1846 from a fragment of a jaw from the London Clay. Other remains were afterwards discovered in France, and lately in great abundance, indicating many species from the size of a Tapir to that of a Rhinoceros, in the Lower and Middle Eocenes of New Mexico and Wyoming in the United States. Coryphodon had forty-four teeth; the canines of both jaws were large and sharp pointed, and the molars had strongly pronounced oblique ridges. general proportions were those of a Bear, but the tail was of moderate length, and the feet short and wide. The femur had a third trochanter; and the cranium was devoid of protuberances.

FIG. 191.-Palatal aspect of the cranium of Coryphodon hamatus, from the Wasatch Eocene of New Mexico. natural size. (After Cope.)

The genus should be regarded as the type of a distinct family Coryphodontidae.

Suborder CONDYLARTHRA.

The term Condylarthra has been proposed by Professor Cope for a number of generalised and mostly comparatively small Ungulates, which were probably allied both to the Perissodactyla and Artiodactyla, but present characters separating them from those divisions as commonly defined. In the structure of the carpus and tarsus these forms (which are chiefly known to us from the Eocene of the United States) come nearer to the Hyracoidea than to any other existing type. As a rule they have the full dental formula; the molars are brachydont, generally bunodont, and in many instances also tritubercular; while the premolars are always simpler than the molars.

The humerus is quite peculiar among Ungulates in having an

entepicondylar foramen; the femur has a third trochanter; and the form and relations of the astragalus are similar to those obtaining in the Carnivora. The feet are usually furnished with five functional digits, of which the ungual phalanges are pointed. In many respects the skeleton of these remarkably generalised Ungulates approximates so decidedly to a Carnivorous type as to have led palæontologists to conclude that the Ungulata and Carnivora are branches of an original common stock.

In this work space only permits of allusion to a few of the more important types of this group. Periptychus, which occurs in the lowest Eocene of New Mexico, is a bunodont type readily distinguished by the vertical flutings of the premolars, and the small size of the incisors and canines. It has been suggested that this genus is closely related to the stock of the bunodont Artiodactyla. Of greater interest is the genus Phenacodus, which is regarded as the lowest factor in the series from which the modern Horse has been evolved, where it holds the position immediately below Hyracotherium or Systemodon (see p. 374). One of the species was about the size of a Bull-dog, while another might be compared to a small Leopard. The structure of the cheek-teeth is such as might readily be modified into that obtaining in Hyracotherium; all the feet had five fully developed digits, and the tail was long. Meniscotherium and Hyracodontotherium are more specialised forms of somewhat later age, with a lophodont dentition; the latter genus being European.

Suborder ToXODONTIA.

In addition to the Macraucheniide and certain other forms noticed under the head of the Perissodactyla, the Tertiaries of South America have yielded some very remarkable forms of mammalian life, the nature and affinities of which have greatly puzzled all zoologists who have attempted to unravel them.

Nesodon and Toxodon.-Among these Nesodon, from Patagonia, has the full typical Eutherian number of teeth; the crowns of the incisors being short, and the molars having a complex rhinocerotic type of structure somewhat intermediate between Homalodontotherium (p. 412) and the following genus Torodon. The typical species of Nesodon was about as large as a Sheep, but nothing more is known of it than the teeth and portions of the skull.

Toxodon is an animal about the size of a Hippopotamus; it was first discovered by Darwin, and many specimens have since been found in Pleistocene deposits near Buenos Ayres, and described by Owen, Gervais, and Burmeister. The teeth consist of large incisors, very small lower canines, and strongly curved molars, all with persistent roots, the formula being apparently i, c {, p †, m } = 38.

The cranial characters exhibit a combination of those found in both Perissodactyles and Artiodactyles, but the form of the hinder part of the palate and the absence of an alisphenoid canal belong to the latter; and the tympanic, firmly fixed in between the squamosal and the exoccipital, ankylosed to both, and forming the floor of a long upward-directed meatus auditorius, is so exactly like that of the Suina that it is difficult to believe it does not indicate some real affinity to that group. These characters seem to outweigh in importance those by which some zoologists have linked Torodon to the Perissodactyla, and the absence of the third trochanter and the articulation of the fibula with the calcaneum tell in the same direction. According to the recent observations of Ameghino the hind feet were certainly tridactylous, and the front feet probably so. The earlier allied genera Protoxodon and Adinotherium are definitely known to have tridactylous front and hind feet, which conform to the Perissodactylate type, the bones of the proximal and distal rows of the carpus interlocking. Acrotherium, which has similar feet, differs from all other Ungulates, and indeed from all Eutherians except some individuals of the existing carnivorous genus Otocyon, in having eight cheek-teeth, five of which have been reckoned as premolars.

