This is probably your ancestor:

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s780/0855.tif&Fln=S165078.pdf

The Enrolled Missouri Militia was not a full time regular militia unit, but basically a reserve unit, called up as and if needed. Here is a Wikipedia page with an explanation:

http://en.wikipedia.org/wiki/Enrolled_Missouri_Militia