Chelonia is a taxonomic order of reptiles that includes turtles, tortoises, and terrapins. It is one of the most ancient reptilian orders, with fossil records dating back to the Triassic period. Turtles are the most well-known members of this order, and they are characterized by their hard shells and webbed feet. Tortoises are similar to turtles, but they have sturdier shells and shorter necks. Terrapins are semi-aquatic turtles that live in brackish water. All three species are found in many parts of the world and play an important role in their respective ecosystems.