Thats shaggy ink caps. Pick them young, have a good look at the gills, you want them when the gills are still all white. They soon turn pinkish, then black, from the bottom up, going all black and inky.
They don't last long at all, even when picked, so get 'em home and make an omelette fast. Very tasty.
Has a cousin called the common ink cap, which you really, really shouldn't eat. That one reacts badly with alcohol; slightist drink of booze two days either way and you'll be violently sick. Looks different; smoother, greyish to brown cap.