A Wet Lichen

This lichen is on the bark of a tree in our yard.  Lichens take on stronger colors when they are wet, as this one was.

I leafed through my book Lichens of North America, by Brodo, Sharnoff, and Sharnoff.  It is a huge, coffee-table book with great photos of lichens–highly recommended if you can find a copy.  I was a little intimidated by the diversity of the group, however, and gave up in my attempts to identify this one.