Thallus: crustose, areolate to subsquamulose, thick, ±continuous; prothallus: absent surface: white to pale brown, dull, rough, epruinose or indistinctly pruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.4-0.6(-1.1) mm in diam., soon sessile margin: black, thin, rarely persistent, soon excluded disc: black, epruinose, plane, sometimes ±convex proper exciple: similar to the dispersa-type, inner excipular hyphae distinct, not reduced, pigmented, prosoplectenchymatous (textura oblita), extending from the deep reddish brown hypothecium (leptoclinoidesbrown, textura intricata), outer excipular hyphae short-celled, cells angular, distinctly swollen (textura angularis) and ± carbonized with various amounts of a brown pigment (cf. elachista-brown, HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, strongly inspersed with oil droplets; paraphyses: simple to moderately branched, apically swollen, with a brown pigment cap (cf. elachista-brown) asci: 8-spored, clavate, Bacidia-type ascospores: soon brown, 1-septate, ellipsoid to ±citriform, usually not or indistinctly constricted, with obtuse or pointed ends, not curved, (15-)16.5-[18.4]-20.3(-21.5) x (7.5-)8-[9.2]-10.5(-11) µm (n=26); proper septum: becoming soon distinctly thickened during spore ontogeny, lateral wall ±thickened (Callispora-type); ornamentation: not visible in DIC 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, 3.5-5 x 0.5-1 µm (n=30) Spot tests: K+ yellow, P-, C- fluorescence: UV- (pale) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin, chloroatranorin, diploicin, dechlorodiploicin, fulgidin, isofulgidin (J. A. Elix, HPLC). According to Marbach (2000) also with traces of norstictic acid, but not detected by HPLC, no crystals formed if thallus or apothecial sections are tested with K. Substrate and ecology: so far only found on decorticated wood of Juniperus or bark of Quercus World and Sonoran distribution: Baja California (Sierra La Laguna, and Guadalupe Island) and Baja California Sur (Bahía Tortugas). Notes: Buellia desertica is very distinct and can hardly be confused with other epiphytic species of Buellia. Most characteristic is its thick, distinctly areolate thallus, its broad, Callispora-type ascospores with their conspicuous septal thickening, and its strongly oil inspersed hymenium. The saxicolous B. regineae sometimes develops a superficially similar thallus, but that species consistently has smaller ascospores, contains different secondary lichen metabolites and has a medulla that reacts strongly blue with Lugol's iodine.