Thallus: umbilicate, monophyllous, thick, rigid, +circular to somewhat irregular in shape, with or without broad folds raising part of the thallus above the substrate, 2-10(-27) cm in diam. upper surface: light gray or pale brown, often with white pruina medulla: white lower surface: black, covered with thin black, often quite eroded rhizinomorphs which release thalloconidia Apothecia: infrequent, black, adnate to sessile, gyrose, up to 3 mm in diam. asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid to broadly ellipsoid, 9-12 x 5-9 µm thalloconidia: multicellular, irregular in shape, dark brown Spot tests: K-, C- or C+ red, KC+ red, P- Secondary metabolites: gyrophoric and ovolic acids. Substrate and ecology: on siliceous rocks, often on vertical rock faces or in crevices where trickle-water runs World distribution: U.S.A. and southern parts of Canada Sonoran distribution: scattered at intermediate elevations of Arizona, California, and Baja California. Notes: Umbilicaria americana seems closely related to the somewhat smaller U. vellea (Poelt and Nash 1993). The two species are mainly distinguished by the rhizinomorphs, which in U. americana are all black with thalloconidia while U. vellea has one set of short, peglike, thalloconidiogeneous rhizinomorphs but also a set of slender, slightly branched ivory-colored rhizonomorphs. In the West U. americana usually occurs at lower elevations than U. vellea.