List of Testudines families

There are fourteen extant families of the order Testudines, an order of reptile commonly known as turtles, tortoises and terrapins. The testudines are some of the most ancient reptiles alive, with only the tuataras considered more primitive. There are approximately 300 extant species and 97 genera of testudines, split into two suborders: the Cryptodirans and the Pleurodirans. The distinction between these two suborders is based on the mode in which they cover their head and neck. The Pleurodirans, also called the side-necked turtles, have long necks, and fold them sideways to align them with the shell. The Pelomedusidae and Chelidae are the only extant families of pleurodires. The Cryptodirans pull their neck straight back to conceal their head within the shell. The , 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). A third order, the Paracryptodirans, are extinct.

Reptiles are classified according to the pattern of in the temporal region of the skull. Testudines are placed in the subclass Anapsida because they lack fenestration. There are suggestions that this lack of fenestration is a secondary characteristic and that turtles belong in Diapsida. Both sides cite strong evidence, and the conflict has yet to be resolved. The shell of testudines distinguishes them from other vertebrates. The shell is not an exoskeleton, but a modified ribcage and part of the vertebral column. Because of the shell, the pectoral and pelvic girdles are located within the ribcage. The limb bones are also modified to accommodate to the shell.

...
Wikipedia