Typotherium.-Typotherium (Fig. 192), also called Mesotherium, from the same locality as Toxodon, was an animal rather larger than

a Capybara, and of much the same general appearance. Its skeleton is completely known, and shows a singular combination of characters, resembling Toxodon or a generalised Ungulate on the one hand, and the Rodents, especially the Leporida, on the other. In the presence of clavicles it differs from all known Ungulates, and in having two pairs of lower incisors from all Rodents. The teeth are i, c8, p, m3 = 24.

From the Tertiaries of various parts of South America a number of forms more or less closely allied to Toxodon and Typotherium have been recently described, but as many of them are very imperfectly known, and there is much doubt as to their generic position, it will be unnecessary to refer to them further.

It will thus be seen that, although our knowledge of many of these forms is still very limited, we may trace among them a curious chain of affinities, which would seem to unite the Ungulates on the one hand with the Rodents on the other; but further materials are required before we can establish with certainty so important a relationship, one which, if true, would alter materially some of the prevailing views upon the classification of mammals.

Group TILLODONTIA.

Here may be noticed a remarkable group of animals, called by Marsh, Tillodontia, the remains of which are found abundantly in

the Lower and Middle Eocene beds of North America. They seem to combine the characters of the Ungulata, Rodentia, and Carnivora. In the genus Tillotherium of Marsh (probably identical with the previously described Anchippodus of Leidy) the skull (Fig. 193) resembled that of the Bears, but the molar teeth were of the Ungulate type, while the large incisors were very similar to those of the Rodents. The dental formula is i, c, p, m3. The first pair of incisors

was very small; the upper molars were tritubercular, while the lower ones had crescentoid ridges as in Palæotherium. The skeleton resembled that of the Carnivores, but the scaphoid and lunar bones were distinct, and there was a third trochanter on the femur. The feet were plantigrade, and each had five digits, all with long pointed claws. In the allied genus Stylinodon all the teeth were rootless. Some forms were as large as a Tapir.

These, with other more or less closely allied animals, such as Calamodon and Psittacotherium, constituting a group called Tæniodonta, are included by Cope in his large order Bunotheria, to which also the existing Insectivora are referred. The dentition of some of these forms makes a remarkable approximation towards a Rodent type, while it has been suggested that there are also signs of remote Edentate affinities. The constantly increasing knowledge of these annectant forms adds to the difficulty so often referred to in this work of establishing anything like a definite classification of the heterodont mammals. An incisor tooth from the Swiss Eocene has recently been referred to Calamodon.

Bibliography of Ungulata.-In addition to the works and memoirs mentioned under the different sections of the order, the following may be referred to:W. Kowalevsky, "Monographie des genus Anthracotherium," Palæontographica 1873; Id. "Sur l'Anchitherium aurelianense et sur l'histoire paléontologique des Chevaux," Mém. de l'Acad. Imp. des Sciences de St. Pétersbourg, 1873; Id. "On the Osteology of the Hyopotamidæ," Philosophical Transactions, 1873; L. Rütimeyer, "Versuch einer natürlichen Geschichte des Rindes,” etc., Noue Denks. der allgem. Schweiz. Gesellsch. für Naturwissenschaften, 1867; Id. “Die Rinder der Tertiär-Epoche," Abhand. der Schweiz. Paläont. Gesellsch. 1877 and 1878; Id. "Beiträge zu einer Natürliche Geschichte der Hirsche," ibid. 1880-1881; C. J. Forsyth-Major, "Beiträge zur Geschichte der Fossilen Pferde," ibid. 1880; M. Schlosser, "Beiträge zur Kenntniss der Stammesgeschichte der Hufthiere und Versuch einer Systematik der Paar-und Unpaarhufer," Morph. Jahrb. 1886; E. D. Cope, "The Perissodactyla," Amer. Natural. 1887; M. Pavlow, “Études sur l'histoire paléontologique des Ongulés," Bull. Soc. Imp. Naturalistes Moscow, 1887-1890. W. B. Scott and H. F. Osborn, "The Mammalia of the Uinta Formation," Trans. Amer. Phil. Soc. vol. xvi. (1889).