The white gills don't look Agaricus to me. The gills together with the slight bump in the center of the cap look like Lepiota or Leucoagaricus (= "white agaricus"). I'll go with Leucoagaricus leucothites just based on photos. I assume you looked closely for any sign of a cup at the base, which would have put it in Amanita. Possibly Amanita ocreata.
Don Loarie
2012-11-07
2012-11-07
United States of America, California, Alexander Valley, CA