I saw this growing on a big leaf maple tree as I was walking to my car at lunch-time yesterday. At first I thought it was a liverwort, then I saw the sporophyte capsules with peristome teeth and I knew I had a moss.

I kind of dreaded trying to key this one out, but this was actually very easy to key out thanks to its complanate, undulated leaves and the funky dentate recurved leaf tips.