In attempting to trace the evolutionary history of the various mammalian groups, it is necessary to bear in mind the inevitable limitations of work of this kind. Speaking of plants, Dr. D. H. Scott says: “Our ideas of the course of descent must of necessity be diagrammatic; the process, as it actually went on, during ages of inconceivable duration, was doubtless infinitely too complex for the mind to grasp, even were the whole evidence lying open before us. We see an illustration, on a small scale, of the complexity of the problem, in the case of domesticated forms, evolved under the influence of man. Though we know that our cultivated plants, for instance, have been developed from wild species within the human period, and often within quite recent years, yet nothing is more difficult than to trace, in any given instance, the true history of a field-crop or garden plant, or even, in many cases, to fix its origin with certainty.”[5] With some mammalian groups the task, though difficult enough, is not so hopeless, because of more complete records, yet in dealing with mammals a very troublesome complication is introduced by the existence within the families, and even within the genera, of two or more parallel phyla, or genetic series. Without complete and perfect material it is impossible to make sure that we are not confusing the different phyla with one another and placing in one series species and genera that properly belong in a different one. Thus, Osborn distinguishes no less than seven such phyla among the true rhinoceroses of the Old and New Worlds, which long followed parallel, but quite independent, courses of development, and five phyla among the American horses. While these phyla add so much to the difficulty of working out the genealogical series, it is possible to simplify the problem and treat it in a broad and comprehensive manner that will sufficiently establish the essential steps of change.

In external appearance and general proportions the different families of existing perissodactyls have very little in common; that tapirs and rhinoceroses should be related is not surprising, but the horses would seem to be as far removed from both of the former as possible. Why, then, should they be included in the same order? A study of the skeleton, however, reveals the community of structure which obtains between the three families, a community which removes them widely from all other hoofed mammals. In all existing perissodactyls, though not in most of the Eocene genera, all the premolars, except the first, have the size and pattern of the molars. The foramina of the skull, or perforations by which blood-vessels and nerves enter and leave the cranium, are arranged in a way characteristic of the order and different from that seen in other hoofed mammals. The femur always has the third trochanter. The number of digits in each foot is usually odd, 1, 3 or 5, but four-toed forms occur, as the tapirs, which have four toes in the front foot, three in the hind; the important character is that the median plane of the foot bisects the third digit, which is symmetrical. The third and fourth, each asymmetrical, together form a symmetrical pair. Especially characteristic is the form of the astragalus and calcaneum (ankle and heel bones); the astragalus has but a single, deeply grooved and pulley-like surface, that for the tibia, the lower end is nearly flat and rests almost entirely upon the navicular, covering but little of the cuboid (see Figs. 146, 148). The calcaneum does not articulate with the fibula and its lower end is broad and covers most of the cuboid.

While the foregoing list includes the most important of the structural features which are common to all perissodactyls and differentiate them from other hoofed animals, there are many others which it is needless to enumerate.

The subjoined table gives the families and principal genera of the American Perissodactyla; extinct groups are marked †.

The earliest perissodactyls of which we have any knowledge are found in the older part of the lower Eocene (Wasatch stage) of Europe and North America, into which they must have migrated from some other region yet unknown, for no probable ancestors of the group are found in the Paleocene of either continent.

I. Suborder Chelodactyla. Normal Perissodactyla.

1. Equidæ. Horses

In order to make intelligible the evolutionary changes which have led up to the modern horses, it will be necessary to say something concerning the dental and skeletal features which characterize these animals. Using the term horses in a broad sense to include all the existing members of the family Equidæ, true horses, asses, zebras and quaggas, we find a greater uniformity in the skeleton and teeth than would be expected from the external appearance. The differences in appearance are, however, largely due to colouring, growth of mane and tail and the size of the ears, which leave no record in the skeleton.

