Tinder Conk - Fomes excavatus

In the photo, you can see a younger conk growing under the older one (which is on top).

If you zoom in, you'll notice trails of short, parallel lines. These trails were made by the radula of a slug (or snail) who was scraping at the surface of the fungus - probably scraping algae off.

This species is well known for its helpfulness in making fire by using the amadou, which is the spongy inner part of the fungus that makes an amazing tinder. In order to get the amadou, the fungus must first be removed from the tree, the hard outer layer scraped off, and then thin strips of the inner spongy layer cut for use as tinder.