Thallus: crustose, discontinuous, thin, hidden, ±endosubstratic (chasmolithic); prothallus: absent surface: usually pale gray, rarely pale brown, dull, not smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.5-1.1(-1.2) mm in diam., soon sessile margin: black, thin, usually persistent, rarely excluded with age disc: black, epruinose, plane, ±becoming slightly convex with age proper exciple: vilis-type, inner excipular hyphae hyaline (deep I+ blue), prosoplectenchymatous, loosely interwoven (textura intricata), transient with the hyaline hypothecium (reacting strongly I+ blue, textura intricata), outer excipular hyphae parallel, thin (textura oblita) and usually very strongly carbonized with large amounts of a dull brown-red to blackish pigment (atra-red, HNO3+ deep purple) epihymenium: dark brown, pigmentation continuous with the outer exciple (HNO3+ deep purple) hymenium: hyaline, not inspersed with oil droplets; paraphyses: simple to moderately branched, apically swollen, with a deep brown to black pigment cap (atra-red) asci: clavate, Bacidia-type, 8-spored ascospores: soon brown, 1-septate, oblong to ellipsoid, usually constricted (with age), with obtuse ends, not curved, (12-)13.1-[14.5]-15.9(-18) x (5-)5.7-[6.6]-7.4(-9) µm (n=60); proper septum: narrow, not thickening during spore ontogeny (Buellia-type); ornamentation: not visible in DIC Pycnidia: unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 4.5-7 x 0.5-1 µm (n=20) Spot tests: all negative (K-, P-, C-, KC-, CK-) fluorescence: UV-(dark) iodine reactions: thallus, hymenium and inner exciple strongly amyloid Secondary metabolites: none detected. Substrate and ecology: chasmolithic (i.e., forming inconspicuous, poorly delimited granules hidden among the mineral grains of the substrate), typically found on small pebbles in exposed habitats (Scheidegger 1993), but specimens from Arizona and New Mexico were found on sandstone, in montane to alpine habitats World distribution: temperate to arctic species throughout the Northern Hemisphere Sonoran distribution: one single collection from Arizona (Mogollon Rim; Coconino Co.). Notes: Because of their chasmolithic growth, Sonoran specimens which clearly belong to Buellia sequax have frequently been misidentified as B. vilis. Within the genus Buellia, B. vilis is unique and is characterized by its deep reddish brown outer exciple, and its hyaline inner exciple and hypothecium. Both inner exciple and hypothecium react very strongly I+ blue (amyloid). In addition, thallus hyphae of the chasmolithic Buellia vilis react deep blue with iodine whereas such a thallus reaction is absent in B. sequax. Buellia vilis is very isolated even within an expanded genus concept of Buellia because of its unique exciple type characterized by a pigment not found in any other species. Like several other species discussed here, B. vilis would have to be placed into a monotypic genus, if all recent segregates of Buellia were accepted. Even though that might be justified in the case of B. vilis, it remains less convincing for taxa like B. trachyspora, B. mamillana, or even B. badia. All these species do have some unusual characters, that could be used to segregate them from Buellia s. str., but newly erected monotypic genera do not necessarily clarify their taxonomic position, and only emphasize their isolated positions.