  Physical quantities obtained from the microcanonical entropy surfaces of
classical spin systems show typical features of phase transitions already in
finite systems. It is demonstrated that the singular behaviour of the
microcanonically defined order parameter and susceptibility can be understood
from a Taylor expansion of the entropy surface. The general form of the
expansion is determined from the symmetry properties of the microcanonical
entropy function with respect to the order parameter. The general findings are
investigated for the four-state vector Potts model as an example of a classical
spin system.
