Life habit: not lichenized Thallus: ± white, lacking a prothallus photobiont: absent Ascomata: irregularly maculiform or rarely slightly lirellate, semi-sessile, 0.2-0.5 x 0.2-0.3 mm, 70-90 µm tall disc: black, ±flat, without pruina; exciple: olivaceous brown, with hyphae c. 2 µm wide, 10-20 µm wide epihymenium: reddish brown, 5-15 µm thick; paraphysoidal end cells: 2 µm wide, without distinct caps hymenium: hyaline, 30-40 µm tall; subhymenium: hyaline, 10-30 µm thick asci: clavate to narrowly ovoid, without distinct stipe, 27-37 x 13-17 µm ascospores: persistently hyaline, (1-)2(-4)-septate (often incised at primary septum), narrowly ovoid, 10-14 x 3-5 µm, with an epispore Pycnidia: c. 0.1 mm wide; wall: olivaceous brown conidia: bacilliform, straight to slightly curved, 5-8 x 1 µm Chemical reactions: ascomatal gels I+ persistently blue, KI+ blue; asci with KI+ blue tholus structure; epispore KI-. Substrate and ecology: on smooth bark or young twigs World and Sonoran distribution: southern California (Channel Islands and adjacent mainland). Notes: Arthonia rhoidis is characterized by its 2-septate ascospores, its rounded ascomata and its epihymenial tips that are often reddish brown rather than olivaceous brown. However, the significance of the latter character is not regarded as high, and similar pigments also exist in other species. Apart from the ascospore septation and ascomatal shape, the species seems to be rather closely related to Arthonia tetramera or A. pinastri. According to Sundin (1999), A. microspermella could be an older synonym of A. rhoidis, but no type material was available.