# Cambridge Natural History Mammalia/Chapter X

CHAPTER X
UNGULATA (continued)—PERISSODACTYLA (ODD-TOED UNGULATES)—LITOPTERNA
Sub-Order 8. PERISSODACTYLA
Fig. 121.—Bones of the manus A, of Tapir (Tapirus indicus). × $^1$⁄$_5$. B, of Rhinoceros (Rhinoceros sumatrensis). × $^1$⁄$_5$. C, of Horse (Equus caballus). × ⅛. c, Cuneiform; l, lunar; m, magnum; p, pisiform; R, radius; s, scaphoid; td, trapezoid; tm, trapezium; u, unciform; U, ulna; II-V, second to fifth digits; V in B, and II and IV in C, represented by rudimentary metacarpals. (From Flower's Osteology.)
These Ungulates derive their name, which is that given by the late Sir Richard Owen, from the fact that the middle digit of the hand and foot is pre-eminent. As will be seen from Fig. 121, the axis of ​the limb passes through the third finger, which is larger than any of the others, and is symmetrical in itself. In this the present group contrasts with the Artiodactyla, where the axis is not "mesaxonic," but where there are two digits, on either side of the axis, which are symmetrical with each other. This arrangement of the limbs is highly characteristic, but appears to be not quite universal. In the Titanotheres, which form a group of the Perissodactyles, the fore-limbs are not quite accurately mesaxonic. Nor on the other hand can all Ungulates which show the Perissodactyle condition be safely included in the present group. The ancient Condylarthra and the Litopterna show precisely the same state of affairs. But other features in their organisation lead to their separation from the Perissodactyles, of which, however, the Condylarthra are probably ancestors. The Litopterna on the other hand, which possess even one-toed members like Equus, are believed to represent a case of parallelism in development. The number of functional toes varies from four to one. In the ankle joint the astragalus either does not, or does only to a comparatively slight extent, articulate with the cuboid as well as with the navicular bone. Moreover the fibula when present does not as a rule articulate with the calcaneum. In the opposed group of Artiodactyles the precise reverse of these conditions obtains. It is usually stated as part of the definition of this group that they do not possess horns of the type of those met with in the Cervicornia and Cavicornia. But the strong bony bosses on the skull of many Titanotheres, so curiously reminiscent of those of the not nearly related Dinoceras and Protoceras, may well have supported horns of the Ox and Antelope pattern.
Fig. 122.—Bones of the manus of Camel (Camelus bactrianus). × ⅛. c, Cuneiform; l, lunar; m, magnum; R, radius; s, scaphoid; td, trapezoid; u, unciform. (From Flower's Osteology.)
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.
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
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.)
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 ​we are to continue the plan of defining the various families of Mammalia.
The genus Equus contains not only the Horse, but the Asses and Zebras. The genus is to be distinguished as regards external characters by the following features:—The body is thickly clothed with hair; there is a more or less bushy tail and mane; the colours are apt to be disposed in stripes of black or blackish upon a yellowish brown ground; this is of course best seen in the Zebras, but the wild Asses also have some traces of it, if only in the single cross-bar of the African Wild Ass, and it is even "reversionary" in the domestic Horse at times. There are no horns upon the forehead or elsewhere; the fore-limbs or both pairs have a callous pad upon the inside, which is possibly to be looked upon as an aborted gland, possibly originally of use as secreting some odorous substance calculated to enable strayed members of the herd to regain their companions. The terminal phalanx of each of the (functionally) single digits is enclosed in a large horny hoof.
The main internal features of structure which divide this genus of Perissodactyles from the Rhinoceros or the Tapir, or from both, are: the existence of strong incisors, three on each side of each jaw; there are canines, but these are small and do not always persist in the full-grown mare. They are popularly known as "tusks" or "tushes." The first of the four premolars (the "wolf tooth") is small and quite rudimentary; it is often absent. As there are three molars, the present genus has the "typical" number of the Eutherian dentition, i.e. forty-four. In the skull the orbit is—as it is not in Tapirs and Rhinoceroses—completely encircled by bone. There is but one functional finger and toe on each hand (Fig. 121 C) and foot; the second and third digits are represented by mere splints, one of which may as an abnormality be enlarged, and reach nearly as far as the well-developed digit. There are even occasionally traces of digit number two.
The Horse, E. caballus, is to be distinguished from its congeners by the small callosities on the hind-limbs which it possesses in addition to the larger ones on the fore-limbs. The hairy covering of the tail is more abundant, as is also the mane. The head too is proportionately smaller, and the general contour ​more graceful. Though Zebra markings are not usual upon E. caballus, there are plenty of examples of—what we may perhaps in this case term—a "reversion" to a striped state. The celebrated "Lord Morton's mare," whose portrait hangs in the Royal College of Surgeons, is an interesting case of this. It was as a matter of fact thought to be an example of that rather doubtfully-occurring phenomenon, "telegony." Its history is briefly this. The animal was the offspring of a mare that had previously produced to a male Quagga a hybrid foal. Afterwards a second foal was produced by the same mare to an Arab sire. This foal, the one in question, was striped, and hence was thought to be an example of male prepotency. But instances are known of unquestioned Horses which show the same stripes, such as a Norway pony which had not even seen a Zebra!
A last remnant of the naked palm of the hand and sole of the foot is left in the shape of a small bare area, smaller in the Horse than in the Asses, known technically as the "ergot," the term being that of the French veterinarians. As already mentioned, the Horse differs from the Asses and Zebras in the fact that the hind-limbs have callosities on the inner side. They are known as "chestnuts," and their nature has been much disputed. It has been suggested that they are the last rudiment of a vanished toe; but in all probability they are, as already suggested, traces of glandular structures, which are common, upon the limbs in many animals (see above, p. 12).
