Is this some sort of Oyster ?

[TonyC]

On a tree stump at Ynys-Hir in Wales in October.

[Leif]

On a tree stump at Ynys-Hir in Wales in October.

Tony: Hard to be sure from the photo but I'm fairly sure they are Honey Fungus - Armillaria mellea. I bet there's a prominent apical ring - i.e. a ring near the top of the stem - and the stem is hollow and brittle. This is a serious pest of living and dead trees, and typically forms clumps at the base. It spreads by long black string like growths that spread out under ground searching for a new host tree. Peel off the bark from a dead infected tree and you will see the black lace like growths. The fruit bodies are edible when well cooked and the cooking water should be discarded. However, before you tuck in, beware that several deadly poisonous Galerina species look similar and grow in a similar manner. In other words, don't even think about eating them unless you know 100% what you have found, otherwise you might not enjoy many more meals!

[cjay]

I concur with that. It looks like Honey fungus to me.