Thallus: crustose, areolate, thin, ±continuous; prothallus: distinct, black, usually surrounding the thallus and growing between the areoles (forming a hypothallus) surface: usually gray to pale brown, rarely dark gray, dull, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.1-)0.2-0.2(-0.5) mm in diam., remaining immersed, not becoming sessile, angular to irregularly circular ("comma"-shaped), predominantly in the center of an areole margin: black, indistinct, reduced, inconspicuous disc: black, epruinose, plane, not becoming convex with age proper exciple: narrow, poorly differentiated, aethalea-type, inner excipular hyphae narrow, hyaline, prosoplectenchymatous (textura oblita), often reduced, similar in structure and orientation to the paraphyses, transient with the hyaline hypothecium (pigment absent in Sonoran specimens; textura intricata), outer excipular hyphae parallel, moderately swollen (textura oblita) and usually strongly carbonized with various amounts of brown and aeruginose pigments (cf. elachista-brown and cinereorufa-green, HNO3+ violet) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3+ violet) 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, broadly ellipsoid, constricted with age, with obtuse ends, not curved, (11-)11.6-[12.8]-14.1(-17) x (5-)7.2-[8.1]-8.9(-10) µm (n=60); proper septum: narrow, not thickening during spore ontogeny (Buellia-type); ornamentation: microrugulate Pycnidia: rare, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 5-5.5 x 1 µm (n=20) Spot tests: thallus and medulla K+ yellow to red (crystals), P+ yellow, C-, KC-, CK- fluorescence: UV- (dark) iodine reaction: medulla non-amyloid [Sonoran specimens, other specimens with variable reaction according to Scheidegger (1993)] Secondary metabolites: norstictic and connorstictic acids. Substrate and ecology: epilithic, on a variety of siliceous (HCl-) rock substrates World distribution: common in temperate climates of northern Europe but rarely reported from North America Sonoran distribution: widely distributed at higher elevations of Arizona, including the San Francisco Peaks and White Moutains. Notes: Scheidegger (1993) reports that hypothecium pigmentation of European specimens varies from hyaline to dark brown. All Sonoran specimens have a hyaline to faintly brown hypothecium. This is rather unusual because Buellia s.str. is generally characterized by a dark hypothecium. Scheidegger (1993) also reports that the medulla reaction with Lugol's iodine varies considerably. All Sonoran specimens are, however, I-. Imshaug (1951) suggested that the species was not found in the USA or Canada, but now its presence has been confirmed.