  The conditions for superintegrable systems in two-dimensional Euclidean space
admitting separation of variables in an orthogonal coordinate system and a
functionally independent third-order integral are studied. It is shown that
only systems that separate in subgroup type coordinates, Cartesian or polar,
admit potentials that can be described in terms of nonlinear special functions.
Systems separating in parabolic or elliptic coordinated are shown to have
potentials with only non-movable singularities.
