Thallus: crustose, rimose to rimose-areolate, moderately thickened, ±continuous; prothallus: absent or delimiting the thallus as a black outline where several different thalli meet surface: pale white to ivory, dull, not smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.4-0.8(-1.2) mm in diam., soon sessile margin: black, thin to moderately thickened, ±persistent, excluded with age disc: black, epruinose, plane, often 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 (leptoclinoides-brown, 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, occasionally with two additional false septa, proper septum becomes soon and distinctly thickened during spore ontogeny, ellipsoid to ±citriform, not constricted, ±tapered into droplet-shaped ends, frequently slightly curved, (16-)19.3-[21.9]-24.4(-26.5) x (7-)8.2-[9.4]-10.6(-11) µm (n=25), with lateral wall ±thickened (Callispora-type); ornamentation: faintly microrugulate (barely seen without DIC) Pycnidia: rare, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: elongate bacilliform, straight, 6.5-9 x 0.5-1 µm (n=30) Spot tests: K+ yellow to orange red (rapidly forming needle-shaped crystals), P- to weakly yellow, C- fluorescence: UV- (pale) iodine reaction: medulla non-amyloid Secondary metabolites: norstictic and ±connorstictic acids, atranorin. Substrate and ecology: on bark or wood of shrubs in subtropical thorn-brush World distribution: North and Central America, originally described from Florida, USA Sonoran distribution: currently only known from thornbrush of the Sierra La Laguna, and the Sierra San Lazaro, near the southern tip of Baja California Sur. Notes: Buellia imshaugiana is recognized by its Callispora-type ascospores with a conspicuously thickened septum, frequently large apothecia, a strongly K+ orange-red reaction (rapidly forming crystals), and elongate bacilliform conidia. According to Marbach (2000), the ascospores have a coarsely ornamented surface ("grob skulpturierte Oberfläche"). Spores of the Sonoran material, however, only have a faintly microrugulate surface.