Can Agaricus sometimes have attached gills, like amanitas do?
What about an odor or any staining reactions?

I agree that this looks a lot more like an Agaricus than an Agrocybe. I disagree with the thought that it might be A. xanthodermus. The Xanthodermati turn yellow and then brown. Given the staining near the annulus, I doubt that all of the staining was done just before the photo was taken and I don’t see any evidence of the brown stains. The key would be the odor. I’m guess these had a sweet almondy odor at least when they were young.

I have to go with Doug on this; this looks like a classic Agaricus. However, if the gills really are attached, it rules this out. No need for a spore print; look closely at the top of the stalk in the photo! I would call that chocolate brown. Going by that distinctive amber-yellow stain in the crack near the bottom of the left stalk, I’d guess this is A. xanthodermus.

You should check this again, this looks more probably Agaricus. You should try a spore print here, which will be brown. But Agaricus will be “chocolate” brown, and agrocybe will be “brown” brown. I know it is annoying, one of those things, that people developed a long time ago this range of “brown” spore prints. Oh, and chocolate brown is the color of dark, unsweetened chocolate, not milk chocolate, brown-brown is lighter.

(I think I got this right, my mind is trying to pull up the different in the color of Agrocybe spores from Agaricus spores… and the details are not coming in… Others can comment if I’m a little off here.)