Thallus: crustose, rimose, often subeffigurate, usually thick; prothallus: sometimes present, black surface: chalky white to gray or ochraceous, esorediate medulla: white, with calcium oxalate (H2SO4+ needle shaped crystals) Apothecia: lecideine, but typically surrounded by a thalline collar (pseudolecanorine), abundant 0.3-1.2 mm in diam., immersed or eventually raised above thallus surface disc: black, often pruinose, with white pruina consisting of calcium oxalate crystals, initially flat, later ±convex margin: indistinct but visible in apothecia raised above thallus surface, in immersed apothecia often obscured by thick thalline rim separated from the surrounding thallus by deep crack proper exciple: <50 µm, frequently inspersed with calcium oxalate crystals (H2SO4+ needle shaped crystals), lacking secondary metabolites; differentiated into a thin, dark brown outer part without carbonized cells (HNO3-), and a pale brown central part, transient with the brown, <240 µm thick hypothecium (HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, not inspersed with oil droplets, 75-125 µm tall; tips of paraphyses: ±6 µm wide with distinct apical caps asci: clavate, Bacidia-type, 52-75 x 15-20 µm, 8-spored ascospores: soon brown, 3-septate, rarely with one or two longitudinal septa, ellipsoid, often curved, with obtuse ends, (14.5-)16-[18.1]-20.2(-24) x (6.5-)6.7-[7.7]-8.7(-9.5) µm (n=120), walls and septa lacking uneven thickenings, proper wall c. 0.3 µm thick, perispore c. 0.65 µm thick, ornamentation: rugulate Pycnidia: rare, immersed, with uppermost part protruding, wall pigmented in upper part conidia: bacilliform, 9-12 x <1 µm Spot tests: thallus and medulla K-, P-, C- [outside North America also K+ yellow turning red (crystals), P+ yellow-orange, C-] fluorescence: UV- iodine reaction: medulla non-amyloid Secondary metabolites: absent [outside North America also with norstictic and connorstictic acids in the medulla]. Substrate and ecology: epilithic, on calciferous substrates, such as calcareous rocks, sandstone, schist, and anthropogenic substrates in open situations; at young stages sometimes parasitic on other lichens, such as Lecanora spp. World distribution: Europe, northern Africa, Asia, North America, and Greenland Sonoran distribution: Arizona, southern Californian coast and islands, Baja California, and Baja California Sur. Notes: Buellia venusta is characterized by its thick, chalky, subeffigurate thallus, its apothecia that erupt from the thallus with a thalline collar and its 3-septate spores with a thick perispore. The species is similar to B. alboatra and B. subdispersa (see those species), and might also be mistaken for species with 1-septate spores, such as B. dispersa. Only norstictic acid deficient specimens have been recorded in North America.