Did you test the iodine reaction? I think it is a lichen with a whitish thallus.

Thanks, Zotto, for your message.
In fact, I thought this was a lichen, but I found no trace of an alga.
The last two sets of photos (except for the  one on the left hand of the last) were taken after join a drop of melzer to the slide. This permits to see clearly the septation of the spores, which was not possible without it.
Thanks again,
zaca

o.k., this would be interesting to look more closely. With oil immersion I am sure you will see the septa, but different. It seems the cells are ocellate, and the septum gets thicker when killed by MLZ.

It is well possible that the asci are inamyloid. Anyhow I would try Lugol for a possible hemiamyloid reaction.

The spores ar very characteristic, I think a lichenologist knows them.

Dear, Zotto,
I will follow your suggestions and try a new preparation, whenever possible. If I'll find anything different, it will be reported here.
Thanks,
zaca

Hi Zaca,

Like Zotto, I think it could be a lichen, with leprose thallus.

You can test chemical reactions on the thallus : K, P, C, KC.

How are ascomatas ? Perithecias ? Apothecias ?

Alain

Problem with solution in sight: Strigula stigmatella.

The ascomata are perithecia. I made the spot tests and all chemical reactions were negative. Moreover, did new microscopic observation and found that there seems to be a brown or golden-green alga (perhaps of the genus Trentepohlia). I checked the reference mentioned below and, taking into account the microscopic data, found two genera of pyrenocarpous to which my specimen can belong: Porina or Strigula. I found, in particular, a species _Strigula stigmatella_ with similar features.
Many Thanks to Zotto and to Alain, for their comments that lead me to the possible solution,
zaca

Ref:
Alan Orange, British and Other Pyrenocarpous Lichens, 2013.
(available at: https://www.museumwales.ac.uk/media/13849/Orange,-A.-(2013)-British-and-other-pyrenocarpous-lichens.pdf)