Thallus on calcareous rocks, well developed, continuous, thin, scurfy, gray-brown, dull, K—, lacking black border lines; associated with green algae.

Ascocarps innate in the substratum or adnate, angular or rounded, to 0.8 mm broad, flat to convex; disk black, epruinose; hypothe-cium reddish brown; epithecium brownish black; hymenium 50-60 /xm, yellowish, not inspersed, 1+ red; paraphyses loose, slightly branched, tips slightly thicker and browned; asci broadly clavate; spores elongate-ovate, hyaline, 2-celled with the cells moderately similar-sized (although in a drawing of the spores of var.fusca, Redinger (1937-1938) shows them as very dissimilar),

11-17 x 4—6 ¡xm. Conidia elongate, 5—6 x

0.8 ¿im.

This species grows on rocks of a wide variety, but especially on those with calcareous content. Previously reported in North America by Fink (1935) from New York, Indiana, Illinois, and Nebraska, it has been collected by I. M. Brodo in the Arctic as far north as Polar Bear Pass on Bathurst Island, by N. McCartney on Somerset Island, and by D. Fahselt on Ellesmere Island. Vainio (1909) reported it from Siberia, and it occurs in Scandinavia, south to the Alps in Europe.

Arthonia rupicola Fink in Hedrick (1933) is probably only this species, as it is differentiated solely by the unequal spore cell sizes. As noted above, this character is also to be found in A. lapidicola.