Petrusaria but not sure

I feel sure that you have arrived at the right genus. It is perhaps surprising that we can recognise members of Pertusaria considering the range of forms that are exhibited, and in particular some which are sterile and sorediate and others that have apothecia. Nevertheless, there is something about the waxy thallus and the disposition of structures which make most Pertusaria specimens recognisable as such (I am using the name Pertusaria in the s. lat. sense, as in LGBI2).

I would expect P. multipuncta to have apothecia with more open discs, the discs covered in coarse crystalline matter and hence somewhat resembling soralia. I can’t detect any such crystalline matter on the apothecial discs in your photos. There is a certain roughness to the thalline margins of the fertile warts, mainly caused where coarse crystals in the upper part of the thallus have eroded. The discs in your photos are hardly exposed.

I don’t get to see any members of Pertusaria very often, they are rare in formerly polluted Eastern regions and most members seem to be reluctant colonists. I did add P. leioplaca to the Bedfordshire list recently and you may be interested to see my investigations to confirm its identity.

https://twitter.com/obfuscans3/status/1491165652585152513

I tentatively suggest that your lichen might also be P. leioplaca but I wouldn’t confirm it as such without microscopy (due to my relative unfamiliarity with the genus). Others may be sufficiently familiar with the genus to reliably identify your photos on sight.

As promised a New Forest Lepra multipuncta (Pertusaria multipuncta) picture

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Lepra multipuncta, Oak, Coomy Hat, New Forest

Fairly distinctive in the pulvinate (pillow like) grey thallus and scattered fertile worts, which are paler at the tips than the thallus. Confirmed by the slow (minutes) K+ chestnut brown to dark reddish reaction of the medulla (visible best on top of the warts). Widespread on well lit trunks and especially branches in the lower canopy in woodlands in the west and south.

Most easily mistaken for Lepra pulvinata (Pertusaria amara f. pulvinata), which is more pulvinate, with a more green-grey thallus and warts (usually sterile), which are whiter and more strongly contrasting with the thallus and are more densely and regularly set. It is K– (brownish after hours) and KC+ violet and tastes bitter. Much rarer and confined to the trunks of veteran trees.

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Lepra pulvinata, old Oak, Mallard Wood, New Forest

Neil