Life habit: lichenized Thallus: effuse, creamy photobiont: a trentepohlioid alga Ascomata: rounded to irregular, 0.2-1 mm wide, immersed to broadly erumpent through uppermost bark layers, disc: dark reddish-brown, plane to slightly convex, sometimes covered laterally with bark remnants, epihymenium: brown, 20-25 µm thick, covered by a 10-20 µm thick hyaline gelatinous matrix that sometimes disintegrates locally, hyphae mostly anticlinally arranged and wavy, often collapsed, ±brown, branched hymenium: hyaline to pale brown, 60-80 µm tall, with closely spaced asci separated by indistinct, anastomosing paraphysoids; subhymenium: hyaline to yellowish brown, 10-20 µm thick asci: clavate, 55-70 x 24-30 µm, 8-spored, with ±thick lateral walls ascospores: hyaline and smooth when mature, but pale brown and verrucose when old, with 5-6(7) transverse septa (incised only at the uppermost septum), upper cell enlarged, obovoid, 25-36 x 9-13 m Pycnidia: ± round, brownish conidia: bacilliform, 7-9.5 x 1 µm Chemical reactions: ascomatal gel I+ blue, later becoming red in epihymenial parts, KI+ blue; brownish pigment in epihymenium K+ olivaceous. Substrate and ecology: on smooth bark in pioneer associations with other lichens World distribution: widely distributed in oceanic habitats of temperate zones Sonoran distribution: central California (Monterey Co., Humboldt Co.) and Sinaloa. Notes: Although Arthonia ilicina has transversely septate ascospores, it has in the past also been treated under Arthothelium. This was based on presumed similarities of ascomatal structures, such as their "stromatic" appearance. Arthonia ilicina is representative of a whole group of Arthonia species with similar spores, such as the reddish species A. picea (=A. macouni) and A. thoracifera, both of which have violaceous K+ reactive pigments, and ascospores with muriform lower parts, as well as foliicolous species, including those assigned to Eremothecella. Further work is needed to clarify the relationships of Arthonia ilicina with the tropical species, A. complanata, which has many slightly variable forms and is also variable in the KI+ reaction of the ascus tholus and degree of epispore formation. Samples of A. ilicina have been determined in the past as A. caudata (now A. hypobela), which is known only from eastern USA and France (Sundin 1999). Arthonia quintaria, known from North America, has also macrocephalic ascospores similar to those of A. ilicina, but is apparently non-lichenized and has branched ascomata.