In the Labyrinthodontia the dermal exoskeleton is in many genera greatly developed. It is generally limited to the ventral surface and consists principally of a buckler formed of three bony plates, one median and two lateral. These plates protect the anterior part of the thorax, and are closely connected with the adjacent endoskeleton. They probably represent the interclavicle and clavicles. Behind this buckler numerous scutes are generally developed, which often cover the whole ventral surface, and may cover the whole body. TEETH[59]. In Amphibia teeth are generally present on the maxillae, premaxillae and vomers, and except in Anura on the dentaries; sometimes they occur on the palatines as in many Urodela, most Labyrinthodontia, and the Gymnophiona; less commonly on the pterygoids as in _Menobranchus_, _Siredon_, some Labyrinthodontia, and _Pelobates cultripes_[60], or on the splenials as in _Siren_ and _Menobranchus_, or parasphenoid as in _Pelobates cultripes_, _Spelerpes belli_ and _Batrachoseps_. In some Anura such as _Bufo_ and _Pipa_ the jaws are toothless. In Gymnophiona, _Menobranchus_, and _Siredon_, the teeth are arranged in two concentric curved rows. The teeth of the outer row are borne on the premaxillae and maxillae if present, (the maxillae are absent in _Menobranchus_), the teeth of the second row on the vomers and pterygoids in _Menobranchus_ and _Siredon_, and on the vomers and palatines in Gymnophiona. In some Gymnophiona there is a double row of mandibular teeth. The vomerine, palatine and parasphenoid teeth of all forms are numerous and are not arranged in rows. The teeth of all living Amphibia are simple conical structures ankylosed to the bone, and consisting of dentine, coated or capped with a thin layer of enamel. In the Labyrinthodontia teeth of more than one size are sometimes present. The dentine of the basal part of the larger teeth is in some genera very greatly folded, causing the structure to be highly complicated. These folds, the intervals between which are filled with cement, radiate inwards from the exterior and outwards from the large pulp cavity. The basal part of the teeth of _Ceratophrys_ (Anura) has a similar structure. ENDOSKELETON. VERTEBRAL COLUMN. Four regions of the vertebral column can generally be recognised in Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral and the caudal regions. In the limbless Gymnophiona, however, only three regions, the cervical, thoracic, and post-thoracic can be made out. The cervical region is limited to a single vertebra which generally differs from the others in having no transverse processes or indication of ribs. It is generally called the atlas, but it commonly bears a small process arising from the anterior face of the centrum which resembles the odontoid process of higher animals, and renders it probable that the first vertebra of Amphibia corresponds to the axis, not to the atlas. Amphibia generally have a single sacral vertebra.