Thallus: thin or indistinct, effuse, often minutely granular; granules 15-70 µm in diam., +immersed, often scattered, with minutely speckled appearance when on light colored bark surface: pale to dark gray brown photobiont: cells 5-9 µm in diam. Apothecia: frequent, broadly adnate, often slightly constricted at base, containing few or abundant algae in lower part, 0.2-0.6(-0.8) mm wide disc: at first concave, later flat or sometimes convex, reddish brown to dull black, epruinose exciple: dark, concolorous with disc or black, thin, at first prominent, later sometimes excluded, dark brown to red or violet-red at edge, hyaline to pale brown within, with radiating, conglutinate hyphae coherent in K, 1.5-2.5 µm wide but thickened by pigment at outer edge, the lumina ellipsoid to oblong, 1.5-3 um wide, thin to thick-walled epihymenium: pale yellow to pale or dark brown, K-, N- hymenium: hyaline, 60-75 µm tall; paraphyses: rather laxly coherent, 0.8-1 µm wide, simple or sparsely branched above; with +swollen apices up to 2-3 µm wide, often with dark brown hood 3(-4) µm wide; hypothecium: hyaline to pale brown, K-, the hyphae conglutinate asci: clavate, 8-spored, the spores biseriately arranged ascospores: hyaline, 1-septate, ellipsoid or oblong-ellipsoid, rounded at the tips, often slightly constricted at the septum, 9-15 x 5-7 µm, with layered walls up to 1.5 µm thick, the outher layer representing a compact halo Spot tests: thallus K-, C-, KC-, P- Secondary metabolites: none detected. Substrate and ecology: on bark or bryophytes on bark, on trunks of mature trees (especially Fraxinus, Quercus), sometimes on mossy branches, rarely on wood of fallen trees (e.g., of pines) World distribution: circumpolar boreal to temperate in Europe, Asia, and North America Sonoran distribution: southern California Notes: C. atropurpurea resembles somewhat Catillaria nigroclavata (Nyl.) Schuler, but the apothecia are mostly dark red-brown. Under the microscope C. atropurpurea is easily distinguished from that much more common species by the ascospores having distinctly layered walls. The closely related Catinaria neuschildii (Körb.) P. James, mainly separated by 16-spored asci, has so far not been found in North America.