The family Iguanidae is almost completely restricted to the Western Hemisphere with its main radiations occurring in North and South America. There are also representatives on Fiji, Tonga, and the Galapagos Islands in the Pacific Ocean. Two distinctly related iguanid genera area also found on Madagascar. These genera, Chalarodon and Oplurus, possess abdominal ribs and are therefore considered to be the most primitive members of the family. Although the iguanid lizards are familiar to most scientists interested in the tropics, their anatomy and evolution are still poorly understood. Because the family Iguanidae is a large and diverse group of lizards, several distinct phylogenetic lines have been recognized. In this study we are concerned with that group of genera belonging to the iguanine line, which includes the following genera: Amblyrhynchus and Conolophus from the Galapagos Islands, Brachylophus from Fiji and Tonga Islands, Enyaliosaurus from Central America, Ctenosaura and Iguana from Central and South America, Cyclura from the West Indies, and Dipsosaurus and Sauromalus from North America. Those iguanid lizards which have a discontinuous distribution all belong to the iguanine line, or are the most primitive members of the family. Explaining the discontinuous distribution pattern between the Western Hemisphere mainland iguanines, the Pacific Island forms, and their Madagascar relatives has proven to be an enigma for zoogeographers and herpetologists. The purpose of this study is to establish the degree of relationship between the iguanines of the Galapagos, Fiji, and Tonga Islands with the mainland genera. We will also attempt to define more completely the relationships between the Madagascar genera and the iguanine line. In order to ascertain these relationships, the anterior osteology and myology of each genus has been investigated along with such specialized features as the tongue, hyoid bones, sterna and hemipenes. Hopefully the morphological relationships between the ten genera can be clarified by the use of these relationships, and the evolution and distribution of the iguanine iguanids can be explained. Of all the genera listed above, only Enyaliosaurus has not been studied in detail as only two skulls and one complete specimen were available for examination.