Chapter IX (1)

UNGULATA--CONDYLARTHRA--AMBLYPODA--ANCYLOPODA--TYPOTHERIA--TOXODONTIA--
PROBOSCIDEA--HYRACOIDEA

ORDER IV. UNGULATA

The existing members of this order can be readily grouped into the Hyracoidea, Proboscidea, Perissodactyla, and Artiodactyla, each of which divisions has quite the value of an order, and all of which are sharply marked off from each other. But as the discovery of so many fossil forms has to a great extent rendered these demarcations less sharp, it is better to regard all these groups as not more than sub-orders of a larger "Order" Ungulata. Even when this conclusion has been necessarily arrived at from a consideration of the more ancient groups of Ungulate animals, the definition of such an order remains a difficult matter for the systematist. For the earliest of these forms, more particularly the Ancylopoda, the Amblypoda, and the Condylarthra, whose peculiarities will be dealt with at length subsequently, are not by any means easily differentiated from the primitive Carnivorous mammals of that date, the Creodonta; these latter, moreover, fade into the Marsupials through the so-called Sparassodonta of Professor Ameghino. To confine ourselves to the Ungulates, we may perhaps define them as terrestrial animals with hoofs rather than claws or nails, and chiefly, if not entirely, vegetarian in habit. The teeth are bunodont or lophodont, the tendency to the production of the latter type being always marked. The walk, although plantigrade in the older types, becomes more and more digitigrade, except in such survivals from antiquity as _Hyrax_. There is, too, as we pass from the ancient types to the modern, a gradual perfection of the limbs as running {196} and not climbing or grasping organs; the number of toes becomes reduced, and culminates twice (in the horse and in the Litopterna) in one toe on each foot; at the same time the ulna becomes rudimentary and fuses with the radius, and the fibula in the hind-limb undergoes a like reduction. The clavicle is absent even in some of the oldest types; its presence in _Typotherium_[120] is highly remarkable. The tail too, an organ which is long in some of the early forms, gets short in their modern derivatives.

FIG. 110.--An early Ungulate. _Phenacodus primaevus._ x 1/12. (After Osborn.)

FIG. 111.--Series of metacarpals and metatarsals of Camelidae, to show secular and progressive increase in size. From left to right the species are _Protylopus petersoni_, _Poebrotherium labiatum_, _Gomphotherium sternbergi_, _Procamelus occidentalis_. F, Fore-foot; H, hind-foot; III, IV, third and fourth metapodials. (After Wortman.)

Coupled with the increasing perfection of the foot as an organ used merely for the support of the body, certain interesting changes have taken place in the arrangement with regard to each other of the several bonelets of the wrist and ankle. It has been held by Cope and others that the truly primitive disposition of these bones was that presented to us by certain early types, such as _Meniscotherium_ or the existing elephant or _Hyrax_. In these animals there is (see Fig. 112) a serial {197} arrangement of these bones, the distal bones only, or very nearly only, articulating with the corresponding bones in the upper series. In the modern types (cf. Fig. 113) there is, on the other hand, an interlocking, so that the bones of the distal series articulate with two of those of the proximal series. By this is produced, as it would appear, a much firmer foot, less liable to "give" under pressure, and thus more fitted for an animal that runs. It is the same principle as that adopted in the laying of bricks. The actual stress and strain of impact has been held responsible for those changes. An equally ingenious and possibly truer explanation of the undoubted facts has lately been advanced by Mr. W. D. {198} Matthew.[121] He has pointed out that in some ancient Ungulates the carpus is not serial but interlocking, even in forms which belong to the earliest Eocene groups, such as the genus _Protolambda_ among the Amblypoda. Now in the fore-foot of _Meniscotherium_ and the living _Hyrax_ there is a separate centrale which is wanting in the greater number of Ungulates. The absorption, that is the practical dropping out of this bone, would restore to an interlocking carpus the serial arrangement; while on the other hand, by the fusion of this bone with the scaphoid, the interlocking disposition would be maintained.

FIG. 112.--Bones of the manus A, of the Indian Elephant, _Elephas indicus_. x 1/8. B, of the Cape Hyrax, _Hyrax capensis_. x 1. _c_, Cuneiform; _cc_, centrale; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _td_, trapezoid; _tm_, trapezium; _s_, scaphoid; _u_, unciform; _U_, ulna. (From Flower's _Osteology_.)

