  In this paper we discuss two general models of random simplicial complexes
which we call the lower and the upper models. We show that these models are
dual to each other with respect to combinatorial Alexander duality. The
behaviour of the Betti numbers in the lower model is characterised by the
notion of critical dimension, which was introduced by A. Costa and M. Farber:
random simplicial complexes in the lower model are homologically approximated
by a wedge of spheres of dimension equal the critical dimension. In this paper
we study the Betti numbers in the upper model and introduce new notions of
critical dimension and spread. We prove that (under certain conditions) an
upper random simplicial complex is homologically approximated by a wedge of
spheres of the critical dimension.
