Thallus: verrucose-subareolate surface: gray, without isidia cortex: 11-24 µm thick, hyaline; without crystals in cortex nor medulla, uniform, without strands of hyphae penetrating into the medulla, borderline of medulla even, parallel with surface fertile verrucae: 1.2-1.8 mm in diam., with uneven surface Apothecia: single in the fertile verrucae or rarely 2-3 mazaedium: 0.7-1 mm in diam., sometimes with a greenish-yellow pruina exciple: 0.23-0.33 mm high, blackish brown hymenium: 55-80 µm tall asci: cylindrical, 8-spored, with uniseriate spores ascospores: 1-septate, constricted at septum, 22-28 x 12-13 µm; surface: smooth or minutely uneven in light microscope Spot tests: thallus K-, C-, KC-, P-; medulla I- Secondary metabolites: thallus having several chemotypes: epanorin and rhizocarpic acid, usnic acid alone, or none detected; yellowish pruina of mazaedium with tetronic acid derivatives. Substrate and ecology: on wood, preferably fence posts, sea level to 600 m World distribution: southern Alaska to California Sonoran distribution: southern California and Baja California. Notes: Contrary to Herre (1910), the thallus of Thelomma occidentale is best described as effuse, without marginal lobation. The exciple is reduced to a dark cushion. As the spore mass pushes against the confining thallus, the adjacent tissue becomes slightly upturned, often creating a distinct white rim. The mazaedium often has a slightly greenish cast.