Although wild populations are now confined to Aldabra, giant tortoises were originally present on many other Indian Ocean islands. All belong to the genus Geochelone but are referable to two distinct subgenera, Cylindraspis in the Mascarene islands and Aldabrachelys on Madagascar, Aldabra, the Seychelles and neighbouring islands. These are distinguishable by skull and nasal structure, degree of shell ankylosis and structure of the plastron. Neither group has obvious close relatives in other areas. Mascarene tortoises, which are completely extinct, comprised several species: G. vosmaeri and the smaller G. peltastes on Rodrigues, G. inepta and G. triserrata on Mauritius, and what should probably be called G. indica on Reunion. All the tortoises of Aldabra, the Seychelles and neighbouring islands seem to be referable to one species, G. gigantea which appears to have shown some geographical variation. Madagascar probably had two species, G. grandidieri and another usually named G. abrupta which may well be conspecific with G. gigantea and was possibly the source of the Aldabra populations. Many of the distinctive features of Indian Ocean and Galapagos giant tortoises are interpretable as adaptations to the peculiar environment of ocean islands, particularly their lack of big predators and competing herbivores.