 Galápagos 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.