428 MAMMALIA [UNGULATA. last lower molar commonly bilobed. 1 Dorso-lumbar vertebrae never fewer than twenty-two, usually twenty-three in the existing species. Nasal bones expanded posteriorly. An alisphenoid canal. Femur with a third trochanter. The middle or third digit on both fore and hind feet larger than any of the others, and symmetrical in itself, the free border of the ungual phalanx being evenly rounded (see fig. 107). This may be the only functional toe, or the second and fourth may be subequally developed on each side of it. In the Tapirs and many extinct forms, the fifth toe also remains on the fore limb, but its presence docs not interfere with the symmetrical arrangement of the remainder of the foot around the median line of the third or middle digit. Traces of a hallux have only been found in some extremely ancient and primitive forms. The astragalus has a pulley- like surface above for articulation into the tibia, but its distal surface is flattened and unites to a much greater extent with the navicular than with the cuboid, which bone is of comparatively less importance than in the Artiodac- tyles. The calcaneum does not articulate with the lower end of the fibula. The stomach is simple, the caecum large and capacious, the placenta diffused, and the mammae inguinal. The very distinct minor groups into which the Perissodactyles are divided in the later periods of the earth s history are, even by the knowledge already gained of the ancient members of the sub order, so closely united by connected forms that it is difficult to make any satisfactory classification of the whole. This is of course what might be expected, and would probably be the case with all other groups if we knew as much of their past history as we do of that of the Perissodactyles. It is necessary, however, for descriptive purposes to have some arrangement ; and perhaps, if not the most natural, the most convenient division (especially as it is chiefly or only by these organs that many are known) is one founded upon the structure of the lower molar teeth. By this character we may make three primary divisions, each of which has a representative at the present time: (A) those in which the crowns of the lower molars are disposed in transverse ridges, as in the Tapirs ; (B) those in which the crowns of the lower molars are formed by a pair of crescents, as in Rhinoceros ; (0} those in which the crowns of the lower molars are formed of a pair of crescents, with the addition of inner lobes or columns, as in the Horses. As these forms are all modifications of the same essential pattern, transitions in certain or all of the teeth must be expected in many cases, and, as before implied, the grouping of the Perisso dactyles into Tapiroid, Rhinocerotic, and Equine sections according to the pattern of their molar teeth may not be a true exposition of the real affinities of the genera, but must be looked upon rather as a convenient provisional arrangement. A. TAPIROID SECTION Lower molars bilophodont. Family LOPHIODONTID.E. Both upper and lower true molars bilophodont. Premolars smaller and simpler than the true molars. Four toes on the anterior and three on the posterior feet. This family includes a large number of more or less imperfectly known forms, all extinct, ranging from the size of a Rabbit to that of an Ox. They are the earliest in time and most generalized in structure of the known Perissodactyles. It is possible that from some either of the known or the still undiscovered members of this group most of the other types of the order have been derived. Their remains have been found in Europe only in the Lower and Middle Eocene, though in North America they appear to have lingered to a somewhat later date. The genus Hyracotherium was established in 1839 by Owen for a small animal, no larger than a Hare, the skull of which was found in the London Clay at Herne Bay. A more perfect specimen apparently of the same species was afterwards (in 1857) described under^the name of Philophus vulpiceps. Closely allied forms from the European continent have been named Pachynolophus and Lophwthcrium. These have all the complete dentition, viz., if, c T, P f , in f = 44. The posterior lower molar has three lobes. The genus Lophiodon (Cuvier, 1822) contains animals of much larger size and later geological period (Middle or Upper Eocene), in which the dentition was so far specialized as to have lost the anterior pre- molar of both jaws, the formula being i f, c , p f, m f = 40. The transverse ridges of the upper molars are placed obliquely, the 1 These dental characters are not strictly applicable to the most ancietit forms. posterior is smaller than the anterior, and they are united by their external borders; those of the mandible are distinct