Life habit: saprobic, not lichenized Thallus: sterile mycelium immersed in the substrate, sometimes rendering it a greenish or greenish gray color, often paler than surrounding wood but usually blackened around the base of the ascomata Apothecia: dark brown to black, rather robust, 0.5-0.8 mm tall, with shining stalk often thickened towards the base capitulum: 0.3-0.6 mm in diam., lenticular to obconical; disc: flat to slightly convex, with strongly incurved margin exciple: 60-90 µm thick, well-developed, consisting of irregularly interwoven hyphae with swollen walls, inner part hyaline, outer part reddish brown to brown, central part often with droplets or amorphous crystals of a yellowish red to red pigment located in the hyphal lumina; a thin layer of spores is often found on the surface of the apothecia stalk: short, 0.10-0.20 mm in diam., the central part consisting of largely periclinally arranged, pale hyphae with swollen walls and moderately branched and intertwined, 2-3 µm in diam., outer part of the stalk with strongly intertwined hyphae with thickened walls, outermost part of stalk reddish brown to brown, reddish parts of the ascomata K+ violet red and H+ slightly intensified reddish hymenium: dark brown above, 80-110 µm tall, lining the inner surface of the exciple and extending upwards along the inner edge of the exciple; paraphyses: simple, filiform, 1-1.5 µm thick; hypothecium: hyaline, poorly developed, obconical asci: cylindrical, 35-60 x 4.5-6 µm, with moderately and uniformly thickened apex c. 1.5 µm thick ascospores: dark brown, simple, fusiform to ellipsoid, 8-11 x 3.5-5 µm, with a minute, irregular ornamentation visible under the light microscope Pycnidia: not observed Secondary metabolites: none detected. Substrate and ecology: on old wood World distribution: South America (Argentina, Brazil, Chile, Colombia, Dominican Republic, and Paraguay) and North America Sonoran distribution: Baja California and Baja California Sur. Notes: Mycocalicium americanum is characterized by its robust ascomata, its incurved exciple edge, its smooth and epruinose exciple margins, its large spores, and the red pigment in the ascomata. It is quite similar to M. calicioides, which differs in having a yellow pruina on the capitulum, a rugose outer surface of the exciple, and a narrower disc. See Tibell (1987; 1996) for further details.