The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia, Ophidia, _Sphenodon_ (fig. 52, B, 4) and some extinct crocodiles, such as _Belodon_. They are completely fused in living Crocodilia and some Lacertilia and Dinosauria. In the gigantic _Polyonax_ they are drawn out into a pair of enormous horns, and the parietals and squamosals are greatly expanded behind. An interparietal foramen occurs in the Theromorpha, the Ichthyosauria (fig. 32, 10), _Sphenodon_, the Sauropterygia and most Lacertilia. The posterior part of the skull is curiously modified in some Chamaeleons, the parietals and supra-occipitals being drawn out into a backwardly-projecting sagittal crest which unites with the two prolongations from the squamosals. In other Chamaeleons (_C. bifidus_) prolongations of the prefrontals and maxillae form large forwardly-projecting bony processes. The roof of the skull is characterised by the development of prefrontals and postfrontals, which lie respectively near the anterior and posterior extremity of the orbit. In Theromorpha, Squamata, Crocodilia, and some Dinosauria lachrymals are developed. There is a ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the Metriorhynchidae among Crocodiles and some Rhynchocephalia, Dinosauria, and Pterosauria. The pro-otic lies in front of the exoccipital and together with the opisthotic forms the hind border of the fenestra ovalis. In Chelonia the opisthotic remains separate, in all other living reptiles it fuses with the exoccipital. The epi-otic fuses with the supra-occipital. The parasphenoid, so important in Ichthyopsids, has very often disappeared completely; it is present, however, in the Ichthyosauria, the Plesiosauridae, and a number of Squamata, in many Ophidia its anterior part forming the base of the interorbital septum. In the Plesiosauridae and most Lacertilia, but not in the Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the parietal or the anterior end of the pro-otic with the pterygoid. A homologous arrangement occurs in the Ichthyosauria and some Chelonia. In most reptiles a transpalatine occurs, connecting the maxillae with the pterygoid, but this is absent in the Chelonia, and some Dinosauria, and in the Typhlopidae among snakes. The quadrate is always well developed, and except in the Squamata is firmly fixed to the surrounding bones. The Chamaeleons also, among the Squamata, have a fixed quadrate, and in them too the quadratojugal is absent. Separate nasal bones do not occur in any living Chelonia. The vomers are generally paired as in Squamata, sometimes unpaired as in Chelonia. [Illustration FIG. 51. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT) VIEWS OF THE SKULL OF THE COMMON SNAKE (_Tropidinotus natrix_). (After PARKER.)