Thallus: crustose, irregularly cracked to rimose areolate, usually in discrete +orbicular thalli up to c. 5 cm wide but sometimes coalescing to form large patches up to 30 cm wide, thin to thick (up to 1 mm); prothallus: absent or thin and black areoles: mostly contiguous and fused but when independent 0.3-1(-1.5) mm wide, irregular, angular, often warted-wrinkled, plane to subconvex upper surface: gray or whitish gray, smooth; cortex: not pigmented, 15-30 µm thick, with hyphae 3-4 µm thick, with an epinecral layer above (5-10 µm thick) medulla: white, I-; algal layer: 50100 µm thick; algae: 8-12 µm in diam. Apothecia: black, immersed to usually becoming sessile, sometimes constricted at the base, round or irregular, 1-2.5 mm in diam. margin: white, biatorine or lecanorine, dull disc: black, flat or slightly concave, shiny, epruinose exciple: hyaline throughout 100 to 200 µm thick, composed of hyphae c. 4 µm in diam. epihymenium: dark red-violet to red-brown hymenium: violet-red or brown above, I+ blue, 50-60 mm tall; hypothecium: dark brown to yellow-brown, 100-150 µm thick asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 10-14 x 6-8 µm conidia: cylindrical to filiform, (9-)12-21 x 1-1.5 µm Spot tests: upper cortex: K+ yellow, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with atranorin, medulla with ±α-collatolic and alectoronic acids (major or minor), +bourgeanic acid (trace). Substrate and ecology: usually non-calciferous rock, or sometimes calciferous rock in open habitats from montane and upper montane habitats as well as coastal regions, also occurring on wood that has been impregnated with dust particles World distribution: fairly cosmopolitan in Antarctica, Africa, temperate Asia, Australasia, Europe, North and South America Sonoran distribution: throughout Arizona, coastal regions of southern California, and montane regions of Chihuahua, Sonora, Baja California and Baja California Sur. Notes: When occurring on wood, Tephromela atra may appear superficially like a member of the Lecanora subfusca group, whose disc is usually brown and whose epithecium lacks the purple pigmentation found in Tephromela species. Along the Sierra Madre Occidental a pale yellow population can frequently be found on rocks. Superficially that population is very similar to Lecidea promontorii Zahlbr., a species from southern Africa that was treated as a synonym of T. atra by Rambold (1989). Eventually the pale yellow population may require recognization as a separate species. However, for the moment it is treated as T. atra s.l. in the key above until its taxonomy can be clarified.