fits well with the group of Minores around semotus and micromegethus, but this doesn’t look like any of them.
Peck’s description of micromegethus is pointing in particular at the strong rusty brown cap center. A good picture of a possible micromegethus can be found here:
http://forestry-dev.org/cgi-bin/matchmaker/latin.asp

(Btw,
Agaricus micromegethus has to be the proper name, because Peck named it so, and also repeated it in several reports)

Fred Stevens’ photo on MykoWeb of A. micromegathus looks a lot more like what I called A. semotus in the obs. I referenced, which makes me think that I have misidentified my collections. It also makes your observation more likely to be A. semotus.
I am at a loss.

It well could be A. micromegathus. I went with semotus because of the habitat ( clearly wooded/no grass), the basically smooth stipe and the rhizos at the base.

A. micromegathus
Here’s a side-by-side I posted about a year ago.
http://mushroomobserver.org/image/show_original/37832?obs=18991&search_seq=1309560&seq_key=1024944