It is a singular fact that there are apparently no wild Horses of this species. The case is curiously analogous to that of the Camel, which also is only known as feral or domesticated. Why the Horse should have become extinct as a wild animal, considering that when it does run wild it can thrive abundantly, is impossible to understand. Sir W. Flower thinks that "the nearest approach to truly wild horses existing at present are the so-called Tarpans, which occur in the Steppe country north of the sea of Azov between the river Dnieper and the Caspian. They are described as being of small size, dun colour, with short mane and rounded obtuse nose." But he adds that there is no evidence to prove whether they are really wild. In favour, however, of their possibly being wild and indigenous European Horses, may be ​mentioned the fact that their general build and appearance is highly suggestive of the wild Horses sketched by primitive man upon ivory.
A really wild Horse, and possibly the ancestor of the European domestic Horse, is E. przewalskii of the sandy deserts of Central Asia. This animal has been believed to be a mule between the Wild Ass and a feral Horse; but if a distinct form, and probability seems to urge that view, it is interesting as breaking down the distinctions between Horses and Asses. The species possesses the four callosities of the Horse, but has a poorer mane and an asinine tail.
There is no question that the Horse has been a domestic animal for very many centuries. Hieroglyphics appear to show that the Egyptians had not originally domesticated the Horse; it seems to have been first introduced among them by the Hyksos or Shepherd Kings. Whatever the date may be, it is certain that considerably anterior to the Egyptians the Assyrians and Phoenicians possessed Horses. In Western Europe the date of the introduction of the Horse seems to have been during the bronze epoch. Lord Avebury has pointed out that out of eighteen cases of graves in which the remains of Horse were found, twelve contained metal implements, i.e. 66 per cent. This does not of course prove that the Horse was domesticated at that period, but it throws doubt upon the earlier occurrence of the Horse in abundance. The Horse, however, does occur on the Continent associated with the remains of man during the Quaternary period.
Messrs. Cuyer and Alix enumerate between fifty and sixty domesticated races of Horse, not counting the supposed wild varieties which have been already referred to. These may be further subdivided; for instance, under the race "pony" we may distinguish the Irish, Scotch, and Shetland varieties, all of which, however, according to Sanson, have originated in Ireland. They are used, remark the authors above quoted, "par les jeunes filles des lords pour leurs promenades." The Arab, the Barb, the Suffolk Punch, etc., are among the numerous races of domestic Horses, into which to enter properly would require another volume, and that of large size. ​
The Asses and Zebras differ from the Horse in the characters mentioned under the description of Equus caballus. In addition to these may be pointed out a feature to which attention has been directed by Mr. Tegetmeier. According to him the period of gestation in the Horse is only eleven months; in the others more than twelve.
Fig. 125.—Asiatic Wild Ass. Equus onager. × $^1$⁄$_{20}$.
Opinions as to the number of species of Asses differ. On the most liberal estimate there are three Asiatic and two African species. The best known of the Asiatic Wild Asses is the Onager, E. onager. It is of a uniform yellowish, "desert" colour, with a dark stripe along the middle of the back, and is found in Persia, the Punjab, and the country of Cutch. The creature is of great swiftness; it has been stated to be untameable, but Mr. Tegetmeier makes the absolutely opposite statement that the Ass occasionally "becomes so tame as to be troublesome"! The Syrian Wild Ass, E. hemippus, hardly, if at all, differs from this.
The Kiang, E. hemionus, seems to have more claims to distinctness. In the first place it has a more limited and a ​different distribution; it is confined to the high tablelands of Thibet at an elevation of 15,000 feet and upwards. In correlation with this habitat it has a thicker and more "furry" coat, which is, moreover, of a darker shade than that of the Onager. This coat is shed in the summer, and replaced by one which is not so dark in hue. It is an interesting fact that the African Wild Asses approach to the zebra type in having at least traces of stripings. There are apparently two species. The best known,
Fig. 126.—Nubian Wild Ass. Equus africanus. × $^1$⁄$_{20}$.
the Nubian Ass, E. africanus, is probably the parent of the domestic donkey. It has a dorsal longitudinal stripe, and another across the shoulder—in legend the marks of the Saviour. The matter of the name of this Ass seems difficult to decide. It has been called also E. asinus and E. taeniopus. It has been observed that this animal has a great aversion to water, and a delight in rolling in the dust—both of which characteristics argue a desert existence. But on the other hand the Kiang will plunge boldly into streams, yet it would seem to be the descendant of a purely desert form. The Ass is a longer-lived ​animal than the Horse. Mr. Tegetmeier calls attention to a donkey living in 1893 which had been ridden fifty-five years previously. The Horse, on the other hand, lives not much more than twenty-five years.
A second species of African Wild Ass, E. somalicus, is distinguished by its greyer colour, by the absence of the shoulder stripe, by the very faint development of the dorsal stripe, and by the presence of numerous cross stripes upon the legs. It has, too, smaller ears, and a longer and more flowing mane. Mr. Lort Phillips, an experienced naturalist and traveller, saw a herd of these Wild Asses in Somaliland, which he regarded as being of quite a new species. A living example in the Zoological Society's Gardens led Mr. Sclater to an identical conclusion, which was supported, as he pointed out, by the fact that this Ass has a different range to the African or Nubian Wild Ass.
Of the Zebras three species are usually allowed; these are E. zebra, the "Mountain" or "Common" Zebra, E. burchelli, E. grevyi, as well as E. quagga. Professor Ewart thinks that the Common Zebra, Burchell's, and the Quagga are not very distinctly marked off from each other. No one, however, has any doubt of the distinctness of E. grevyi. This latter differs from the rest in its larger size, in the large head and ears, and in the marked hairiness of the ears. It would seem to be a primitive type of Zebra, if the fact that the occasional reversion of hybrids to a parent form be allowed; for Professor Ewart found a cross-bred Zebra to present several characteristics in the face-marking of this, the finest of the Zebra tribe. Only four specimens of E. grevyi have been exhibited alive in Europe—two in Paris, and two in the Zoological Society's Gardens in London. The latter were presented to Queen Victoria by King Menelek of Abyssinia. The species was named by Professor A. Milne-Edwards in honour of a late President of the French Republic, from an example also sent by King Menelek.
