The molar teeth approximate to the _Palaeotherium_ type, but have a more or less strongly developed median longitudinal cleft. The three-toed type is represented by _Diadiaphorus_, in which the dental formula is i.½, c.(0/1), p.(4/5), m.(3/8), and the feet are very like those of _Hipparion_. The cervical vertebrae are of normal form, the orbit (as in the second family) is encircled by bone, the last molar has a third lobe, the single pair of upper incisors are somewhat elongated, and have a gap between and behind them, while the outer lower incisors are larger than the inner pair, the canines being small. The skull has a short muzzle, with elongated nasals. Remains of this and the other representatives of the group are found in the Patagonian Miocene. In _Proterotherium_, which includes smaller forms having the same, or nearly the same, dental formula, the molar teeth differ from those of _Diadiaphorus_ by the deeper median longitudinal cleft, which completely divides the crown into an inner and an outer moiety, the two cones of the inner half being united. According to the description given by Argentine palaeontologists, this genus is also three-toed, the single-toed representative of the family being _Thoatherium_, in which the lateral metapodials, or splint-bones, are even more reduced than in the _Equidae_. In the second family--_Macraucheniidae_--the dentition is complete (forty-four) and without a gap, the crowns of nearly all the teeth being of nearly uniform height, while the upper molars are distinguished from those of the _Proterotheriidae_ by a peculiar arrangement of their two inner cones, and the elevation of the antero-posterior portion of the cingulum so as to form an extra pit on the crown. To describe this arrangement in detail is impossible here, but it may be stated that the two inner cones are closely approximated, and separated by a narrow V-shaped notch on the inner side of the crown. The elongated cervical vertebrae are peculiar in that the arch is perforated by the artery in the same manner as in the llamas.