From such a form the line leading to the horses began to lengthen the middle digits and lift the side toes off the ground; in the upper molar teeth the oblique cross-crests became markedly twisted, while the main outer cusps became broadly crescentic ; the upper premolars gradually became more molariform until three of them equalled or even surpassed the molars in size and complexity. The subsequent evolution of the horse family is treated elsewhere (see EQUIDAE ; HORSE).
In the line leading to the tapirs the number of toes did not change but the animals grew larger, heavier, and their feet broadened. In the cheek teeth the cross-crests became less oblique, more transverse, finally becoming two high, straight, sharp-edged ridges, shearing with similar ridges on the lower molars. The most distinctive feature is the trunk or proboscis, in which the nose and upper lip are combined into a mobile, more or less protrusile organ. The proboscis of the tapir, however, has no finger-like projection of the tip and lacks the mobility and adaptability of that of the elephant. The corresponding alterations in the skull of the tapir are far less than in the case of the ele phant, involving only a moderate retreat or retraction of the nasal bones. The middle septum or cartilage of the nose also is far more conspicuous and extended further forward (see' TAPIR). Thus the skeleton as a whole retains much that was characteristic of the earliest Eocene members of the order. The feet, for in stance, have the same number of digits and arrangement of all the elements as in the very earliest Perissodactyla, and the rest of the skeleton is also primitive except for the modifications of the nasal region for the support of the proboscis. Even the cross crested molars are much like those of the Eocene lophiodonts (family Lophiodontidae). which were, however, intermediate between the tapirs and the rhinoceroses.
The- family of the palaeotheres (Palaeo theriidae) was characteristic of the Eocene of Europe and was closely related to the horse family. The typical genus, Palaeo therium, resembled a tapir in general proportions but had three instead of four toes on each forefoot; it had at most only the beginning of a proboscis ; the upper molar crowns comprised a W-shaped outer wall and two oblique cross-crests united with the two conical inner cusps. The palaeotheres are connected with the horses through such small light-limbed forms as Palaeoplo theriunt.
The titanothere family (Titanotheriidae) ranged from the Lower Eocene of Wyoming to the summit of the Lower Oligocene in South Dakota, Nebraska and Saskatche wan. Representatives have also been found in the Eocene of Europe and in the Upper Eocene and Lower Oligocene of Mon golia. The oldest known titanotheres (Eotitanops) were about as large as a sheep but shaped more like a tapir except that they had no proboscis, while the nose and lips were possibly somewhat horse like. The most distinctive character is found in the upper molar teeth, the crowns of which had a W-shaped outer wall, two conical internal cusps and vestigial cross-crests. The lower molar crowns
were W-shaped. This general type of molar was retained through out the history of the family, the chief modification being that in the later forms the W-shaped outer wall of the upper molar crowns became higher and more curved toward the mid-line of the palate, while the internal cusps remained conical and low.
During the Eocene and Lower Oligocene the titanotheres be came differentiated into numerous genera and over 'co species. The fossil remains of the Oligocene were formerly abundant in the "Big Badlands" on the White river in South Dakota and Nebraska. The primitive perissodactyl numbers of four toes on the forefeet and three on the hindfeet were retained throughout the family. In the main lines leading to the Lower Oligocene end forms the size of the body increased until a bulk surpassing that of the largest recent rhinoceros was finally attained. Of the middle-sized genera of the Middle and Upper Eocene the most notable was the genus Palaeosyops, comprising animals larger than tapirs, with widespreading feet, broad, rounded skull and low crowned, circular-cusped molars. At the opposite extreme was the genus Dolichorhinus, with an extremely elongated skull and snout, laterally compressed but not long feet, and upper molars with deepened outer wall and flattened outer cusps. The genus Manteoceras ("prophet-horn") was more or less intermediate be tween these extremes, but possessed a small bony swelling above and in front of the eye on each side of the skull. In later forms this swelling became more and more pronounced, until in the Oligocene titanotheres it grew into huge transversely-paired bony outgrowths from the naso-frontal junction, above and in front of the eyes. The shape of these bony protuberances differed in the numerous genera and species. In the more primitive Oligocene forms the "horns" were more or less three-sided, with a pointed tip, but in the more advanced genera they were broad transversely and compressed anteroposteriorly (Brontotherium), or erect with cylindrical section (Megacerops), or club-shaped (Brontops robus tus), or three-faced with a rounded, recurved tip (Menodus). The skulls of these genera were relatively long (Brontotherium, Menodus), or wide (Brontops robustus, Megacerops), or of inter mediate proportions (Allops). The skeleton of the largest titano theres abounded in adaptations for the support of the huge bodies. Whereas the earliest forms had rather narrow hands and feet, the later giants of the race had very broad four-toed forefeet, parallel to those of the hippopotamus, and broad, short, three-toed hind feet, parallel to those of the rhinoceros. The angles of the elbows and knees, which were well bent in the earlier running forms, opened out in the larger forms as in other "graviportal" mammals, while the ilium spread widely across the back to support the weight of the viscera. The great monograph on the titanotheres by H. F. Osborn affords one of the clearest and most interesting demonstrations of the facts of evolution yet recorded.