Western pearlshell
The name Margaritifera margaritifera is normally used for specimens east of the continental divide, but I have concluded that all Montana populations are M. falcata. They likely crossed the divide with the west-slope cutthroat trout, which is the native salmonid of the upper Missouri River drainage. This is a headwater species occuring very near the continental divide on both sides. It does not seem to occur anywhere in the cental part of the continent. All of our Margaritifera have purple nacre, while the true M. margaritifera of the Atlantic slope has white nacre. I can find no differences in the shells of our east and west slope populations. While I am quite convinced, further study would be good.
In our trout streams and trout rivers west of the divide and in the Missouri headwaters. Outside of Montana this species is reported from the Pacific drainage from southern Alaska to central California.