FIG. 113.--Bones of the manus A, of Rhinoceros, _Rhinoceros sumatrensis_. x 1/5. B, of Pig, _Sus scrofa_. x 1/3. Letters as in Fig. 112. (From Flower's _Osteology_.)

The gradual perfecting of the fore- and hind-limbs as running organs has been put down to the advent of the grasses, and the formation of large plains covered with this herbage. The same reason would also be in harmony with the equally gradual change in the shape of the molar teeth, from a tubercular form calculated for a mixed or even a carnivorous diet, to the flatter crushing surfaces exhibited by the lophodont teeth of later Ungulates. Strong {199} canines would in the same way cease to be useful, and even become encumbrances to such grazing creatures; and their disappearance is one of the salient features in the history of the Ungulata, that is of the modern representatives of the order. The extraordinary hypertrophy of these teeth in such a line as that of the Amblypoda, which has left no descendants, was one of the reasons perhaps for the decay of those great pachyderms of mid-Tertiary times; their excessive armature became an encumbrance, since it was not accompanied by improvements in other necessary directions. Some of the features of the Tertiary Ungulates have, however, been dealt with in our general sketch of the mammalian life during that epoch, and need not be again referred to here. Of existing Ungulates there are no clear indications of the descent of the Elephants or of the Hyracoidea. Their structure proclaims these two divisions to be of ancient descent, and not to be modern twigs of the Ungulate stem. As to the Perissodactyla and the Artiodactyla we cannot bring them together nearer than in quite early Tertiary times. The order Condylarthra seems to be the starting-point of both these sub-divisions. _Euprotogonia_ has been considered to be an ancestor of the Perissodactyle branch, and _Protogonodon_ or _Protoselene_ of the Artiodactyla. If this be true, {200} the likenesses which _Titanotherium_ shows to the Artiodactyla must be either purely superficial and secondary, or a cropping out of ancient characters which had been dormant for many generations.

HORNS.--The Ungulata are the only order of mammals which possess horns; as they are on the whole a more defenceless group than the Carnivora, it may be that the horns are a counterpoise to the teeth and claws of the latter; need for defence and for armature in the combats with their own kind for the favours of the does has led to a different kind of protective and aggressive mechanism. Horns as weapons are, however, particularly effective in this group wherever they exist. A Ruminant is most frequently a large and heavy animal without the agility and litheness of the Carnivore. It is precisely to this sort of animal, where weight is an important consideration, that horns are the most suitable weapons. This is further shown by the fact that although the general term horn is used to describe the weapons of the Ungulate mammals, there is more than one kind of structure included under this general term; it is indeed probable that the extreme terms in the series of horns have been independently acquired by their possessors. There is but little in common between the horns of a Giraffe and of a Rhinoceros. In the Rhinoceros we have one or two horns, in the latter case one placed behind the other, which are purely epidermic growths; they may indeed be regarded as matted masses of hair, borne, it is true, upon a boss of bone, which however is not a separate structure. The Giraffe supplies us with the simplest term in that series of horns which are partly epidermal and partly bony. The paired horns of this animal have often been contrasted with those of the Deer, for example; but there is no fundamental difference between them. In the Giraffe a pair of bony outgrowths, originally separate from the skull which bears them, but ultimately ankylosed to it, are covered by a layer of entirely unmodified skin. A distinction of undoubtedly practical importance is usually drawn between the Hollow-horned Ruminants, _i.e._ Oxen, Goats and Antelopes, and the Deer tribe. There is nevertheless no fundamental distinction. In the Antelopes there is a core of bone, the "os cornu" as it has been termed, which is covered by a horny layer, the horn proper, variously modified in shape and size according to the genus or species. In the Deer there is the {201} same os cornu, which may however be branched, but which is in the same way covered by a layer of modified integument; this is known as the "velvet"; it only lasts for a certain period, and is then torn off by the exertions of the animal itself, leaving behind the bony core, which is popularly termed the horn. It will be clear that here is only a difference of comparative unimportance; the same essential features are present in both groups of animals, but the modification of the epidermis has progressed along different lines. Both can be referred back to the primitive conditions seen in the paired horns of the Giraffe. Even the difference, such as it is, is bridged over by the Antelope _Antilocapra_, where the os cornu is bifid and the horn is periodically shed, as is the velvet of the stag; but in the stag the bony part of the horn is also shed, a state of affairs which has no parallel in the Hollow-horned Ruminants. The great _Sivatherium_ may conceivably be an annectant form between the two types of compound horns, _i.e._ those of the Antelope and those of the Deer. This creature had two pairs of horns, of which, naturally, only the bony cores remain; the hinder pair of these were branched. But although so far they resemble the Deer's horns rather than the Antelope's, Dr. Murie has thought that they were covered by a horny sheath and not by soft skin as in the Deer. In any case these horns were apparently never shed, which is a point of likeness with the Antelope and of difference from the Deer. Apart therefore from the nature of the covering of the bony cores, there are good grounds for looking upon them as intermediate between those of the Deer and those of the Antelopes.

