Thallus: crustose, rimose-areolate, flat to verrucose, rarely subsquamulose; prothallus: not seen surface: white to greenish gray, smooth, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine, abundant, often tightly crowded, 0.2-0.7 mm in diam., first immersed but soon becoming sessile disc: black, epruinose, first flat, later ▒convex margin: black, distinct on young apothecia, later excluded proper exciple: 15-55 Ám thick, lacking secondary metabolites; differentiated into a thin, pale to dark brown outer part with carbonized cells (<6 Ám, HNO3-), and a broad, dark brown inner part, transient with the brown, <90 Ám thick hypothecium (HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, not inspersed with oil droplets, 60-70 Ám tall; tips of paraphyses: ▒5.5 Ám wide with distinct apical caps asci: clavate, Bacidia-type, 45-60 x 13-20 Ám, 8-spored ascospores: soon brown, 3-septate, rarely with longitudinal septa on either side of the median septum, ellipsoid, sometimes curved, (14-)16.1-[17.7]-19.3(-22) x (5.5-)6.6-[7.2]-7.9(-9) Ám, walls and septa lacking uneven thickenings, proper wall c. 0.55 Ám thick, perispore c. 0.2 Ám thick, ornamentation: rugulate Pycnidia: rare, immersed, with uppermost part protruding, wall pigmented in upper part conidia: bacilliform, c. 4-6 x 1 Ám Spot tests: thallus K-, C-, P- fluorescence: UV- iodine reaction: not checked Secondary metabolites: absent or traces of atranorin. Substrate and ecology: on bark and wood of broad-leaved and coniferous trees, mainly branches and twigs, in open situations across wide elevational ranges, often closely associated with Lecanora, Candelariella, and Rinodina spp. World distribution: southwestern Europe and western North America Sonoran distribution: montane regions of Arizona, southern California, and Baja California (Guadalupe Island). Notes: Buellia triseptata is characterized by its small, often crowded apothecia and its 3-septate spores. The species is similar to B. triphragmioides on the basis of spore characters, but differs by having a grayish white thallus, a different secondary chemistry (lacking arthothelin), its smaller apothecia and spores, and its longer conidia.