Not Agaricus Augustus or smithii, but Agaricus subrufescensThis is a new one for me here. Also called The Prince.Possibly A.smithii, but the season is wrong and the stem (stipe) does not bulge at the base. It seems to have features of both A.agustus and A.smithii so I'll guess at A.Augustus. Now id'd as Agaricus subrufescens. Found today (5/25/2013) under a sequoia by the bridge on our driveway. This is what those little button mushrooms you buy in the store aspire to be, but they will never get near as tasty.
We've positively identified around 135 different fungi here, but only a handful are edible and fewer still are really delicious. This is one of the winners even with an unsure i.d.. The cap is about 23cm's wide. This will give you a better feel for it's size:
Those are regular paper dinner plates. (white for the spore print)
Which was dark chocolate brown.
I'll cook them up now.
The sample was very tasty