A description of the heart of Sphenodon punctatus (Gray) based on gross dissection, serial sections and wax plate reconstruction of six specimens is given. They were found to resemble most the hearts of lacertilian reptiles. The bulbar region is only partially absorbed into the ventricle. A primary, oblique septum is present, dividing the cavity of the ventricle into a cavum dorsale and a cavum ventrale. Both of these have the same relationship to the various arterial trunks as in other non-crocodilian reptiles. Of the numerous muscle trabeculae and ridges in the cuvum dorsale none is developed to a greater extent than the others. There is, therefore, no secondary septum in the ventricle. The coronary arteries originate from the left systemic arch to supply the bulbus and ventricle. A gubernaculum cordis is not always present but, if it is, apical coronary vessels pass through it to the ventricle.
Consideration of the comparative anatomy of the heart of Sphenodon and those of other existing reptiles leads to the conclusion that it is the most primitive.