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: pale gray to white, without a prothallus photobiont: absent Ascomata: punctiform to elongate (lirellate), immersed to erumpent, 0.2-1mm long and 0.1-0.3 mm wide, disc: grayish black, rough, occasionally with pruina epihymenium: olivaceous brown, 10-15 µm thick; paraphysoid tips: olivaceous brown, up to 3 µm thick, with a brownish gelatinous matrix, with granules hymenium: hyaline, 40-50µm tall; paraphysoids: branched and anastomosed, c. 1 µm thick; sub-hymenium: hyaline, 10-20 µm thick, hyphae 1-3 µm thick asci: ovoid to subglobose, 30-40 x 20-26 µm, 8-spored, without distinct stipes, ascospores: hyaline, 5(-7) septate (slightly incised at median septum), cells beneath the median septum often larger than their adjacent outer cells, narrowly ovoid, slightly curved, with a thin epispore Pycnidia: c. 0.1 mm wide, with brownish walls conidia: bacilliform 3-4.5 x 1 µm Chemical reactions: ascomatal gels I+ blue, KI+ blue; asci with KI+ ring-structure; epispore KI+ blue. Substrate and ecology: on smooth bark of coniferous and deciduous trees World distribution: Mediterranean Europe and North America Sonoran distribution: Baja California Sur. Notes: Arthonia sexlocularis is especially noteworthy for its peculiar ascospores. The septa in the upper and lower spore half are closer to each other than to the central septum. This septation-type is also present in another dark colored Arthonia, A. polymorpha, but that poorly known tropical species seems to differ in having deep brownish black colored ascomata with a smooth surface. Another unidentified specimen from the Sonoran Desert area (southern California, Santa Barbara Co.) with similar ascospores differs by having an I+ red hymenium. Arthonia quintaria has branched ascomata and is distinguished by macrocephalic ascospores.