  We explicitly determine the symplectic structure on the phase space of
Chern-Simons theory with gauge group $G\ltimes g^*$ on a three-manifold of
topology $R \times S$, where $S$ is a surface of genus $g$ with $n+1$
punctures. At each puncture additional variables are introduced and coupled
minimally to the Chern-Simons gauge field. The first $n$ punctures are treated
in the usual way and the additional variables lie on coadjoint orbits of
$G\ltimes g^*$. The $(n+1)$st puncture plays a distinguished role and the
associated variables lie in the cotangent bundle of $G\ltimes g^*$. This allows
us to impose a curvature singularity for the Chern-Simons gauge field at the
distinguished puncture with an arbitrary Lie algebra valued coefficient. The
treatment of the distinguished puncture is motivated by the desire to construct
a simple model for an open universe in the Chern-Simons formulation of
$(2+1)$-dimensional gravity.
