Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Life habit: not lichenized Thallus: disrupting surface layers of bark, making host bark paler photobiont: absent Ascomata: narrowly lirellate, often branched, immersed to erumpent, 0.1-2 mm long, up to 0.1 mm wide disc: black, plane, epruinose; exciple: reddish brown, 5-20µm wide epihymenium: reddish brown, 5-10 µm thick; paraphyoidal end cells: with pigment caps, 2-3 µm wide hymenium: hyaline, 30-50 µm tall; subhymenium: hyaline, 20-40 µm thick asci: clavate, without distinct stipe, 25-33 x 12-16 µm, 8-spored ascospores: persistently hyaline, 3-septate (slightly incised at primary septum), narrowly ovoid, 10-14 x 3-5 µm, without an epispore Pycnidia: ± round, 0.1 mm wide, wall reddish to olivacous brown conidia: bacilliform, straight, 7-8 x 1 µm Chemical reactions: ascomatal gels I+ KI+; ascus tholus with KI+ bluish ring-like structure. Substrate and ecology: on smooth bark of young Quercus twigs World and Sonoran distribution: coastal, known from southern and central California, Baja California, and Baja California Sur. Notes: Arthonia tetramera differs from other non-lichenized species primarily by its narrowly lirellate ascomata. Arthonia radiata f. angustata, cited by Hasse (1913) and in Fink (1935), belongs here.