Thallus: umbilicate, usually monophyllous, circular to irregular in outline, sometimes with deep incisions, thick, rigid, with or without broad folds, 3-8(-15) cm in diam. upper surface: white to light gray, smooth, dull, with a few tiny fissures, often pruinose medulla: white lower surface: dark brown to black, +trabeculate around the umbilicus, scabrous, covered with short, stout, often ball-tipped brown-black rhizinomorphs, interspersed with numerous slender, mostly branched, ivory rhizinomorphs Apothecia: rare, black, sessile, gyrose, up to 3 mm in diam. asci: clavate, 8-spored ascospores: hyaline, ellipsoid, simple, broadly ellipsoid, 9-12 x 6-8 µm Thalloconidia: large, irregular, multicelluar, released mostly from the tips of the stout, short rhizinomorphs Spot tests: medulla K-, C+ red, KC+ red, P- Secondary metabolites: gyrophoric, hiascic, umbilicaric, and lecanoric acids. Substrate and ecology: on siliceous rocks, vertical surfaces in sheltered places on large boulders and cliff faces, often in fissures with trickle-water; above the snow in winter World distribution: Europe, Asia, Africa, and North and South America Sonoran distribution: scattered, on the high peaks of Arizona. Notes: In North America Umbilicaria vellea is more common at higher latitudes and higher elevations than U. americana. Many earlier reports (e.g. Baja California) of U. vellea have proved to be the more recently described U. americana (see notes for that species for the differences).