Collection location: Mountain View, Santa Clara Co., California, USA [Click for map]
Ok these are Armillaria mellea as far as I can tell. But I looked at:
And I don’t really understand the details. It seems that mellea is a east coast only one? But I’m not sure what will be a west coast caespitose Armillaria that is not on conifers…
In any case, I am going with mellea, it has all the features.
One difference with the “sinapina” I found in the hills this weekend, was that the “sinapina” has a creamy light yellow spore print, and this mellea has a white-white, as white as the bright white paper I use, spore print.
The photos here are like a time-lapse series of the same cluster over 4 days.