Tuataras grow to two feet long, and are the largest reptiles in New Zealand. They are a greenish brown color, which can change as they grow older and can vary between individuals. A row of spines runs down their back, from their head to the end of their tail.
Tuataras have a unique feature called a parietal eye, which is a sort of third eye in their forehead. It has a lens, cornea-like structures, and a retina like a normal eye. However, it is completely coveredi n the adult tuatara, and only visible in juveniles. Its function is not known, but it may absorb ultraviolet rays.
Tuataras have very primitive hearing organs, and no external earhole is visible. Their teeth are unique, with two rows in the upper jaw and one row in the lower jaw, with the lower row fitting between the two upper rows when the mouth is closed.