Modern turtles are placed into one of two suborders within the Testudines -- Peurodira (side-necked) and Cryptodira (hidden neck). The namesake difference between the two is the method of head retraction. In pleurodires, the neck vertebrae flex laterally, allowing the neck to bend and pull the head in sideways. In cryptodires, the neck vertebrae flex vertically, allowing the head to be drawn straight back within the shell. The Pelomedusidae and Chelidae are the only extant families of pleurodires. The Carettochelyidae, Cheloniidae, Chelydridae, Dermatemydidae, Dermochelyidae, Emydidae, Kinosternidae, Testudinidae, and Trionychidae are all cryptodires, although the ability to retract the head has been lost in the sea turtles (Cheloniidae and Dermochelyidae)., The fourth known longest living species is the Western and Kings Canyon National Parks at altitudes between 7,500 and 11,500 feet. have found a number of uses , The elements of the shell are present, only in greater numbers than exist in current species. Three important differences between Proganochelys and modern turtles are the presence of the palatal teeth (lost in modern species), the inability to retract the head within the shell, and the lack of a trochlear pulley in the jaw closing anatomy..