I've only very recently started collecting mushrooms so my expertise tends to begin and end with being able to correctly spell "mushroom" three out of four times but to my inexperienced eyes that has the look of a green spored parasol (AKA green spored lepiota, false parasol etc). If so it's the most common cause of mushroom poisoning in the USA.
I spotted a cluster of them yesterday when scouting possible stand locations in the woods behind our house. One was 10-11" across.
Maybe someone more experienced in mushroom ID will come along with a different ID though.
I am glad you posted as I was looking up via Google to determine what kind and that is exactly what I arrived at. There are about 6 more around this one when I went out this morning and it has now opened up flat. Beautiful to look at but not to eat!
As I said, I'm not an expert but it's always better safe than sorry with mushrooms. As I understand things, green spored parasols won't kill you or cause you to need a liver transplant but hey will make you quite sick. They tend to grow in clusters and often in "fairy rings". Seeing multiple specimens at the same time which are in different stages of development can be a big help identifying most mushrooms.