The teeth (Figs. 45, p. 95; 154, p. 306) are extremely high-crowned, or hypsodont, and do not form roots till an advanced age; the incisors have a deep, enamel-lined pit, the “mark” in the centre of the grinding surface; the first premolar in each jaw is very small and of no functional importance; the other premolars have the same pattern as the molars, which is excessively complex in the arrangement of the enamel ridges and the areas of dentine and cement.

The skull (Fig. 154, p. 306) is long, especially the facial portion, the eye-socket (orbit) being shifted behind the teeth, which otherwise, on account of their great height, would press upon the eye itself; the orbit is completely encircled in bone. The lower jaw is deep vertically and the ascending ramus (see p. 66) very high, on account of the hypsodont character of the teeth, which thus necessitates a remodelling of the skull in several respects. The neck is long, each of its seven vertebræ being elongate; except in the atlas and axis, the anterior face of each centrum is strongly convex and the posterior of all except the atlas is deeply concave; the odontoid process of the axis (see p. 71) is spout-shaped, concave on the upper and convex on the lower side, lodging and protecting the spinal cord. The spines of the anterior dorsal vertebræ are very high, making a low hump at the withers between the shoulder-blades; the trunk-vertebræ are so arranged as to make the back almost straight and horizontal. The limbs and especially the feet are very long. The two bones of the fore-arm, the ulna and radius, are coössified into a single piece (Fig. 30, p. 81), but the limits of each are still plainly to be seen, especially in a young animal; and it is evident that the ulna is greatly reduced in size and has lost its middle portion, while all the weight is borne by the radius. Similarly, in the hind leg the enlarged tibia, or shin-bone, alone supports the weight; and only the two ends of the fibula are preserved (Fig. 38, p. 87), and these are indistinguishably fused with the tibia in the adult animal, but may be made out in the colt. The thigh-bone has a very characteristic shape, which is difficult to describe without an undue use of technical terms, but the unusual prominence of the great trochanter (Fig. 35, p. 85) and of the rotular groove is an important factor in producing this appearance.

The very long and slender feet are so raised from the ground that the animal walks upon the very tips of the toes, the wrist being what horsemen call the “knee” and the heel is the “hock,” and the gait is thoroughly unguligrade. Each foot has but a single functional toe, the third or middle one of the primitive five-toed foot; and, as this toe has to carry the whole weight supported by its leg, it is necessarily much larger than in animals which distribute the weight among several digits. The horses are therefore said to be monodactyl, or single-toed, but the term is not strictly accurate, for on each side of the functional digit is a rudimentary or vestigial one, the 2d and 4th of the original five. These rudimentary digits, which are not visible externally, have no phalanges and are merely “splint-bones,” metapodials (see p. 90) which have very slender shafts and end below in blunt points. The single functional metapodial has encircling its lower articular end a prominent ridge or keel, which fits into a corresponding groove on the upper end of the first phalanx and serves to prevent lateral dislocation. In most mammals this keel is merely a projection from the lower articular surface and is confined to the posterior side, so as not to be visible from the front. The terminal or ungual phalanx is much enlarged to carry the great weight which it supports and is enclosed in the characteristic hoof, unlike that of any other mammal.

In brief, the whole structure of the horses is pre-eminently adapted to swift running; they are admirable “cursorial machines,” as they have been called, and every part of the skeleton has been modified and specialized to that end; the narrow, rigid hoofs fit them for walking on firm ground and they speedily are made helpless in quicksand or bog. Did we know nothing of their mode of life, we might confidently infer from their teeth that the horses were grazers, feeding principally upon grass. A long-legged, grazing animal must needs have a neck of sufficient length to enable the mouth to reach the ground easily, unless a long proboscis is developed; and so we shall find in the history of the horses that the elongation of the head and neck kept pace with the lengthening of the legs and feet.

Though it can hardly be doubted that the horses passed through most of their development in North America, yet the immediate ancestry of all the existing species must be sought in the Old World, none of the many Pleistocene species of the western hemisphere having left any descendants. In North America all of the known Pleistocene forms belonged to the genus Equus, but the True Horse, E. caballus, was not among them. The more abundant and important of these species have been sufficiently described in Chapter VII (p. 199); it need only be recalled that there were ten or more distinct forms, ranging in size from the great E. †giganteus of Texas to the minute E. †tau of Mexico, while the plains and forests were the feeding grounds of moderate-sized species, about 14 hands high.