The horns of the Ruminants are frequently a secondary sexual character; this is especially the case with the Deer. The Reindeer is, however, an exception, both the stags and the does having horns. That they are associated with the reproductive function is shown by their being shed after the period of rut, the destruction of the velvet at that period, and also by the effect upon the horns which any injury to the reproductive glands produces. Some useful facts upon this latter head have been amassed by Dr. G. H. Fowler,[122] who noticed in a series of stags, horns showing various degrees of degeneration in the antlers produced by varying degrees and periods of gelding. From the facts {202} here collected it is clear that a direct effect is produced. If we are to regard horns as secondary sexual appendages which have been subsequently handed on to the female by heredity, we should expect to meet with examples of animals now horned in both sexes, of which the earlier representatives had the horns confined to one sex. This is most interestingly shown by the extinct and Miocene Giraffe, _Samotherium_, of which the male alone had a pair of short horns, while the skull of the female was entirely hornless; the modern _Giraffa_, as is well known, has horns in both sexes.

It is interesting to note that the existing Perissodactyles and Artiodactyles are to be distinguished by their unpaired or paired horns. But while there are no Artiodactyles with unpaired horns (save occasional sports) the Perissodactyles have more than once tried, so to speak, paired horns, which ultimately proved fatal to them. The Rhinoceros _Diceratherium_ apparently inherited and improved upon the small paired horns of _Aceratherium_, but it has left no descendant. The paired horned Titanotheria offer another instance of the same apparent incompatibility between the Perissodactyle structure and the persistence of paired horns.

SUB-ORDER 1. CONDYLARTHRA.

This group is characterised by the following assemblage of characters. Extinct, often plantigrade Ungulates, with five-toed limbs. Bones of carpus and tarsus not always interlocking, but sometimes lying above each other in corresponding positions. The humerus has an entepicondylar foramen. Dental formula quite complete; the molars brachyodont and bunodont. The premolars are simpler than the molars. The canines are small. As with other early types, the zygapophyses are flat and do not interlock. The astragalus is like that of the Creodonta. This group was American and European in range, the remains of its rather numerous genera being of Eocene time. The best-known genus is _Phenacodus_, of which some account will be given before discussing the, in many cases, more fragmentary remains of other allied forms.

