A vote here for
Lepiota procera, Parasol Mushroom
Scaly surface, egg shaped babby, shape of mature specimen, pronounced ring on stalk, distinctive bump on the parasol ...
Gills white?
Spore print white?
Agaricus augustus seems to have smaller scales, less of a wide flattened shape, no bump, more vestigial ring on stalk, and brown gills?
Habitat in open woods and pastures (that's why I think it's not
Lepiota rhacodes Shaggy Parasol, which grows in woods)
According to Roger Phillips' mushroom bible, uncommon and excellent eating :)
also good pics here, scroll down a bit
http://www.ilmyco.gen.chicago.il.us/Taxa/Lepioproce213.html