A strange white fungus, possibly Hyphodontia arguta, growing on the decaying stump of a silver birch tree (Betula pendula). [Note added 2012-12-09: this more likely to be a Schizopora sp, maybe Schizopora paradoxa.]
As I was looking at these pictures again while writing this post, it occurred to me that this might just be the remains of a more 'traditional' form of fungus that has been eaten by some animal, maybe a deer or a rabbit. However, I have never seen any deer in the area, the fungus is too high for rabbits, and there are no signs of teeth marks on the fungus or tree, and there were no fragments of the fungus on the ground at the foot of the tree. So I probably have to accept that this is the natural form for this fungus.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-01-30.