The Common Zebra has closer and darker stripes than Burchell's, but not quite so close as in E. grevyi. It has also a very characteristic arrangement of stripes on the withers in the form of a gridiron. This latter is wanting in both the other species. In E. grevyi, in fact, this part of the back is white. E. zebra has also a dewlap in front. E. burchelli has fewer and broader ​stripes, and between them lie in many cases shadow-stripes of a faint brown.
Fig. 127.—Burchell's Zebra. Equus burchelli. × $^1$⁄$_{20}$.
All these animals, and the Quagga too, are absolutely confined to Africa. Mr. R Crawshay, in describing what he considered to be a new variety, remarked upon the curiosity of E. burchelli. "They remain out in the sun on the plains all day long, not retiring into covert at all. They are then an intolerable nuisance to any one in pursuit of other game; indeed this may be said of them at all times. If once they notice you, they draw in and mob you in their curiosity—only, however, when one takes no interest in them, for when they fancy they are the object of the intruder's attention, no animals are more watchful and cunning in safeguarding themselves. If only their curiosity were manifested in silence it would not so much matter, but it vents itself in snorts and thundering stampedes, which puts every beast within earshot on the qui vive."
Whether Burchell's Zebra can be further subdivided into species or sub-species appears to be doubtful. Dr. Matschie considers that Equus boehmi may be regarded as a valid form, and in addition to this two sub-species, E. burchelli granti and ​E. burchelli selousi, have been proposed for what are at most local races. But it is at present far from certain whether their distribution favours this subdivision.
The Quagga was more striped than is sometimes represented in illustrations. According to Dr. Noack, from whose paper upon the animal I quote here, the transverse stripes reached back as far as the buttocks; they were, however, completely absent from the legs. The animal is, as every one knows, probably completely extinct. In the year 1836 it was still abundant; in 1864 the last specimen ever exhibited was received by the Zoological Society. Mr. W. L. Sclater thinks that it may have survived in the Orange River Colony as late as 1878, but admits that any certainty is difficult, as it was frequently confounded by the Boers with Burchell's Zebra. Its rarity is emphasised by the fact that it is not mentioned in the recent work of that most skilful of hunters, Mr. F. Selous. Gaudry places the Quagga nearest of all living Equidae to the Hipparion gracile of Pikermi.
Fossil Equidae.—The existing Equidae all belong to the genus Equus, though there are some who would (quite unnecessarily) divide off the Zebras as a genus Hippotigris. The genus Equus itself goes back in time to the Pliocene, during which epoch there lived in India E. sivalensis, the same species according to some with the E. stenonis of Europe. None of these species, Old World or New, are easily to be separated from E. caballus. But many names have been given to them. It is of course perfectly conceivable that they may have differed among themselves as much as do the existing Zebras and Asses, the separation of which would be hardly possible did we know their bones only. There are, however, extinct genera, undoubtedly related so closely to Equus as to be placed in the same family, though clearly separable as genera. Hipparion is one of these genera; its remains are known from Europe, Asia, and North Africa, from beds of Miocene and Pliocene times. A large number of different species have been described. It was a beast of about the size of a Zebra. The principal characters are that each foot has three toes, of which, however, the two side ones are smaller than the central toe. There is a marked round fossa on the maxillary bone, a feature shared by the South American Onohippidium. The pattern of ​the molar teeth is, too, a little different from that of Equus. Protohippus of the North American Pliocene is also three-toed, but the two additionally-developed toes are smaller than in Hipparion. Other forms are dealt with below in connexion with the ancestry of Perissodactyles. It is a curious fact about Hipparion, which is not now regarded as on the direct line of equine descent, that the edges of the enamel plates of the molars may show a complicated folding very like that presented by that clearly terminal form of Perissodactyle life, the gigantic Elasmotherium. This is indicative of high specialisation, which ended in extinction.
Ancestry of the Horses.—The Lophiodontidae and the Palaeotheriidae are two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a classificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relationship to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse. Equus, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse."
In the Eocene of both Europe and America are met with a number of forms from which we may start. Hyracotherium, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dentition with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder blade ​has a well-marked coracoid process. The radius and ulna are separate; so too are the tibia and fibula. Eohippus, belonging to the same sub-family, is slightly more primitive; for the hind-feet have a rudiment of digit I. Orohippus is a little nearer to the Horses in that the molar teeth have acquired a little further advance towards the equine type. Instead of the tubercles of the teeth remaining for the most part separate, they have fused into a set of ridges, of which, however, the pattern is less complex than in the modern Horses. In other respects Orohippus is much like Hyracotherium. Pachynolophus seems to be but a synonym.
The next stage is shown by Mesohippus, a Lower Miocene form, usually referred to the neighbourhood of Palaeotherium. It has nearly lost one of the toes of the fore-foot, a rudiment only remaining; the metapodials, at any rate of the fore-feet, seem to be slightly increased in length. The orbit is not encircled by bone, but there is a strong process from the frontal, which nearly meets the zygomatic arch.
Anchitherium, from the Upper Miocene, is not far removed in structure from the last-mentioned form; it is a trifle nearer the existing Horse in several points. The ulna is further reduced and fused with the radius below: the rudiment of digit V is still more rudimentary; the two lateral digits are smaller in proportion to the central one than they are in Mesohippus; the fibula is fused below with the tibia. From this form to Equus is a small series of steps, characterised by the still further reduction of all the digits except III, by the still further reduction of the already rudimentary ulna and fibula, and by the increasing depth of the molar teeth, which are of course, in Equus, hypselodont.
Another interesting conclusion may seem to follow when we consider the geographical range of the ancestral Horses. Hyracotherium and Pachynolophus occurred both in the Old and New World. From them may have arisen the Horses of both hemispheres. After that point there is a division. Mesohippus is American, and we get at Equus in that continent through Desmatippus and Protohippus. On the other hand there are no remains known of Mesohippus in Europe; and unless subsequent researches prove the existence of Mesohippus, we have to rely upon forms which are placed with Anchitherium and Hipparion.
