  We consider a class of spin systems on randomly triangulated surfaces as
discrete approximations to conformal matter fields coupled to 2d gravity. On
the basis of certain universality assumptions we argue that at critical points
with diverging string susceptibility the model either exhibits mean field
behaviour or it can effectively be described by a conformal matter system with
central charge less than or equal to 1 coupled to 2d gravity. As a particular
consequence we conclude in the unitary case that the string susceptibility
exponent is limited to possible values of the form 1/n, n=2,3,4,..., where n=2
corresponds to mean field behaviour.
