Thallus: crustose, rimose, moderately thin, ±continuous; prothallus: absent or delimiting the thallus as a black outline surface: usually pale ivory, rarely gray, dull or ±shiny, smooth or slightly roughened (as a result of surface damage in herbarium specimens?), epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.3-)0.4-0.6(-0.9) mm in diam., soon sessile margin: black, thin, ±persistent, sometimes excluded with age disc: black, epruinose, plane, sometimes ±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 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, (9-)11.9-[13.5]-15(-16) x (5-)6.2-[7.3]-8.3(-9.5) µm (n=109); proper septum: narrow, not thickening during spore ontogeny, (±Physconia-or Orcularia-type); ornamentation: microrugulate (conspicuous in DIC at an early stage of the ontogeny) Pycnidia: rare, globose, unilocular, at maturity lined with short barely branched conidiophores; ontogeny similar to the Roccella-type conidiogenous cells: terminal, not intercalary (conidiophore-type III) conidia: filiform, 7-43 x 0.5-1 µm (n=106) Spot tests: usually all negative (K-, P-, C-, KC-, CK-), rarely K+ yellow-orange (crystals), C+ orange fluorescence: usually UV+ bright yellow to orange, sometimes faint (thin specimens) iodine reaction: medulla non-amyloid Secondary metabolites: no substances, or with various amounts of ± norstictic and connorstictic acid, ±atranorin and / or several xanthones: 4,5-dichloro3-O-methylnorlichexanthone, thuringione, 4,5-dichlorolichexanthone, and 2,4,5-trichlorolichexanthone (J. A. Elix, HPLC). Substrate and ecology: epilithic, on siliceous mineral-poor coastal rock (generally HCl-) World distribution: probably cosmopolitan but restricted to coastal areas Sonoran distribution: southern California, Baja California, Baja California Sur, Sonora, and Sinaloa. Notes: In general, B. prospersa can be easily recognized by its pale, distinctly epilithic thallus and ascospores with a persistent septal thickening. Buellia pullata is microscopically similar, but has a deep brown to dark gray thallus, that does not react with UV-light. In B. prospersa, septal thickenings are persistent and thus easily observed. Spore septa of Buellia pullata also become slightly thickened, but this thickening becomes soon reduced. This may be one of the reasons why Imshaug (1951) thought B. pullata was a synonym of B. punctata. Poorly developed thalli of B. prospersa sometimes appear ±chasmolithic, and these specimens are then difficult to separate from forms of B. sequax with a well developed thallus. If filiform conidia can be found, the material is easily recognized, because B. sequax has bacilliform conidia; however, in both species pycnidia are quite rare. Young ascospores of B. sequax are usually narrowly oblong and all stages of the spore ontogeny lack septal thickenings. Buellia sequax is widely distributed throughout the Sonoran Region, but B. prospersa is restricted to coastal habitats.