We believe this is Montaphe elrodi, a species native to the northwestern United States. Millipedes and centipedes both have a lot of legs, but they have some key differences. Millipedes are slow-moving, have two pairs of legs on most segments, and are harmless decomposers. Centipedes are fast, venomous predators that have one pair of legs per segment. This particular order of millipedes (Polydesmida) is the largest, containing about 3,500 species worldwide, with 1,400 of those species in North America north of Mexico. If you were to count the body rings there would be 18 to 22, with 20 being the most common. Flat-backed millipedes are so called for their wing-like lateral extensions of the body segment (called paranota), which give a flattened appearance to many species. Many in this group are brightly colored, warning of their toxic nature. Some secrete cyanide as a defense. 

Size: This individual was approximately an inch long. 

Photo by: Glenn Marangelo on 10/7/21 in Missoula, MT