  In many models of Gamma-Ray Bursts (GRBs) relativistic shells are responsible
for the overall envelope of emission. We use kinematics and symmetry to
calculate the time history and spectral evolution expected from a relativistic
shell including effects from intrinsic variations in the shell's intensity and
spectra. We find that the decay phase of an envelope is produced by photons
delayed by the shell's curvature. These delayed photons are produced by regions
that are off-axis such that the spectra evolve according to a universal
function (T^{-1}) regardless of intrinsic variations in the rest frame of the
shell. We compare our predictions to the overall envelope of emission of GRBs.
The observed spectra evolve faster. Intrinsic variations cannot make the
spectra evolve that fast, which adds strength to the ``shell symmetry''
problem: models, in particular, the external shock model, that involve
relativistic shells must either confine the material to narrow pencil beams, be
very inefficient, or break the local spherical symmetry so that the shell acts
like a parallel slab. In the case of the internal shock models involving winds
(i.e., central engines), it will probably be easier to break the local
spherical symmetry, but then one must postulate nearly continuous energy
generation at 10^{51} erg s^{-1} lasting up to hundreds of seconds at the
central site.
