Yellow Waxcap Mushroom
The Yellow Waxcap Mushroom - Hygrocybe species - comes in many sub-varieties. They are bright yellow in color and feature, as you might guess, a waxy surface. This one has a flat, bright yellow top but I've also seen photos of them with a more rounded shape. I'm not sure if they are a different sub-variety or if the mushroom changes shape as it grows and ages.
This Yellow Waxcap Mushroom was photographed in the Sutton State Forest in August 2012.

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Mushrooms Photos from Sutton Massachusetts
Photos in Sutton Massachusetts