It seems that in America the next genus in the direct line of equine descent to Mesohippus is Miohippus. It is smaller in ​size than Anchitherium, to be considered immediately. The odontoid process of the axis is just beginning to assume the characteristic spout-like shape of that of the existing Horse and many modern Ungulates. The median digit of both fore- and hind-limbs has become greatly enlarged as compared with the corresponding digit of earlier forms.
It is held, however, that Anchitherium is not on the direct line of descent either in America or in Europe, in both of which it occurs. Its teeth are in some respects less Horse-like than in some of the more ancient genera, to which the converse would be expected on the descent theory. Its hoofs are much elongated and flattened, a mark of specialisation and not appropriate to a creature holding an intermediate position in the equine series. Both the American (A. equinum) and the European species (A. aureliense) are of very large size, larger than its successors, and such "alternations in bulk are unlikely."
The genus Desmatippus of Professor Scott fills in the gap between Miohippus and Protohippus. The molars and premolars are brachyodont, but there is a thin deposit of cement in the tooth valleys, leading towards the more complete filling of these valleys with cement, which is found in Protohippus. This genus of Horses, of which there is at present but one species, D. crenidens, was three-toed, and "the lateral digits, so far as can be judged by fragmentary remains, were still fairly developed, and though much more reduced than in Miohippus, appear to be somewhat less so than in Protohippus."
To recapitulate, the following is the probable series of equines in America—Mesohippus, Miohippus, Desmatippus, Protohippus.
The development of the limbs of the Horse shows a most interesting series, of stages, which correspond in part to the ancestral forms which palaeontology seems to prove to be the line of the descent of our existing Equidae. This matter has recently been elucidated by Professor Ewart, who details the following facts and comparisons:—
In the youngest embryo (about 20 mm. in length) the humerus is somewhat curved, and considerably longer than the radius and carpus taken together. The first-named bone is shorter in the adult, and the proportions of that bone in the young as well as its curvature are suggestive of that ancient ​Ungulate Phenacodus (see p. 202). In the next stage (an embryo of 25 mm.) the humerus has slightly decreased in proportionate length, and has come to be more like that of Hipparion. In both of these embryos it should be noted that the ulna is complete and separate from the radius. In the second of the two it has more distinctly acquired the form which it will possess in the adult. The second metacarpal—one of the splint bones of the adult—is tipped with a small nodule of cartilage, which is clearly the representative of one or more of the phalanges belonging to that digit.
Fam. 2. Tapiridae.—The Tapirs may be distinguished from the Horse and from the Rhinoceros tribe by a few characters, which are as follows:—
The dentition is generally the full one of forty-four teeth. The premolars in the more ancient forms are unlike the molars, but like them in more recent forms. The molars of the upper jaw have two crests parallel and united by an outer crest. The fore-feet have four, the hind-feet three toes.
The family is fully as ancient as that of the Equidae, but the specialisation of the toes never advances so far. The modern representatives of the order are, so far as the feet are concerned, in the condition of very early representatives of the equine stock. Nor do the teeth of the Tapirs ever reach the complicated pattern of that presented by at least the modern Horses, or indeed of the Palaeotheres. Apart from this it is not an easy matter to distinguish accurately between these several families, including the Lophiodontidae, which, as already mentioned, is placed nearer to the Tapiridae than to the Palaeotheriidae. Indeed the differentiation of these two families, the Tapiridae and the Lophiodontidae, seems to be a matter of the greatest difficulty. The difficulty is well emphasised by the fact that naturalists disagree most profoundly as to the relations of various genera of extinct Tapir-like animals. For Mr. Lydekker the genus Lophiodon includes also the American genera Isectolophus and Systemodon, which are placed by Zittel in the sub-family Tapirinae as opposed to Lophiodontinae, which contains Lophiodon and Helaletes. The existing Tapirs can be differentiated from the existing Horses with great ease, as the following account of the existing genera will show.
The genus Tapirus is now met with only in South and ​Central America, and in the Malay Peninsula and the islands of Java and Sumatra. This animal is in many respects the most ancient of existing forms referable to the Perissodactyle order. It has four toes on the front-feet, though only three on the hind-feet. The number of teeth is 42—nearly the typical Eutherian number. The Tapirs are always moderately-sized animals, entirely covered with hair, and usually of a brownish-black colour.
Fig. 128.—American Tapir. Tapirus terrestris. × $^1$⁄$_{10}$.
The Malayan Tapir is, however, banded broadly with white—a single band; the young of the Tapir is spotted, and striped with white. The nose and upper lip conjoined are produced into a short trunk, precisely comparable with that of the Elephant. As in the Rhinoceros—and in this both contrast with the other existing Perissodactyle genus Equus—the temporal fossa is not separated from the orbit by bone. Of existing Tapirs there are at any rate T. terrestris, T. roulini (the "Tapir Pinchaque" of Cuvier), T. dowi and T. bairdi in America (the last two being sometimes separated into a distinct genus, Elasmognathus, on account of the prolongation of the ossified mesethmoid), and T. indicus in the East. The tapir, probably T. terrestris, is described by Buffon as "a dull and gloomy animal." It is certainly mainly nocturnal in habit. The name terrestris was given by Linnaeus, who placed it in the same genus as ​Hippopotamus amphibius; hence the epithet applied to the Tapir. But as a matter of fact it loves marshy neighbourhoods, and is in a way amphibious. This does not of course apply to the Andesian T. roulini, which inhabits the cordillera of Ecuador and Colombia. The distribution of existing Tapirs is, as is so often the case, restricted when compared with that of their extinct congeners and allies. In Europe the remains of the genus Tapirus are abundant from Pliocene strata, and its remains are there known from as far back as the Miocene. The genus is thus one of the very oldest forms of Mammalia at present inhabiting the earth.
Fig. 129.—Malayan Tapir. Tapirus indicus, young. × $^1$⁄$_{10}$. (From Nature.)
