Collection location: Bon Tempe Reservoir, Marin Co., California, USA [Click for map]
Growing on an oak leaf, these don’t seem to fit any of the listed California species of Marasmius. The closest match seems to be Marasmius quercophilus but these seem to lack the pruinose, more wrinkled cap. The conditions were fairly wet, which could be a factor.?.
M. quercophilus seems to work.