Thallus: lacking or very scarcely developed (then rimose, whitish, thin); prothallus: indistinct; epilithic thallus if present, white, esorediate medulla: white, I- (in the specimens recorded from the Sonoran region) to I+ deeply violet (most of the specimens world-wide) Apothecia: black, loosely attached with strongly constricted base, 0.40.8(-1.4) mm in diam. disc: black, flat to slightly convex, dull to shiny, epruinose margin: black, well developed, persistent, dull to shiny, often undulate exciple: with a 15-22 µm thick blackish to dark brown rim and hyaline interior, weakly obscured by crystalline masses, C+ red epihymenium: green-black, c. 10 µm thick hymenium: very pale green, 30-55 µm tall, I+ blue; paraphyses: simple, occasionally scarcely branched and anastomosing, apical cells 4-5 µm wide subhymenium: somewhat more intensely green than hymenium, 10-50 µm thick hypothecium: ochre to pale brown to brown, rarely pale ochre to almost hyaline asci: clavate, 25-45 x 7-10 µm, 8-spored ascospores: hyaline, simple, narrowly oblong, (5-)7-12.2 Pycnidia: immersed, slightly graphidoid conidia: bacilliform to cylindrical, straight, 11-16 x c. 1 µm [studied in 1 specimen] Secondary metabolites: 2'-Omethylanziaic acid (exciple). Substrate and ecology: in open habitats on granites, schist and other acidic rocks in montane and alpine regions World distribution: bipolar in predominately alpine areas Sonoran distribution: rare in the alpine and montane zone of Arizona, and in California. Notes: Lecidea diducens is a member of the Lecidea auriculata group and is characterized by its C+ red exciple (due to 2'-O-methylanziaic acid). The rather similar Lecidea cerviniicola de Lesd., also showing a C+ red exciple, has slightly larger spores and anziaic and perlatolic acid stored in the exciple (instead of 2'-O-methylanziaic acid). It is not yet known to occur in North America. The few Sonoran specimens differ in having an I- medulla. We hesitate however at this time to separate these populations taxonomically, because very rarely specimens without an amyloid medulla are found in the European Alps too. Most of the specimens of Lecidea diducens we have studied were collected in alpine regions, in contrast to most of the Sonoran specimens. However, in NW Europe Lecidea diducens was often collected near to the sea-shore (including the type); thus, there is also no striking ecological difference for separating the Sonoran specimens. Most of the older collections preserved in North American herbaria labeled "L. diducens" are misidentified and belong often to e.g. Lecidea auriculata (mostly arctic or high alpine origin) or to Lecidea laboriosa. The etymology is not clearly understood by us; "diducere" (Latin) means to separate, to thin out.