Collection location: Bon Tempe Reservoir, Marin Co., California, USA [Click for map]
I believe these were identified at the MSSF Fungus Fair as E. rhodopolium and they do match the description in Mushrooms Demystified.
However, due to Nathan’s comments and additional information, I think Entoloma ferruginans is the correct id.
He says they are 5-6 sided and distinctly angular. Based on the spore drawings, you could say they have one prominent appendage. I would guess it’s where the spore attaches to the basdia, but that’s not clear from the drawing. The ‘appendage’ is roughly triangular and fairly close to equilateral.
That’s what I’d call them in southern California. According to Largent is has been collected in Marin Co. He says:
“__E. ferruginans can be recognized by the unique combination of a dark grey brown to nearly black pileus which is lubricous, grey to dark grey lamellae, and the nitrous or chlorinaceous order which becomes cucumber-like when the basidiome is cut.”