Hoof fungus
Fomes fomentarius.

Its been a poor year for fungi so far so we were pleased to find this Hoof fungus in the woods this morning.
You can see how it got its name. But it is also sometimes called Tinder fungus as it has a traditional use for lighting fires. 
It infects its host tree through cracks in the bark and causes rot. It will continue to live on its host for many years after the tree is dead.