The teeth of the Perissodactyles are lophodont, more rarely bunodont. The selenodont Artiodactyle form of molar is not met with. The dental formula, moreover, is at least near the
complete one, the more modern forms as usual being the more deficient in numbers of teeth.
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.
Fig. 123.—Anterior aspect of right femur of Rhinoceros (Rhinoceros indicus). × ½. 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.
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; i1, i2, i3, incisors; L, lachrymal; m1, m2, m3, molars; Ma, malar or jugal; Mx, maxilla; Na, nasal; oc, occipital condyle; Pa, parietal; pm1, situation of the vestigial first premolar, which has been lost in the lower, but is present in the upper jaw; pm2, pm3, pm4, remaining premolars; PMx, premaxilla; pp, paroccipital process; Sq, squamosal. (After Flower and Lydekker.)
Fam. 1. Equidae.—This family, which includes the living Horse, Zebras, and Asses, as well as a number of extinct genera agreeing with those types in structure, may be defined by the possession of but one functional toe, the two lateral ones being mere splints, or but little more. The molar teeth are hypselodont, and
the premolars, with the exception of the first, resemble the molars in their pattern. The orbit is completely surrounded by bone. The incisors are chisel-shaped, with a pit on the free surface. The canines are rudimentary if present. The radius and ulna are fused, as are the tibia and fibula. Although for the sake of uniformity a family, Equidae, is here separated from its allies, it is quite impossible owing to the full state of our knowledge of this group to draw a really hard-and-fast line between this family and the Palaeotheriidae. We shall deal presently with the conjectured pedigree of the Horse, which naturally involves that family, and which presents an unbroken series from four-toed Perissodactyles to the present one-toed Horse, the various bones and teeth becoming modified in the course of the descent "with the regularity of clockwork." We are compelled to draw the line at functional second and third toes; directly these are no longer used the animal is a Horse in the strict sense! This is irrational and regrettable, but necessary for practical purposes, if