Thallus (Fig. 31) areolate ; areoles (1–) 3–10 (–15) per mm^-2 ; areoles flat to distinctly convex, rounded, grey to light brown; medulla I-, often with calcium oxalate. Prothallusindistinct, black. Apothecia(Fig. 31) irregularly arranged, (0.3–) 0.5–1.0 (–1.5) mm diam; disc flat to convex, often with white pruina, otherwise black; margin (0.04–) 0.07–0.11 (0.14) mm thick, indistinct when fully mature, black; exciple in section Atra-red (K+ purple, N- or N+/- intensifying, HCl+/- intensifying). Hymeniumhyaline, sometimes with a Macrocarpa-green tinge (K-, N+ red, HCl+ bright blue), (125–) 160–250 (–320) μm high; epihymenium Atra-red, often intermixed with Macrocarpa-green, with or without calcium oxalate. Asci8-spored. Ascospores (Fig. 8F) narrowly ellipsoid to ellipsoid, (25–) 30.5–40.5 (–43) X (12–) 14–19 (–22) μm, eumuriform, with (5–) 9–16 (–17) cells in optical view; length/breadth-ratio (1.5–) 2–2.5 (–3.5).

Chemistry: Lichen products not detected.

Ecology:R. roridulumhas been found on siliceous rocks, rarely on schistose rock, in the inundation zone of lakes, on irrigated rocks, and on damp rock walls.

Distribution:R. roridulumhas a scattered distributed in the northern and eastern areas of the Nordic countries where it occurs in the slightly continental and the indifferent sections of the boreal and alpine zones (Fig. 32). It has been found up to an altitude of c.1100 m (Kongsvoll, Oppdal, Sor-Trondelag).

Discussion:R. roridulumis the only species which frequently has a pruinose apothecial disc, and is then an easily recognized species. When this pruina is absent, it macroscopically resembles R. amphibium, but it is distinct in having convex and circular areoles (in R. amphibium, they are flat and polygonal), and apothecial margins which are distinct (in R. amphibium, the margin is distinct when young and later indistinct).  R. roridulumhas never been found with the Macrocarpa-green pigment in the upper outer part of the exciple, whereas it is sometimes present in R. amphibium.  When the pruina of the apothecial disc is absent, it is also very similar to R. lavatum. However, R. roridulum differs from that species by its Atra-red exciple (reacting K+ purple), by having more rounded and convex areoles, and by having fewer cells per ascospore (there are 12–21 cells per ascospore in optical view in R. lavatum).  R. roridulumis apparently new to Finland, as it is not listed in the checklist of Vitikainen et al. (1997).