Egghead Mottlegill - Panaeolus semiovatus

Pale buff-colured cap; egg-shaped and not flattening out as it matures.  The gills are grey and become black with age.  The stem is whitish with a ring zone that becomes black with shed spores when the mushroom is mature.  

other Mottlegills and Dung Roundhead Stropharia semiglobata

Photograph from top down, in side-view and underneath to show gills; ensure stipe with ring-zone is clearly shown; note habitat and substrate.

On weathered dung of herbivores

Found from late spring to early winter.

Widespread but infrequently recorded in Britain

Occasional in Leicestershire and Rutland.

https://www.first-nature.com/fungi/panaeolus-semiovatus.php