Life habit: lichenized Thallus: ±crustose or sometimes granular, areolate, rimose, or entirely without cracks, rarely discontinuous, composed of discrete to contiguous areoles; prothallus: often absent, when present usually thin, endosubstratal, white or pale gray, sometimes as a distinct episubstratal blackish line surface: smooth, wrinkled, or warted, without isidia, soredia, or goniocysts cortex: usually present as a phenocortex and an epinecral layer, both often inspersed with minute crystals medulla: very thin, sometimes indiscernable photobiont: primary one chlorococcoid green alga, secondary one absent Ascomata: apothecial, biatorine, up to c. 2 mm wide, usually orange, orange-, red-, or purple-brown, or black, sometimes with white pruina, containing various combinations of a green to blue-green (K-, N+ purple), brown (K+ purplish, N+ orange), pale yellow to orange (K+ intensifying, N+ intensifying), blue-violet to purple-brown (K+ green, N+ purple), red to red-black (K-, N-), brown to red-brown (K+ purple-red, N-), and a bronze-yellow (K-, N-) pigment; proper exciple: annular, prosoplechtenchymatic, composed of radiating ±branched and anastomosed hyphae with very thick, gelatinized, conglutinated walls, and thin, narrowly cylindrical lumina [thinner in the lower (older) than in the upper (younger) part of the exciple]; terminal 1-2(-8) cells of the excipular hyphae forming a zone along the outer edge of the exciple; crystals: absent or present epithecium: containing pigment and sometimes crystals hymenium: hyaline, 55-130 µm tall; paraphyses: straight, 0.8-1.5(-2) µm wide in mid-hymenium, ±conglutinated, unbranched or sparingly branched in upper part, often also anastomosed; apical cells: often swollen hypothecium: colorless to almost black, without crystals and oil droplets, not chondroid asci: clavate, surrounded by a gelatinous, amyloid sheet, with a well-developed, amyloid tholus containing an axial body, a well-developed, conical ocular chamber, and sometimes a deeper amyloid, conical zone around the axial body (Bacidia- or Biatora-type), 8-spored ascospores: hyaline, transversely 3- to many-septate, clavate to acicular, straight, curved or sigmoid, without a halo or perispore Conidiomata: pycnidial, ±immersed, upper part of wall containing same pigments as apothecia, unilocular, with short, sparingly branched conidiophores and ampulliform conidiogenous cells conidia: acrogenous, (1) filiform, curved, non-septate (rarely with up to 9 septa), or (2) short-fusiform to ellipsoid to oblong, straight, non-septate (very rare) Secondary metabolites: atranorin, rarely zeorin and orcinol meta-depsides Geography: arctic to tropical regions of the world Substrate: mostly bark, in some cases rock or soil. Notes: Traditionally, the genus Bacidia has included numerous unrelated taxa with biatorine apothecia, transversely 3- or more septate ascospores, and a chlorococcoid photobiont, some of which are now referred to other families, e.g., Pilocarpaceae, Ectolechiaceae, and Fuscideaceae. Ekman (1996a) attempted a more restricted delimitation of the genus, but a recent molecular investigation (Ekman 2001) strongly indicates that even that delimitation was far too generous. In fact, among the species treated here, only B. campalea, B. heterochroa, and B. salazarensis belong in Bacidia s.s. The remaining taxa should probably be referred to Toninia s. lat. (Bacidia bagliettoana, B. circumspecta, B. coruscans, B. reagens, B. subincompta, B. veneta, and B. vermifera), Bacidina s. lat. (Bacidia medialis), Biatora s. lat. (Bacidia beckhausii), and other unknown genera outside the Ramalinaceae (B. jacobi). The generic description above has been adapted from Ekman (1996a) to fit Bacidia in the very strictest sense. Bacidia is classified here in the Ramalinaceae, which, according to Ekman (2001), includes the Bacidiaceae. Bacidia kingmanii Hasse, sometimes appearing in the literature, belongs in Lecidea s. s. The key below has been designed to work for specimens with normal apothecium pigmentation. However, +pigment deficient specimens have been observed from the Sonoran region in a number of species, viz, B. bagliettoana, B. coruscans, and B. subincompta. Some taxa are repeatedly misidentified as species of Bacidia, notably Cliostomum griffithii and members of the Arthoniales (particularly Bactrospora spp.). Fresh specimens of the former sometimes possess 2-3 guttulae in each of the two cells of the ascospores. Sometimes, these 4-6 guttulae are interpreted as cells and the specimen consequently identified as a Bacidia. Members of the Arthoniales being confused with Bacidia possess a Trentepohlia photobiont and often a continuously pigmented, brown-black proper exciple and and hypothecium. In addition, Bactrospora has asci that are easily detached from the ascogenous hyphae when squashed in K.