Butter Caps

This morning I found a number of Russula's, some with purple caps others maroon. The Russula group is rather large and I feel like I'm unlikley to ever identify more than a few of the more obvious ones in the field. The other fungi which seems omni-present at the moment is the Butter Cap or Greasy Tough-shank. Its allegedly easily identified by the 'buttery feel' of its cap (rather than the greasy feel of its tough shanks, I presume) and its distinct 'umbo'. I rather like the word 'umbo' (the little knobbly bit in the middle of the cap).