[Illustration FIG. 50. PREPARATION OF PART OF THE RIGHT MANDIBULAR RAMUS OF _Crocodilus palustris_ × 1/2. (Brit. Mus.) 1. tooth in use. 2. fairly old germ of future tooth. 3. symphysial surface of the mandible.] Teeth have been detected in embryos of _Trionyx_, but otherwise no teeth are known to occur in Chelonia, or in _Pteranodon_ (Pterosauria), while the anterior part of the jaw is edentulous in _Iguanodon_, _Polyonax_ and some other Dinosaurs, and in _Rhamphorhynchus_. ENDOSKELETON. VERTEBRAL COLUMN. The vertebral column is commonly divisible into the usual five regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among Lacertilia, only into caudal and precaudal regions. In the Chelonia there are no lumbar vertebrae. The form of the vertebral centra is very variable. A large proportion of extinct reptiles,--several entire orders,--and the earlier and more primitive forms in some of the other groups have amphicoelous vertebrae. Vertebrae of this type occur in the Theromorpha, Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many Dinosauria, also in some of the early Crocodilia such as _Belodon_, _Teleosaurus_ and _Goniopholis_, and the Geckonidae among Lacertilia. The majority of living reptiles have procoelous vertebrae. Thus they occur in the Lacertilia (excluding the Geckos), the Ophidia, and the Crocodilia, also among extinct forms in the Pterosauria and many Dinosauria. On the other hand some Dinosauria such as _Iguanodon_ have opisthocoelous cervical vertebrae, while others have opisthocoelous thoracic vertebrae. The vertebrae of the Ceratopsidae and some Sauropterygia, the thoracic vertebrae of _Iguanodon_, and the sacral vertebrae of Crocodilia have flat centra. The first caudal vertebra of modern Crocodilia is biconvex, and in the Chelonia all types of vertebral centra are found. The cervical vertebrae of _Sphenodon_ are noticeable for the occurrence of a small pro-atlas, which may represent the neural arch of a vertebra in front of the atlas. In most reptiles the vertebrae are fully ossified, but in some of the more primitive forms the notochord persists in the centre of the vertebra (i.e. intervertebrally), this is the case for instance in many of the Theromorpha and Rhynchocephalia, and also in the Geckos. In other reptiles it persists longest intravertebrally. The centrum of each of the caudal vertebrae of most Lacertilia is traversed by an unossified septum along which it readily breaks. Chevron bones occur below the caudal vertebrae in Lacertilia, Chelonia, Ichthyosauria, many Dinosauria, and _Sphenodon_, articulating with quite the posterior part of the centrum which bears them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a well-marked odontoid process. The ventral portions of the intervertebral discs are sometimes ossified, forming wedge-shaped inter centra, as in Geckos, and the cervical vertebrae of _Sphenodon_.