Green-brown lichen growing on top of flat limestone tombstone. Possibly Collema polycarpon.
Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
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Great to see you, Alan. I hope all is well.
I'll be really surprised if this isn't C. polycarpon - and a bit deflated, because I thought I'd got to grips with these two species in recent months, though by no means with all of Collema (ha! no chance). But I agree that the spores need looking at.
Aimed at no one in particular:
Just spotted a fine example in the first photo of a 'clean zone' around the thallus, where algal growth has been inhibited by lichen substances (allelochemicals). Exciting stuff, no?
In the absence of spore details we will have to agree to differ on this. I have looked again, and also gone through the excellent series of Collema photos in F. Schumm's Kalkflechten der Schwäbischen Alb, ein mikroskopisch anatomischer Atlas* and I still think this is C. cristatum.
There again, I have numerous Collema collections of my own that I am not naming until I gather them together and do the microscopy. I also admit that you have more recent experience of limestone lichens than I have. But to me, it looks like the common species, not the rare one.
* Very nice book on limestone lichens. I think I got it off Amazon.
Google Books won't hand me the C. cristatum pages, but the superb photomicrographs I can see make this book very tempting.
I should have tried to explain my justification for favouring C. polycarpon, but it isn't easy to express. The lobes don't look right (including being too thick?) for my fledgling concept of C. cristatum, and some of the apothecia are (sub)stipitate.
My encounters with C. polycarpon are dwarfed, as they should be, by those of C. cristatum, but the former species seems quite common in some pockets of the Yorkshire Dales but not in other limestone areas (I have yet to find it in the Arnside/Silverside area). However, I must admit that my most recent records aren't backed by microscope evidence because I was sure I understood the morphology. Given how awkward C. cristatum is, in all its myriad forms, I could easily be wrong.
Thanks to both Phil and Alan for keeping me on my toes!
Lat/Lng: 52.0209, -2.036
OS grid ref: SO976358