The Malayan Tapir is to be distinguished from the American (T. terrestris—the other species have not been dissected) by the greater development of the valvulae conniventes in the intestine, the absence of a moderator band in the heart, and the less elongated caecum, which is sacculated by only three bands, there being four in T. terrestris. The animal frequents the most retired spots among the hill woods, by which habit it seems ​largely to escape the Tiger, its most formidable foe in those regions of the world. Its quickness of senses enables it also to slip away with rapidity. It can proceed at a great pace when disturbed, and can readily push its way through obstacles. The young animal, like that of the American species, is dark brown with yellowish spots. It is stated by Mr. H. N. Ridley that the young animal lies during the hot part of the day under bushes, in which situation "its coat is so exactly like a patch of ground flecked with sunlight that it is quite invisible." It is interesting to note that here, as with some other animals, it is the young that are especially protected by such mechanisms. Moreover, some of the spots are round and some are more elongated, so that the resemblance to spots of sunlight which come in a direct and in a slanting direction is greatly increased. Even the colours of the adult are not so conspicuous when it is in its native haunts as might be supposed. The breaking up of the ground colour into tracts of two different colours prevent it from striking the eye so plainly as if it were of one colour throughout. "When lying down during the day it exactly resembles a grey boulder, and as it often lives near the rocky streams of the hill jungles, it is really nearly as invisible then as it was when it was speckled."
Fam. 3. Rhinocerotidae.—This family is to be distinguished from the preceding by a number of characters, which though not universal are general. In the first place, there are commonly horns, or a horn, consisting of what appears to be an agglomeration of hair-like structures fixed upon a roughened patch of bone on the surface of the nasals. The incisors are diminished or defective, and the upper canines are often wanting. The molars and premolars are alike. The fore-feet are four- or three-toed, but are functionally tridactyle; the hind-feet are three-toed. The skeleton in this family is massive, and the limbs relatively short. The skull, as in the Tapirs, has a confluent orbit and temporal fossa. The upper lip is generally more or less prehensile; the body is as a rule—to which the Pleistocene Hairy Rhinoceros is of course an exception—rather sparsely covered with hair. In this feature the Rhinocerotidae contrast both with the Tapiridae and the Equidae. The family in reality contains but one existing genus, though three have been instituted, viz. ​Rhinoceros, Ceratorhinus, and Atelodus. As there are so few existing species the subdivision of animals which agree in so many and such highly-characteristic features seems to be an unnecessary procedure. The existing Rhinoceroses are but a fragment of the total number of known forms from past epochs. The family is very markedly on the wane.
The genus Rhinoceros is characterised by its heavy build and thick, almost smooth, skin—smooth, that is to say, so far as concerns the slight development of hair—which is often thrown into folds. There is one or there are two horns on the fore-part of the head, which are, as has already been pointed out, structures sui generis, and not exactly comparable with the horns of other living Ungulates. There are three nearly equal toes on both fore- and hind-limbs. The canine teeth of existing species have disappeared; the incisors are, or are not, present; the molars and premolars are three and four in each half of each jaw.
The visceral anatomy of the Rhinoceros has been much investigated so far as concerns the Asiatic forms. A curious feature, which serves to discriminate some of the Asiatic species from others, is to be seen in the small intestine. In Rh. indicus this gut is furnished with numerous long cylindrical narrow outgrowths "like tags of worsted"; in the allied Rh. sondaicus these tags are present, but are flatter and broader; while in the two-horned Rh. sumatrensis there are no tags at all, but only smooth valve-like folds. Another mark by which these species can be distinguished depends upon the variation in the presence or absence of certain glands imbedded in the integument of the foot—the so-called "hoof glands." These occur in Rh. indicus and Rh. sondaicus, but are absent in Rh. sumatrensis.
Sir W. Flower studied some years since the skull features which serve to differentiate the existing forms.
In Rh. sumatrensis the two long downward processes of the squamosal bone, termed respectively post-glenoid and post-tympanic, do not unite below the auditory meatus. In this the species in question agrees with the African forms but not with the one-horned Asiatic species, where the two processes completely fuse. Again, another character, though perhaps less important, ​is the sloping backwards instead of forward of the occipital crest in all two-horned species, whether African or Asiatic.
The Asiatic Rhinoceroses have, what the African animals have not, functional incisor teeth throughout life. It has been proposed on these and other grounds to separate generically the African and Asiatic forms.
Fig. 130.—Indian Rhinoceros. Rhinoceros indicus. × $^1$⁄$_{40}$.
The Asiatic Rhinoceroses include three well-differentiated species, in all of which the skin is much thrown into folds. Rh. indicus is the largest form. It is one horned, and has enormous folds of skin at the neck and hanging over the limbs. So like artificial armour is this thick plating, that Albrecht Dürer may be excused for having given the beast the appearance of being actually mail-plated in a sketch which he made of a specimen sent over to the King of Portugal in 1513. This particular beast, one of if not the first sent over to Europe, proved so intractable in disposition that the king sent it as a present to the Pope. But "in an access of fury it sunk the vessel on its passage"! The horn of this and of other species was held until almost our times to have medicinal and other more curious values. So recently as 1763 it was gravely asserted that a cup made of its horn would fall to pieces if poison were poured into it. "When wine is poured therein," wrote Dr. Brookes in the year referred to, "it will rise, ferment, and seem to boil; but when ​mixed with poison it cleaves in two, which experiment has been seen by thousands of people." John Evelyn also wrote of a well in Italy which was kept sweet by a Rhinoceros' horn. This species seems to be long-lived, even in captivity; a specimen now to be seen in the Zoological Society's Gardens has been there since the year 1864.
Rhinoceros sondaicus, the Rhinoceros of the Sunderbunds, has a much wider range than the last species or Indian Rhinoceros. This is unknown out of India itself, and is there limited to a small region; the Sondaic form is found in Bengal and in the Malayan Islands. It is a smaller species, and the armour has a tesselated appearance. The female generally, if not always, is hornless.
