So I found three of these popping out of a lawn on the side of the road on my street today. It is possible that it was near conifers, as my area is thick with them, but it is also possible that it wasn't. They were in a nicely trimmed lawn. I believe them to be from the Agaricus family, which is indicative of their gills.
The gills are free and brown. The central stalk is obviously white and does not bruise, or does so quite slowly. When dissected, it is white and fleshy, without stains. On its exterior it contains an intermediate annulus, which means that a partial veil was present; however, there is no vulva at its base.
The cap, as you can see, is a dark yellow-ish beige in the center then it fades to white and then to a brownish-black color towards the edges. Over time, as the mushrooms have been sitting on the table, the black circumference of the cap has continually gotten bigger, expanding into the center.
I did the spore print on both white and red paper, in case the print was white. It was actually a smoky-black, but as I am new to this it could technically be "chocolatey-brown". The mushrooms has a mild fungal odor and did not seem to me to be too acrid, but then again it could be to my amateur status that led me to mistake the faint phenolic odor. Thus it could be in the general Agaricus (edible), Hortenses (edible), or Xanthodermati (poisonous). I ruled out 4 other subgroups due to its lack of rufescence (turning red) and the form of its annulus. I found some cleaner in the house that I could use to perform a KOH test. When it touched the mushroom, the flesh became yellow indicating that it is positive for KOH, which overrules the Agaricus and Hortenses, leaving the Xanthodermati. Although they could potentially be A. osecanus, or Giant Horse Mushrooms, because they are supposed to stain yellow with KOH; I want to say it is the A. xanthodermus because it looks more like this one than the other in the book's pictures. When I googled pictures of this species, none looked like what was in the book OR what I had picked. Oh well.
Thoughts on my find? Disagreements?