The dorso-lumbar vertebrae are as a rule twenty-three; but the extinct Titanotheres are again an exception; for, at least in _Titanotherium_, there are but twenty of these vertebrae--an Artiodactyle character. The femur has a third trochanter. There are so few recent Perissodactyles that an enumeration of the distinguishing characters of the viscera may very probably be useless for purposes of classification. But the living genera at any rate are to be separated from the living Artiodactyles by the invariable simplicity of the stomach coupled with a very large and sacculated caecum. The liver is simple and not much broken up into lobes, and the gall-bladder is always absent. The brain is well convoluted. The teats are in the inguinal region. The placenta in this group is of the diffused kind. [Illustration] FIG. 123.--Anterior aspect of right femur of Rhinoceros (_Rhinoceros indicus_). x 1/2. _h_, Head; _t_, great trochanter; _t_', third trochanter. (From Flower's _Osteology_.) The living Perissodactyles belong to three types only, indeed to three genera only (in the estimation of most), which are the Horses, Tapirs, and Rhinoceroses. But taking into account the extinct forms, they may be divided primarily (according to Professor Osborn) into the four following groups:--(1) Titanotherioidea, including but one family, Titanotheriidae; (2) Hippoidea, including the families Equidae and Palaeotheriidae; (3) Tapiroidea, with two families, Tapiridae and Lophiodontidae; and (4) Rhinocerotoidea with families Hyracodontidae, Amynodontidae, and Rhinocerotidae. It is conceivable, according to the same writer, that the Chalicotheres (here treated of as a separate sub-order, Ancylopoda) should be added to the Perissodactyle series. [Illustration] FIG. 124.--Side view of skull of Horse with the bone removed so as to expose the whole of the teeth. _c_, Canine; _Fr_, frontal; _i_^1, _i_^2, _i_^3, incisors; _L_, lachrymal; _m_^1, _m_^2, _m_^3, molars; _Ma_, malar or jugal; _Mx_, maxilla; _Na_, nasal; _oc_, occipital condyle; _Pa_, parietal; _pm_^1, situation of the vestigial first premolar, which has been lost in the lower, but is present in the upper jaw; _pm_^2, _pm_^3, _pm_^4, remaining premolars; _PMx_, premaxilla; _pp_, paroccipital process; _Sq_, squamosal. (After Flower and Lydekker.)