Verrucaria nigrescens
Description : Thallus dark-brown, green-black or black (often bright-green when moist), areolate. Areolae 0.2–0.8 mm wide, usually smooth, flat to subconvex, occasionally with sorediate or isidiate margins. Prothallus black, visible also between areolae and forming a black rim to areolae. Perithecia compound, ½–¾ immersed, apex flat to hemispherical. Involucrellum 0.2–0.4 mm diam., dimidiate or extending down to prothallus, black. Centrum globose, 0.15–0.25 mm diam. Exciple dark-brown. Ascospores (17–)20–28(–30) × 8–15 μm.
S: Canterbury (Milford). On limestone. Known also from Great Britain, Scandinavia, central and southern Europe, the Ukraine, Turkey, Morocco, Tunisia, North America, and Australia (Zschacke 1934; Egea 1996; John 1996; Kondratyuk et al. 1996a; Seaward 1996; Hawksworth et al. 1992; McCarthy 2001j: 190; 2003c; Santesson et al. 2004; Elvebakk & Bjerke 2006).
Cosmopolitan
Illustrations : Zschacke (1934: 300, fig. 143); Dobson (1992: 358; 2000: 408; 2005: 457).
Verrucaria nigrescens is characterised by: the basicolous (limestone) habit; the chocolate-brown to black, thick, areolate thallus with a black prothallus visible between areolae, the areolae sometimes sorediate at the edges; small perithecia often in groups, partially immersed in thallus; and ascospores 17–30 × 8–15 μm.