The genus _Phenacodus_ was first described so long ago as 1872, from a few scattered teeth. Since then several nearly complete skeletons have been obtained, and we are in full possession of {203} the details of its osteology. It was not a large creature (see Fig. 110, p. 196), about 6 feet in length, with a small head. The feet were more or less plantigrade, and five-toed. The last phalanges of the toes show that they carried hoofs and not claws; yet the fore-feet look a little as if they could be used as grasping organs. The third digit of both hind- and fore-feet exceeds the others, and thus a Perissodactyle-like foot characterised this Eocene creature. The tail is exceedingly long, and must have reached the ground as the animal walked. This is of course by no means an Ungulate character. Still, in the totality of its organisation the animal was decidedly Ungulate, though Professor Cope spoke of _Phenacodus_ as not merely an ancestral Ungulate but as the parent form of Insectivores, Carnivores, Lemurs, Monkeys, and Man himself! The scapula indeed is from its breadth and oval contour rather like that of a Carnivore. The clavicles as in other Ungulates are absent. The femur is Perissodactyle rather than Artiodactyle in the presence of a third trochanter. The creature had fifteen pairs of ribs and five or six lumbar vertebrae. The two bones of the leg which lie below the femur are perfectly distinct and separate. A cast of the brain-case shows that the cerebral hemispheres were smooth and small, the cerebellum of course completely uncovered and nearly as large as the cerebrum. The olfactory lobes were also large. The complete skeleton of _Phenacodus_ has lately been excavated more fully from the enveloping matrix by Professor Osborn,[123] and mounted in what is regarded as the natural position of the beast. It appears that though five-toed it went upon the three middle toes only, and furthermore that of these the middle one was the more prevailing, so that _Phenacodus_ was distinctly "Perissodactyle," at least in habit. Moreover its "long hind-quarters, the long powerful tail ... are reminiscent of Creodont ancestry." The genus was European and American in range.

_Meniscotherium_ ( = _Hyracops_[124]) comprises several forms of about the size of a fox; they are both European and American in range. The teeth are more distinctly Ungulate in form than those of _Phenacodus_, with a [125]-shaped outer wall. The skull is described as possessing "indifferent, primitive characters," permitting a comparison with those of Opossums, Insectivores, and {204} Creodonta. It has, as in _Phenacodus_, no orbital ring. The humerus resembles that of a Carnivore rather than that of an Ungulate. The carpus and tarsus are serial. The fibula articulates with both the calcaneum and the astragalus, which is not the case with _Phenacodus_. It is suggested that these animals are ancestral forms of the Chalicotheres. In the brain the hemispheres do not cover the cerebellum.

More primitive apparently than _Phenacodus_ was the less-known genus _Euprotogonia_, or _Protogonia_[126] as it has been called. The best-known species is _E. puercensis_, so called from its occurrence in the Puerco beds of the American Eocene. It was a slender, long-limbed creature, smaller than _Phenacodus_, with a long and heavy tail as in that animal. Like _Phenacodus_ it was semiplantigrade, and shows more likenesses to the Creodonta. The skull is only known by a part of the lower jaw with teeth, and by the teeth of the upper jaw. The vertebrae are not entirely preserved, but enough remain to show that the animal had a tail of 16 or 17 inches, which is a considerable length when compared to its height, about a foot at the rump. In the fore-limb the most noteworthy point is that the ulna has a convex posterior border as in the Creodonts, the same border in _Phenacodus_ being concave. The humerus is slender, with less-marked tuberosities. The fifth digit seems to have been less reduced. The phalanges seem to have borne horny sheaths somewhat intermediate between hoofs and claws. The pelvis is described as being, as is also that of _Phenacodus_, rather like that of the Creodonta. The right hind-limb is known in all its details. It appears that the bones are not serial but interlocking; this, however, on the views with regard to the relations of these two forms of tarsus mentioned on p. 198, does not militate against regarding _Euprotogonia_ as the ancestor of the genus _Phenacodus_. The third toe is the pre-eminent one, the animal thus being Perissodactyle. The lateral digits are larger than in _Phenacodus_, and the metatarsals and the phalanges are slightly curved, which is again a Creodont character as compared to the perfectly straight corresponding bones of _Phenacodus_. It seems evident that this animal is to be looked upon as a more ancient type than _Phenacodus_, even if not as its actual ancestor.

Another group of the Condylarthra contains the genus _Pertipychus_ and some others. _Periptychus_ has the full dentition {205} of forty-four teeth, the molars being of course bunodont, with the three chief tubercles most developed. The bones of the tarsus interlock and are not serial, as they are in many other members of the Condylarthra. The astragalus has a shorter neck than in _Meniscotherium_, for example. It has in this a likeness to the same bone in the Amblypoda, to the primitive members of which, such as _Pantolambda_, this animal bears much resemblance. "Astragali and many skeletal bones of _Periptychus rhabdodon_ and _Pantolambda bathmodon_ are almost indistinguishable," observes Mr. Matthew. The fore-feet of this genus are unknown, but it would seem that it was plantigrade from the evidence of the hind-feet. There are several species of the genus.

