Thallus: crustose, areolate to subsquamulose, moderately thick, ±dispersed; prothallus: absent or, if present, indistinct and not prominent surface: usually pale beige, rarely pale gray, dull or ±shiny, smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.3-0.5(-0.7) mm in diam., remaining immersed, not becoming sessile, angular to irregularly circular, predominantly along the margin of an areole margin: black, indistinct, reduced, inconspicuous disc: black, epruinose, plane, angular, rarely becoming slightly convex proper exciple: narrow, poorly differentiated, of the aethalea-type, inner excipular hyphae narrow, hyaline, prosoplectenchymatous (textura oblita), often reduced, similar in structure and orientation to the paraphyses, transient with the hyaline to faintly brown hypothecium (pigmentation ± absent, textura intricata), outer excipular hyphae parallel, moderately swollen (textura oblita) and usually strongly 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, oblong to ellipsoid, usually not constricted, with obtuse ends, not curved, (12-)12.1-[13.7]-15.2(-18) x (6-)6.3--7.7(-9) µm (n=60); proper septum: narrow, not thickening during spore ontogeny (Buellia-type); ornamentation: microrugulate Pycnidia: rare, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 4- 7 x 1-1 µm (n=20) Spot tests: thallus and medulla C+ pink (fleeting), KC+ pink or C-, KC-, K-, P- fluorescence: UV+ pale beige to yellow iodine reaction: medulla non-amyloid Secondary metabolites: lecanoric and 5-O-methylhiascic acids (J. A. Elix, HPLC). Substrate and ecology: epilithic, on a variety of siliceous (HCl-) rock substrates in subalpine to alpine habitats World distribution: southwestern USA, including Lake Peak and Sierra Blanca Peak (New Mexico) Sonoran distribution: currently only known from Arizona, including the San Francisco Peaks and the White Mountains. Notes: Buellia eganii is very similar to B. aethalea, and they are most reliably distinguished by their different secondary chemistries (e.g. lecanoric in B. eganii and norstictic acid in B. aethalea) and the reaction of epihymenium with HNO3. In B. eganii apothecia tend to be formed along the margin of thallus areoles, but in B. aethalea they are usually formed in the center. In both species, apothecia usually remain immersed and often become irregular in outline due to pressure from adjoining areoles.