Life habit: non-lichenized Thallus: effuse, in uppermost bark layers photobiont: absent Ascomata: irregularly rounded, more or less immersed; up to 0.5 mm wide, cracked when large disc: pale gray due to white pruina of bark remnants, underneath blackish; peripherical structures indistinct epihymenium: opaque due to large amounts of minute greyish crystals, 20-30 mm thick, with spaced, brown hyphal tips, to 2 µm thick hymenium: hyaline, 55-65 µm tall, paraphysoids hradly distinguishable in water, branched and anastomosed, to 2 µm wide; subhymenium: hyaline to pale brownish, 20-30 µm thick asci: clavate, 55-60 x 25-28 µm, 8-spored, with thick lateral walls ascospores: persistently hyaline, muriform, with 6-8 transverse septa and 3-4 septa in each transverse segment (3 primary transverse septa, which are more or less irregularly subdivided), ±ellipsoid, 20-25 x 10-12 µm, epispore indisctinct in water Pycnidia: not observed Chemical reactions: ascomatal gels I+ deep blue, KI+ blue; small crystals in epihymenium K insoluble; spores with K+ distinct epispore (KI-); asci without KI+ reactive tholus structures, vegetative hyphae KI+ faint blue. Substrate and ecology: found on smooth bark of Sambucus or Malvastrum World and Sonoran distribution: known with certainty only from the type in southern California (Santa Monica Range). Notes: The relationships and morphological variability of Arthonia pruinascens are still poorly understood to date. The species seems closely related to A. beccariana, and might eventually be included in that taxon if transitional forms are found (see Sundin 1999). However, several specimens have incorrectly been identified as A. pruinascens in the past. Most of these actually represent A. sanguinea, for example when the latter hardly developed its typical red pruina. One additional specimen is anatomically very close to A. pruinascens, but differs by clearly convex ascomata without pruina.