TITANOTHERIIDAE (also known as Menodontidae and Brontotheriidae)  , a family of large rhinoceros-like perissodactyl ungulate mammals from the Oligocene and Eocene strata of North America . The cheek-teeth are low-crowned, with the external cones of the upper molars fused into a W-like outer wall, and the inner ones retaining a regular conical form; while in the lower teeth the crown is formed of crescentic ridges, of which there are three in the last and two each in the other teeth . There is generally little gap between the canines and the premolars . Titanotherium, of the Oligocene of the Dakotas and neighbouring districts, was a huge beast, with the hinder upper premolars similar in character to the molars, a pair of horn-cores, arising from the maxilla, overhanging the nose-cavity, four front and three hind toes, only twenty dorso-lumbar vertebrae, and an almost continuous and unbroken series of teeth, in which the canines are short; the dental formula being i . I, c. p . -, m . The muzzle probably formed a snout in life; and there is presumptive evidence that these animals were very long-lived . Brontops seem scarcely separated from the type genus; but the name Brontotherium is applied to species with two pairs of incisor teeth in both jaws . The length of the largest species was about 14 ft.; and the height about 8 ft . The alleged occurrence of remains of members of the group in the Balkans apparently rests on insufficient evidence . A second group is typified by Palaeosyops, of the Bridger Eocene of North America; P. paludosus being an animal about the size of a tapir . The skull, which has a longer face than in Titanotherium, lacks horn-cores, while all the upper premolars are simpler than the molars, and the full series of 44 teeth was present .

The limbs were relatively slender, and the

brain was small . In the lower, or Wasatch, Eocene the group was represented by the still more primitive Lambdotherium . On the other hand, Palaeosyops is connected with Titanotherium by means of Telmatotherium of the upper Bridger and Washakia Eocene, a larger animal, with a longer and flatter skull, showing rudiments of horn-cores, only two pairs of lower incisors, and a general approximation in dental character to Titanotherium . Another of these titanotheroid forms is Diplacodon, from the Upper or Uinta Eocene; an animal the size of a rhinoceros, with the last two upper premolars molar-like . It was probably off the direct ancestral line of Titanotherium . These intermediate forms render the reference of the group to a distinct family—Palaeosyopidaeunnecessary . Professor H . F . Osborn, who recognises four genera, Titanotherium, Megacerops, Symborodon and Brontotherium, in the typical section of the family, considers that each of these represents a distinct line of descent from the Palaeosyops-like group . The whole assemblage forms one of the four main sections of the Perissodactyla, namely the Titanotheroidea . See H . F .

Osborn, " The

Cranial Evolution of Titanotherium." Bull . Amer . Mus . (1896), viii., 137, and the " Four Phyla of Oligocene Titanotheres," op. cit . (1903), xvi . 91; C . H . Earle, " A Memoir on the Genus Palaeosyops and its Allies," Journ . Acad . Philadelphia (1892), ix . 267 . (R .