Mycelium: I- Apothecia: for a long time half-immersed in the host thallus, later becoming superficial, 200-400 µm in diam. disc: black, often covered by a greenish pruina, applanate to convex epihymenium: medium to dark olivaceous or green, K+ green, often covered by a dense crystalline layer (= pruina), dissolving with K in a yellow solution hymenium: lower part pale brownish, upper part pale olivaceous, K+ green hypothecium: reddish brown, K- asci: 8-spored ascospores: dark brown, verrucose, 1septate, cells slightly unequal in width, strongly constricted at the septum, (13.5-)15-19(-19.5) x (5.5-)6-7.5(-8) µm Pycnidia: not known with certainty. Hosts: thallus of Sticta species World distribution: Europe and North America Sonoran distribution: once collected in California. Notes: Abrothallus welwitschii is characterized by the large, frequently pruinose apothecia, which are partly immersed for a long time in the thallus of Sticta species. A second species described from Sticta, Abrothallus stictarum Etayo, is distinguished by ascospores becoming 3-septate at maturity.