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Paired Sand Gaper valves. Left valve shown uppermost with the more pointed posterior on the right. There is evidence of the periostracum so a fairly fresh shell. The spoon shaped chondrophore which is typical of Mya species can be seen in the second picture. The pointed posterior rules out Mya truncata

My fourth, maybe final, observation of this delight.
Partnered with -
https://www.inaturalist.org/observations/18435310
and
https://www.inaturalist.org/observations/18270468

It was once known once as the Hartlepool Horn Limpet.
HERE
It is worth reading on a little to get a feel for the language used in those days.
This was at a time when it was fashionable for biologists to vie with each other, often re-describing a well-known species again and again in the hope of attribution in the Nature journals of the day. Known then by a few as the Horn-shell, it was found on a beach in Hartlepool UK by James Petiver (c. 1665 – c. 2 April 1718)

The little jeweil-of-a-shell was known to, and named by, Linnaeus in 1758.
But I suspect he did not handle the late stage shell which, for a few years, was considered a separate species with various popular names. One, which hung on for years, was Patina pellucida leavis (Pennant, 1777)
But in 1778 Emanuel Mendes da Costa wrote
"This species of Limpet in its states of young and old is extremely different that it seems two distinct species and for such authors have both figured and described them."
The rest, of course, is history

I have visited 10 distilleries where this fungus is rampant. Usually expresses as sooty staining on external walls, nearby trees, signposts and street furniture. It forms on structures up to 600 metres (downwind?) away from Distilleries and Bond Warehouses. It is known to cause unsightly staining on nearby modern dwellings.
It is granular and can be scraped from the surface and cultivated. Here we see examples on structures at the Blair Athol Distillery, as well as one or two from other locations..
3 Pictures showing the distinctive spore chains
https://en.wikipedia.org/wiki/Baudoinia_compniacensis

(Edited 22 Dec)
At first glance you may think I have made a mistake here. But ALL the shells are of Patella pellucida.
These are late-stage shells. Probably in their third, up to fifth year, the young molluscs make their way from the frond of, mostly, kelp to the lower parts of the stipes (stems). There, they feed into the tough tissue and take on a horn-like colour and grow to an absolute max of 3cms (2.76mm in Orkney). It is these specimens that were given another name Patina pellucida leavis because they were seen for a long time as a separate species but they are not.
At this stage they loose their delightful jewelled appearance and take on the shape of the rounded stipe (stem) but retain their overall patella appearance but without ridges.
Nice specimens show remnants of the blue rays but also elusive underlying red ones.
Get your eye in and you will find this shell mixed in with common limpets, on the beaches of the north-eastern Atlantic,
the one USA record has an interesting story.
See the record - https://www.biodiversitylibrary.org/page/27054800#page/267/mode/1up
.
Follow the story here -
https://www.inaturalist.org/observations/18435310
.
See - https://www.marlin.ac.uk/species/detail/1298