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. The Artiodactyla have a number of well marked characters, one of the most obvious being the fact that many of the most characteristic forms have large paired outgrowths on the frontal bones. These may be (1) solid deciduous bony _antlers_, or (2) more or less hollow bony outgrowths which are sheathed with permanently growing horn. The premolar and molar teeth are usually dissimilar, the premolars being one-lobed and the molars two-lobed; the last lower molar of both the milk and permanent dentitions is almost always three-lobed. The grinding surfaces of the molar teeth have a tendency to assume one of two forms. In the Pigs and their allies the crowns are bunodont[1], while in the more highly specialised Ruminants the crowns are selenodont[125]. The nasals are not expanded posteriorly, and there is no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always nineteen. The symphysis of the ischia and pubes is very elongated, and the femur has no third trochanter. The limbs never have more than four digits, and are symmetrical about a line drawn between the third and fourth digits; the digits, on the other hand, are never symmetrical in themselves. The astragalus has pulley-like surfaces both proximally and distally, and articulates with the navicular and cuboid by two nearly equal facets. The calcaneum articulates with the lower end of the fibula when that bone is fully developed.