  Lax-Phillips evolutions are described by two-space scattering systems. The
canonical identification operator is characterized for Lax-Phillips evolutions,
whose outgoing and incoming projections commute. In this case a (generalized)
Lax-Phillips semigroup can be introduced and its spectral theory is considered.
In the special case, originally considered by Lax and Phillips (where the
outgoing and incoming subspaces are mutually orthogonal), this semigroup
coincides with that introduced by Lax and Phillips. In the more general case
the existence of the semigroup is not coupled with the (global) holomorphic
continuability of the scattering matrix into the upper half plane. The basic
connection of the Lax-Phillips semigroup to the so-called characteristic
semigroup of the reference evolution is emphasized.
