  We present the results of a deep narrow-band [OII] 3727 \AA emission-line
search for faint ($g <$ 27), star-forming galaxies in the field of the $z=0.37$
MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line
sources relative to the field at $z \sim$ 0.4 (Hogg et al. 1998), and therefore
conclude that the MS1512.4+3647 sample is dominated by field [OII]
emission-line galaxies which lie along the $\sim$ 180 Mpc line of sight
immediately in front and behind the cluster. This is surprising, given that the
previously surveyed $z=0.41$ cluster Abell 851 has 3-4 times the field
emission-line galaxy density (Martin et al. 2000). We find that the
MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 $<
g-i <$ 2.0) and implied star-formation exponential decay timescales $\tau \sim$
100 Myr - 1 Gyr that dominate the Abell 851 emission-line galaxy population.
Instead, the majority of [OII] emission-line galaxies surrounding the
MS1512.4+3647 cluster are blue ($g-i \leq 1.0$) and forming stars in bursts
with $\tau < $ 100 Myr. In both samples, galaxies with the shortest
star-formation timescales are preferentially among the faintest star-forming
objects. Their i luminosities are consistent with young stellar populations
$\sim 10^8 - 10^9 \Msun$, although an additional factor of ten in stellar mass
could be hiding in underlying old stellar populations. We discuss the
implications for the star-formation histories of dwarf galaxies in the field
and rich clusters.
