Life habit: not lichenized Thallus: ±white, disrupting outermost bark layers photobiont: not present Ascomata: dispersed, ±round to slightly elongate, simple, rarely slightly branched, 0.2-0.5 mm in diam., in section 80-100 µm thick, slightly protruding, with integrated bark fragments disc: black, with a rough surface, often covered by bark remnants; peripheric hyphal structures: indistinct epihymenium: olivaceous brown or occasionally orang-ish brown due to scattered pigment granules, 10-15 µm thick, expanded into thallus; hyphae irregularly branched and partly with brown walls, tips of the hyphae not constantly embedded in gel; cells: c. 2 µm wide hymenium: hyaline, c. 30 µm tall, asci closely to moderately spaced; paraphysoids: occasionally branched, anastomosing, c. 1 µm thick; subhymenium: hyaline to pale brown, 10-20 µm; cells ±round, c. 3 µm in diam. asci: broadly clavate to subglobose, 40-60 x 22-28 µm, 8-spored, indistinctly stipitate, occasionally asci without well developed spores but often with orange brown, granular amorphous pigments ascospores: initially hyaline but becoming brown, muriform, with 5-7 transverse and with 1-3 longitudinal septa in each segment, obovate to ellipsoid, 18-23 x 9-12 µm, with thin gelatinous sheath when young, with c. 0.7 µm thick walls Pycnidia: 0.05-0.1 mm wide; wall: olivaceous brown, 2-3 µm thick conidia: bacilliform, straight, (3-)4-5 x 1 µm Chemical reactions: thallus hyphae I-; ascomatal gels with Idil+ blue patches; I+ deep blue except for I+ orangish red ascus gels, all gels KI+ blue.; brownish red pigments in the epihymenium and in asci K+ purple, not dissolving, becoming fine granular; asci with KI+ blue ring structure in the tholus. Substrate and ecology: on smooth bark of various trees in coastal areas World distribution: throughout Mediterranean, Eritraea Sonoran distribution: mainland southern California and Santa Catalina, Santa Rosa, and Santa Cruz Islands, Baja California Sur, and Sonora. Notes: Some Californian specimens previously determined as Arthothelium orbilliferum belong to Arthonia beccariana (Sundin 1999). Some material (determined as Arthothelium pruinascens) tends to have more lirellate and pruinose ascomata. Also included here are specimens determined as A. subcyrtodes. Amorphous reddish-brown pigments may be found in A. beccariana, and these are irregularly distributed in the ascomata and may occur in the epihymenium as well as inside the asci. They are clearly different pigments from the extracellular crystallized pigments present in certain other Arthonia species (A. sanguinea, A speciosa, and A. redingeri).