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A greenish-gray foliose lichen with black, unbranched cillia. Apothecia have a hole in the middle--this gives the lichen its species name and common name, and also distinguishes it from the similar appearing P. michauxianum.
Most commonly on trees. Occasionally on rocks.
I have a tough time identifying lichens, but I'm confident I got this one right. Nonetheless, feel free to correct me (especially if you'll take the time to explain how you ID'd it).