Very crinkly lobes with masses of soredia (I think that is what they are) along edge.
White underneath lobes.
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Thanks all for help with ID.
I think Parmotrema perlatum is very close to my observation.
... the left of the image looks to me like P. glauca with a covering of alga. The right does resemble P. perlatum in some ways, apart from the pale undersides and rhizines (usually black).
Sorry for Tuckermannopsis chlorophylla. The image and description in Dobson seemed a good match, but looking online, I can't find any images that are. The alga (I'm assuming that's what it is) adds to the confusion.
I want to void my ID, and it's a difficult call. On balance, I'm leaning towards P. glauca. If AlanS pops in, he might have a final say.
Lichen on LH side looks a pretty good match with this UK Lichens photo:
But its a species I've not seen myself.
I'm wondering if it might be Cetrelia olivetorum to the right but (having a difficult lichen ID day) could well be wrong! There's no Parmotrema perlatum there though
Thanks for that - so it does. I'm certainly having a difficult time with this one, but rather than change my mind again, I think I'd better give up!
I'd hold tough on your ID of Tuckermannopsis as it doesn't look like Platismatia glauca - its just that not having come across it yet myself I don't feel ok about adding an 'I agree'
You've persuaded me! It's done. Thanks.
I'm new to this so I hope you will forgive me if I have taken a confusing photo.
I've had another look at the sample and I think that the left hand side and the right hand side of the photo are different species as has been suggested above. The left hand specimen has a large 'leafy' form. The largest lobe is approx the diameter of a 10 pence piece. It does appear to have green algae covering the surface in areas making the lichen look greener than it actually is.
The specimen to the right has much smaller lobes in comparison and they are very crinkly with highlighted white edges.
Both appear to be white underneath gradually turning to dark brown when in contact with the substrate.
I am only having a quick look in here today as it is more interesting than the pile of exam marking that lurks just a few feet away.
But after looking at this, maybe I prefer my marking!
To be honest, I don't agree with any of the suggestions so far, but I don't really have one of my own. I usually say we need a closer view, but in this case it might have been good to have one from a little further away. We are kind of looking in at what seems to be an unusual state of something that is probably quire common.
Oh wait a bit - penny drops - I should have known what this is; I wrote a webpage on it a couple of weeks ago! Revision about to be added ...
Thanks for putting us out of our misery. Now I'm kicking myself for choosing something unfamiliar, only to find it was something I had encountered and photographed only last month. Aaargh!
There is a lobe underside that is much too pale for H. afrorevoluta, and some lobes have margins with very fine soredia. I shall come back to this after I have checked something at home.
Thanks again to everyone. Especially to Alan for the detective work. I'll look out for the algae coat in future!
Lat/Lng: 51.94601, -1.19098
OS grid ref: SP557278