Thallus: crustose, usually rimose to rimose-areolate, rarely becoming thickened and distinctly rimose-areolate to subsquamulose, thin to moderately thickened, ±continuous; prothallus: absent surface: usually deep brown, rarely pale brown, dull or rarely ±shiny, usually smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.3-0.5(-0.8) mm in diam., soon sessile margin: black, conspicuously thickened, usually persistent, rarely excluded with age disc: black, epruinose, plane, rarely becoming slightly convex proper exciple: narrow, poorly differentiated, aethalea-type, inner excipular hyphae narrow, hyaline, prosoplectenchymatous (textura oblita), often reduced, similar in structure and orientation to the paraphyses, transient with the deep reddish brown hypothecium (leptoclinoides-brown, 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, broadly ellipsoid to almost globose, never constricted, with obtuse ends, not curved, (9-)10.3-[11.7]-13.2(-15) x (6-)6.8-[7.7]-8.6(-10) µm (n=81); proper septum: narrow, not thickening during spore ontogeny, wall evenly thickened in premature spores (Buellia-type); ornamentation: not visible in DIC Pycnidia: rare to common, globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 2-4.5 x 1-1.5 µm (n=69) Spot tests: all negative fluorescence: UV-(dark) iodine reaction: medulla non-amyloid Secondary metabolites: no substances detected or traces of norstictic and connorstictic acids (occasionally detected by TLC and very rarely forming orange, needle-shaped crystals in K). Substrate and ecology: epilithic, on siliceous mineral-poor coastal rock (HCl-) World and Sonoran distribution: known only from coastal southern California, Baja California, and Baja California Sur. Note: Buellia christophii is superficially similar to B. pullata and differences are discussed in the description of that species.