  We give a characterization, in terms of Galois infinite comatrix corings, of
the corings that decompose as a direct sum of left comodules which are finitely
generated as left modules. Then we show that the associated rational functor is
exact. This is the case of a right semiperfect coring which is locally
projective and whose Galois comodule is a projective left unital module with
superfluous radical.
