UNGULATA, a great and heterogeneous group of hoofed mammals which first appeared in the Eocene period as small marsh-dwelling forms scarcely distinguishable from the con temporaneous ancestors of the Carnivora, but which early became differentiated into several quite distinct series that have continued to diverge until in the living fauna they terminate in quite isolated groups. Under these circum stances it is not surprising that the opinions of zoologists differ greatly concerning the classifi cation of these animals. Without considering the changes in classification that have followed increased knowledge of the living forms, it may be said that if the latter alone were con sidered they would fall naturally into three distinct groups ; one typified by the horse, ox, pig, etc., another by the elephant, and the third by the coney (Hyrax). But with the great palkontological discoveries of recent years, especially in the Eocene of Wyoming and other Western States, and in the later formations of Patagonia, not only have these groups been con nected by annectent forms, but several totally new types have been found, and generalized un gulates have been shown to intergrade with the Carnivora on the one hand and with the Roden tia Quadriamana on the other. By some zoolo gists, as Cope, each of these principal groups, or radiating descent-series, has been given a dis tinct ordinal name. Others have grouped all together as a single order, with numerous sub divisions. As the latter arrangement is best adapted to the needs of the present article it is here followed.
In this broad sense the Ungulata have on each foot from one to five terminated by broad, blunt nails or hoofs. With a very few exceptions among extinct primitive forms they are digitigrade and many of them walk on the very tips of the toes. The scaphoid and lunar bones of the wrist are always distinct. Well marked milk and permanent dentitions are al ways developed, and with a few exceptions these animals are strictly terrestrial and herbivorous. Evolution in this group has particularly affected the feet and teeth, with the other digestive or gans. Correlated with the replacing of marshy
forests by dry, grassy plains these animals in creased in size and developed greater speed in running by a reduction in the number of digits from five to two or one, by the union of cer tain bones of the foot and lower leg, and by a change in the position of the carpal and tarsal bones, so that they alternate and interlock. As the soft succulent vegetation of the swamps gave way to the wiry grasses of the plains the teeth gradually changed from a tuberculate low crowned form to one with crescentic and then complexly-folded enamel ridges, and a high persistently growing crown. Although similar changes have been traced in many families, they are shown in greatest perfection in the horses, where every step in the evolution from the four-toed, low-tooth-crowned Orohippus of the Eocene, with well-developed ulna and fibula, to the one-toed, complexly high-crowned modern Equus with the greatly reduced ulna and fibula completely ankylosed with the radius and tibia respectively. ' See HORSE, EVOLUTION OF.
A subdivision of all the Ungulata into Un gulata Vera, or true ungulates, and Subutigu Iota is generally followed, though this, as in deed every sharp subdivision of the ungulates, is obscured by known fossil species. The Sub ungulata is a rather heterogeneous assemblage of the more primitive forms, in which the bones of the carpus and tarsus are not at all or only partially interlocking, the toes often five in number, and the feet sometimes sub plantigrade. Some of the extinct forms have well-developed clavicles. In the living species the mammary glands are, at least in part, situated on the breast, and the placenta forms a zone or broad band. Besides the many re markable extinct groups placed by Cope in the orders Taxeopoda (Condytarthra), Toxodontia and Amblypoda, the Hyracoidea and Pro boscides, represented in the modern fauna by Hyrax and the elephants, belong here.