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.
The best known genus is Zeuglodon, which is found in beds of Eocene age in various parts of Europe, and in Alabama.
Suborder (2). Mystacoceti or Balaenoidea.
These are the Whalebone Whales or True Whales.
Calcified teeth representing the milk dentition occur in the foetus, but the teeth are never functional, and always disappear before the close of foetal life. There is a definite though small olfactory fossa. The palate is provided with plates of baleen or whalebone. The skull is symmetrical, and is extremely large in proportion to the body. The nasals are moderately well developed, and the maxillae do not overlap the orbital processes of the frontals. The lachrymals are small and distinct from the jugals. The tympanics are ankylosed to the periotics, and the rami of the mandible do not meet in a true symphysis. The ribs articulate only with the transverse processes, and the capitula are absent or imperfectly developed. Only one pair of ribs meets the sternum, which is composed of a single piece.
The group includes among others the Right whale (Balaena), Humpbacked whale (Megaptera), and Rorqual (Balaenoptera).