The perissodactyls, or odd-toed animals, are mammals in which the central toe on each limb is greatly enlarged. Modern representatives include the horses, rhinoceroses, and tapirs. Extinct members of the Perissodactyla include the _titanotheres_, _chalicotheres_, and _baluchitheres_, all of which grew to tremendous size and took on many unusual body forms. HORSES.— One of the first perissodactyls was _Hyracotherium_ (also called _Eohippus_), which is the earliest known horse (Pl. 47). This small animal, whose remains have been found in Big Bend National Park, was about 1 foot high and his teeth indicate a diet of soft food. Following the first horse, there is a long series of fossil horses which provide much valuable information on the history of this important group of animals. The record of the development of the horse is well represented in Texas, and the bones and teeth of fossil horses are common in certain parts of the State. Fossils of this type have been reported from the Tertiary of the Trans-Pecos, Gulf Coastal Plain, and High Plains regions of Texas, and the teeth of Pleistocene horses have been found in sand and gravel pits in many parts of the State. Horse teeth (fig. 26) are particularly useful fossils as they may be accurately identified and used to determine the age of the rocks in which they are found. [Illustration: Fig. 26. Typical Pliocene horse tooth. Top view (a) and lateral view (b) of molar tooth (×½).] TITANOTHERES.— This group of odd-toed mammals appeared first in the Eocene, at which time they were about the size of a sheep. By Middle Oligocene time they had increased to gigantic proportions but still had a small and primitive brain. _Brontotherium_ (Pl. 48) was slightly rhinoceros-like in appearance and is believed to be the largest land animal that ever inhabited the North American continent. This animal was about 8 feet tall at the shoulders; a large bony growth protruded from the skull and this was extended into a flattened horn, which was divided at the top. [Illustration: Plate 48 TERTIARY MAMMALS] UINTATHERIUM × ¹/₄₅ BRONTOTHERIUM × ¹/₃₅ Although the titanotheres underwent rapid development during the early Tertiary, these huge beasts became extinct during the middle of the Oligocene epoch. Titanothere remains have been reported from the Trans-Pecos region of Texas. CHALICOTHERES.—