Field Mushroom (Agaricus campestris)
The Field Mushroom was once common but is now declining. It was the original edible mushrooms but they now much harder to find and cultivated species now for the basis of supplies in our supermarkets.
They occur on fairly rich soils, usually amongst grass and often in troops (large groups) in the summer and in to the autumn. They first appear as 'buttons' but soon grow to have the distinctive smooth, white cap with black or very dark brown gills. The cap can be anything from 5 to 10 cms across.
As I said, they are good to eat but you need to be absolutely certain that what you are picking are Field Mushrooms!
Find out more about the Field Mushroom in Dorset here: http://www.natureofdorset.co.uk/species_panel/Field%20Mushroom?vdt=species_master%7Cpage_1