The cervical vertebrae are much compressed, but they are never ankylosed together. In _Manatus_ there are only six cervical vertebrae. The caudal vertebrae have well-developed chevron bones. The humerus is distinctly articulated to the radius and ulna, and these two bones are about equally developed, and are often fused together. There are no clavicles, and the pelvis is vestigial, consisting of a pair of somewhat cylindrical bones suspended at some distance from the vertebral column. In living forms there is no trace of a posterior limb, but in _Halitherium_ there is a vestigial femur connected with each half of the pelvis. _Order 3._ CETACEA[121]. In these mammals the general form is more fish-like than is the case even in the Sirenia. The skin is generally almost completely naked, but hairs are sometimes present in the neighbourhood of the mouth, especially in the foetus. In some Odontoceti vestiges of dermal ossicles have been described, and in _Zeuglodon_ the back was probably protected by dermal plates. The anterior limbs have the form of flattened paddles, showing no trace of nails, the posterior limb bones are quite vestigial or absent, and there is never any external sign of the limb. Teeth are always present at some period of the life history, but in the whalebone whales they are only present during foetal life, their place in the adult animal being taken by horny plates of baleen. In all living forms the teeth are simple and uniform structures without enamel; they have single roots, and the alveoli in which they are imbedded are often incompletely separated from one another. As in some forms traces of a replacing dentition have been described, it has been concluded that the functional teeth of Cetacea belong to the milk dentition. The texture of the bones is spongy. The cervical vertebrae are very short, and though originally seven in number, are in many forms completely fused, forming one solid mass (fig. 67). The odontoid process of the axis is short and blunt, or may be completely wanting. The lumbar and caudal vertebrae are large and numerous, and as zygapophyses are absent, are very freely movable on one another; zygapophyses are also absent from the posterior thoracic vertebrae. The lumbar vertebrae are sometimes more numerous than the thoracic. The epiphyses are very distinct, and do not unite with the centra till the animal is quite adult. None of the vertebrae are united to form a sacrum, but the caudal vertebrae have large chevron bones. [Illustration FIG. 67. CERVICAL VERTEBRAE OF A CA'ING WHALE (_Globicephalus melas_) × 1/4. (Camb. Mus.) 1. centrum of seventh cervical vertebra. 2. neural arch of seventh cervical vertebra. 3. transverse process of atlas. 4. foramen for exit of first spinal nerve. 5. transverse process of axis. 6. fused neural spines of atlas and axis.]