This dun was collected in proximity to this spinner, and to this female that looks like the same species.
Among western species, it most closely resembles Ephemerella aurivillii, of which I reared a specimen from a positively-identified nymph and photographed the male dun previously in Alaska. In the photos, this dun does not seem to have the same evidence of the nymph's dorsal (Dorsal: Top.) stripe, but that's due to the lighting. A faint stripe is apparent under the microscope, as it is in the female linked above.
They do appear to be a bit short for aurivillii, with the male at 8 mm and the female at 9 mm, but the spinners could be a bit longer.
Among the other Ephemerella mayflies documented in Montana, Ephemerella alleni and Ephemerella mucronata are the only two others that seem to not be ruled out.
This mayfly was collected from the Madison River on July 1st, 2019 and added to Troutnut.com on July 18th, 2019.
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