Thallus: areolate-verrucose, thick, coriaceous areoles: sterile verrucae: 1-1.5 mm in diam. surface: pale yellow with a distinctly olivaceous brownish tinge, smooth, slightly shiny cortex: consisting of three layers: the outermost one: hyaline, paraplectenchymatous, 6-11 µm; middle layer: 40-50 µm, well delimited, with small, irregular crystals; innermost layer: very irregular and with prosoplectenchymatous hyphae penetrating deep into the medulla fertile verrucae: 1.4-2.1 mm in diam., constricted at base, almost peltate, strongly convex Apothecia: developing in older verrucae, fully immersed mazaedium: black, epruinose, not or slightly protruding, 0.31-0.42 mm high; thalline margin: cortex of three layers as in T. mammosum; outermost layer: 6-11 µm; middle layer: 40-50 µm exciple: central part 0.23-0.36 mm high, dark brown hymenium: 70-90 µm tall asci: cylindrical, 31-38 x 4-5 µm when mature, with uniseriate spores, 8-spored ascospores: non-septate, spherical, 14-16 µm in diam., with coarse, irregular ornamentation; semi-mature spores: sometimes appearing faintly striate Spot tests: thallus K-, C-, KC-, P-; mazaedium K+, P+ Secondary metabolites: thallus with divaricatic acid; mazaedium containing norstictic and salazinic acids. Substrate and ecology: on rocks near the sea coast World and Sonoran distribution: coastal Baja California Sur to southern California. Notes: Thelomma santessonii differs from T. mammosum in having a thicker, distinctly yellowish olivaceous thallus, larger fertile verrucae (1.4.-2.1 mm in diam.), and being KC- and containing divaricatic acid.