Fig. 131.—Sumatran Rhinoceros. Rhinoceros sumatrensis. × $^1$⁄$_{15}$. (From Nature.)
The Sumatran species, Rhinoceros sumatrensis, is to be distinguished from the last two by its two horns. It is also covered ​by a much thicker coat of hairs, which are sometimes blacker and sometimes redder. On account of its two horns it has been proposed to separate it from the other Oriental species into a distinct genus, Ceratorhinus. The animal has much the same range as the last species, but extends to Borneo. A variety of this species with hairy ears, from Assam, has been separated as a distinct form, under the name of Rh. lasiotis, by Mr. Sclater. The animal upon which that species was founded was until quite recently living in the Zoological Society's Gardens.
Fig. 132.—Hairy-eared Rhinoceros. Rhinoceros lasiotis. × $^1$⁄$_{30}$.
There are only two certainly-known species of Rhinoceros in Africa. These are the White Rhinoceros (Rh. simus) and the Black Rhinoceros (Rh. bicornis). The origin of the names is not easy to understand, since the "white" animal is, if anything, darker in colour than the Black Rhinoceros. It is stated, however, that in past years the specimens of Rh. simus found in the south-west of Cape Colony were "paler and whiter in colour than those in the north-east." At present there are no grounds for distinguishing the species by their colour characters. But they are plainly distinguishable on other grounds. Rhinoceros simus has a square upper lip, and in relation to this crops the herbage upon the ground. Rh. bicornis has a prehensile upper lip projecting beyond the lower, and in a corresponding fashion feeds principally upon the branches of shrubs, It has been pointed out by Mr. ​Coryndon that the calf of Rh. simus "always runs in front of the cow, while the calf of Rh. bicornis invariably follows its mother." Both animals of course have two horns, and upon the varying proportions of the horns a large number of "species" have been made in the past. It is stated that the longest horn of the "White Rhinoceros" known measures 56½ inches; while that of R. bicornis is shorter, 40 inches being apparently the maximum. But the animal is smaller.
Fig. 133.—Head of Rhinoceros bicornis.
The possible third African species of Rhinoceros has been provisionally named after Mr. Holmwood, and is based upon two horns 41 and 42 inches long, which may be abnormal horns of Rh. bicornis; but they are thinner and have a smaller pedicel.
Extinct Rhinocerotidae.—The existing Rhinoceroses are thus confined to Africa, to certain parts of the continent of Asia, and to some of the large islands lying to the south of that continent. But formerly the genus, and allied genera, had a wider range. As far back as the Miocene we meet with remains of Rhinoceroses closely allied to existing forms. The more ancient forms have, as is natural, more ancient characters. Thus in Rh. schleiermacheri of the Miocene, canines appear to have been present. The Miocene Aceratherium, primitive in the absence of horns as its ​name denotes, had also canines and, in one species, six incisors in the lower jaw. This Aceratherium had, moreover, four toes in the fore-feet. In the Miocene and later the Rhinoceros existed in Europe and America. There was even a purely northern form, the Rh. tichorhinus, which possessed a woolly covering and had the same range as the Mammoth. This Rhinoceros was two-horned.
The post-Pliocene and European Elasmotherium was a colossal rhinocerotine creature. This great beast had two horns and a body 15 feet long. Its limbs are not known, and as the teeth are different from those of Rhinoceroses in general, it may not have belonged to this group at all, though Osborn is inclined to derive it from Aceratherium, admitting at the same time that the evidence is "decidedly slender." The teeth in fact are like those of a Horse in being hypselodont and prismatic in form. As to the two horns, they were apparently not exactly like those of typical Rhinoceroses; there was an enormous horn posteriorly, supported on a huge boss of bone, and in front of this a roughened spot suggests a smaller or at least a much more slender horn.
It is important to notice that fossil Rhinoceroses belonging to the restricted genus Rhinoceros were in Europe invariably two-horned; it is only in India, where they still exist, that one-horned forms are met with in a fossil state.
The Rhinoceroses of America were mostly hornless. Diceratherium is an exception; but in many cases it had two parallel not successive horns, and these were, to judge from the slight prominences, but feeble in development, and perhaps hardly exactly comparable with the formidable weapons of the Old-World forms. Aceratherium tridactylum, with indications of paired horns, may be ancestral to Diceratherium. The American forms have weak and slender nasals in correspondence with the absence of horns; the sagittal crest is retained in contradistinction to the great flattened surface of the skull in the horned Rhinoceroses. Aceratherium of both divisions of the globe probably represents the ancestral group of the horned and the hornless forms. This being the case it is highly interesting to note a distinct convergence in the quite ​separate American genera towards the European horned genera. A genus sometimes united with Aceratherium, but still differing from it in some points, is Aphelops (Teleoceras). This animal is more nearly approximated to "the modern standard" of Rhinoceroses than is its possible ancestor Aceratherium.
Fig. 134.—Skeleton of Hyracodon nebrascensis. × $^1$⁄$_{12}$ (After Scott.)
The skeleton in general is more robust, even surpassing that of modern forms, and approaching the Hippopotamus. There is a reduction in the upper incisors, which are limited to two pairs, and the lower molars ​
Fig. 135.—Skeleton of Aphelops (Teleoceras) fossiger. × $^1$⁄$_{15}$. (After Osborn.)
The most ancient rhinocerotine types are the Hyracodonts and the Amynodonts. They both date from the Eocene, and became extinct in the succeeding Oligocene. Hyracodon (Fig. 134) was "an agile, light-chested, and rather long-necked" type, resembling a Horse in build. There were no horns present, but the hoofs were more like those of the Horses than of the existing Rhinoceroses. These animals were apparently plain dwellers and defenceless, which is held to account for their compact hoofs and outward similarity to a Horse. The genus is Oligocene. The dental formula is I 3/3 C 1/1 Pm 4/3 M 3/3.
