  The galaxy population in the intermediate-redshift ($z=0.228$) rich cluster
Abell 2390 is investigated. We present velocities, colors, and morphological
information for an exceptionally large sample of 323 galaxies (216 cluster
members) in a 46$^\prime \times 7^\prime$ (6 $h^{-1}$ Mpc $\times$ 1 $h^{-1}$
Mpc) strip centered on the cD galaxy. This sample of confirmed cluster members
is second only to that for the Coma cluster in terms of sample size and spatial
coverage in the cluster rest frame, and is the first to trace the transition
between a rich cluster and the field at intermediate redshift. The galaxy
population in the cluster changes gradually from a very evolved, early-type
population in the inner 0.4 \hmpc\ of the cluster to a progressively later-type
population in the extensive outer envelope of the cluster from 1 to 3 \hmpc\/
in radius. Radial gradients in galaxy $g-r$ color, 4000 \AA\/ break, H$\delta$
and [O II] line strengths and morphology are seen in the cluster, and are
investigated by comparing the data to models computed with the GISSEL spectral
synthesis package. The results suggest that the cluster has been gradually
built up by the infall of field galaxies over $\sim 8$ Gyr and that star
formation has been truncated in infalling galaxies during the accretion
process. The morphological composition of the cluster is shown to be consistent
with such a scenario. If true for other clusters, infall-truncated star
formation as seen in Abell 2390 may explain both the Butcher-Oemler effect and
the large fraction of S0 galaxies in clusters. Only $\simlt$5\% of the galaxies
observed in Abell 2390 exhibit evidence for star formation at levels stronger
than those seen in typical late-type systems. This suggests that starbursts do
not play a major role in driving cluster galaxy evolution at the redshift of
