The structure and unlabelled enumeration of toroidal graphs with no K3,3ʹs

We characterize the toroidal graphs with no K3,3-subdivisions as canonical compositions in which 2-pole planar networks are substituted for the edges of non-planar cores. This structure enables us to enumerate these graphs. We describe an explicit enumerative approach that requires unlabelled enumeration of 2-connected planar graphs.