Life habit: lichenized Thallus: inconspicuous, pale, 250 µm thick, upper layer with hyaline grains photobiont: Trentepohlia; cells: 15-20 µm Ascomata: round to subangulate, up to 0.5 mm wide, sessile, c 130 µm thick disc: dark brown to blackish, smooth to rough; exciple: up to 100 µm thick, made of ±parallel brown-walled hyphae separated by intercellular spaces containing hyaline grains (up to 15 µm in diam.) epihymenium: pale brown, 15-20 µm thick, with end cells having pigment caps, slightly protruding or irregular in height hymenium: hyaline, 70-90 µm tall; paraphysoids: branched and anastomosing, 3 µm thick; sub-hymenium: hyaline, 30-50 µm thick, with rounded cells 2-3 µm in diam, with ascogenous hyphae up to 7 µm in diam. and thick-walled asci: clavate, apical endotunica thickened, c. 55 x c. 17 µm ascospores: hyaline, 1-septate, 10-14 x 5-6 µm Pycnidia: globose, 60-80 µm wide, with brownish black walls conidia: straight, 5-8(11) x 1 µm Chemical reactions: ascomatal gels and vegetative hyphae I+ blue, KI+ blue; asci with very faint KI+ ring-structure and bluish tholus flanks. Substrate and ecology: on smooth bark World and Sonoran distribution: southern California (San Nicolas and Catalina Islands). Notes: Arthonia lecanactidea is readily recognized by its adnate, sessile ascomata. Its relationships to other species are not entirely clear, but the rather distinct epihymenium excludes a close relationship with other species assigned to section Coniangium, as was suggested by Zahlbruckner (1902).