Runemark, H 1956: Studies in RHIZOCARPON. I. Taxonomy of the yellow species in Europe. Opera Botanica 2(1): 1-152.

Description.  Thallus 1-5 cm in diam.  Areolae and apothecia close together forming a continuous thallus, or +/- dispersed.  Prothallus usually visible between the areolae.

Areolae 0.3-1.0 mm in diam., 0.2-0.6 mm thick, usually strictly angular, plane to convex, usually intensely yellow, rarely grayish-yellow, usually smooth, glossy, rarely matt.

Apothecia 0.3-0.9 mm in diam., 0.2-0.6 mm thick, strictly angular, plane, with a thin distinct margin.  Hymenium 110-160 µ thick.  Spores muriform, with few septa, 20-32 X 10-14 µ.

Ca. 310 collections examined.  Analyses of lichen substances in 25 specimens.

Variability.  Ssp. frigidum is rather invariable.  It varies mainly in the place of areolae.  Especially in specimens from the Arctic areolae, or groups of areolae and apothecia are +/- dispersed on a prominent prothallus.  Transitional trypes between ssp. frigidum and ssp. vulgare are often met with in montainous (sic) districts.

Ecology.  On exposed rocks and boulders mainly in the alpine belt.  Usually on acid rock.

Distribution.  In high mountains of Europe (in northern Scandinavia also in lowlands), Spitsbergen, Novaya Zemlya, arctic Siberia, arctic Canada, Greenland.