I'm not terribly good at IDing mushrooms. But this mushroom is big, growing on grass, has darker flakes on the cap, a big, pronounced ring on the stipe, and picture two looks like it's got green spore dust on the gills. So my guess is Chlorophyllum molybdites. http://en.wikipedia.org/wiki/Chlorophyllum_molybdites
Hard to tell the colour of the spores or gills in these photos, but Chlorophyllum molybdites is certainly a possibility. The cap shape and scale pattern look more like Agaricus to me, so if the gills mature to pinky brown with purple-brown spores instead of creamy white with green spores then it could be one of the large meadow Agarics. Proper identification could be done using a good mushroom guide (for your area), plus spore prints, plus examining the shape and colour of cap and gills at various stages of maturity.
Update: mushrooms have made a return appearance! Same yard, same location---half a dozen in a row that meanders several feet across the grass. At first I thought that they were different from the above-pictured: looked almost like shaggy manes. But sure enough the next day it was obvious that this new crop were younger fungal siblings. Mushroom transformation from juvenile to mature form never fails to amaze me. So rapid, so profound.
