There are lots of these Shaggy Ink Caps growing on our lawn at the moment. They start off almost perfectly oval and white, and gradually the gills deliquesce from the bottom up, dripping away like black ink.
Apparently they are edible. But not to be confused with the Common Ink Cap, which is poisonous if consumed with alcohol up to three days afterwards, hence - according to the website where I gleaned this fascinating information - its common name of Tippler's Bane. Hah! Tippler's Bane? Surely someone made that up?
Well, there's no way I'm going to risk eating one of these. What if I mix up shaggy with common, a sip of wine passes my lips within the next three days (it's Friday tomorrow, after all) and I'm rushed off to Accident and Emergency with the tell-tale symptoms of Tippler's Bane? I'd never be able to show my face at the school gate again!
Your photo reminds me to water my new plant! ;)