Thallus: large, well developed, 0.3-0.4 mm thick, placodiform, with distinct, radiating lobes at the periphery, chinky to areolate and verrucose, continuous surface: faintly yellowish gray to gray, minutely granular cortex: distinct, 30-40 µm thick, consisting of perpendicular hyphae densely interspersed with minute granular crystals fertile verrucae: conical, 2-2.5(-3) mm in diam.; cortex: of fertile verrucae consisting of three layers, the outermost layer hyaline, 10-15 µm thick, paraplectenchymatous with very indistinct hyphae; middle layer: densely interspersed with minutely granular crystals, 50-60 µm thick; lowest layer: very irregular, prosoplectenchymatous, with strands of hyphae penetrating deep into the verrucae, giving the outer border of the algal layer a strongly folded appearance Apothecia: immersed; disc: 1.1-1.7 mm in diam., flat to slightly convex, black exciple: thin, becoming crenate mazaedium: 0.5-0.7 mm high; central part of exciple: 0.2-0.4 mm thick hymenium: 80-110 µm tall asci: 8-spored, uniseriate ascospores: 15-20(-25) x 10-12(-18) µm; semi-mature spores: with a distinct striation pattern as seen in light microscopy; mature spores: 1-septate, slightly constricted at septum, with an ornamentation of minute ridges interrupted by irregular fissures Spot tests: thallus K-, C-, KC+ rose-red, P- Secondary metabolites: 3chlorodivaricatic acid, an unknown metabolite, skyrin and variolaric acid also reported. Substrate and ecology: on siliceous rocks, particularly on sandstone, and on wood of old posts and fences, in open situations World distribution: coastal California and Macaronesia Sonoran distribution: southern California at 0-800 m. Notes: Thelomma californicum is easily distinguished from the other species of Thelomma by its radiating, plicate, marginal lobes, covered by a dense, rough pruina, and by its one-septate spores. The steep-sided, conical, fertile areolae lose the pruina eventually and take on a brownish tan color similar to that of T. mammosum. In structure the apothecia of T. californicum are similar to those of T. occidentale, the black excipular wall not extending up the sides but confined to a rudimentary cushion below the hyaline hypothecium. The report of this species from Florida by Fink (1935) is based on misidentifications.