Rhizocarpon postumum
≡Lecidea postuma Nyl., Flora 51: 345 (1868).
Description : Thallus areolate, spreading in irregular patches 1–3 cm diam., with a distinct, black, marginal prothallus. Areolae flat to subconvex, angular to rounded, 0.3–0.5(–0.6) mm diam., creamish or grey-white. Apothecia scattered, angular to rounded, plane, black, matt, epruinose, with a thin, concolorous proper margin, 0.03– 0.07 mm wide, or becoming immarginate, 0.3–0.6 mm diam. Exciple in section atra -brown, K−. Epithecium yellow-brown to blue-black. Hymenium colourless, 80–145(–170) μm tall. Hypothecium brown. Asci clavate, 8-spored, 80–100 × 20–30 μm. Ascospores colourless, narrowly ellipsoidal to ellipsoidal, 3-septate at first, becoming dark greenish blue and submuriform when mature, 17–25(–28) × 9– 13 μm.
Chemistry : Thallus K+ yellow or −, C−, KC−, Pd+ yellow; containing stictic acid.
S: Otago (Poolburn Reservoir). On exposed, sunny schist outcrops in alpine grassland. Known also from Great Britain; Europe, Scandinavia (Feuerer 1978; Santesson 1993; Scholz 2000; Hafellner & Türk 2001; Coppins 2002b; Ihlen 2004a; Santesson et al. 2004).
Bipolar
Illustrations : Ihlen (2004a: 540, fig. 7B; 554, fig. 24).
Rhizocarpon postumum is characterised by: the areolate, brown thallus, 1–3 cm diam., with a marginal, black prothallus; scattered, black apothecia; an exciple that is K−; a yellow-brown to blue-black epithecium; a hymenium, 80–145(–170) μm tall; 8-spored asci; colourless, 3-septate ascospores, 17–25(–28) × 9–13 μm; and stictic acid in the medulla.
