Thallus: foliose, single lobed attached by a single umbilicus and with 9-19 mm wide lobes upper surface: gray, with epinecral layer consisting of air filled hyphae upper cortex: 70-170 µm thick, hyaline with outermost algal layer: 30-170 µm thick, photobiont regularaly scattered in upper cortex and occasionally also in medulla medulla: 60-290 µm thick, hyaline, consisting of hyphae 2-3 µm in diam. lower cortex: 30-90 µm thick, hyaline with the outermost (c. 10 µm) part brown lower surface: dark brown to almost black, in most cases distinctly reticulate Perithecia: broadly obpyriform to globose, 130-490 µm high and 90-460 µm wide, with hyaline exciple which is brown in the uppermost part; ostiolum: often slightly prodruding, black asci: clavate or cylindrical, 8-spored ascospores: simple, hyaline, 1320 x 5.5-8 µm Pycnidia: not found in Sonoran material, elsewhere rare, immersed, ostiolum similar to that of the perithecium conidia: bacilliform, 4-6 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on rocks, often basalt and usually in seepages (at least seasonal) World distribution: Europe and western North America Sonoran distribution: scattered throughout Arizona. Notes: Dermatocarpon bachmannii is close to Dermatocarpon deminuens Vain. and molecular analysis indicate that the two taxa possibly are conspecific. In a cladistic analysis of the ITS area Heiômarsson (2003) included two specimens of D. bachmannii and one of D. deminuens, were one of the D. bachmannii specimens originated in Oregon. His results indicated that the North American specimen belongs to a different taxon than the European specimens of D. bachmannii and D. deminuens, which are monophyletic in the analysis. Further study of North American material is, however, necessary before it can be concluded that D. bachmannii in North America belongs to a new species different from the European one.