birch polypore

Fomitopsis betulina (previously Piptoporus betulinus), commonly known as the birch polypore, birch bracket, or razor strop, is a common bracket fungus and, as the name suggests, grows almost exclusively on birch trees. The brackets burst out from the bark of the tree, and these fruit bodies can last for more than a year.

Excerpt from the Wikipedia article Fomitopsis betulina, by its authors, under the CC BY-SA 4.0 licence.