The Copulatory Organs of Reptiles
Reptiles display three significantly different patterns of sex organ development. Lizards and snakes (collectively called squamates) have paired hemipenes. Turtles and crocodiles, including alligators and caiman, have a single penis and the tuatara have no copulatory organ at all.
Tuatara are ancient reptiles - they are sometimes called living fossils because they are the only survivors of a group of reptiles that roamed the Earth at the same time as dinosaurs. Tuatara have not changed much in 220+ million years. They are found only on 30 small islands in New Zealand. They have a mysterious third eye.
The tuatara was believed to be a lizard because it looks like one, but it is not. The tuara are endangered, a situation made worse by their slow breeding cycle, one of the slowest in reptiles.