The Collaborative International Dictionary of English v.0.48:

Mississippian \Mississippian\ prop. n.
   1. A geological period extending from from 310 million to 345
      million years ago; it was associated with an increase of
      land areas, the presence of primitive ammonites, and
      emergence of winged insects; called also Missippian
      period and Lower Carboniferous period.

   Syn: Missippian period, Lower Carboniferous, Lower
        Carboniferous period.
        [WordNet 1.5]

   2. A resident of Mississippi.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

Mississippian
    n 1: from 345 million to 310 million years ago; increase of land
         areas; primitive ammonites; winged insects [syn:
         Mississippian, Missippian period, Lower
         Carboniferous, Lower Carboniferous period]
    2: a native or resident of Mississippi