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

I. M.  Brodo, Sharnoff, S. Duran, Laurie-Bourque, S., and Sharnoff, S., Lichens of North America. Yale University Press, 2001.

J. W.  Hinds and Hinds, P. L., The Macrolichens of New England, vol. 96. China: The New York Botanical Garden Press, 2007.