  We present a new, objectively selected, sample of galaxy overdensities
detected in the Hubble Space Telescope Medium Deep Survey. These
clusters/groups were found using an automated procedure which involved
searching for statistically significant galaxy overdensities. The contrast of
the clusters against the field galaxy population is increased when
morphological data is used to search around bulge-dominated galaxies. In total,
we present 92 overdensities above a probability threshold of 99.5%. We show,
via extensive Monte Carlo simulations, that at least 60% of these overdensities
are likely to be real clusters and groups and not random line-of-sight
superpositions of galaxies. For each overdensity in the MDS cluster sample, we
provide a richness and the average of the bulge-to-total ratio of galaxies
within each system. This MDS cluster sample potentially contains some of the
most distant clusters/groups ever detected, with about 25% of the overdensities
having estimated redshifts z > 0.9. We have made this sample publicly available
to facilitate spectroscopic confirmation of these clusters and help more
detailed studies of cluster and galaxy evolution.
  We also report the serendipitous discovery of a new cluster close on the sky
to the rich optical cluster CLl0016+16 at z=0.546. This new overdensity, HST
001831+16208, may be coincident with both an X-ray source and a radio source.
HST 001831+16208 is the third cluster/group discovered near to CL0016+16 and
appears to strengthen the claims of Connolly et al. (1996) of superclustering
at high redshift.
