INTRODUCTION Currently, New Guinea Stegonotus with 17 scales at midbody and divided subcaudals are identified as S. modestus. But this will not account for the disparity in dentition within "S. modestus", nor the colour differences even at one locality, nor that in East and West Sepik Districts there are three quite distinct "S. modestus": (1) a large form with high ventral count and high tooth count; (2) a small form with low ventral count but high tooth count; and (3) a medium-sized form with high ventral count but low tooth count (fig. 1). The material in the British Museum and American Museum of Natural History indicates that scale count does not correctly identify the species, and that such features as dentition, form of the palatine bone, scale pits, size, and coloration are more reliable. GENERAL FEATURES OF NEW GUINEA STEGONOTUS New Guinea Stegonotus are very closely related to the Oriental genus Dinodon; the supposed dental differences between these genera do not hold and the maxillary dentition of (for example) Stegonotus diehli more closely resembles that of Dinodon orientale than it does that of Stegonotus cucullatus. I find only one character that will separate New Guinea Stegonotus from Dinodon: in Dinodon, the membranous dorsal wall of the trachea is expanded as a large and simple sac continuous with the lung 1); but in all New Guinea Stegonotus, dorsal tracheal perforations lead into a series of separate membranous sacs, as described by Brongersma (1957). Stegonotus and Dinodon show a general similarity in form and a sexual dimorphism in ventral count with males exceeding females (the reverse of the usual dimorphism). The pattern of the head of juvenile S. diehli