Life habit: not lichenized Thallus: inconspicuous, in the uppermost bark layers photobiont: absent Ascomata: ±circular to lirellate, erumpent, 0.1-0.7 mm wide, 50-100 µm tall; disc: brown, plane to ±convex, sometimes covered by remnants of the substrate, epruinose or pruinose; exciple: olivaceous brown, with hyphae 2-3 µm wide, 10-25 µm wide epihymenium: olivaceous brown, 5-15 µm thick, with olivaceous brown paraphysoidal tips without distinct pigment caps hymenium: hyaline, 40-70 µm tall, with granules; subhymenium: hyaline, 5-20 µm thick asci: ovoid, without distinct stipe, 25-35 x 19-23 µm, 8-spored ascospores: persistently hyaline, 3(4)-septate (often incised at the septae), narrowly ovoid ellipsoid, 13-20 x 4-6 µm, with an epispore Pycnidia: c. 0.1 mm wide; wall: olivacous brown conidia: bacilliform, straight, (2-)3-6(-8) x 1µm Chemical reactions: ascomatal gels I+ blue turning red, KI+ blue; asci with eloganted ring-structure in the tholus. Substrate and ecology: on smooth bark World distribution: Europe and North America Sonoran distribution: southern California (San Diego Co., Los Angeles Co., Santa Monica Range) and Baja California. Notes: As Sundin (1999) noted, this species has often been called A. epipastoides, which is a superfluous name for A. dispersa (Schrad.) Nyl. [A. dispersa (DC.) Dufour is a synonym of A. ruana]. However, the latter species is a (sometimes poorly) lichenized species with 1-septate ascospores. Arthonia galactites, a similar species known from Europe, differs by having more roundish, epruinose ascomata and by frequently having a yellowish, K+ red pigment. Arthonia pinastri lacks this pigment and differs by having I+ persistently blue ascomatal gels.