Zwackhia sorediifera

Thallus thin, smooth or somewhat scurfy, occasionally evanescent with a narrow, dark prothallus, pale to medium brown; soralia 0.4–1.2 mm diam., occasionally confluent to 3 mm diam., punctiform, pale orange-yellow or buff when fresh, fading to cream in dried collections; soredia farinose, 12–26 μm diam. Apothecia 0.3–0.6 (–1.3) × 0.12–0.3 mm, 0.06–0.14 mm tall, frequent, scattered, short, semi-immersed to sessile, unbranched; disc a slit; exciple K+ olive-green; epithecium pale; hymenium 90–120μm, tall, I+ red. Ascospores 30–40 (–58) × 4–5 μm, 10- to 14-septate, elongate-fusiform, with a distinct gelatinous sheath. Conidia 4–6 × 0.6–0.8 μm, bacilliform. Soralia C+ pink-red, K–, KC+ red, Pd–, UV– (gyrophoric acid, ± unidentified UV+ red pigments).

Often sterile and then distinguished from Thelopsis corticola and Francisrosea bicolor by the much smaller, usually discrete soralia and chemistry (both are C–). Rare, juvenile forms of Gyrographa gyrocarpa on bark differ in chemistry. Porina multipuncta (previously included in Opegrapha) has a more extensive thallus with numerous, fleck-like soralia, and is also C–. Z. viridis apothecia are similar but the thallus lacks soralia and is often richly fertile.