hi and welcome to WAB
it's a bolete that has been attacked by the parasitic mould
Hypomyces chrysospermus; as it develops the yellow colour implied by the name "chrysospermus" will become evident
so two fungi for the price of one - (and it never did have gills of course

); and don't expect every fungus you find to be in your books (unless you have a veritable mycological library . . . . )
cheers
Chris