  We consider a family of linearly viscoelastic shells with thickness
$2\varepsilon$, clamped along a portion of their lateral face, all having the
same middle surface $S=\mathbf{\theta}(\bar{\omega})\subset \mathbb{R}^3$,
where $\omega\subset\mathbb{R}^2$ is a bounded and connected open set with a
Lipschitz-continuous boundary $\gamma$. We show that, if the applied body force
density is $\mathcal{O}(1)$ with respect to $\varepsilon$ and surface tractions
density is $\mathcal{O}(\varepsilon)$, the solution of the scaled variational
problem in curvilinear coordinates, defined over the fixed domain
$\Omega=\omega\times(-1,1)$, converges in ad hoc functional spaces as
$\varepsilon\to 0$ to a limit $\mathbf{u}$. Furthermore, the average
$\overline{\mathbf{u}(\varepsilon)}= \frac1{2}\int_{-1}^{1}\mathbf{u}
(\varepsilon) dx_3$, converges in an \textit{ad hoc} space to the unique
solution of what we have identified as (scaled) two-dimensional equations of a
viscoelastic generalized membrane shell, which includes a long-term memory that
takes into account previous deformations. We finally provide convergence
results which justify those equations.