In the latest Pliocene, and no doubt earlier, species of the modern genus Equus had already come into existence; and in association with these, at least in Florida, were the last survivors of the three-toed horses which were so characteristic of the early Pliocene and the Miocene. However, little is known about those earliest recorded American species of Equus, for the material so far obtained is very fragmentary. In the absence of any richly fossiliferous beds of the upper Pliocene generally, there is a painfully felt hiatus in the genealogy of the horses; and it is impossible to say, from present knowledge, whether all of the many species of horses which inhabited North America in the Pleistocene were autochthonous, derived from a purely American ancestry, or how large a proportion of them were migrants from the Old World, coming in when so many of the Pleistocene immigrants of other groups arrived. It is even possible, though not in the least likely, that all of the native American stocks became extinct in the upper Pliocene and that the Pleistocene species were all immigrants from the eastern hemisphere, or the slightly modified descendants of such immigrants; but, on the other hand, it is altogether probable that some of these numerous species were intruders. Unfortunately we are in no position yet to distinguish the native from the foreign stocks.

In the middle Pliocene, which also has preserved but a meagre and scanty record of its mammalian life, we again meet with horses in relative abundance, but of a far more primitive type. They are still incompletely known, but it is clear that they belonged to three parallel series, or phyla, of three-toed grazing horses, with teeth which, though high-crowned, had not attained to the extreme degree of hypsodontism seen in the species of Equus and had a somewhat less complex pattern of the grinding surface, though distinctly foreshadowing the modern degree of complication. One of the genera (†Pliohippus) was not improbably the ancestor of a very peculiar horse (†Hippidion) of the South American Pleistocene. These middle Pliocene genera were much smaller animals than the Pleistocene horses, aside from the pygmy species of the latter, of light and more deer-like proportions, and with three functional toes or digits. The median digit (3d of the original five) was much the largest and carried most of the weight, on hard ground practically all of it; the lateral digits (2d and 4th) which in existing horses are represented by the rudimentary metapodials, or “splints,” though much more slender than the median digit, yet had the complete number of parts and each carried a small hoof. Mere “dew-claws” as these lateral toes were, they may have been of service in helping to support the weight in mud or snow. In all parts of the skeleton there are little details which show that these species of the middle Pliocene were not so advanced and differentiated as are their modern successors, but it would be unprofitable to enumerate these details, which are of interest only to the anatomist.

In the lower Pliocene the horses were very much more numerous and varied than in the middle portion of the epoch. The same three genera of grazing animals, represented by less advanced and modernized species, are found; and, in addition, there was an interesting survival (†Merychippus) from the middle Miocene of an intermediate type, together with several species of browsing horses (†Parahippus and †Hypohippus). In these browsing forms the teeth were all low-crowned and early formed their roots, and the crowns were either without cement or with merely a thin film of it in the depressions of the grinding surface. The pattern of the grinding surface is so very much simpler than in the high-crowned, prismatic teeth of the grazers that it requires close analysis to detect the fundamental identity of plan. Such teeth imply that their possessors must have fed habitually upon a softer and less abrasive diet than grass, probably the leaves and soft shoots of trees and bushes and other succulent vegetable substances, very much in the fashion of existing deer, and must therefore have been chiefly inhabitants of the woods and groves and thickets along streams, as the grazing species were of the plains and open spaces. “This assemblage of the progressive and conservative types of horses was certainly one of the most distinctive features of Lower Pliocene time in North America” (Osborn).

