Champignon Mushroom

It's more likely to be Coprinopsis atramentaria, because of the grey cap, which is also larger (4 to 8cm high, 3 to 6cm in diameter) than that of Coprinellus micaceus (3.5cm high, 5cm across when spread max). Though to be sure which of the Coprinopsis species it is, you'll need a microscope. It is also very difficult to tell without seeing the stipe and details of the capsurface. Definitely not Agaricus bisporus, because of the inkforming of the cap, which does not happen to Agaricus species.

These look more like old glistening inky caps, I've seen a ton of them all over my neighborhood in various stages of development.