C. J. B.  Hitch and Millbank, J. W., ?Nitrogen Metabolism in Lichens. VII. Nitrogenase Activity and Heterocyst Frequency in Lichens with Blue-Green Phycobionts?, New Phytologist, vol. 75, pp. 239-244, 1975.

J. Walter Thomson and Brehmer, B., American Arctic Lichens I. The Macrolichens. Columbia University Press, 1984.

O. W.  Purvis, Moore, D. M., Coppins, B. J., Hawksworth, D. L., and James, P. W., The Lichen Flora of Great Britian and Ireland. London: Natural History Museum Publications, 1992.

T. D. V.  Swinscow and Krog, H., Macrolichens of East Africa. Dorchester: Henry Ling Ltd, The Dorset Press, 1988.

B.  McCune and Geiser, L., Macrolichens of the Pacific Northwest. Corvallis: Oregon State University Press, 1997.

O.  Gilbert, Lichens. London: HarperCollins, 2000, p. 288.

?Index Fungorum [taxonomy information]?, 2008.