I think this is Coprinus comatus, the shaggy mane (US) or shaggy ink cap (British) mushroom. It gets its common name from the scales that flare down and out on the entire cap. The scales are white, sometimes becoming brown at the tip. As the mushroom grows older, the cap begins to flare out a little on the bottom. Eventually, it deliquesces (appears to melt) into an inky, black liquid. Only the stem is left behind.