Growing on lawn near oak.  Growing singly and in clumps.  When in clumps they are fused together.

Bill N. is pretty sure this is Xanthoconium affine (or X. purpureum)

"One way to separate these two is to put a drop of Ammonia on
the cap cuticle. X. purpureum will turn greenish blue."
I tried this later on 7/14 when I found a suspected purpureum in Westboro.  The reaction surprised me.  Where the drop of ammonia hit the cap, the brick red color of the cap was changed to light yellow, and a ring of blue/green formed around this spot.  Almost immediately this blue/green ring started fading and eventually only the yellow bleached spot remained.

The caps a dazzling orange red but turn brown after being in the house a while.   Flesh white.  Pores white turning to ocher/yellow in older specimens.  Stalk short so that it looks like the caps are laying on the cut grass.  They fuse together where they touch.  Largest single cap 4.5".

Here is what the color of the cap looks like the next day.

Spore print on grass, rusty red.

Underside

Stem solid.  White flesh.