It is surmised by Professor Scott that the number of dorso-lumbar vertebrae was twenty-three or twenty-four. The radius and ulna are complete and separate bones, but the latter is somewhat reduced. There are four metacarpal bones, of which, however, the fifth is much reduced. The animal is only three-fingered. The tibia and the fibula are distinct, and show no tendencies towards fusion; but the fibula is much reduced. There are only three metatarsals and three toes. Had this line, which is to be regarded as a side branch of the Rhinoceros stem, not died out, it would probably have resulted, thinks Professor Scott, in monodactyle—very Horse-like types. It is later than the next genus to be described, Hyrachyus, of which it is possibly a descendant. An intermediate type, Triplopus, appears to bind together Hyracodon and Hyrachyus.
In Hyrachyus agrarius the skull is long and narrow, the facial region being markedly longer than in existing Rhinoceroses. The mastoid portion of the periotic bone is widely exposed upon the outer face of the skull, which is, as has been said, not the case with the existing genus Rhinoceros. The dentition is the complete Eutherian dentition of forty-four teeth. The upper ​molar teeth are strikingly like those of the genus Rhinoceros. The fore-feet are pentadactyle, but functionally tetradactyle; the hind-feet tridactyle. The ulna is less reduced than in Hyracodon, and the dorso-lumbar vertebrae are twenty-five.
Fig. 136.—Skeleton of Metamynodon planifrons. × $^1$⁄$_{22}$. (After Osborn and Wortman.)
The Amynodonts were short, heavy types, probably marsh-haunting in habit, and possibly with a proboscis like that of the Tapir. The orbit is higher than it is in the purely terrestrial ​Hyracodonts, and it is suggested that when swimming it was raised above the surface as with the Hippopotamus. "This feature," observes Professor Osborn, "with the long curved tusks, undoubtedly used in uprooting, suggests the resemblance between the habits of these animals and those of the hippopotami." There were no horns in the Amynodonts. The face is shorter than in the Hyracodonts, and the mastoid is covered as in recent Rhinoceroses. The canines are very strongly developed into tusks, but the incisors show signs of disappearance. We know of the genera Amynodon, Metamynodon, and Cadurcotherium. All except the last, which is European, are American in range.
Fam. 4. Titanotheriidae.—These Oligocene Ungulates, often attaining to large dimensions, are nearly peculiar, so far as is at present known, to the North American Continent, and are at least most abundant in it. Many generic names, such as Titanotherium, Brontotherium, Brontops, Titanops, and Menodus, have been given to them; but a recent study of the entire material accessible for description or already described has led Professor Osborn to the opinion that there was but a single genus, to which the name Titanotherium must be applied. Of this genus there are some thirty well-characterised species, of which the gradual evolution can be traced from the lowest strata of the White River beds where their remains occur. An entire skeleton of T. robustum enables us to understand the osteology of these forms and to compare them with other Perissodactyles. This animal was more than 13 feet long, standing some 7 feet 7 inches in height. It seems to have presented during life the aspect of a Rhinoceros with perhaps a touch of Elephant. The skull is not unlike that of a Rhinoceros in general dimensions and shape; but there are a pair of apparent horn cores anteriorly, which are smaller in the more ancient forms and acquire a large size, a forward direction with a divergence of the two in the later forms. A glance at the accompanying figures of skulls (Fig. 137) of early and later Titanotheres will exhibit the changes in this particular which the skulls underwent in the lapse of time occupied by the deposition of these Oligocene beds. The nasals are short in the later, longer in the more early species, such as T. heloceras and T. coloradense. The zygomatic arch projects much, and is "shelf-like" in the later forms, the skull thus getting an immense breadth, which, ​
Fig. 137.—Three figures showing the cranial evolution of Titanotherium. Upper figure, T. trigonoceras; middle figure, T. elatum; lower figure, T. platyceras. (After Osborn.)
The genus further agrees with the Artiodactyles in the structure of the carpus. The toes of the fore-limb are four, those of the hind-limb three; but while the hind-limb is undoubtedly Perissodactyle in the arrangement of its component parts, the fore-limb shows a hint of an Artiodactyle mode of structure. This limb is paraxonic, the axis of the limb passing between the two middle digits. It may be that this genus represents more nearly than any other Perissodactyle or Artiodactyle the primitive stem from which both have diverged, though, of course, it is not old enough to be very near to the actual ancestor. The molar dentition is the typical one; the incisors seem to vary as to their presence or absence, and, if present, in their numbers. In comparing the older with the more recent forms it is noteworthy that there has been an increase of size exactly as there has been during the evolution of the Camels and some other groups of Ungulates. As already mentioned, the size of the horn cores also increases until it culminates in the extraordinary species, T. platyceras and T. ramosum, in which these are half as long as the skull, flattened in form, and connected at their bases by a "web" of bone. Arrived at this amount of specialisation the genus Titanotherium apparently exhausted its capacities for modification and ceased to be. The many generic names may be explained by sexual differences on the one hand and an incomplete knowledge of connecting links on the other.
Palaeosyops is somewhat like a Tapir in build, the skull especially resembling that of the Tapir. As in Titanotherium the molar teeth, instead of having an outer wall formed by fused cusps, have a -shaped outer wall on one side and two or one cusps on the opposite side. It is, moreover, an Eocene form, and in correspondence with its greater age is more primitive in some points of structure, for example, in the absence of horns and in the full dental formula. The fore-limbs are four-toed, the hind ​three-toed. It was intermediate between a Tapir and a Rhinoceros in size. It has been shown, too, from casts of the interior of the skull, that the cerebral hemispheres are much less convoluted than were those of Titanotherium.
