In snakes, Theropod Dinosaurs, and the iguanas among lizards, the neural arches are provided with _zygosphenes_, and _zygantra_. The neural arches are usually firmly ankylosed to the centra, but in the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the suture between the centrum and neural arch persists at any rate till late in life. In the Ichthyosauria the neural arches were united to the centra by cartilage only. The thoracic vertebrae of some of the Theromorpha (_Dimetrodon_) are remarkable for the extraordinary development of the neural spine, and those of Chelonia for the absence of transverse processes. In living reptiles the number of sacral vertebrae is nearly always two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as five or six bones may be ankylosed together in the sacral region. In Crocodiles the two halves of the pelvis sometimes articulate with different vertebrae. The vertebrae of some of the great Sauropoda are remarkably hollowed out, having a large vacuity on each side of the centrum communicating with a series of internal cavities. The whole structure of these vertebrae shows a combination of great strength with lightness. THE SKULL. The reptilian skull is well ossified and the bones are noticeable for their density. The true cranium is often largely concealed by a secondary or false roof of membrane bones, which is best seen in the Ichthyosauria and some of the Chelonia. In other reptiles the false roof is more or less broken up by vacuities exposing the true cranial walls. The ethmoidal region is the only one in which much of the primordial cartilaginous cranium remains. The lateral parts of the sphenoidal region are also as a rule not well ossified. In some reptiles, such as most Lacertilia and Chelonia, the orbits are separated only by the imperfect interorbital septum, while in others, such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity extends forwards between the orbits. In the occipital region all four bones are ossified. The great majority of reptiles have a single convex occipital condyle, but some of the Theromorpha such as _Cynognathus_ have two distinct condyles as in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the exoccipitals, as well as the basi-occipital, take part in the formation of the single condyle; sometimes, as in Crocodiles, it is formed by the basi-occipital alone, as in birds. The relations of the bones to the foramen magnum vary considerably, in Chelonia the basi-occipital generally takes no part in bounding it, and in the Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded. The parietals are paired in Geckos and Chelonia alone among living forms, and in the extinct Ichthyosauria and some Theromorpha; in all other reptiles they are united.