I didn't find anything in Ancestry nor in FamilySearch for Cass and Matilda. Mississippi didnt record deaths until 1912, therefore I didn't expect to find their death records but it's odd they don't appear on a Census.
I'd contact the following and ask if they have a local death index, indexed obits or inventoried cemetery records:
Southwest Mississippi Genealogical Society, http://swmgs.com/
Franklin County Public Library, www.llf.lib.ms.us/webpage_7.htm