the development of the canines into projecting tusks; and the loss of the anterior premolars.
2. As regards the limbs. Reduction of the ulna from a complete and distinct bone to a comparatively rudimentary state in which it coalesces more or less firmly with the radius. Reduction of the fibula till nothing but its lower extremity remains. Reduction and final loss of outer pair of digits (second and fifth), with coalescence of the metacarpal and metatarsal bones of the two middle digits to form a cannon-bone. Union of the navicular and cuboid, and sometimes the ectocuneiform bone, of the tarsus.
3. Change of form of the odontoid process of the second or axis vertebrae from a cone to a hollow half-cylinder.
4. Development of horns or antlers on the frontal bones, and gradual complication of form of antlers.
5. By inference only, increasing complication of stomach with ruminating function superadded. Modification of placenta from simple diffused to cotyledonary form.
 | |||
| Fig. 1.—Bones of Right Fore Feet of existing Artiodactyla. | |||
| A, | Pig (Sus scrofa). | l, | Lunar. |
| B, | Red deer (Cervus elaphus). | s, | Scaphoid. |
| C, | Camel (Camelus bactrianus). | u, | Unciform. |
| U, | Ulna. | m, | Magnum. |
| R, | Radius. | td, | Trapezoid. |
| c, | Cuneiform. | ||
| In the Sheep and the Camel the long compound bone, supporting the two main (or only) toes is the cannon-bone. | |||
The primitive Artiodactyla thus probably had the typical number (44) of incisor, canine and molar teeth, brachyodont molars, conical odontoid process, four distinct toes on each foot, with metacarpal, metatarsal and all the tarsal bones distinct, and no frontal appendages.
As regards classification, the first group is that of the Pecora, or Cotylophora, in which the cheek-teeth are selenodont, but there are no upper incisors or canine-like premolars, while upper canines are generally absent, though sometimes largely developed. Inferior incisors, three on each side Pecora.with an incisiform canine in contact with them. Cheek-teeth consisting of p.33, m.33, in continuous series. Auditory bulla simple and hollow within. Odontoid process of second vertebra in the form of a crescent, hollow above. Lower extremity of the fibula represented by a distinct malleolar bone articulating with the outer surface of the lower end of the tibia. Third and fourth metacarpals and metatarsals confluent into cannon-bones (fig. 1 B), and the toes enclosed in hoofs. Outer toes small and rudimentary, or in some cases entirely suppressed; their metacarpal or metatarsal bones never complete. Navicular and cuboid bones of tarsus united. The skull generally lacks a sagittal crest; and the condyle of the lower jaw is transversely elongated. Horns or antlers usually present, at least in the male sex. Left brachial artery arising from a common innominate trunk, instead of coming off separately from the aortic arch. Stomach with four complete cavities. Placenta cotyledonous. Teats 2 or 4.
The group at the present day is divided into Giraffidae (giraffe and okapi), Cervidae (deer), Antilocapridae (prongbuck), and Bovidae (oxen, sheep, goats, antelopes, &c.). (See Pecora.)
The second group is represented at the present day by the camels (Camelus) of the Old, and the llamas (Lama) of the New World, collectively constituting the family Camelidae. They derive their name of Tylopoda (“boss-footed”) from the circumstance that the feet form large cushion-like pads, Tylopoda.supporting the weight of the body, while the toes have broad nails on their upper surface only, instead of being encased in hoofs. The cheek-teeth are selenodont, and one pair of upper incisors is retained, while some of the anterior premolars assume a canine-like shape, and are separated from the rest of the cheek-series. Auditory bulla filled with honeycombed bony tissue. Odontoid process of second vertebra semi-cylindrical; skull with a sagittal crest; and the condyle of the lower jaw rounded. Third and fourth metacarpals and metatarsals (which are alone present) fused into cannon-bones for the greater part of their length, but diverging inferiorly (fig. 1, C) and with their articular surfaces for the toes smooth, instead of ridged as in the Pecora. Navicular and cuboid bones of tarsus distinct. No horns or antlers. Stomach, although complex, differing essentially from that of the Pecora. Placenta diffuse, without cotyledons. Teats few. (See Tylopoda.)
In the same sectional group is included the North American family of oreodonts (Oreodontidae), which are much more primitive ruminants, with shorter necks and limbs, the full series of 44 teeth, all in apposition, and the metacarpal and metatarsal bones separate, and the toes generally of more normal type, although sometimes claw-like. (See Oreodon.) The Eocene American genus Homacodon is regarded as representing a third family group, the Homacodontidae (= Pantolestidae), in which the molars were of a bunodont type, and approximate to those of the Condylarthra from which this family appears to have sprung, and to have given origin on the one hand to the Oreodontidae, and on the other to the Camelidae. The family is represented in the Lower, or Wasatch, Eocene by Trigonolestes, in the Middle (Bridger) Eocene by Homacodon (Pantolestes), and in the Upper (Uinta) Eocene by Bunomeryx.
The third group is that represented by the chevrotains or mouse-deer, forming the family Tragulidae, with Tragulus in south-eastern Asia and Dorcatherium (or Hyomoschus) in equatorial Africa. The cheek-teeth are selenodont, as in the two preceding groups; there are no upper incisors, but Tragulina.there are long, narrow and pointed upper canines, which attain a large size in the males; the lower canines are incisor-like, as in the Pecora, and there are no caniniform premolars in either jaw. Cheek-teeth in a continuous series consisting of p.33, m.33. Odontoid process of axis conical. Fibula complete. Four complete toes on each foot. The middle metacarpals and metatarsals generally confluent, the outer ones (second and fifth) slender but complete, i.e. extending from the carpus or tarsus to the digit. Navicular, cuboid and ectocuneiform bones of tarsus united. Auditory bulla of skull filled with cancellar tissue. No frontal appendages. Ruminating, but the stomach with only three distinct compartments, the maniplies or third cavity of the stomach of the Pecora being rudimentary. Placenta diffused. (See Chevrotain.)
In this place must be mentioned the extinct Oligocene European group typified by the well-known genus Anoplotherium of the Paris gypsum-quarries, and hence termed Anoplotherina, although the alternative title Dichobunoidea has been suggested. It includes the two Anoplotherina.families Anoplotheriidae and Dichobunidae, of which the first died out with the Oligocene, while the second may have given origin to the Tragulina and perhaps the Pecora. There is the full series of 44 teeth, generally without any gaps, and most of the bones of the skeleton are separate and complete; while, in many instances at any rate, the tail was much longer than in any existing ungulates, and the whole bodily form
