Ganoderma applanatum. I found these in Giant City State Park. This is one of my favorite mushrooms, and these specimens were particularly lovely. The cap color is variable, but the creamy brown of these is what I see the most. The thin crust, which can be chipped off with a knife, is concentrically sulcate. The pore surface appears to be solid, but with a hand lens you can see four to six circular pores per millimeter. The key identifying feature is the easily bruising pore surface which can be drawn on.
Here you will find a record of my forays into the woods looking for mushrooms. If you're interested in what kinds of mushrooms grow in south central Illinois and when they grow, then please join me.