The Order Sphenodontia contains only one living genus, Sphenodon, which represents the two living species of tuatara [1].
The order also contains several extinct groups, the Gephyrosauridae and the Pleurosauridae.
Links:
[1] http://www.untamedscience.com/biodiversity/animals/chordates/reptiles/tuataras/tuataras/sphenodon/tuatara