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.

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.

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

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

T. H.  Nash_, III, Ryan, B. D., Gries, C., and Bungartz, F., Lichen Flora of the Greater Sonoran Desert Region, vol. 1. Tempe: Lichens Unlimited, 2001.

?Index Fungorum [taxonomy information]?, 2008.