Rhizocarpon subpostumum
≡Lecidea subpostuma Nyl. in F. Arnold, Verhandl. Zool.-bot. Ges. Wien 27: 554 (1877).
Description : Thallus areolate, spreading in irregular patches 1–3 cm diam., without a 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, 0.3–0.6 mm diam., plane, black, matt, epruinose, with a thin, concolorous proper margin, 0.06–0.1 mm wide, or becoming immarginate. Exciple in section atra -red (K+ purple). Epithecium yellow-brown to blue-black. Hymenium hyaline, sometimes with a brownish or greenish tinge, 80–120 μm tall. Hypothecium brown. Asci clavate, 80–100 × 20–30 μm. Ascospores colourless, narrowly ellipsoidal to ellipsoidal, 3-septate at first, becoming blue-black and submuriform when over-mature, 17–25 × 9–13 μm.
Chemistry : Thallus K−, C− or C+ fugitive red; containing occasional accessory gyrophoric acid.
S: Otago (Poolburn Reservoir). On exposed, sunny schist outcrops in alpine grassland. Easily overlooked and probably more widespread in Central Otago. Known also from Great Britain, Europe and Scandinavia (Feuerer 1978; Santesson 1993; Scholz 2000; Hafellner & Türk 2001; Coppins 2002b; Ihlen 2004a; Santesson et al. 2004).
Bipolar
Illustrations : Feuerer (1978: 99); Ihlen (2004a: 562).
Rhizocarpon subpostumum is characterised by: a creamish or grey-white, areolate thallus; small, scattered apothecia; a K+ purple exciple; colourless, narrowly ellipsoidal to ellipsoidal ascospores that are 3-septate at first, becoming submuriform at maturity, 17–25 × 9–13 μm; and lacking stictic acid that is characteristic of R. postumum (q.v.).
