  We discuss the galaxy population of the rich cluster MS1621.5+2640 at
z=0.4274, based on spectra and imaging in a field of size 9 arcmin by 23 arcmin
(~2 by 5 h**-1 Mpc). The sample comprises 277 galaxies, of which 112 are
cluster members, 7 are `near-members', and 47 are field galaxies in the
redshift range 0.37<=z<=0.50. The results are analyzed and compared with the
z=0.2279 rich cluster Abell 2390. MS1621.5+2640 has a higher blue fraction, a
younger stellar population, and is a less evolved cluster. We do not find
strong evidence of significant excess star formation compared with the field,
although there is a small population of outlying near-members that is unusually
blue and that may be affected by the cluster. There is a substantial population
of red galaxies with significant Hdelta absorption, which are not easily
explained by any simple form of modeled star formation history. We detect two
distinct populations of cluster galaxies: those where star formation ceased
some time ago, and those with a gradual decrease over many Gyr. Our data
suggests that the cluster formed by accretion from the field, with truncation
of the star formation beginning at very large radii (>2 times the virial
radius). The truncation process does not seem to be a sharp one though, in that
lower-luminosity early-type galaxies in the inner core of the cluster are seen
with significant Hdelta absorption, indicating some star formation within the
last 1-2 Gyr. Some combination of stripping of gas from the outer parts of the
galaxy, together with gradual exhaustion of the gas in the inner parts would be
consistent with our data.
