Thallus: crustose, areolate, thin to moderately thickened, ±continuous; prothallus: conspicuous black, in most specimens strongly developed and growing between the areoles (forming a hypothallus), rarely only surrounding the thallus outline surface: usually white to whitish gray, rarely dark gray, dull or ±shiny, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.3-0.4(-0.5) mm in diam., immersed to adnate, rarely sessile margin: black or color masked by grayish remains of necrotic thalline material (thalline veil), thin, ±persistent, rarely excluded with age disc: black, epruinose, plane, rarely becoming slightly convex with age 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 (leptoclinoidesbrown, textura intricata), outer excipular hyphae parallel, moderately swollen (textura oblita) and strongly 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-)8.7-[9.9]-11.1(-13) x (4.5-)4.8-[5.5]-6.1(-7) µm (n=60); proper septum: narrow, not thickened 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, 3.5- 4 x 0.5-1 µm (n=20) Spot tests: K+ yellow (sometimes weak), P- or + faintly yellow, C-, KC-, CK- fluorescence: UV-(dark) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin, 2'-O-methylperlatolic and confluentic acid. Substrate and ecology: epilithic, on siliceous rock (generally HCl-), usually close to the coast but not confined to seashore habitats World distribution: widely distributed throughout the Northern Hemisphere Sonoran distribution: coastal areas of southern California, Baja California, and Baja California Sur. Notes: Buellia stellulata is very similar to B. spuria, but contains 2'-O-methylperlatolic acid, not stictic or norstictic acid, and the thallus medulla is I-. Specimens reported from inland localities as B. stellulata belong to B. spuria.