1 De Vaucouleurs et al., 1991, Cat. 2 confirmation of redshift provided by P. Rosati, 1999, private communication, and Ebeling et al., 2000, ApJ, submitted 4 Stocke et al., 1991, Cat. 5 additional spectroscopy provided by I. Del Antonio 7 Abell et al., 1989, Cat. 9 additional redshift information available in Perlman et al., 1998, Cat. 10 Longhetti et al., 1998A&AS..130..267L 13 Carballo et al., 1995MNRAS.277.1312C 14 Huchra et al., 1990, Cat. 15 Slinglend et al., 1998, Cat. 16 Struble & Rood, 1987ApJS...63..543S 23 Zabludoff et al., 1993, Cat. 27 Fairall et al., 1992AJ....103...11F 28 additional redshift information available in Boyle et al., 1995MNRAS.272..462B 29 spectroscopy provided by D. Turnshek and E. Monier 30 spectroscopy provided by I. Del Antonio 31 Perlman et al., 1998, Cat. 32 also known as EXSS 1646.5+8238 (Tucker et al., 1995ApJ...444..532T) 33 also known as "Cluster B" (Reimers et al., 1997A&A...326..489R). 34 Hewitt & Burbidge 1993, Cat. 36 Hickson et al., 1992, Cat.