David, nice fossil. If this is definitely from the Madison Limestone then it is Mississippian in age — far too young to host Rafinesquina or Strophomena. I do concur that it is a strophomenide brachiopod, most likely an orthotetidine. This article is a good jumping-off point:
Thanks for the link to the PDF. I saw that article before. I didn’t save the article for future use. I wasn’t sure the age of the limestone. I found this article “Type Sections of the
Madison Group (Mississippian)
and its Subdivisions in Montana
GEOLOGICAL SURVEY PROFESSIONAL PAPER 842”
This article identifies the area to the mile where I found the brachiopod. I will now use the linked article you’ve given to narrow the species. I will be looking at the ones you named to see how they line up with what I have.