Spotting

I thought Calvatia didn't have stems; this has a substantial one. Did you cut it open? The flesh and lack of gills would be telling; I almost think this could be a young Agaricus, if there turned out to be gills. In other respects it really does look like a Calvatia gigantea.

One of the giant puffballs, genus Calvatia. Perhaps C. cyathiformis: http://www.mushroomexpert.com/calvatia_c...