In the upper Miocene very much the same conditions prevailed and, for the most part, the same genera of horses, with different and somewhat less advanced species, were found as in the lower Pliocene, so that no particular account of them is needed. In the middle Miocene, however, there was a change, the typically grazing horses being very rare or absent and those with intermediate forms of teeth taking their place. Evidently, it was about this time that the horses with more plastic organization and capable of readjustment to radically different conditions began to take to the grazing habit, while other phyla, less capable of advance, retained the ancient, low-crowned type of grinding teeth and, after persisting, as we have seen, into the lower Pliocene, became extinct before the middle of that epoch. It is of great interest to observe that in the genus (†Merychippus) intermediate between the browsing and grazing types, the milk-teeth retained the older and more primitive character of low crowns without covering of cement, while the permanent grinders had much higher, cement-covered and complex crowns. In the lower Miocene, the variety of horses was much diminished and all had the low-crowned, cement-free, browsing type of teeth. Reversing the statement, we see that in the middle and still more in the upper Miocene the primitive and more or less distinctly homogeneous phylum branched out into several series, like a tree, some of the branches continuing and further subdividing through the Pliocene and Pleistocene, while others, less progressive and less adaptable, underwent but little change and had died out before the middle Pliocene.

The Oligocene horses deserve more particular attention, for they were almost the half-way stage of development in the long backward ascent to the earliest known members of the family in the lower Eocene. We may pass over the John Day horses (†Miohippus), which were somewhat larger than those of the White River, but otherwise very like them, merely noting the presence of a slightly different genus (†Anchitherium) which was the probable ancestor of †Hypohippus and the other non-progressive types of the Miocene and Pliocene. The genus (†Mesohippus) which characterizes the White River, or lower Oligocene, was a group of species of different sizes, becoming smaller as we go back in time, the commonest one being considerably smaller than a sheep and differing more or less in all its parts from the horses of the upper Miocene and all subsequent formations. The teeth were very low-crowned and fitted only for the mastication of soft vegetable tissue; but it is of particular interest to observe the beginnings of the “mark” in the upper incisors in the form of a low enamel-ridge arising behind the cutting edge of the tooth; the lower incisors still had the simple chisel-like crowns of the more ancient genera; all the premolars, except the first, had already acquired the molar-pattern.

The skull resembled that of a very small modern horse, but with many differences of detail, the most obvious of which is the shallowness of the jaws, for depth was not needed to carry the very low-crowned teeth, and, for the same reason, the ascending ramus of the lower jaw was short. The face was relatively short and the eye-socket, which was incompletely surrounded by bone, was directly above the hindmost teeth; the cranium was proportionately large and capacious and the brain, as is shown by the cast, was richly convoluted. The neck was relatively far shorter than in the Miocene genera, the ball-and-socket joints between its successive vertebræ were less elaborated and the odontoid process of the axis was in the first stage of assuming the spout-like form, being semicylindrical, with convex lower and flat upper surface. The trunk was proportionately long and the back sloped forward, owing to the greater length of the hind legs. The limbs and feet were elongate and very slender, but the fore-arm bones are only partially coössified, and the ulna, though greatly attenuated, was still complete. The same is true of the bones of the lower leg; the shaft of the fibula was hardly more than a thread of bone, but its full length was preserved. In the fore foot there were three functional digits, the median one enlarged and supporting most of the weight, but its hoof was much thinner and flatter than in the corresponding digit in the Miocene and subsequent genera; the lateral digits touched the ground and were not entirely functionless and, in addition, there was a small splint, the rudiment of the fifth digit. The hind foot was three-toed, without splint.

The little Uinta horse (†Epihippus) is still very incompletely known, but gives us one point at least of greater primitiveness than the White River genus in that only the last two premolars had taken on the molar-pattern, the forward two being smaller and simpler. The known species of the Uinta genus was very much smaller than any of the White River forms and even smaller than some of those of the preceding Bridger formation; but it should be remembered that the Uinta has been but partially explored and much remains to be learned regarding its fauna.

