Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.

Thallus: subcrustose-lobulate or crustose, finally pulvinate-subumbilicate, small to very small, up to 5 mm wide, rounded or somewhat angular lobes: few, usually repeatedly branched, 0.1-0.2 mm thick when dry, 0.2-0.45 mm thick when moist, short, 0.5 mm long, ±contiguous, terete or somewhat flattened, adnate or finally pulvinate, not distinctly swollen upper surface: dark olive-green or black, dull, epruinose isidia: absent, but lobes finally appearing somewhat 'isidioid' lower surface: somewhat paler, loosely affixed in the center Apothecia: at first ±globose, later flattened and disciform, semi-immersed to sessile and with constricted base, small, 0.3-0.7 mm wide disc: plane to concave, red or red-brown to black, dull or somewhat glossy, smooth, epruinose thalline margin: thin, usually entire and smooth, inconspicuous or slightly prominent, persisting or finally ±disappearing, usually lacking a pseudocortex true exciple: rather thick, c. 100 µm wide, euparaplectenchymatous or subparaplectenchymatous; cells: isodiametric or oblong, 3-8.5 µm wide hymenium: hyaline, 90-150 µm tall asci: narrowly clavate to subcylindrical, 8-spored ascospores: ellipsoid to subglobose, with acute or rounded ends, usually submuriform, rarely 4-celled, ±constricted at septa, 17-26 x 8.5-10.5(-13) µm Pycnidia: laminal, immersed, small, c. 130 µm wide, pale conidia: bacilliform to slightly swollen at the ends, 4-5 x 1-1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: usually on bare calcareous rock (the Arizona collection on sandstone) World distribution: North America, Greenland, Europe, Mediterranean, India, and Japan Sonoran distribution: a single collection from mountainous northeastern Arizona. Notes: Collema callopismum is distinctive because of its small thallus size with clusters of short, cylindrical branches, its conspicuous, thick, euparaplectenchymatous proper exciple and its 4-6-celled ascospores.