The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
See the main Ephoron page for details about this species, which is not known to differ in any important ways (besides location) from the other species.
On page 243 of Hatches II there is a passage from an 1802 speech before the American Philosophical Society in which this mayfly was introduced to science. It was the first mayfly species described in the United States, so it is ironic that it went unnoticed through so many of the early decades of our sport.