  Wide-field optical and near--IR ($JHK$) imaging is presented for two rich
galaxy clusters: Abell~370 at $z=0.374$ and Abell~851 (Cl0939+47) at $z=0.407$.
Galaxy catalogs selected from the near--IR images are 90\% complete to
approximately 1.5 mag below $K^\ast$ resulting in samples with $\sim$100
probable member galaxies per cluster in the central $\sim$2 Mpc. Comparison
with $HST$ WFPC images yields subsamples of $\sim$70 galaxies in each cluster
with morphological types. Analysis of the complete samples and the $HST$
subsamples shows that the $z\sim 0.4$ E/S0s are bluer than those in the Bower
et al.\ (1992) Coma sample in the optical$-K$ color by $0.13$~mag for Abell~370
and by $0.18$~mag for Abell~851. If real, the bluing of the E/S0 populations at
moderate redshift is consistent with that calculated from the Bruzual and
Charlot (1993) models of passive elliptical galaxy evolution. In both clusters
the intrinsic scatter of the known E/S0s about their optical$-K$ color--mag
relation is small ($\sim 0.06$ mag) and not significantly different from that
of Coma E/S0s as given by Bower et al.\ (1992), indicating that the galaxies
within each cluster formed at the same time at an early epoch.
