Thallus: crustose, usually continuous, smooth, areolate, rimose, or entirely without cracks areoles: discrete, convex; prothallus: often present as a thin black line surface: almost white, ±gray, rarely gray-green or grayish yellow, warted, or wrinkled Apothecia: at first flat, remaining so or rarely becoming convex, 0.5-1.1 mm in diam. disc: basically purple-brown to black but often mottled with paler pinkish, brownish, or purplish hues, epruinose or rarely with a thin white pruina on a few young apothecia margin: concolorous with or paler than disc (especially below), at first distinct, at first raised above disc, soon level with with disc, persistent or rarely excluded; upper part of margin exciple: laterally 50-120 µm wide, without crystals or rarely with radiating clusters of crystals, interior colorless, yellowish or orange-brown to brown-orange, fading below, upper part of edge darker than interior, brown to red-brown (K+ purplish, N+ orange), fading below; edge: with single cell layer of enlarged cell lumina (up to 6 µm wide) epithecium: ±brown (K+ purplish, N+ orange) in distinct layer; pigment mostly confined to paraphysis walls hymenium: hyaline, 75-115 µm tall; paraphyses: 0.8-1.6 µm wide in mid-hymenium, apices ±clavate or not swollen, 1.2-4 µm wide, with distinct hoods of brown pigment hypothecium: colorless or pale yellow (K+ intensifying, N-) asci: clavate, 8-spored ascospores: hyaline, (3-)7-15-septate, acicular, straight to curved to sigmoid, 32-73 x 2.5-4.3 µm Pycnidia: ±immersed in thallus, brownish or black, 40-100 µm in diam. conidia: simple, filiform, curved, 5-18 x 0.8-1 µm Spot tests: all negative Secondary metabolites: none detected. Habitat and ecology: on bark of various broad-leaved trees World distribution: pantropical, extending into warm-temperate, coastal areas of the world, known from all continents and possibly the most widespread species of Bacidia on earth Sonoran distribution: known from a few localities on the Channel Islands and adjacent mainland areas of coastal, southern California. Notes: The description of Bacidia heterochroa above has been adjusted to fit western specimens of this species. Specimens from the eastern U.S. contain atranorin (Ekman 1996a).