Brown Rollrim, Paxillus involutus s. l.

 

Paxillus involutus (Batsch) Fr. is a very common and also highly toxic mushroom indeed. It'll quite likely get you in the end, destroying your red blood corpuscles after your own immune system is eventually triggered to fatally turn on you.

It is related to the poroid fruiting bodies. The gills become more pore-like as they become decurrent down the stem, and it is said that they are detachable as a layer by pushing with a finger tip at the apex of the stem. 

I couldn't really detect much downiness in either cap, although this is noted in the literature to reduce as the caps age.

Note the brown staining when handled or otherwise bruised. There is indeed a slight umbo in the shallow central depression of the cap in this quite regular cap, possibly not too old a specimen. 

In the second photo, where the cap has been turned upside down, the fluting around the edge as reported in the literature is very clear.

And another larger, somewhat more distorted (a partial rose-comb aberration perhaps?), cap nearby. 

The fluting around the rim of the cap is far more limited in extent in this specimen.

Paxillus involutus s. l. (Batsch) Fr. is a species complex of at least four species, distinguishable by DNA analysis. These fruiting bodies might be the nominate species, as they were growing in more acidic soil and close to Birch, Paxillus involutus s.s.