We have carried out additional spectroscopic observations in the field of cluster Cl 1324+3011 at z = 0.76 .
Combined with the spectroscopy presented in Postman , Lubin & Oke ( 2001 , AJ , 122 , 1125 ) , we now have spectroscopically confirmed 47 cluster members .
With this significant number of redshifts , we measure accurately the cluster velocity dispersion to be 1016 ^ { +126 } _ { -93 } km s ^ { -1 } .
The distribution of velocity offsets is consistent with a Gaussian , indicating no substantial velocity substructure .
As previously noted for other optically-selected clusters at redshifts of z \gtrsim 0.5 , a comparison between the X-ray luminosity ( L _ { x } ) and the velocity dispersion ( \sigma ) of Cl 1324+3011 implies that this cluster is underluminous in X-rays by a factor of \sim 3 - 40 when compared to the L _ { x } - \sigma relation for local and moderate-redshift clusters .
We also examine the morphologies of those cluster members which have available high-angular-resolution imaging with the Hubble Space Telescope ( HST ) .
There are 22 spectroscopically-confirmed cluster members within the HST field-of-view .
Twelve of these are visually classified as early-type ( elliptical or S0 ) galaxies , implying an early-type fraction of 0.55 ^ { +0.17 } _ { -0.14 } in this cluster .
This fraction is a factor of \sim 1.5 lower than that observed in nearby rich clusters .
Confirming previous cluster studies , the results for cluster Cl 1324+3011 , combined with morphological studies of other massive clusters at redshifts of 0 \leq z \lesssim 1 , suggest that the galaxy population in massive clusters is strongly evolving with redshift .
This evolution implies that early-type galaxies are forming out of the excess of late-type ( spiral , irregular , and peculiar ) galaxies over the \sim 7 Gyr timescale .