Smith, C.W., Aptroot, A., Coppins, B.J., Fletcher, A., Gilbert, O.L., James, P.W. and Wolseley, P.A. (2009) The Lichens of Great Britain and Ireland - under Polychidium dendriscum.
Also some in Dobson, Frank S. (2011) Lichens, An Illustrated Guide to the British and Irish Species and in Goward, Trevor (1999) The Lichens of British Columbia, Illustrated Keys, Part 2 - Fruticose Species