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Some fungi are very poisonous so a mistaken ID could have serious consequences.
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Sorry Lynzeylou; this is an amendment to the comment I made earlier. I thought I had worked out the name changes for Xanthoria, but, thanks to Jenny's helpful comment, I now see that I got it completely wrong. Do read Jenny's comment,
This foliose lichen without apothecia and without isidia looks like a lichen that was, until recently, known as X. calcicola but is now called X. aureola. As you will see if you read Alan's comments, this change of name has caused confusion. http://www.lichens.lastdragon.org
However, your location, a churchyard near Milton Keynes, makes this unlikely, as it is not near the coast.
It wasn't X. calcicola which had its name changed to X. aureola!
The mainly coastal lichen you mean can be found in older reference books as Xanthoria ectanoides but is now known as Xanthoria aureola
Many thanks to your helpful comment, Jenny, I thought I had worked out the name changes for Xanthoria, but I see that I got it completely wrong. In addition, I may have misled Lynzeylou, so I have amended my original comment to deal with that.
So, is Lynzeylou's original suggestion of X. calcicola correct?
Lat/Lng: 52.0476, -0.7886
OS grid ref: SP831394
Village churchyard dating from the 13th century, now in large urban conurbation, close to main railway line and busy trunk road.