Arthothelium norvegicum

Thallus immersed, white or greenish grey, usually forming small (<3 cm diam.) patches; photobiont trentepohlioid. Apothecia 0.3–0.6 mm diam., ± rounded, slightly convex, black, not pruinose, in section 90–130 µm tall; epithecium reddish brown, K+ greenish; hymenium 70–100 µm tall, colourless or pale straw, K+ greenish yellow; hypothecium 10–30 µm tall, colourless or patchily reddish-brown (K+ greenish); paraphysoids 0.7–1.5 µm diam., numerous, pigmented and to 2.5 µm diam. in the epithecium; apices often with a dark hood or cap. Ascospores 29–36 × 12–15 µm, obovoid-ellipsoidal to obovoid-cylindrical, muriform with 7 to 8 (–9) transverse septa, at maturity most transverse cells with 2 or 3 longitudinal septa, when old brown and warted.

Norwegian material has ascospores 31–42 µm, with mostly 9 transverse septa. Distinguished from A. spectabile by the smaller apothecia and K+ greenish epithecium. Frisch et al. (2014) found that sequences identified as this species clustered with Diarthonis spadicea in a clade quite distinct from that occupied by Arthonia sensu stricto; however relationships between the two species need further investigation before a new combination would be appropriate.