  [Heavily Abbreviated] In this paper we analyze previously published spectra
with high signal-to-noise ratios of E/S0 galaxies in the rich cluster CL1358+62
at z=0.33, and introduce techniques for fitting stellar population models to
the data. Here we focus on the 19 E and S0 galaxies with an homogeneous set of
eight blue Lick indices. We explore the galaxy properties using six-parameter
stellar population models from the literature, and describe an approach for
fitting the models differentially, such that the largest systematic errors are
avoided. We find: (1) no differences between the stellar population parameters
of Es and S0s, at fixed sigma; (2) the stars in the Es and S0s are uniformly
old, consistent with previously published results using M/L ratios; (3) a
significant correlation of [Z/H] with sigma, in a manner consistent with the
observed B-V colors of the galaxies; (4) no significant correlation of
[alpha/Fe] with sigma; and (5) a significant anti-correlation of [alpha/N] with
[Z/H], which we interpret as the signature of secondary nitrogen. Neither
[alpha/C], nor [alpha/Ca] shows significant variation. While the differences
between our conclusions and the current view of stellar populations may point
to serious deficiencies, our deduced correlation of mean metallicity with sigma
does reproduce the B-V colors of the galaxies, as well as the slope of the
local Mg-sigma relation. In matching the inferred population trends with
published data on nearby galaxies, the line strength-line width relations match
well, save for the narrow iron indices. Taken together, these results reduce
early-type galaxies in clusters to a family with one-parameter, velocity
dispersion, greatly simplifying scenarios for their formation and evolution.
