Sphenodontia

The Sphenodontia is a diverse group of reptile. It includes only one living genus, the tuatara ("Sphenodon"). An alternative name for the order is the Rhynchocephalia. 
They are an offshoot from the evolutionary line leading to the true lizards and snakes. They had many families during the late Triassic and Jurassic periods but then declined, and are the least successful of the lepidosaurs. 
The only surviving member today is the New Zealand tuatara.