Thallus: within its substrate, not visible, rarely partly epilithic and then thin, granulose, chalky white, ecorticate Apothecia: occurring singly, constricted at base, concave, cup-like, or flat when mature, 0.4-1 mm in diam. disc: pale brown, rarely blackish brown, smooth, plane, slightly or heavily pruinose to scabrose margin: prominent and usually thick, fissured with 3-5 cracks ('crenulate'), epruinose or pruinose, white, without a parathecial ring amphithecium: present, with algae filling the medulla, 90-170 µm thick, corticate; cortex: distinctly delimited, clearly expanded at the base, 30-70 µm thick laterally and 50-130 µm thick at base, with gelatinous hyphae +obscured by granules (pol+, insoluble in K, soluble in N) parathecium: distinct, prosoplectenchymatous, with granules on top, up to 20 µm wide epihymenium: shades of brown, granular (pol+), or not at all granular, granules superficial, coarse, insoluble in K and soluble in N hymenium: hyaline or pale yellow, 50-60(-80) µm tall; paraphyses: slender to moderately thick, somewhat branched throughout, with few anastomoses, slightly expanded apically, not pigmented or pigmented brown; subhymenium: hyaline, distinct; hypothecium, 45-80 µm thick: hyaline or almost so, composed of prosoplectenchyma, clear, without granules but often with numerous droples of oil, 45-80 µm thick asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, narrowly ellipsoid to oblong (7.4-)8.9-11.8(-13.3) x 4.5-5.9 µm Pycnidia: not seen Spot tests: all negative Secondary metabolites: none observed. Substrate and ecology: on calcareous rocks World distribution: widespread in the Northern Hemisphere Sonoran distribution: northern Arizona, southern California, and Baja California. Notes: Lecanora crenulata has distinctive apothecia with white, cracked margins and usually heavily pruinose apothecial discs. A similar species is L. flowersiana, which has similar, distinctly cracked apothecial margins; however, its discs are epruinose or rarely and only slightly pruinose. The two taxa are easily distinguished also by their spores, which are much longer in case of L. flowersiana and by their paraphyses, which are thicker, capitate and often submoniliform on tips in L. flowersiana. Another related taxon is L. hagenii, which has smaller apothecia with much thinner apothecial margins, that are entire to dentate rather than cracked. The latter species has also much thinner amphithecial cortex compared to L. crenulata.