Rhizocarpon lavatum
≡Lecidea atroalba var. lavata Fr., Nov. Sched. Critic.: 18 (1827).
Description : Thallus pale-grey to pale-brown, rimose to areolate, in spreading patches, 2–6(–8) cm diam., on a black prothallus. Areolae 0.5–0.8(–1) mm diam., rounded to angular, not peltate, 0.3–0.7 mm diam., brown, red-brown or grey-brown, smooth, matt. Apothecia scattered, sessile, rounded, black, matt, epruinose, 0.8–2(–2.5) mm diam., the disc sometimes with a central umbo. Epithecium brownish, grey or olive-brown. Hymenium colourless, 120–160(–200) μm tall. Hypothecium brown to brown-black. Asci clavate, 8-spored, 100–160 × 30–50 μm. Ascospores colourless, muriform, 33.5–50 × 12.5–21 μm.
Chemistry : TLC−, all reactions negative
C: (Mt Azimuth). On subalpine to alpine rock outcrops, in damp habitats or subject to periodic inundation (Fryday 2004a). Known also from Great Britain, Europe, Scandinavia, Svalbard, Morocco, the Ukraine, and North America (Feuerer 1978; 1991; Purvis et al. 1992; Nimis 1993; Esslinger & Egan 1995; Wirth 1995a; Egea 1996; Kondratyuk et al. 1996a; Elvebakk & Hertel 1997; Diederich & Sérusiaux 2000; Scholz 2000; Hafellner & Türk 2001; Llimona & Hladun 2001; Coppins 2002b; Nimis & Martellos 2003; Ihlen 2004a; Santesson et al. 2004).
Bipolar
Illustrations : Feuerer (1978: 90); Foucard (1990, pl. 280); Fryday (2000b: 212, fig. 1D); Dobson (2000: 342; 2005: 387); Ihlen (2004a: 537, fig. 1; 540, fig. 8C; 550, fig. 21).
Rhizocarpon lavatum is characterised by: the pale-grey to pale-brown, rimose to areolate thallus; rather large, 0.8–2(–2.5) mm diam., and sometimes umbonate apothecia, with a thick, tumid margin; large, colourless, muriform ascospores with numerous septa, 33.5–50 × 12.5–21 μm; and no detectable secondary chemistry.
