown pigment (cf. elachista-brown, HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, not inspersed with oil droplets; paraphyses: simple to moderately branched, apically swollen, with a brown pigment cap (cf. elachista-brown) asci: clavate, Bacidia-type, 8-spored ascospores: soon brown, 1-septate, narrowly-oblong to ellipsoid, usually not constricted, with obtuse ends, not curved, (9-)11.9-[13.8]-15.7(-19) x (4-)6.4-[7.4]-8.3(-9) µm (n=79); proper septum: soon becoming briefly thickened during spore ontogeny, (±Physconia-type); ornamentation: microrugulate (often faint and inconspicuous in DIC) Pycnidia: rare, urceolate to globose, unilocular; ontogeny similar to the Umbilicariatype conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 5-6 x 1-1.5 µm (n=20) Spot tests: thallus typically K+ yellow, P+ yellow, C-, KC-, CK- (reactions can be weak according to the concentration) fluorescence: UV- (dark) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin, 2-O-methylperlatolic and ±confluen-tic acid (J. A. Elix, HPLC). Substrate and ecology: epilithic, on a wide variety of siliceous (HCl-) rock substrates, rarely with minor traces of carbonates (HCl+, weak reaction) World distribution: very widely distributed throughout semi-arid to arid regions of the Northern Hemisphere, Mediterranean Europe, northern Africa, and southwestern North America Sonoran distribution: probably the most common species throughout the Sonoran Desert Region, occurring in Arizona, southern California, Baja California, Baja California Sur, Sonora, and Chihuahua. Notes: Thallus morphology of B. dispersa is extremely variable, but the taxon has a rather uniform anatomy and secondary chemistry. Three different morphotypes were distinguished by Bungartz et al. (2002a). These morpho-types may, to some extent, correspond to separate species, but preliminary molecular studies were consistent with the inference, that eventually even more taxa may have to be distinguished. In the current treatment, we adopt a very wide species concept for B. dispersa, as further studies are necessary.