chelonian
n 1) any reptile of the order Chelonia, including the tortoises and turtles, in which most of the body is enclosed in a protective bony capsule
adj 2) of, relating to, belonging to, or characteristic of the Chelonia
Etymology: C19 from New Latin Chelōnia, from Greek khelōnē tortoise [CED]

Chelonians
The order of the testudines differentiated itself from the rest of the reptile world during the Triassic Period. Today it comprises marine turtles and terrestrial turtles. The species of this order are unique. They are covered with shells that consist of a dorsal carapace and a ventral plastron. These shells are so much a part of these animals that their thoracic vertebrae and ribs are included in them. Since these rigid shells do not allow turtles to expand their chests to breathe, these animals use their abdominal and pectoral muscles like diaphragms.

Reptiles and Dinosaurs, Britannica Illustrated Science Library, 2008.