The Bridger horses are fortunately much better known. There are several species of the genus †Orohippus, which form a connected and progressive series; and, though much smaller than the smallest and oldest of the White River forms, they were somewhat larger than the known representative of the Uinta, †Epihippus, but distinctly more primitive in all other respects. The incisors were simple cutting teeth, with no trace of even an incipient “mark,” and only one premolar in each jaw, the hindmost one, had taken on the molar-pattern. The orbit was farther forward in the skull and less enclosed behind than in †Mesohippus, the cranium narrower and less capacious; the neck was even shorter and the odontoid process of the axis still retained the primitive peg-like form. The limbs and feet were conspicuously shorter in proportion than those of the White River genus; the ulna and fibula were stouter and less reduced and entirely separate from the radius and tibia respectively. The front foot had four functional toes; the fifth digit, which in †Mesohippus had been reduced to a splint, was completely developed in the Bridger horses, but the hind foot was three-toed.

Passing over, for lack of space, the transitional forms of the Wind River, we come finally to the most ancient known horses, the Wasatch species comprised in the genus †Eohippus, the “Dawn Horse,” as its name signifies; these were little creatures ranging in size from a cat to a small fox. Despite an unmistakably equine look in the skeletons of these diminutive animals, it is only the long intermediate series of species and genera, together forming a closely linked chain, which we have traced back from the Pleistocene to the lower Eocene, that leads us to regard †Eohippus as the ancestral type of the horses. Were only the two ends of the chain known, he would be a daring speculator who should venture to connect them. In these little Wasatch horses we have the evidence of a still more ancient form with five fully developed toes in each foot, since the front foot had four functional digits and indication of a splint, and splints, as the whole history of the long series teaches, always are found to be functional digits in the ancestor; the hind foot had three toes and perhaps two splints. This preceding form is hardly to be looked for in America or Europe; it will be found, if ever, in the region whence the great migration came. In all other respects, as well, †Eohippus was what we should expect the forerunner of the Wind River and Bridger horses to be. The premolars were all smaller and simpler than the molars and the latter in the upper jaw are particularly interesting, for they had no crests and ridges of enamel, but four principal conical cusps, arranged in two transverse pairs, and between the cusps of each pair was a tiny cuspule no bigger than the head of a pin. These cuspules were the first step in the formation of the transverse crests, which were destined to assume such importance in the subsequent members of the series. The neck was very short, the body long, with curved or arched back, the limbs and feet short, and the hind limb much longer than the fore, making the relative proportions of the various parts of the skeleton very different from what they afterwards became.

Reviewing this marvellous history of steady and long-continued change, beginning with the most ancient genus, †Eohippus, the following modifications may be noted:

(1) There was a nearly constant, if somewhat fluctuating, increase in size, leading by slow gradations from the diminutive horses of the lower Eocene to the great animals of the Pleistocene.

(2) The molar teeth, originally made up of conical cusps, changed to a highly complex pattern of crests and ridges, and the premolars, one by one, assumed the size and pattern of the molars; the low-crowned, rooted and cement-free teeth, fitted only for browsing, became very high-crowned, prismatic and cement-covered, admirably adapted to grazing. Beginning in the upper incisors of the White River †Mesohippus, the “mark” became established as an enamel-lined pit, growing in depth as the teeth increased their length.

(3) The face grew relatively longer, the eye-socket being shifted behind the teeth and becoming completely encircled in bone, and the jaws were greatly increased in depth to accommodate the very long teeth.

(4) The short neck was greatly elongated and the individual vertebræ modified so as to give flexibility with no loss of strength. The primitive peg-like odontoid process of the axis became first semicylindrical and then spout-shaped.

(5) The arched back was straightened and the neural spines, especially of the anterior dorsals, elongated.

(6) The limbs grew relatively much longer; the bones of the fore-arm and lower leg were fused together, the one on the inner side (radius and tibia) enlarging to carry the entire weight and the external one (ulna and fibula) becoming more or less atrophied.

