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.
This order includes a great and somewhat heterogeneous group of animals, a large proportion of which are extinct. They all (except certain extinct forms) agree in having the ends of the digits either encased in hoofs or provided with broad flat nails. The teeth are markedly heterodont and diphyodont, and the molars have broad crowns with tuberculated or ridged surfaces. Clavicles are never present in the adult except in a few generalised extinct forms such as Typotherium, and it is only recently that vestigial clavicles have been discovered in the embryo[122]. The scaphoid and lunar are always distinct.
The order Ungulata may be subdivided into two main groups, Ungulata vera and Subungulata.
Section I. Ungulata vera[123].
The cervical vertebrae except the atlas are generally opisthocoelous. The feet are never plantigrade[124]. In all the living and the great majority of the extinct forms the digits do not exceed four, the first being suppressed. In the carpus the os magnum articulates freely with the scaphoid, and is separated from the cuneiform by the lunar and unciform. In the tarsus the cuboid articulates with the astragalus as well as with the calcaneum, and the proximal surface of the astragalus is marked by a pulley-like groove. All the bones of the carpus and tarsus strongly interlock. These characters with regard to the carpus and tarsus do not hold in Macrauchenia and its allies. The humerus never has an ent-epicondylar foramen.
The group is divided into two very distinct suborders:—
Suborder (1). Artiodactyla.