To say that I don't know my lichens would be one of the vastest understatements in natural history; to say that I should know my lichens is also true, since I have on my bookshelf, the uber-book about these combinations of fungi and algae, Lichens of North America, the almost-800-page behemoth by Irwin Brodo, Sylvia Sharnoff, and Stephen Sharnoff. Alas, I haven't given the volume sufficient study, and so when I see a lichen—an everyday occurrence, since lichens are just about everywhere—the identity is almost always a mystery. So it was with this crustose specimen that covered part of a boulder. I was about ready to give up on a name when I got lucky in the LNA compendium. (It's way too big and weighty to be called a field guide.) The gray color, the habit of encrusting on rock, and, especially, the whitish edge, which is known as a prothalline border, makes me lean in the direction of Cryptothecia striata, a member of the Christmas Lichen clan. I'm sending this shot out for expert opinion; I'll let you know.