Injected ion drift tube techniques, including ion mobility measurements and annealing and fragmentation studies, have been used to examine even-numbered carbon cluster cations containing 100-320 atoms. The drift time distributions reveal the presence of several families of geometric isomers. The single-shell fullerene remains the dominant isomer for clusters containing up to 320 atoms. In addition, a small component of nested cage fullerenes may be present for the larger clusters. The polycyclic polyyne ring isomers, which are abundant for clusters with less than 70 atoms, vanish at around 160 atoms. There is no evidence for the presence of long polyacetylene chains. A new family of isomers, a broad distribution with mobilities smaller than the fullerene but larger than the polycyclic rings, appears at around 100 atoms and slowly increases in relative abundance with increasing cluster size. These new isomers are probably the coalescence products of fullerenes and rings. When collisionally excited, a small fraction of these isomers readily dissociate, primarily by loss of C-18 and larger even-numbered fragments, while the remainder appear to anneal into the fullerene structure. The estimated activation energy for this process is similar to the activation energy for conversion of the smaller polycyclic ring isomers into fullerenes.