Possibly, but not at all certainly, the Mioclaenidae, with the genera _Mioclaenus_ and _Protoselene_, are to be referred to this same order of primitive Ungulates. It is only necessary to mention them here, because they show very clearly the primitive form of dentition of these early Eocene mammals. The teeth are quite complete and unbroken by a diastema. The canines are but little pronounced. The molars are not strictly tritubercular, but have a prevailing trituberculy. The nature of the feet is not known. Since the genus _Protoselene_, as its name denotes, shows an indication of a commencing selenodonty, it has been suggested that this group is the stock whence the Artiodactyles have been derived.

In any case, whether the particular comparisons that have been made as to the relationship of various forms of Condylarthra are valid or not, it seems to be plain that this group represents the earliest Ungulate stock, but little differentiated from the contemporaneous Creodonts.

SUB-ORDER 2. AMBLYPODA.

This group of extinct mammals has the following principal characteristics:--

They are large, semiplantigrade Ungulates, of heavy build and apparently elephantine gait. The dentition is for the most part complete as in other ancient groups, and the canines are in the later forms big tusks. The back teeth are brachyodont and ridged (lophodont). Both radius and ulna in the fore-limb, and tibia and fibula in the hind-limb, are well developed. The bones {206} in the carpus are alternating in position. The toes are five in both feet, and are very short. There is a hint of commencing "perissodactylism" in the fore-feet at any rate. The brain is small and the hemispheres smooth.

The Amblypoda, or Amblydactyla, are so called on account of their short and stumpy feet and toes. They were held by Professor Cope to be on the direct line of ancestry of both Perissodactyles and Artiodactyles, a view which is on the whole not accepted at present.

FIG. 114.--Skull of _Protolambda bathmodon_. x 3/4. _e.a.m_, External auditory meatus; _m_, mastoid; _m.f_, mastoid foramen. (After Osborn.)

As is the case with other groups, the Amblypoda commenced existence as a sub-order with relatively small forms such as _Pantolambda_, the most ancient type known, which is in many respects a transition between the later forms and other groups of mammals such as the Creodonta.[127] The race culminated and ended in the giant _Dinoceras_ and _Coryphodon_, and spread into the Old World. In spite of their smooth and diminutive brain, these mammals were able to hold their own and to multiply into many species and genera; in this they were perhaps aided by their formidable tusks and by the horns which many of them possessed. The teeth seem to imply an omnivorous diet, which was quite possibly an additional advantage in the struggle for existence. It does not seem to be necessary to divide off the Dinoceratidae into a sub-order equivalent to the Coryphodontidae as was done {207} by Professor Marsh; the numerous points in common possessed by the members of both families forbid their separation more widely than as families.

The earliest types of Amblypoda belong to the genus _Pantolambda_, of which the species _P. bathmodon_ was about four feet in length. As restored it seems to have had proportionately short fore- and hind-limbs, and it had a long tail. It was apparently plantigrade, and would have had not a little likeness to a carnivorous type. The skull has no air cavities, such as are developed in the later types from the Lower Eocene, e.g. _Coryphodon_; _Pantolambda_ is from the basal Eocene. The frontal bones show no trace of the horns that are developed in subsequent forms; the nasals are comparatively long; the zygomatic arch is slender. The molar teeth are in the primitive form of trituberculy, and the premolars, as is so often the case with primitive animals, are unlike the molars in form, being less markedly selenodont. As to the vertebral column, the dorsal vertebrae appear to have had short spines, which argues, as it does also in the case of the larger and heavier _Coryphodon_, a feebleness in the development of ligaments and muscles supporting and moving the head. The scapula seems to have the same peculiar leaf-like form that it has in the later _Coryphodon_.[128] This primitive type shows an entepicondylar foramen in the humerus. It is interesting to observe that the posterior border of the ulna is convex, as in the Creodonts, and in the early Condylarthrous form _Euprotogonia_. In the subsequently-developed Amblypoda, as in the later Condylarthra, that bone acquires a concave outer border. In the carpus the os centrale is distinct. In the femur the third trochanter is well formed; it gradually dies out in later Amblypoda. The fibula articulates with the calcaneum. This species, according to Osborn, "typifies the hypothetical Protungulate, being more primitive than either _Euprotogonia_ or _Phenacodus_."[129]

FIG. 115.--Skeleton of _Coryphodon radians_. x 1/10. (After Osborn.)