(7) The feet were much elongated and the median (3d) digit of each gradually enlarged until it carried the whole weight, at the same time modifying the shape of the hoof so as to fit it to be the sole support of the body. The other toes gradually dwindled and became functionless, though often retained as splints. The first digit (pollex and hallux) was first lost, then the fifth, then the second and fourth were reduced to dew-claws and finally to splints. Thus the pentadactyl horses of the lower Eocene were transformed into the monodactyl species of the Pliocene and Pleistocene.

In South America the story of the horses was a brief one, for they were among the immigrants from the north and did not reach the southern continent till the Pliocene, probably late in that epoch, for none of the three-toed genera have been found in South America. So far as known, these southern equines were small and medium sized animals, with large heads, relatively short feet and somewhat ass-like proportions. There were two well-defined groups of these animals: (1) species of the genus Equus, which thus, at one time or another, inhabited every one of the continents, Australia excepted; (2) three genera peculiar to South America and developed there from northern ancestors, probably †Pliohippus. Two of these genera (†Hippidion and †Onohippidium) displayed curious modifications of the nasal bones, which were extremely slender and attached to the skull only at their hinder ends, instead of being, as is normally the case, supported for nearly their whole length by lateral articulation with other bones. What can have been the significance and function of these excessively slender, splint-like nasals, it is difficult to conjecture. The third genus (†Hyperhippidium) was a small mountain-horse, with extremely short feet, which were well adapted to climbing.

This is the merest outline sketch of a most wonderful series of gradual and progressive modifications, a sketch that might readily be expanded into a volume, were all the details filled in. While each set of organs, teeth, skull, neck, body, limbs and feet, might appear to advance independently of the others, in reality there was no such independence, for at every stage of the progression all the parts must have been so coördinated into a harmonious whole, that the animal could thrive and hold its own in the stress of competition. Could we but discover all the facts of environment, on the one hand, and organization, on the other, we should doubtless learn that the little †Eohippus was as exquisitely fitted to its place in the Wasatch world, as are the horses, asses and zebras of the present day to theirs. It was the response to changing needs, whether of food, climate, disease or competition, that was the main factor of development.

2. †Titanotheriidæ. †Titanotheres

This family, all of whose members vanished from the earth ages ago, was a comparatively short-lived group and nearly the whole of its recorded history was enacted in North America; only a few belated stragglers reached the eastern hemisphere, though the family may, nevertheless, have originated there.

In the lowest of the three substages of the White River Oligocene the most conspicuous and abundant fossils are the †titanotheres, the latest members of which were huge animals of almost elephantine proportions. They belonged to four parallel, or rather slightly divergent, phyla, differing in the development of the horns, in the shape of the head and in the relative length and massiveness of the limbs. The teeth were all low-crowned, or brachyodont, the canines much too small to have been of any service as weapons and the incisors had curious little, button-shaped crowns, which can have had little or no functional importance, since they show hardly any wear, even in old animals. With such front teeth, a prehensile lip and long tongue would seem to have been necessary for gathering and taking in food.

The †titanotheres were one of two perissodactyl families in which the premolars never became so large and complex as the molars. The upper molars had a longitudinal outer wall, composed of two deeply concave cusps, and two internal conical cusps, but no transverse ridges; the lower molars were composed of two crescents, one behind the other, a pattern which was very widely distributed among the early and primitive artiodactyls and perissodactyls.

The so-called “horns” were not strictly such, but a pair of bony protuberances from the front of the skull and, from their shape, could hardly have been sheathed in horn. The long, immensely broad and massive head resembled that of some fantastic rhinoceros, as did also the body and limbs. The brain was quite absurdly small, the cavity for it, lost in the huge skull, would hardly contain the fist of an ordinary man; these great beasts must have been incredibly dull and stupid, surpassing even the modern rhinoceroses in this respect. As is generally true in mammals which have horns, antlers, or similar weapons borne upon the skull, or very large tusks, the bones of the brain-case were made enormously thick and yet lightened by an intricate system of communicating cavities or “sinuses,” separated by many bony braces and supports connecting the inner and outer denser layers, which form the surfaces of the bones. In this way the skull is made strong enough without any proportionate increase of weight to endure the severe shock of impact, when the horns or tusks are made use of. The principle is the same as the engineer employs in designing a steel truss-bridge. The upper profile of the head was deeply concave, just as it is in those rhinoceroses which are armed with nasal horns.

