Not sure what this is, but for a second I got my hopes up for oyster.

Oysters grow out of wood, for the most part. And you can see how it has gills instead of ridges. Ridges would gradually get narrower and less deep as they go down the stem. There is no sudden cut off between ridges and stem like there is between gills and stem.

There are better ways of identifying mushrooms than that, but that requires some books.