Thallus: umbilicate (foliose), (3-) 5-10 (-20) cm diam., attached only by the umbilicus, otherwise free and raised from substrate, often becoming markedly folded, leathery upper surface: yellow to yellowish green or dull greenish yellow, dull to somewhat shiny, usually with a network of ridges that are rounded and not very prominent (generally less than 1 mm thick), with conspicuous papillae (verruculae) developing in circular rows; papillae less than 0.5 mm wide or tall, emaculate; pseudocyphellae: punctiform, occurring in the center of the papillae around distinct (10 x magnification) linear cracks through the upper cortex, rare in specimens with large numbers of pycnidia upper cortex: densely paraplectenchymatous (sensu Hale, but perhaps better characterized as prosoplechtenchymatous sensu Henssen), with a narrow, frequently pigmented outer layer composed of densely aggregated hyphae medulla: white, with strongly developed radial support tissue, cell walls with isolichenan lower cortex: thin, prosoplectenchymatous lower surface: dark blue-green to black (lighter around the umbilicus), pigment HNO3+ red, dull, smooth to slightly rugose around the umbilicus, developing centrifugally-formed, irregular, sharp-edged ridges towards the margin; papillae: numerous, frequently with discolored areas (pseudocyphellae), and developing into complex, knobby, finger-like tubercles, erhizinate Apothecia: very common and conspicuous, up to 10mm diam.; thalline margin: concolorous with thallus, predominantly smooth with occasional pseudocyphellae; disc: red-brown, plane to slightly convex; exciple: 50-60 µm thick, prosoplectenchymatous; inner part 50-70 µm thick; epihymenium: reddish brown, not granular or inspersed, with brownish pigment in the paraphyses tips; hymenium: hyaline, c. 70-90 µm high asci: clavate, 8-spored Ascospores: simple, ellipsoid, 9-11 x 5-6 µm Pycnidia: sometimes very abundant, at least partially immersed, surrounded by blue pigments conidia: fusiform to weakly bifusiform, c. 8 x 1 µm Spot tests: upper cortex K-, C-, KC+ yellow, P-; medulla K+ yellow then orange-red, C-, KC-, P+ orange Secondary metabolites: cortex with usnic acid; medulla with a ß-orcinol depsidone (norstictic acid). Substrate and ecology: on exposed acidic rocks and cliff faces, locally abundant, at higher elevations, in the upper montane zone (mixed conifer forest) World distribution: southwestern USA (Arizona, Colorado & New Mexico) Sonoran distribution: eastern and southern Arizona, rather rare, 2680-3360 m. Notes: As the only large yellow to yellow-green, umbilicate lichen with lecanorine apothecia, Omphalora is unique in the Sonoran region. Only junveniles would have the same size as the crustose genus Rhizoplaca. The species of the basionym refers to a South African species, that together with two other South African species was separated into the genus Xanthomaculina by Hale (1985) based on the presence of a vaulted cortex (not found in Omphalora), a feature that appears as prominent maculae from the upper surface. In addition, Xanthomaculina has lichenan in its fungal cell walls, has a brownish lower surface, and a different ascal structure. With the erection of Xanthomaculina, our species was left in the genus Omphalodium, the type species of which occurs in southern South America. Nash et al. (1990) separated these two Omphalodium species into monotypic genera Omphalora and Omphalodium based on differences of anatomy, morphology, cell wall chemistry and ascal dehiscence. Subsequently the generic separation was supported by Henssen (1992), who demonstrated that major differences in ascal ontogeny occur between the two. In contrast to Omphalora arizonica, Omphalodium pisacomense is a brown species with a very different surface topology (upper and lower).