Mushrooms in the Grass

Several years ago we took out a struggling, weak-branched tree that was too closely surrounded by two other large trees. Doing so has certainly created healthier conditions for the trees on either side of it. We recently noticed that the removed tree has left a legacy in the soil: a crop of mushrooms thriving on the decaying root structure after the stump was ground up. As mushrooms frequently seem to do, they sprang up seemingly overnight, disappeared again to all appearances, and reappeared.

Unless these are two different kinds of mushroom, it appears that the tops are almost flat and white with an oyster-like wavy edge when newly emerged and then undergo a separation and furling-up so that the dark brown undersides are exposed in a pattern like flower petals or fat asterisks.

I know very little about identifying fungi, so I'm certainly not going to try to eat these. Here's a visual identification guide to mushrooms, but I understand it can take careful examination to be sure of an identification except in some more obvious cases. Perhaps someone more knowledgeable than I will have an opinion about what type these are.