Thallus: crustose, areolate, thin to moderately thickened, ±continuous; prothallus: distinct, delimiting the thallus as a black outline, rarely also between the areoles (forming a hypothallus) surface: usually ivory, rarely pale beige, dull, not smooth, pruinose, phenocorticate, esorediate medulla: white, filled with large mineral crystals (40-60 µm in diam.; H2SO4-) Apothecia: lecideine; (0.2-)0.3-0.6(-0.8) mm in diam., soon adnate to sessile margin: prominent, black or rarely masked by grayish remains of necrotic thalline material (thalline veil), ±persistent, rarely excluded with age disc: black, epruinose or faintly pruinose, plane, becoming only slightly convex proper exciple: dispersa-type, inner excipular hyphae distinct, not reduced, pigmented, prosoplectenchymatous (textura oblita), extending from the deep reddish brown hypothecium (leptoclinoides-brown, textura intricata), outer excipular hyphae short-celled, cells angular, distinctly swollen (textura angularis) and ±carbonized with various amounts of brown and aeruginose pigments (cf. elachista-brown and cinereorufa-green, HNO3+ violet) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3+ violet) hymenium: hyaline, not inspersed with oil droplets; paraphyses: simple to moderately branched, apically swollen, with a brown pigment cap (cf. elachista-brown) asci: clavate, Bacidiatype, 8-spored ascospores: soon brown, 1-septate, oblong to ellipsoid, usually not constricted, with obtuse ends, not curved, (8-)8.8-[10.2]-11.6(-13.5) x (4-)4.1-[4.8]-5.5(-6) µm (n=39); proper septum: narrow, not thickened during spore ontogeny (Buellia-type); ornamentation: not visible in DIC Pycnidia: rare, globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: short to elongate bacilliform, 6-9 x 1-1.5 µm (n=20) Spot tests: usually K+ yellow to red (crystals), P- or + yellow, C-, KC-, CK- fluorescence: UV- (dark) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin with chloroatranorin, norstictic with connorstictic. Substrate and ecology: epilithic, on siliceous mineral-poor coastal rock (generally HCl-) World and Sonoran distribution: currently known only from the coast of Sinaloa and in Baja California Sur. Notes: Because of its plane apothecia with a thick, ±persistent margin, B. sheardii superficially resembles B. subdisciformis. However, the latter species has a rimose-areolate thallus (rather than a distinctly areolate thallus), ascospores with median septum thickenings, and does not contain an aeruginose, HNO3+ violet pigment in its exciple or epihymenium.