Hello Mushroom Guru
False Parasol - Chlorophyllum molybdites does sometimes grow in a ring, very much like the Fairy Ring mushroom. The false Parasol is normally confused with the "Shaggy Parasol", as they look very similar. The gills on the false parasol have a slight greenish tinge to them which helps you identify it - the shaggy parasol has pure white gills.
Here are some clearer photographs from Wikipedia:
Be sure to look for the green tinge in the gills if you are not sure! Green BAD!!!