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 $zsim 0.4$ 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.
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