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Added: 15th January 2013 Description:
Grey areolate crustose lichen with dark prothallus (possibly Aspicilia caesiocinerea) growing on siliceous rock seems to be being taken over by a larger structured light yellow crustose lichen with bigger areoles and with black apothecia. Parasitic lichen possibly Rhizocarpon viridiatrum.
Identifications Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
Confidence: It might be this.
Confidence: It might be this. Notes: This is a speculative ID, but, bearing in mind that Phil has described it as "light yellow" (colour evidently lost in the digital photography, as happens), which I am extending to an assumption of light brown, then I think it is plausible. I do now think it is a Lecidea at least (advance on my comment below). It is very variable, in colour, and in the areoles, that are completely flat, or convex as shown here. Note that the photo in current Dobson is indeed of this species, but past photographs and identifications are very often of L. grisella, which has been separated off from L. fuscoatra and appears to be the more common. As to whether it invades other lichens, I have no knowledge of this, but it's the sort of thing that crustose lichens do when they think no one is watching.
No interactions present.
Other observations of