Great find. This mycorrhizal fungus is distinctive with it'sgreyish colouration and striations around the cap margin. You've also captured one of its distinctive characters, the grey margins of the gills. If you'd had a gentle scratch around the base you should have found a membranous saccate volva at the base of the stem (stipe). I've updated the information on this species in the species list.. It used to be known as Amanita punctata.