Life habit: lichenized Thallus: squamulose, pulvinate, or peltate-umbilicate (to foliose or fruticose), monophyllous to strongly polyphyllous; attachment: by a single umbilicus or stalk per thallus or subunit, sometimes also with rhizinose strands; margins: entire to deeply lobed, upper surface: ± pale greenish yellow, to slightly brownish, or white; often blue-green-black; thallospores: sometimes present along edges; isidia, soredia and similar structures: absent upper cortex: usually with structure difficult to see, due to gelatinization and to inspersion by granules medulla: white, loose to ± hollow towards center, but often filled with granular or crystalline material; cell walls: with isolichenan photobiont: primary one a chlorococcoid alga, secondary photobiont absent lower cortex: usually thicker (and with thicker and more gelatinized hyphae) than upper one lower surface: pale to dark brown, to blue-green black near edges Ascomata: lecanorine apothecia, laminal to marginal, rounded to irregular, immersed to more often sessile or substipitate; exciple: ± hyaline; hymenium: ± hyaline or yellowish to orange; paraphyses: not (or scarcely) branched or anastomosing asci: ± clavate, Lecanora-type, c. 8-spored ascospores: non-septate, mostly ± ellipsoid (8-13 x 4-8 µm), but globose (10-12 x 7-10 µm) in one species and ovoid-oblong to crescent-shaped (15-20 x 4-5 µm) in another; wall: hyaline, smooth, non-halonate, without distinctly developed endospore thickening, not amyloid Conidiomata: pycnidia, laminal or lateral, immersed conidia: formed acrogenously, filiform, c. 15-40 x < 1 µm, ± curved Secondary metabolites: upper cortex: with usnic acids; medulla: with aliphatic acids, ± ß-orcinol depsides, ß-orcinol depsidones, or triterpenoids Geography: circumpolar to temperate in both hemispheres, extending to the tropics in alpine areas, in ± dry areas Substrate: non-calciferous or weakly to moderately calciferous rocks; some taxa also occurring on soil in cool, open and windy areas. Notes: The usual lack of distinctly radiating marginal lobes, and the loose medulla and well-developed lower cortex, help to distinguish Rhizoplaca from Lecanora. The systematics and nomenclature of Rhizoplaca were reviewed briefly by Ryan and Nash (1997b), and more comprehensive and definitive treatments are in preparation by us. Information on distribution of chemotypes is given by Leuckert et al. (1976) and McCune (1987), but those treatments included R. subdiscrepans s. lato (and occasionally members of the Lecanora opiniconensis complex) under R. chrysoleuca (or occasionally R. melanophthalma). The here broadly treated taxa R. chrysoleuca and R. melanophthalma are extremely variable, often with several “morphotypes” or “color phases” appearing quite distinct when growing side by side, but no attempt is made here to account for the numerous published infraspecific taxa, nor any but the most commonly cited synonyms. Morphs growing loose on soil do not occur in the Sonoran region and are therefore not included in the present account.