As our knowledge of the western Ephemerid fauna, particularly in its nymphal stages, increases, cur conception of the individual species becomes more accurate, necessitating in some instances corrections of previous statements, and enabling us to differentiate closely allied species in various groups which here-tofore had been lumped under one name. The basis for the following notes has been the rearings carried on by my assistant, Mr. A. N. Gartrell, in the Okanagan region of British Columbia in 1934.