UNG ULATA,] MAMMALIA teeth, forming an uninterrupted series. The cervical vertebra resemble those of the Camels in the position of the vertebrarterial canal, but the ends of the centra are flat, and not opisthocoelous as in the allied forms. In some of the limb characters it resembles the Equidse, but in the articulation of the fibula with the calcan- eum it agrees with the Artiodactyles. The structure of the feet is, however, distinctly Perissodactyle, there being three toes on each. Families CHALicOTHERiiDJE and MENODONTID^B. These families, with not very distinctly defined boundaries, con tain a large number of extinct forms from Eocene and Miocene formations of both the Old and the New World. Among the latter the most remarkable is a-group of animals of gigantic size, to the first-known fragment of which the name of Menodus was given in 1849 by Poinel, but of which more perfect remains have since been described by Leidy as Titanotherium and Megacerops, by Marsh as Brontotheriiim, and by Cope as Symborodon, some of which appear to represent distinct generic modifications, but the syuonymy of the group is at present much confused. The head was large and much elongated, as in the Rhinoceros ; but they had a pair of stout diverging osseous protuberances like horn-cores on the maxillaries in front of the orbits. Their molar teeth were of a simple palreotheroid type, and the incisors and canines were very much reduced. Their fore feet had four and their hind feet three short, stout toes. Their remains abound in the Lower Miocene strata of North America. Family PAL.EOTHERIID.E. The structure of the type of this family, Palseothcrmm, was made known by Cuvier, from specimens found in the Paris gypsum beds (Upper Eocene). Fig. 108 gives an idea of its general appear ance, not unlike that of a Tapir, which also it resembled in size. It had, however, but three toes on the fore feet. The dentition was i f, c Y, p , in f = 44. Many species and allied genera (as FIG. 108. Restoration of Palxotherium (Upper Eocene). Cnvier. Palaplotheriwm, an earlier form from the Middle Eocene, and Anchitherium, a later one from the Miocene) have been discovered both in Europe and North America. To some of these the ancestral form of the modern Horses may be traced, the transition from this to the next family being formed by almost imperceptible gradations. 0. EQUINE SECTION. Lower molars formed of a pair of crescents complicated by the addition of columns or lobes at the inner extremities. Upper molars a modification of the palseotheroid type, but gradually passing as time advanced from the brachyodont to the hypsodont form. Outer digits becoming gradually reduced, until, as in the modern Horses, there is but one (the third) functional digit on each foot. To these alone the family Equidse is restricted by some authors, but in few groups is the artificial nature of the boundaries placed between such divisions so apparent as in the Perissodactyles, for the simple reason that their palseontological history is better known than that of most others, and so many of the intermediate forms have been preserved. For the history, characters, and pre sent distribution of the Equidse, see the article HORSE, vol. xii. p. 172 sq. SUBORDER ARTIODACTYLA. This is an equally well-defined group, traceable from the Early Eocene period, though then apparently by no means so numerous as the Perissodactyles. Some of its types, as that represented in the existing Swine, have retained to the present time much of the primitive character of the group ; but others have been gradually becoming more specialized and more perfected in structure, and its latest modification, the Cavicorn Ruminants or Bovidx (Antelopes, Sheep, and Oxen), are now the dominating members of the great Ungulate order, widespread in geographical range, rich in generic and specific variation, and numerous in individuals, forming in all these respects a great contrast to such decadent types as those represented by the Tapirs and Rhinoceroses. The principal anatomical characters by which they are distinguished from the Perissodactyles are as follows. The premolar and molar teeth not alike, the former being single and the latter two-lobed. The last lower molar of both first and second dentition almost invariably three- lobed. Nasal bones not expanded posteriorly. No alisphenoid canal. Dorsal and lumbar vertebrae together always nineteen, though the former may vary from twelve to fifteen. Femur without third trochanter. Third and fourth digits of both feet almost equally developed, and their ungual phalanges flattened on their inner or con tiguous surfaces, so that each is not symmetrical in itself, HI. FIG. 109. Bones of Fore Foot of existing Artiodactyles. A, Pig (Sus scrofa), X|; B, Red Deer (Cervus etaphus), X} ; C, Camel (Came/us bactrianus), XJ. U, ulna; R, radius; c, cuneiform; ?, lunar; s, scaphoid ; Cuneiform; m, mag num; td, trapezoid ; tin, trapezium. From Osteology of Mammalia. but when the two are placed together they form a figure symmetrically disposed to a line drawn between them. Or, in other words, the axis or median line of the whole foot is a line drawn between the third and fourth digits, while in the Perissodactyles it is a line drawn down the centre of the third digit. Distal articular surface of the astragalus divided into two nearly equal facets, one for the navicular and one for the cuboid bone. The calcaneum with an articular facet for the lower end of the fibula. Stomach almost always more or less complex. Colon convoluted. Caecum small. Placenta diffused or cotyledonary. Mamma) few and inguinal, or numerous and abdominal. In treating of many sections of mammals, it is only from the exist ing species that our characters and classification can be derived, and to these chiefly our observations upon the group must be directed, the extinct forms being so little known that they can only be referred to incidentally. With the Ungulata, however, it is quite otherwise. As with the last section, the history of the Artio- dactyla throughout the Tertiary period is now well known, and throws great light upon the position and relations of the existing groups. The principal modifications which have taken place in the type from its earliest known and most generalized manifestation have been the following : 1. As regards the teeth. Assumption of the grinding surfaces of the molar teeth either of a distinctly tubercular (bunodont) or of a
crescentic ridged (selenodont) form. Modification of the latter from