Thallus: crustose, areolate, thin to moderately thickened, ±continuous; prothallus: usually distinct, black, fimbriate along the thallus outline surface: dull, and smooth, not shiny, usually white, rarely pale gray, often pruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.3-0.4(-0.5) mm in diam., immersed, rarely emergent margin: black (or color masked by grayish remains of necrotic thalline material, thalline veil), thin, rarely persistent, usually excluded with age disc: black, often whitish pruinose, plane, rarely becoming 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 (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. elachistabrown) asci: clavate, Bacidia-type, 8-spored ascospores: soon brown, 1-septate, oblong to ellipsoid, usually not constricted, with obtuse ends, not curved, (9-)9-[9.8]-10.6(-12) x (4-)4.6-[5.2]-5.7(-6.5) µm (n=60); proper septum: narrow, not thickening during spore ontogeny (Buellia-type); ornamentation: distinctly rugulate Pycnidia: infrequent, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: bacilliform, 6-11 x 1-1.5 µm (n=20) Spot tests: usually K+ yellow, rarely red (±crystals), P- or + faintly yellow, C-, rarely C+ fleeting pink, KC-, rarely KC+ fleeting pink, CK- (reactions often weak) fluorescence: UV-(pale) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin and chloroatranorin, and divaricatic, nordivaricatic, and 3-chlorodivaricatic, 2'-0-methylperlatolic; 3,5-dichloro-2'-O-methylanziaic, and 3,5-dichloro2'-O-methylnorimbricaric acids; rarely also with norstictic, cryptostictic, and stictic acids, and an unknown trace substance (J. A. Elix, HPLC). Substrate and ecology: epilithic, a distinctly maritime species, typically found on siliceous rocks (HCl-) close to the seashore World distribution: Mediterranean regions of Europe and northern Africa and west coast of North America Sonoran distribution: seashores of southern California, Baja California, and Baja California Sur and extending inland to Arizona, Sonora, and Chihuahua. Notes: Rico et al. (2003) considered the species as very closely related to Dimelaena radiata. Emending the genus concept of Dimelaena to include taxa with a fimbriate margin is, however, problematic. A lobate margin is the major criterion currently used to segregate Dimelaena from Buellia. Although the two taxa are very similar they, nevertheless, maintain a distinctly different thallus morphology when growing together in the same habitat. This is consistent with the inference that the different morphologies do not represent environmental variation, and that the two taxa are indeed separate species.