Thallus: crustose, subsquamulose to distinctly squamu-lose, thin to moderately thickened, ±continuous or becoming dispersed; prothallus: absent surface: pale brown to olive brown, dull, smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: similar to the mamillana-type, i.e., initially lecanorine, but soon becoming lecideine; (0.1-)0.2-0.4(-0.6) mm in diam., soon adnate to sessile margin: brown, blackening with maturity, thickening with maturity, inconspicuous in immature apothecia, but soon becoming distinct, initially emerging from the thallus with a ±well developed pseudothalline margin that becomes excluded with maturity disc: dark brown to blackened, epruinose, plane, sometimes ±convex proper exciple: fist thin, in-distinct but soon thickening, i.e., inner excipular hyphae distinct, hyaline but becoming pigmented with age, pros-oplectenchymatous (textura oblita), extending from the hyaline subhymenium and the brown hypothecium (lepto-clinoides-brown, textura intricata), outer excipular hyphae short-celled, cells angular, ±swollen (textura angularis) and ±carbonized with various amounts of a brown pigment (cf. elachista-brown, HNO3-) 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, ellipsoid, not constricted, with obtuse ends, not curved, (9-)9.8-[11.7]-13.6(-15) x (5-)5.2-[5.8]-6.5(-7) µm (n=25); proper septum: soon but only very briefly becoming thickened during spore ontogeny (Buellia-or indistinct Physconia-type); ornamentation: microrugulate Pycnidia: infrequent, urceolate to globose, unilocular, at maturity lined with short barely branched conidiophores; ontogeny similar to the Roccella-type conidiogenous cells: terminal, not intercalary (conidiophore-type III) conidia: filiform, 14-22.5 x 0.5-1 µm (n=30) Spot tests: all negative (K-, P-, C-, KC-, CK-) fluorescence: UV- (dark) iodine reaction: medulla non-amyloid Secondary metabolites: none detected (J.A. Elix, HPLC). Substrate and ecology: currently only known from the bark of Fouquieria species World and Sonoran distribution: possibly a subtropical species but currently known only from coastal Baja California Sur. Notes: Buellia fouquieriensis is one of the species with a hyaline subhymenium that is distinctly differentiated from the dark hypothecium. The only other species from the Sonoran region, where this phenomenon can be observed, is the saxicolous B. lacteoidea. The two species have otherwise a very different growth morphology and anatomy, and are clearly not related. The thallus morphology of B. fouquieriensis resembles that of B. badia. However, the thalli have not been observed to grow parasitically during any of their life stages, and they are a paler, sordid brown rather than the dark brown "chocolate" color typical for B. badia. Buellia badia has bacilliform conidia and lacks the thalline margin that is present in young apothecia of B. fouquieriensis. Thalli of B. fouquieriensis also resemble exuberantly developed forms of B. dakotensis, both species with filiform conidia and immature apothecia that initially develop a thalline margin, which later becomes reduced. Spores of B. fouquieriensis are not as broad as in B. dakotensis and do not become conspicuously constricted with age. In the Sonoran Region, B. dakotensis is restricted to montane to subalpine forests, whereas B. fouquieriensis is currently known only from coastal Baja California; B.badia is widely distributed throughout the entire region, though usually not very abundant. Marbach (2000) identified one specimen from Baja California Sur as B. extenuata, a problematic taxon not clearly distinguished from B. punctata (see the discussion there). This specimen in hb. Kalb from Punta Conejo, BajaCalifornia Sur, growing on Fouquieria, has a well developed, subsquamulose thallus and therefore clearly belongs to B. fouquieriensis.