Related to Palaeosyops is another primitive Titanothere, the genus Telmatotherium. This is also Eocene, from the Uinta Basin, the uppermost of Eocene strata. The skull of these creatures was rather elongated, and not unlike that of a Titanothere in general aspect. The dentition was complete and the canines not very large. The horns, which acquire so prodigious a development in the later Titanotheres, are just recognisable in at any rate many species of this genus Telmatotherium, the name being thus by no means an apt one. Better was that proposed by Dr. Wortman, of Manteoceras or "prophet horned." The horns are small elevations upon the frontals just at the junction of these with the nasals, and, indeed, lying partly upon the latter bones. In T. cornutum the horns are chiefly borne upon the very long nasals, whose size contrasts with the same bones in the more highly-developed Titanotherium. It appears to be quite possible that Titanotherium was evolved from the genus Telmatotherium.
Sub-Order 9. LITOPTERNA.
Whether the Macraucheniidae should be considered as a separate group of Ungulata is a matter of dispute. Cope placed them in a special order of Ungulates which he called Litopterna. Zittel, on the other hand, regards them as definitely Perissodactyles. One curious point of resemblance to existing Horses is shown—that is the presence of a pit in the incisor teeth. This matter seems to be so important as to need a placing of these forms in the neighbourhood of the Perissodactyles, even of the Equidae; it is so peculiar a character, and apparently so little related to any obvious similarity in way of life, that it seems to mark a special affinity. Not so the fact that in Macrauchenia at any rate the orbit was entirely surrounded by bone as in the Horse. We find that condition so frequently acquired in many groups,—a development from an earlier condition where the cavity for the lodgment of the eye is in continuity with the temporal ​fossa, that it cannot be regarded as anything more than a mark of specialisation. It is, in fact, the case that the Macraucheniidae are in many points specialised, while retaining many primitive features of structure.
The chief primitive features are: the non-alternating positions of the wrist- and ankle-bones; these, of course, interlock in the Perissodactyles of to-day and in many extinct families. Then the absence of a diastema in the tooth series, coupled with the presence in Macrauchenia of a complete dentition. The small brain may be referred to the same category. Macrauchenia must have been a strange-looking animal. It walked upon three toes on each limb; the skull was Horse-like in general form, but the nostrils are removed to a point about as far back as in the Whales or nearly so, the nasal bones being correspondingly reduced. This it is thought argues a proboscis. The humerus is particularly compared by Burmeister to that of a Horse. The radius and ulna though both well developed are fused. The neck is long, and, as in the Camel, the vertebral arteries run inside the neural arches. Since the fore-legs seem to have been rather longer than the hind-legs, though only very slightly, and the neck was long, the animal may have presented some likeness to the Giraffe. It is interesting to note that in the proportions of humerus to ulna this animal is more Lama-like than Horse-like. On the other hand, the proportions of femur to tibia are more Horse-like. The remains of the creature are limited to South America, and to quite superficial deposits. It is evidently a specialised type, and has pursued a course parallel to that of the Horse. Much nearer to the Horse however, but apparently by convergence only, is the genus Thoatherium, usually placed in a separate family, the Protorotheriidae. In this creature, which has many archaic characters, the toes are reduced to one in each foot. In an allied form, Protorotherium, we have the two lateral toes diminishing just as in Anchitherium.
* ↑ Sir W. H. Flower, The Horse, London, 1890.
* ↑ See Ewart, The Penicuik Experiments, Constable and Co., 1899.
* ↑ The Horse, London, 1890.
* ↑ Cuyer and Alix, Le Cheval, Paris, 1886.
* ↑ Lubbock, Prehistoric Times, London, 1865.
* ↑ J. Geikie, Prehistoric Europe, London, 1881.
* ↑ Horses, Asses, and Zebras, London, 1895.
* ↑ Proc. Zool. Soc. 1884, p. 540.
* ↑ Proc. Zool. Soc. 1895, p. 688.
* ↑ See Pocock, Ann. Nat. Hist. (6) xx. 1897, p. 33.
* ↑ "Das Quagga," Zool. Garten, 1893, p. 289.
* ↑ Of this Horse, remains have been lately discovered (see Lönnberg, Proc. Zool. Soc. 1900, p. 379) in the cave which produced the remains of Glossotherium. A piece of skin covered with Fox-red hair, possibly spotted with paler areas, is believed to be a relic of Onohippidium.
* ↑ Trans. American Phil. Soc. xviii. 1896, p. 55.
* ↑ T. leucogenys and T. ecuadorensis are probably not distinct, the latter being in reality T. terrestris, the former T. roulini.
* ↑ See Beddard, Proc. Zool. Soc. 1889, p. 252, and other papers there cited, for the anatomy of the Tapir.
* ↑ Natural Science, vi. 1895, p. 161.
* ↑ Garrod, Proc. Zool. Soc. 1873, p. 92; ibid. 1877, p. 707. Beddard and Treves, Trans. Zool. Soc. xii. 1887, p. 183.
* ↑ Proc. Zool. Soc. 1876, p. 443.
* ↑ Proc. Zool. Soc. 1894, p. 329. See also Mr. Selous' paper in Proc. Zool. Soc. 1881, p. 275.
* ↑ P. L. Sclater, Proc. Zool. Soc. 1893, p. 514.
* ↑ Quite recently, however, a species, A. incisivum, preserved at Darmstadt, has been found by Professor Osborn to possess a slight rugosity upon the frontal bones, which probably indicates the presence of a rudimentary horn, and the same author is apparently inclined to place in Aceratherium the horned Teleoceras (see p. 261).
* ↑ Osborn, Bull. Amer. Mus. Nat. Hist. x. 1898, p. 51.
* ↑ See Osborn, Mem. American Mus. Nat. Hist. vol. i. pt. iii. 1898.
* ↑ Scott, in Gegenbaur's Festschrift, ii. 1896, p. 351.
* ↑ Remains of the genus have been met with in the Balkans.
* ↑ See especially Osborn and Wortman, Bull. Amer. Mus. Nat. Hist. vii. 1895, p. 333, and Osborn, ibid. viii. 1896, p. 157.
* ↑ See Osborn, Bull. Amer. Mus. Nat. Hist. vii. 1895, p. 82.
* ↑ N. Acta Acad. Caes. Leop. Car. xxvii. 1885, p. 238.