Thallus: umbilicate, usually monophyllous, +orbicular, thick, stiff, 1-5 cm in diam. upper surface: light gray to dark gray or brownish gray, scabrous, with a white necral layer centrally, with sharp ridges radiating from the umbo, fading into a reticulate white pattern of weak ridges peripherally medulla: white lower surface: sooty black, paler marginally, smooth, covered with thalloconidia, sometimes with radiating folds from the umbilicus Apothecia: rare, stipitate, black, omphalodisc (with central ring) asci: clavate ascospores: hyaline, simple, ellipsoid, 6-14 x 4-8 µm thalloconidia: dark brown to black, simple, spherical to oviform, 7.5 x 7 µm Spot tests: medulla K-, C+ red or C-, KC+ red or KC-, P- Secondary metabolites: gyrophoric, lecanoric and norstictic acids. Substrate and ecology: on siliceous rocks, vertical surfaces on large, wind-swept boulders in the middle to high alpine vegetation zones, usually above the snow in winter World distribution: world-wide in Europe, Asia, Africa, and North and South America, Australasia and Antarctica Sonoran distribution: locally common, on San Francisco Peaks in Arizona. Notes: Umbilicaria decussata is one of the few Sonoran Umbilicariae with omphalodiscs (also found in U. krascheninnikovii and U. virginis. With its black, simple thalloconidia on the lower surface, U. decussata can readily be separated from U. krascheninnikovii that lacks thalloconidia. In U. decussata this option has led to a reduction in sexual reproduction and thus apothecia are not common. The occurrence of rhizomorphs in U. virginis is unique among the three species. The subspecies U. decussata ssp. darrowi was described by Frey (1936) but is in fact ordinary U. kraschenninikovii (see that species below).