Concerning the reptilian pubis there are considerable difficulties. Sometimes there is only a single pubic structure present, sometimes there are two. The reptilian pubis is best understood by comparing the arrangements met with in the various other groups with that in the Orthopod Dinosaurs such as _Iguanodon_. In _Iguanodon_ the pubis consists of two portions, viz. of a moderately broad pre-pubis directed downwards and forwards, and of a narrow greatly elongated post-pubis directed backwards parallel to the ischium. The pubis is united to both ilium and ischium, the acetabulum has a large unossified space, and neither pre-pubes nor post-pubes meet in ventral symphyses. The arrangement bears a great resemblance to that of Ratite birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria together with Theropod and Sauropod Dinosaurs the pubis corresponds to the pre-pubis of _Iguanodon_ and is a more or less cylindrical bone expanded at both ends, meeting its fellow in a ventral symphysis. In Chelonia and Lacertilia the pubis bears a lateral process which is homologous with the post-pubis of Iguanodon. In Lacertilia and sometimes in Chelonia there is a cartilaginous epipubis attached to the anterior border of the pubic symphysis; this is well developed in the Chamaeleons and Geckos. In Crocodilia there is, forming the anterior and ventral portion of the acetabulum, a patch of cartilage (fig. 49, 3) which is probably the true pubis homologous with that of lizards and with the pre-pubis of _Iguanodon_. The large bone generally called the pubis in Crocodiles is probably an epipubis. THE POSTERIOR LIMB. The posterior limb is entirely absent in some Lacertilia and in most Ophidia, though traces occur in _Python_, _Tortrix_ and _Typhlops_. In the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs form swimming paddles and have been already referred to. The arrangement of the proximal and middle segments of the limb is fairly constant in all reptiles with limbs adapted for walking, and the tibia and fibula are always separate. The pes is however subject to a considerable amount of variation, especially as regards the tarsus. In some Chelonia the tarsus like the carpus has an extremely simple arrangement, consisting of a proximal row of three bones, the tibiale, intermedium and fibulare, a centrale, and a distal row of five tarsalia. In most living reptiles, however, the tibiale and intermedium are as in mammals united, forming the astragalus. In Crocodiles (fig. 48, B, 15) the centrale is also united with the tibiale while the distal tarsalia are very slightly developed. The calcaneum in Crocodiles is drawn out into a long process forming a heel in a manner almost unique among Sauropsida. In _Sphenodon_ and Lacertilia the tibia and fibula articulate with a single large bone representing the whole proximal row of tarsalia.