Could it be Agaricus sp? The gills are white, so probably not? Growing in semi-circle in grass under a Pine tree. The spore test failed, probably because the mushroom was over-soaked with water. In this usually very dry area we have so far received 35 cm of rain since the beginning of October, 43 cm since the middle of September (comp. with 4 mm of rain in October 2013). Rather alarming!