Birch polyphore or Razorstrop Fungus

This is Piptoporus betulinus found growing on a decaying birch stump on Chailey Common. They are multi-layered because the fruiting bodies persist from one year to the next with a new one forming each year. You can see the remains of the oldest ones rotting away in the top left photo. The original would have been much prettier; toasted on top and creamy white underneath. It is a very common fungus found on birch throughout the British Isles.
Interestingly bits of this fungus were found amongst the possessions of the 5500 year old mummy discovered in the Italian/Swiss Alps back in the 1990s. Otzi may have been carrying them to sharpen blades or possibly for its antibiotic properties.