Thallus: areolate, emerging from areoles of Dimeleana oreina or independent but scattered or forming small patches up to 2 cm across among areoles of D. oreina areoles: mostly 0.5-0.8 mm wide, less than 0.4 mm thick, mostly angular, becoming convex upper surface: brown, very dark with black undertone to mahogany, dull to glossy, epruinose, smooth to deeply fissured or warty lower cortices: paraplectenchymatous, 30-50 µm thick; syncortex: 10-12 µm thick; eucortex, dark pigmented narrow upper layer, hyaline lower layer algal layer: even and not interrupted medulla: white, prosoplectenchymatous lower surface: dark attachment: broad, continuous with attaching hyphae, not truly stipe-forming, but attaching hyphae can be long and attenuated, from competition for probably light with areoles of D. oreina Apothecia: one or two per areole, often lacking disc: black, rough and uneven to almost gyrose with ridges, epruinose parathecium: thin or poorly developed, but apparently expanding rarely to form blackened parathecial crown epihymenium: dark brown, 10-15 µm thick hymenium: hyaline, (60-)80-120 µm tall; paraphyses: 1.8-2 µm wide at mid-level, apically mostly expanded, 3-4 µm wide subhymenium: 20-30 µm thick; hypothecium: indistinct asci: cylindrical, 50-70 x 10-15 µm, 50-60-spored ascospores: mostly globular, 3-6 µm wide, with distinct perispores Pycnidia: not seen Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Ecology and substrate: independent or emergent in thalli of Dimeleana oreina World Distribution: North America (Nevada, New Mexico and Wyoming) Sonoran distribution: not yet known, but expected to occur as it is documented from NW and E of Arizona. Notes: Four specimens collected from Nevada and Wyoming by T. H. Nash were studied and a neotype selected from among them. The holotype was deposited in the Bouly de Lesdain in Dunkirk and was apparently lost during WW II. This description above differs from protologue in one important fact: specimens were not C+ red in cortex. The author thinks C+ spot test reaction in holotype was from chemistry derived from the host which was definitely C+ red according to Magnusson (1930). Acarospora succedens has not yet been found independent of D. oreina. While several other species have been observed growing out of thalli of other lichens (A. dispersa, A. interspersa, A. obnubila), they do not appear to be obligatory or host-specific juvenile parasites. Further specimens of A. succedens need examination to establish the º of host-specificity. Its round spores with a perispore, low number per ascus, C- reaction, and well-developed syncortex are a distinct suite of characters for this western North America species. Several other species of Acarospora are common among thalli of D. oreina so caution is required in making a determination.