Thallus: crustose, flat, continuous or rimose, occasionally flaking-off and in parts becoming subsquamulose, thin to thick; prothallus: sometimes present, black surface: chalky white to ochraceous, pale gray or pale brown, esorediate medulla: white, with calcium oxalate (H2SO4+ needle shaped crystals) Apothecia: lecideine, but initially often with a thalline collar (pseudolecanorine), initially immersed, later ±sessile, abundant, 0.3-1(-1.5) mm in diam., usually <0.5 mm disc: black, often pruinose with white pruina consisting of calcium oxalate crystals, flat to ±convex margin: black, moderately to poorly developed, excluded in strongly convex apothecia but in young apothecia with a thalline veil or rim or with thalline granules proper exciple: <50 µm thick, often poorly developed and frequently inspersed with calcium oxalate crystals (H2SO4+ needle shaped crystals), lacking secondary metabolites; differentiated into a thin, dark brown outer part without carbonized cells (HNO3-), and a pale central part, transient with the hyaline to deep brown, <190 µm thick hypothecium (HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, not inspersed with oil droplets, 60-110 µm tall; tips of paraphyses: ±6 µm wide with distinct apical caps asci: clavate, Bacidia-type, 36-63 x 15-17 µm, 8-spored ascospores: soon brown, submuriform, with 3 transversal and usually few longitudinal septa, thus 6-8-celled in optical section, ellipsoid, (13-)14.7--17.3(-20) x (6.5-)7.5-[8.3]-9.1(-10) µm (n=224), walls and septa lacking uneven thickenings, proper wall c. 0.3 µm thick, perispore c. 0.4 µm thick; ornamentation: rugulate Pycnidia: rare, immersed, with uppermost part protruding, wall pigmented in upper part conidia: bacilliform, 6-10 x 1 µm Spot tests: thallus and medulla K- or + yellow turning red (crystals), P- or + yellow-orange, C- fluorescence: UV- iodine reaction: medulla not amyloid, or rarely amyloid Secondary metabolites: absent or norstictic acid and connorstictic acid in the medulla. Substrate and ecology: epilithic, on calciferous and siliceous rocks, bark and lignum close to the sea-shore as well as at high elevations, usually at N-facing, overhanging surfaces; at young stages often parasitic on various lichens, particularly Caloplaca spp. World distribution: Africa, Asia, Australia, Europe, North America, and South America Sonoran distribution: montane localities in Arizona, southern California coastal mountains and the Channel Islands, Baja California (Guadalupe and Cedros Islands), and Sinaloa. Notes: Buellia alboatra is characterized by its brown, submuriform, and relatively small spores, and small, often pruinose apothecia that are usually surrounded, at least initially, with a thalline collar. The species is similar to Buellia venusta and B. subdispersa, but these mainly differ by having 3-septate spores. It is often placed in the genus Diplotomma (e. g., Esslinger 1997, Purvis 1992). Norstictic acid containing specimens are often regarded as a separate taxon, Buellia chlorophaea (Diplotomma chlorophanum). Also, specimens growing on Xanthoria elegans and Caloplaca teicholyta have been designated as separate taxa, as Buellia nivalis (Diplotomma nivale) and Diplotomma murorum respectively.