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Family: ArthoniaceaeGenus: Arthonia
Vegetative hyphae colourless, branched, sinuous, cells 4–7 × 1–2(–3) μm, forming a dense net in the hymenium of the host.
Anamorph: not known.
Teleomorph: Ascomata usually merging in large numbers into composite apothecia to 2 mm diam, black, rarely with brown tinge, matt to slightly glossy, convex to tuberculose. Epihymenium distinct, medium grey- or brown-olive, ca 10 μm tall, K–, I+ red, with numerous small granules in KI. Hymenium pale grey- or brown-olive (particularly above) to almost colourless, 40–50 μm tall, I+ red, K/I+ blue. Subhymenium indistinct. Hypothecium to 150 μm tall, pale yellow-brown to almost colourless, I+ and K/I+ blue. Paraphyses more or less unbranched below, straight and 2–3 μm diam, often branched above, sinuous and 3–5(–8) μm diam; the upper cells slightly to strongly inflated, sometimes constricted at the septa, clearly pigmented, but without a pigment cap. Asci (28–)33–43(–48) × 11–16(–20), clavate, often with a stipe, wall I+ reddish, K/I+ pale blue, no K/I+ blue ring observed in the tholus, 8-spored. Ascospores irregularly arranged, (7.5–)9.5–12.5(–15.0) × (3– )3.5–4.5(–5.0) μm, l/b = (1.8–)2.4–3.1(–4.0), persistently hyaline, smooth, 1- septate, sometimes slightly constricted at the septum, clavate with the upper cell wider, no gelatinous sheath observed.