 Originating at 75,789 square miles, its original boundary under Washington Territory contained the southern portion of Idaho County, Idaho's 34 southern counties, part of Ravalli County, Montana, and parts of Fremont, Lincoln, Park, Sublette, and Teton counties in Wyoming.Boise was partitioned off in January 1863 with the Payette River being the primary dividing line.In 1864, two separate acts transferred the portion in Montana to Missoula County, established the southern boundary at 44° 30' latitude, and made slight adjustments in the northern boundary to define the county as one of Idaho Territory's seven original counties.