Thallus: crustose, areolate; prothallus: black, sometimes also developed between areoles (forming a hypothallus) surface: grayish white, often with a faintly yellowish tinge, smooth, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine, immersed within the thallus or between areoles, level with the thallus surface or slightly protruding, abundant, irregularly angular to circular, <0.5 mm in diam. disc: black, epruinose, flat margin: black, thin, almost entirely reduced when immersed in the thallus proper exciple: 15-40 µm thick, lacking secondary metabolites; differentiated into a broad, aeruginose outer part without carbonized cells (<5 µm, cinereorufa-green, HNO3+ violet), and a pale central part, transient with the hyaline to blackish brown, <230µm thick hypothecium (cinereorufa-green, HNO3+ violet) epihymenium: aeruginose, pigmentation continuous with the outer exciple (HNO3+ violet) hymenium: hyaline in middle part, aeruginose in upper and lower part, not inspersed with oil droplets; tips of paraphyses: 2-4 µm wide with indistinct apical caps asci: clavate, Bacidia-type, 50-64 x 15-26 µm, 60-85 µm tall, 8-spored ascospores: soon brown, transversally 3-septate, usually with one longitudinal septum on either side of the median septum, with 4-6 cells in optical section, ellipsoid, (18-)18.2-[19.4]-20.6(-22) x (8.5-)9.3-[10.1]-10.9(-11.5) µm (n=20), walls and septa lacking uneven thickenings, proper wall ca. 0.4 µm thick, perispore ca. 0.25 µm thick, ornamentation: rugulate Pycnidia: rare, immersed, with uppermost part protruding, wall pigmented in upper part conidia: bacilliform, 9-11 x c. 1 µm Spot tests: thallus K+ yellow, C-, P+ yellow; medulla K+ yellow turning red (crystals), C-, P+ yellow-orange fluorescence: UV- iodine reaction: medulla non-amyloid Secondary metabolites: norstictic acid (major), atranorin (minor), and connorstictic acid (trace). Substrate and ecology: epilithic, on volcanic or granitic rocks in the alpine zone World distribution: central Mexico and Arizona Sonoran distribution: northern and eastern Arizona (Mt. Humphreys and Mt. Baldy). Notes: Buellia mexicana is characterized by its areolate thallus, its immersed, often angular apothecia, and its aeruginose pigment in the proper exciple and epihymenium. In the field it might be confused with Buellia species with 1-septate spores related to B. aethalea, but differs on the basis of its pluriseptate spores.