and only con nected by a feeble diagonal crest, the last bearing a talon or rudiment of a third lobe. On the premolars the anterior ridge only is developed. Nearly allied was the American genus Hyrachyus, the structure of which is now very completely known from well-preserved remains. The skeleton closely resembles that of the Tapir, though the dentition is more like that of Lophiodon, except that the last lower molar has but two lobes. As many as nine species have been already de scribed, all from the Upper Eocene. Another form from the same deposits, Colonoccras of Marsh, is said to have had an attachment for a dermal horn on each nasal bone. Triplopus, otherwise closely similar to Hyrachyus, wants the fifth digit of the manus, and henco is placed by Cope in a distinct family, Triplopides. Family TAPIRID^E. Both upper and lower true molars bilophodont. Posterior pre molars above and below resembling the true molars. This family is connected with the last by the Middle Miocene genus Listriodon. The genus Tapirus, in which as many as three pre molars resemble the true molars, and in which the last lower molar has no talon, appears first in the Upper Miocene of Europe, and has continued with scarcely any appreciable change until the present time, being represented by several species in Central and South America, and one in the Malay Peninsula and adjacent islands. It is therefore probably the oldest existing genus of mammals. One of the American species differs from all the others in the great an terior prolongation of the ossification of the mesethmoid cartilage, and has been separated generically by Gill under the name of Elasmognathus. See TAPIR. B. RHINOCEUOTIC SECTION. Lower molar teeth with the ridges, instead of being transverse, curved in a cresceiitic manner. The outer extremity of each ridge is curved forwards so that the hinder ridge abuts against the ex ternal surface of the ridge in front of it. An unworn lower molar of a Rhinoceros has thus externally two convex areas separated by a vertical groove, and internally two principal sinuses, correspond ing to the projections externally. The entrances to these sinuses are bordered by three conical pillars the first of comparatively little importance, representing the anterior talon of the Tapir s tooth, the second, the largest, representing the antero-iuternal principal cusp, and the third the postero-internal principal cusp. The upper molars of all the animals of this section resemble those of Lophiodon in principle, the transverse ridges being joined by an outer wall and placed obliquely, their inner ends inclining back wards and their posterior surfaces being more or less concave. There are two further chief modifications of this type: (1) that in which the free edge of the outer wall acquires a strongly zigzag or bi- crescentic character, being deviated inwards opposite each of the principal outer cusps, and outwards at the anterior and posterior angles of the tooth and in the middle between the cusps, as in PaliKothcriuin ; and (2) that in which the outer wall is greatly developed, and in the main flat or smoothly convex, though with slight elevations and depressions corresponding with those so regular and well-marked in the last section ; this is the character of the teeth of Rhinoceros and its allies. Family HYRACODONTIDJE. Separated by Cope from the next, and containing the genus Ifyracodon of Leidy, a primitive or simple Rhinoceros-like type, from the Lower Miocene of North America, with the full number of teeth, but only three digits on each foot. Family RIIINOCERONTID.E. A very extensive group, of which many modifications, form ing a gradual series, showing increasing specialization from primitive Lopkiodon-ik& animals, have been discovered both in North America and in the Old World. One of the most remarkable of these specializations has been the development of dermal horns over the nasal bones, either in laterally placed pairs as in some of the early forms, or in the median line, either single or double. In America they all became, extinct before the close of the Pliocene period; but in the Old World, although their geographical distribution has become greatly restricted, at least five well-marked species survive. See RHINOCEROS. Family MACRAUCIIEKID^E. This contains one extinct genus, Macrauclicnia, with two pecies M. patachonica and M. bolivicnsis, both from South America, and apparently from Pliocene formations. They are very singular and specialized forms, quite out of the line of descent of any of the existing Perissodactyles, and the steps by which they are connected with the rest of the group have not yet Liecn discovered. Of the larger species, M patachonica, the skeleton
is completely known. It had the full number of forty-four