Thallus: crustose, rimose to rimose-areolate, thin to moderately thickened, ±continuous; prothallus: absent or delimiting the thallus as a black outline where several different thalli meet surface: pale white to ivory, dull, smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.2-)0.4-0.6(-0.7) mm in diam., soon sessile margin: black, thick, ±persistent, sometimes excluded with age, rarely with a thalline veil when emerging from the thallus disc: black, epruinose, plane, rarely becoming 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 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: clavate, Bacidia-type, 8-spored ascospores: soon brown, 1-septate, occasionally with two additional false septa, narrowly ellipsoid, usually not constricted, with pointed ends, sometimes slightly curved, (12-)15.6-[18.5]-21.3(-21.5) x (5.5-)6.6-[7.7]-8.9(-10) µm (n=26); proper septum: narrow, not thickening, but young spores often with a ±thickened endospore during spore ontogeny; lateral wall: ±thickened (Callispora-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: bacilliform, 2.5-5 x 0.5-1 µm (n=30) Spot tests: K+ yellow, P-, C- fluorescence: UV- (pale) iodine reaction: medulla non-amyloid Secondary metabolites: atranorin, fulgidin, often accompanied by traces of fulgoicin and norfulgoicin. Sonoran specimens examined by J. A. Elix, Canberra, with HPLC also contained placodiolic acid, isousnic acid, brialmontin 1 and 2, gyrophoric, norstictic, 5-O-methylhiascic acid, and traces of 4,5-di-O-methylhiascic acid. Substrate and ecology: on a variety of trees, rarely also on wood World distribution: temperate areas in the northern USA and Europe Sonoran distribution: infrequent in woodlands at higher elevations in Arizona, but in southern California occasionally found at an altitude as low as 500 m, and on the Channel Islands. Notes: Marbach (2000) regarded B. parasema as a synonym of B. disciformis, although the type of that taxon contains norstictic acid. In contrast, Kalb and Elix (1998) suggested that both taxa are distinctly separate species. We agree with Marbach (2000), as the Sonoran material appears chemically quite variable. Slight differences in spore size and ontogeny may not be sufficient to treat both as separate species. More material should, nevertheless, be studied. In the Southwest, material of B. chloroleuca has consistently been misidentified as B. disciformis, even though the two species are quite distinct. Most significant are differences in thallus chemistry and oil inspersion of the hymenium. Thalli of B. chloroleuca that distinctly react C+ orange can easily be distinguished from B. disciformis. However, if xanthone concentrations are low, no obvious C reaction may be observed. In most cases, these specimens of B. chloroleuca can, nevertheless, be distinguished from B. disciformis, even without further chemical analysis. The hymenium of B. disciformis is very strongly inspersed with oil droplets. Oil inspersion is consistently absent from the hymenium of B. chloroleuca, even though occasionally a few, sparse oil droplets may be observed within subhymenium and hypothecium. In squash preparations, these oil droplets will obviously be observed throughout, but the amount of droplets is significantly lower! In B. disciformis, the inspersion strongly obscures the entire hymenium. In contrast, hymenial details can be easily studied in apothecial sections of B. chloroleuca.