  We have constructed a large, statistically complete sample of galaxy clusters
serendipitously detected as extended X-ray sources in 647 ROSAT PSPC pointed
observations. The survey covers 158 square degrees with a median sample flux
limit of 1.2 x 10^-13 erg cm^-2 s^-1 (0.5-2.0 keV). Our sample consists of 201
clusters of galaxies characterized by a median redshift of z=0.25 and a maximum
of z=1.26. With 22 clusters at z > 0.5, the 160 Square Degree ROSAT Survey is
the largest high-redshift sample of X-ray-selected clusters published to date.
Here we describe the revised sample which features spectroscopic redshifts for
99.5% of the clusters and discuss the implications for evolution in the cluster
abundance.
