Artiodactyle Ungulates. Section B. Artiqdactyla. - In this section of the Ungulates the number of the toes is even - either two or four - and the third toe on each foot forms a symmetrical pair with the fourth (fig. 388, C) . The dorso-lumbar vertebra are nineteen in number; and there is no third trochanter on the femur. If true horns are present, these are always in pairs, and are supported by bony horn-cores. The antlers of the Deer are also paired, but they are not to be regarded as true horns. The stomach is always more or less complex, or is divided into separate compartments, and the caecum is comparatively small and simple. By Kowalewsky the Artiodac-tyla, recent and extinct, are divided into two great groups or sections, in accordance with the nature of the teeth. These two sections were differentiated at a very early period, and they are known respectively as the Bunodonta and Selenodonta. In the "Bunodont" section are comprised only the Pigs and their allies, and the Hippopotamus, in all of which the molars and praemolars have tuberculated crowns (fig. 398). In the
Fig. 398. - Grinding-surface of the molar and praemolar teeth of a Peccary (Dicotyles labiatus), showing the bunodont type of dentition. (After Giebel.)
"Selenodont" section of the Artiodactyla the praemolars and molars (fig. 399) have the grinding-surfaces of their crowns divided each into two crescentic lobes, the convexities of which are turned inwards in the upper and outwards in the lower teeth. Some fossil forms, which are otherwise allied to the Bunodont Artiodactyles, show teeth of a "selenodont" character, and thus form a transition between these otherwise sharply separated divisions of even-toed Ungulates.
Fig. 399. - Grinding-surface of the molar and praemolar teeth of the Giraffe (Camelo-pardalis Giraffa), showing the selenodont type of dentition.
The section Artiodactyla comprises the Hippopotamus, the Pigs, and the whole group of the Ruminants, including Oxen, Sheep, Goats, Antelopes, Camels, Llamas, Giraffes, Deer, etc.
Besides these there is an extensive series of fossil forms commencing in the Eocene or Lower Tertiary period, and in many respects filling up the gaps between the living forms.