At first glance gill attachment of this fungus is 'free', it has dark coloured spores and a weak, transient ring on stipe. The cap is about 4cm diam and the stipe a little longer.
Trying to key this to 'genus' leads to Agaricus but no species in that genus is anything like this!
A more careful look shows the gills have neatly torn a portion of stipe apex resulting in a form of collar ---- the gill attachment is 'broadly adnate'.
The spore print is brown and size and shape of spores fit Agrocybe molesta. This is further complicated by name changes. Funga Nordica make this a synonym of A.dura but FRDBI does not yet accept this! Take your pick ----pictures on the web look the same and it has been given a common name, 'Cracked-cap Agaric', which describes the appearance quite well. Foud on fixed dunes at Pembrey saltings --- in one of the cattle compounds.