At the latest NWL meeting at Evergreen, I collected Fellhanera on Gaultheria leaves. On the Fellhanera there were tiny (0.1-0.2 X 0.1 mm) lirellae. A smash mount showed 3- septate spores measuring 12.4-14.8 x 3.7-4.1 um, 8/ ascus. The upper two cells are enlarged, but the penultimate cell seems slightly larger. 

It is not clear whether this is an Arthonia or an Opegrapha, but the cell dimensions suggest Arthonia.

In the British flora it comes closest to the Arthonia graphidicola / thelotrematis pair. However, the host is wrong, and there are no pigments in the hymenium. TThe Sonoran flora fails in both genera that I considered. 

The lichenicolous website lists no fungi on Fellhanera.

Does anyone have an opinion on the correct genus? And a suggestion about who might be working on lichenicolous species in that genus?

I have never understood how the term exciple applies to Arthonioid lirellae. Brodo's expanded keys point to the black margins ("lips") as exciple. So I guess I don't know how to decide what is well developed or not. I did find some that I could make a clean section on, and those are attached.

As to who is growing on whom, I do not find the Arthonia on any bare part of the leaf, and I do see it growing over the edge of Fellhanera apothecia. So I am pretty sure that the Arthonia is growing over the Fellhanera. 

Although it is virtually impossible to take a good picture of it, these lirellae mound on top of each other forming piles 2-3 layers thick. The new habit photo shows the mound somewhat, as well as the lirellae partially covering a Fellhanera apothecium.

very interesting, I will keep an eye out for this