  We present a catalog of 200 clusters of galaxies serendipitously detected in
647 ROSAT PSPC pointings covering 158 square degrees. This is one of the
largest X-ray selected cluster samples, comparable in size only to the ROSAT
All-Sky Survey sample of nearby clusters (Ebeling et al. 1997). We detect
clusters in the inner 17.5 arcmin of the ROSAT PSPC field of view using the
spatial extent of their X-ray emission. Cluster X-ray luminosities range from
10^42 erg/s to ~5x10^44 erg/s, i.e. from poor groups to rich clusters. The
cluster redshifts range from z=0.015 to z>0.5. The catalog lists X-ray fluxes,
core-radii, spectroscopic redshifts for 73 clusters and photometric redshifts
for the remainder. We have optically confirmed 200 of 223 X-ray sources as
clusters of galaxies. Of the remaining 23 sources, 18 are likely false
detections arising from blends of unresolved point X-ray sources, and for 5 we
have not obtained deep CCD images. Above a flux of 2e-13 erg/s/cm**2, 98\% of
extended X-ray sources are optically confirmed clusters. The log N-log S
relation derived from our catalog shows excellent agreement with previous
results: EMSS and ROSAT All-Sky survey BCS in teh bright end and WARPS at
intermediate fluxes. Our cluster counts appear to be systematically higher than
those from a 50 deg^2 survey of Rosati et al. (1998). In particular, at a flux
of 2e-13 erg/s/cm**2, we find a surface density of clusters of 0.57+/-0.07 per
square degree, which is a factor of 1.3 more than found by Rosati et al. This
difference is marginally significant at the ~2 sigma level. The large area of
our survey makes it possible to study the evolution of the X-ray luminosity
function in the high luminosity range inaccessible with other, smaller area
ROSAT surveys.
