Verrucaria margacea
≡Thelotrema margacea Wahlenb. in E. Acharius, Methodus: 30 (1803).
Description : Thallus medium to dark grey-green, dark-grey or greenish black, somewhat gelatinous when moist, 20–40(–60) μm thick. Perithecia immersed, except around ostiole, dominated by a brown-black involucrellum, (0.25–)0.4(–0.65) in diam., that may or may not be overgrown by a thalline layer 20–60 μm thick. Centrum subglobose to globose, 0.16–0.3 mm diam. Exciple dark-brown above and ±colourless below. Ascospores large, ellipsoidal, broadly ellipsoidal or subglobose, 18–35 × 9.5–18.5 μm.
S: Southland (Borland Valley, Garvie Mts), Otago (Nenthorn, North Waikouaiti River, Old Man Ra., Rock & Pillar Ra.). On stones on gravel bed of streams in alpine and upland localities. Known also from Great Britain, Scandinavia, western Eurasia, North America, India and Australia (Swinscow 1968; McCarthy 1995f; 2001j: 187; 2003c; Santesson et al. 2004; Elvebakk & Bjerke 2006).
Cosmopolitan
Illustrations : Zschacke (1934: 213, fig. 94); Swinscow (1968: 48, fig. 6); Foucard (1990: fig. 339): Johnson & McCarthy (1997: 15, fig. 3); McCarthy (2001j: 186, fig. 56).
Verrucaria margacea is a distinctive aquatic species characterised by: a thin, dark greenish brown to black thallus; perithecia 0.25–0.65 mm diam.; and large ascospores, 18–35 × 9.5–18.5 μm. It is similar to V. hydrela, the main difference between the two taxa being in the dimensions of the perithecia. Both have a thin, subgelatinous thallus, with perithecia largely immersed in it but forming conical-hemispherical or flattened-hemispherical projections of it, and an involucrellum that spreads laterally into the thallus. Perithecia and ascospores are significantly larger in V. margacea than in V. hydrela (Swinscow 1968: 48).
