The skull is peculiarly modified; the bones forming the occipital segment show a specially strong development, and the cranial cavity is short, high, and almost spherical. The supra-occipital is very large and rises up to meet the frontals, thus with the interparietal completely separating the parietals from one another. The frontals are expanded, forming large bony plates, which roof over the orbits. The zygomatic process of the squamosal is extremely large and extends forwards to meet the supra-orbital process of the frontal; the zygomatic process of the jugal is on the contrary very slender. The face is drawn out into a long rostrum, formed of the maxillae and premaxillae surrounding the vomer and the mesethmoid cartilage. The maxillae are specially large, and extend backwards so as to partially overlap the frontals. The nasals are always small, and the anterior nares open upwards between the cranium and rostrum. The periotics are loosely connected with the other bones of the skull and the tympanics are commonly large and dense. The mandible has hardly any coronoid process, and the condyles are at its posterior end. There are no clavicles, but the scapula and humerus are well developed. The humerus moves freely in the glenoid cavity, but all the other articulations of the anterior limb are imperfect; the various bones have flattened ends, and are connected with one another by fibrous tissue, which allows of hardly any movement. Frequently the carpus is imperfectly ossified. The number of digits in the manus is generally five, sometimes four, and when there are four digits it is the third and not the first that is suppressed. The number of phalanges in the second and third digits almost always exceeds that which is normal in mammals, and the phalanges are also remarkable for having epiphyses at both ends. The pelvis is represented by two small bones which lie suspended horizontally at some distance below the vertebral column; in some cases vestiges of the skeleton of the hind limb are attached to them. The Cetacea are divided into three suborders. _Suborder_ (1). ARCHAEOCETI. The members of this group are extinct; they differ from all living Cetacea in having the dentition heterodont and in the fact that the back was probably protected by dermal plates. The skull is elongated and depressed, and the brain cavity is very small. The temporal fossae are large, and there is a strong sagittal crest. The nasals and premaxillae are a good deal larger than they are in living Cetacea, and the anterior nares are usually far forward. The cervical vertebrae are not fused with one another, and the lumbar vertebrae are unusually elongated. The limbs are very imperfectly known, but while the humerus is much longer than in modern Cetaceans, it is nevertheless flattened distally, indicating that the limb was paddle-like, and that there was scarcely any free movement between the fore-arm and upper arm.