Thallus: crustose, rimose to rimose-areolate, thin to moderately thickened, ±continuous; prothallus: absent or delimiting the thallus margin as a black outline surface: usually ivory to pale brown, rarely grayish brown, usually dull or ±rarely shiny, smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.4-0.6(-0.8) mm in diam., soon adnate to sessile margin: black, prominent, usually persistent, rarely excluded with age disc: black, epruinose, plane, rarely becoming convex with age proper exciple: distinct, leptocline-type, exciple thick and not distinctly differentiated into an inner and outer part, hyphae thin-walled (mesodermatous), prosoplectenchymatous and usually ±densely interwoven (textura ±intricata), dull fuscous brown throughout, becoming ±carbonized by various amounts of a brown pigment (cf. elachista-brown, HNO3-), hypothecium deep reddish brown (leptoclinoidesbrown, textura intricata) 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, oblong to ellipsoid, usually not constricted, with obtuse ends, not curved, (9-)11.3-[12.8]-14.2(-16) x (5-)5.8[6.7]-7.6(-8) µm (n=60); proper septum: becoming distinctly thickened during spore ontogeny (Physconia-type); ornamentation: microrugulate to rugulate Pycnidia: rare, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: short to elongate bacilliform, 7-14.5 x 1-1.5 µm (n=20) Spot tests: K+ yellow (±crystals), P+ yellow, C-, rarely C+ pink, KC-, rarely KC+ pink, CK- fluorescence: UV-(pale) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin, and gyrophoric, norstictic, and secalonic acids, and eumitrin x, eumitrin x2, eumitrin y (J. A. Elix, HPLC). Substrate and ecology: epilithic, on exposed siliceous rocks (HCl-) World distribution: widespread in the Mediterranean region of Europe (Scheidegger 1993) and southwestern North America Sonoran distribution: very rare, currently known only from one locality in Sinaloa. Notes: Scheidegger (1993) distinguishes the following morphologically, anatomically and ecologically similar taxa: Buellia subdisciformis (atranorin, norstictic and connorstictic acids, medulla I-), Buellia sardiniensis (atranorin, gyrophoric, norstictic and connorstictic acids, medulla I+ blue) and Buellia saxorum (atranorin, gyrophoric and lecanoric acids, medulla I+ blue). Based on the negative iodine reaction of the medulla, the material is assigned here to B. subdisciformis, even though the Sonoran material regularly contains atranorin, and both norstictic and gyrophoric acid. Buellia sardiniensis might better be treated as a synonym of B. subdisciformis. However, Scheidegger (1993) also suggested that Buellia sardiniensis could be treated as a chemical strain of Buellia saxorum. It is therefore possible that all three taxa belong to the same species and the name B. saxorum would then have taxonomic priority. Without additional specimens available from the Sonoran Region this conclusion seems, however, premature.