The neck was of moderate length and the body, as indicated by the long, arched ribs and the greatly expanded hip-bones, was extremely bulky and massive. The spines of the anterior dorsal vertebræ were excessively long, forming a great hump at the withers. The limbs and feet were columnar, like those of an elephant; the feet were supported on a great pad, while the hoofs were mere excrescences on the periphery of the foot. The bones of the fore-arm were entirely separate and the ulna was very stout; in the lower leg also the bones were not coössified, but the fibula was but moderately heavy. This is a sharp contrast to the arrangement found in the horses and in those hoofed animals generally which are swift runners and have slender, elongate limbs and feet, such as deer, antelopes, camels, etc. Heavy, slow-moving animals, like elephants, tapirs, rhinoceroses, etc., almost always have separate fore-arm and leg-bones and generally a heavy ulna. The number of digits was four in the front foot and three in the hind. The genera differed in the proportions of limbs and feet, one having them longer and less ponderous than another, and, no doubt, the former was of swifter gait.

At a certain level in the White River beds the †titanotheres abruptly cease, disappearing with what seems like startling suddenness. In all probability, however, the extinction was more gradual and its apparent abruptness was due, partly at least, to the break in the deposition of the beds, which is very obvious. Such a break, or “unconformity,” as geologists call it, almost always implies an unrecorded lapse of time, which may have been very long. However it came about, gradually or suddenly, the extinction of these great animals is difficult to explain; no Carnivora of the time could have been formidable enemies and they had no rivals in their own walk of life. Their stupidity may have been a factor, but it seems more likely that the onset of some new infectious disease, perhaps imported by incoming migrants from the eastern hemisphere, gave the coup de grace. In the lower substage, beneath the unconformity, where the remains of †titanotheres are so abundant, successive changes may be observed. The species with great “horns,” rounded, flattened or triangular, are confined to the upper levels; in the middle section other species, somewhat smaller and with shorter “horns,” are found, while in the bottom levels the animals are much smaller and have still smaller “horns.”

The Uinta †titanotheres were much more numerous and varied than those of the White River; in the upper part of these beds are found two genera (†Diplacodon and †Protitanotherium) which already had quite prominent bony protuberances on the nose; their canines were large enough to be of value as weapons and the incisors were well developed and functional. Evidently, there was a change here in the manner of feeding, the front teeth were used for cropping and browsing, a function which in the White River members of the family must have been largely taken over by the lips and tongue, while the growth of the horn-like protuberances on the skull rendered the canines superfluous as weapons. This latter change is one which recurs frequently in different phyla of the hoofed animals, in which the earlier and more primitive members had canine tusks, and the later, more advanced representatives developed horns, the tusks diminishing as the horns increased. While this rule is a general one, it is not entirely without exceptions.

In the lower Uinta and in the Bridger the †titanotheres were extremely abundant and numerically they are the commonest of all fossils in those beds; no less than five series or phyla may be distinguished, three of them being added in the upper Bridger. The differences between the phyla, however, principally concern the forms of the teeth and the shape of the skull; in some the head was short and broad, in others long and narrow, and in others again of medium proportions; some had broad and extremely low-crowned grinding teeth, which in others were higher and more erect. But these are matters of minor detail, useful as they are in pointing the way to a proper arrangement of the various species; in essentials, the forms all agreed and constituted several series of closely allied genera. Comparing these Bridger animals with the great †titanotheres of the lower White River, the first and most obvious difference that strikes the observer is the very much smaller size of the more ancient types. With some variation in this respect, hardly any of the Bridger species exceeded a modern tapir in stature and very much resembled one in proportions. The canine teeth were tusks as large as those of a bear and must have been very effective weapons; the molar-pattern was identical with that found in the great Oligocene beasts, but the premolars were simpler and relatively smaller.