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.

J. W.  Thomson and Ahti, T., ?Lichens Collected on an Alaska Highway Expedition in Alaska and Canada?, The Bryologist, vol. 97, pp. 138-157, 1994.

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

?Index Fungorum [taxonomy information]?, 2008.