  We examine the coexistence of spherical and $\gamma$-unstable deformed
nuclear shapes, described by an SO(5)-invariant Bohr Hamiltonian, along the
critical-line. Calculations are performed in the Algebraic Collective Model by
introducing two separate bases, optimized to accommodate simultaneously
different forms of dynamics. We demonstrate the need to modify the
$\beta$-dependence of the moments of inertia, in order to obtain an adequate
description of such shape-coexistence.
