I am not sure if this is P. piluliformis. What I have been finding in Marin and San Francisco which I identified as either P. piluliformis or perhaps P. laevissima, are much darker. The pileus is typically deep purple (almost black) when young, purple when mature, and pinkish/lilac when dry. The stipe tends to be pinkish, and much more sturdy than any other Psathyrella I’ve found.
It is unclear to me whether P. hydrophila was replaced with P. piluliformis or if P. piluliformis was split from it… I think that they were split, however, I do not know for certain what the taxon is for the Northern CA species pictured here.
I hope to get Dennis some dried specimens of the dark purple Psathyrella in early February, so hopefully the classification of these related species will become clearer soon.