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_). _Suborder_ (3). ODONTOCETI. Teeth always exist after birth and baleen is never present. The teeth are generally numerous, but are sometimes few and deciduous; the dentition is homodont (except in _Squalodon_). The dorsal surface of the skull is somewhat asymmetrical, there is no trace of an olfactory fossa, the nasals are quite rudimentary, and the hind ends of the maxillae cover part of the frontals; in all these respects the skull differs from that of the Mystacoceti. The lachrymal may either be united to the jugal or may be large and distinct. The tympanic is not ankylosed to the periotic. The rami of the mandible are nearly straight and become united in a long symphysis. Some of the ribs have well developed capitula articulating with the vertebral centra. The sternum is almost always composed of several pieces as in other mammals, and several pairs of ribs are connected with it. There are always five digits to the manus, though the first and fifth are usually very little developed. The suborder includes the Sperm Whale (_Physeter_), Narwhal (_Monodon_), Dolphin (_Delphinus_), Porpoise (_Phocoena_), and many other living forms as well as the extinct _Squalodon_ which differs from the other members of the suborder in its heterodont dentition. _Order 4._ UNGULATA.