 Galapagos tortoises possess two main shell forms that correlate with the biogeographic history of the species group . They exhibit a spectrum of carapace morphology ranging from " saddleback " ( denoting upward arching of the front edge of the shell resembling a saddle ) to " domed " ( denoting a rounded convex surface resembling a dome ) . When a saddleback tortoise withdraws its head and forelimbs into its shell , a large unprotected gap remains over the neck , evidence of the lack of predation during the evolution of this structure . Larger islands with humid highlands over 800 m ( 2 @,@ 600 ft ) in elevation , such as Santa Cruz , have abundant vegetation near the ground . Tortoises native to these environments tend to have domed shells and are larger , with shorter necks and limbs . Saddleback tortoises originate from small islands less than 500 m ( 1 @,@ 600 ft ) in elevation with dry habitats ( e.g. Española and Pinzón ) that are more limited in food and other resources . Two lineages of Galapagos tortoises possess the Island of Santa Cruz and when observed it is concluded that despite the shared similarities of growth patterns and morphological changes observed during growth , the two lineages and two sexes can be distinguished on the basis of distinct carapace features . Lineages differ by the shape of the vertebral and pleural scutes . Females have a more elongated and wider carapace shape than males . Carapace shape changes with growth , with vertebral scutes becoming narrower and pleural scutes becoming larger during late ontogeny . 
