Flat Creek Hatch

These mayflies were landing on me this morning and molting (shedding) from duns to spinners.  Only sparse on the water, but they brought up a few nice fish to cast to.  They were too big to be tricos (16-18ish), not mahoganys, not callibaetis, didn't look like any pmd's I've been seeing this summer.  Kinda fat for mayflies.  Beats me!  ???