Mary R.Bishop;91;formerly of Belle Vernon;died Thursday;Dec.4;1997;in the Lawson Nursing Home;Jefferson Borough.  She was born Oct.15;1906;in Lonaconing;Md.;a daughter of the late Frederick and Agnes Fisher Merrbaugh.  She was a member of the Marion Presbyterian Church;Belle Vernon;and the Order of the Eastern Star Chapter 196.  She was a member of the former Washington Township Lions Auxiliary.  She resided in Belle Vernon since 1953.  Survivors include three sons;Theodore L.Bishop of Grover;Mo.;Fred R.Bishop pf Post Vue;and Raymond Bishop pf McKeesport;11 grandchildren and 12 great-grandchildren.  Friends will be received Sunday from 3 to 8 p.m.in the Ferguson Funeral Home Inc.;700 Broad Ave.;Belle Vernon.