The whole skeleton and especially the skull and ribs is remarkably massive and heavy. The dentition varies; in the two living genera _Manatus_ and _Halicore_, incisor and molar teeth are present, in one extinct genus, _Rhytina_, teeth are entirely absent, while in another, _Halitherium_, the dentition is more decidedly heterodont than in living forms. In the two living genera the dentition is monophyodont, but in _Halitherium_ the anterior grinding teeth are preceded by milk teeth. The tongue and anterior part of the palate and lower jaw are covered with roughened horny plates. The skull is noticeable for the size and backward position of the anterior nares, also for the absence or small size of the nasal bones. There is no union of certain of the vertebrae to form a sacrum, and in living forms the centra are not terminated by well-formed epiphyses[120].
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.
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.