The genus _Coryphodon_ is known by a large number of species, of which the first was discovered in this country, and was represented merely by a jaw with some teeth. This was named by Sir R. Owen _C. eocaenus_, and was dredged up from the bottom of the sea off the Essex coast. A second specimen consisted of a single {209} canine tooth only, and was brought up from a depth of 160 feet during the making of a well at Camberwell. More abundant remains have since been found in North America.

This genus had a large head, and in some specimens traces of the "horn cores," so marked in the related _Dinoceras_, are to be noticed. The skull is broad behind and narrowed in front; the roofing bones show the cellular spaces so characteristic of the Elephant. The jugal bone, however, is not, as it is in the Elephant, placed in the middle of the somewhat massive zygomatic arch. As in some other primitive Ungulates (e.g. _Phenacodus_) there are twenty dorso-lumbar vertebrae, of which fifteen bore ribs.

The scapula seems to have possessed a peculiar leaf-like form, swelling in the middle and ending almost in a point above. It has a well-marked spine, and the acromion projects much. The fore- as well as the hind-feet are in a state of transition between plantigradism and digitigradism. It was at one time held that the animal was digitigrade as to the fore-feet and plantigrade as to the hind-feet. Though, as has been pointed out, it is a fact that the hind-feet are often on a different plane of evolution from the fore-feet, it seems that this amount of difference does not characterise any Ungulate, not excepting the genus now under consideration.

The toes are very spreading. The pelvic girdle is of great strength and broadness. The femur, as in the Perissodactyles, has a well-developed third trochanter; but whereas in this particular the hind-limb is Perissodactyle, it is Artiodactyle in the fact that the tibia and the fibula articulate with the astragalus and calcaneum. The ridged teeth have given the name to the genus.

A curious feature in the structure of the genus are the slender spines of the dorsal vertebrae, which contrast with the enormous ones of some other Ungulates--more curious in this genus, which is of heavy build, than in the lighter _Pantolambda_. The back of the animal is short, and the limbs are very spreading, so that the gait was doubtless shuffling. The large head, and short and heavy limbs and limb girdles added probably to its cumbrous walk or trot. The canines are great tusks, and spread out on both sides of the mouth.[130]

The late Professor Cope, in 1874, described the probable appearance of the _Coryphodon_ in the following words:--"The general appearance of the Coryphodons, as determined by the skeleton {210} probably resembled the bears more than any living animals, with the important exception that in their feet they were much like the elephant. To the general proportions of the bears must be added the tail of medium length. Whether they were covered with hair or not is of course uncertain. Of their nearest living allies, the elephants, some were hairy and others naked.... The movements of the Coryphodons doubtless resembled those of the elephant in its shuffling and ambling gait, and may have been even more awkward from the inflexibility of the ankle."

The most recent members of this sub-order come from the Middle Eocene beds, and are chiefly referable to the genus _Dinoceras_, with which _Tinoceras_ and _Uintatherium_ are at least very nearly related, if not identical. These creatures were of great size, larger than the earlier types which have been considered. They show a certain superficial resemblance to the Titanotheriidae, on account of the massive horn cores upon the skull. These horn cores are large upon the maxillae and the parietals, and are paired; on the nasals are smaller horns. The bones of the skull have air cavities. The incisors of the upper jaw are absent; the canines are enormous tusks, and the lower jaws are flanged downwards near the symphysis where these tusks border them. Contrary to what is found in the older types, where the position of the condyle of the lower jaw is normal, this prominence faces backwards in the Dinocerata. The same shortness of the spines of the dorsal vertebrae prevails in this group as in the other Amblypoda, though it is perhaps hardly so marked. The scapula has not the peculiar acuminate form that exists in _Coryphodon_, but is triangular and broad above. The limbs are elephantine, in that the angle between the humerus and the femur respectively, and the bones which follow, is not marked. The hind-limbs are especially straight. The tail is short as compared with that of the primitive Amblypoda. The Dinocerata are purely digitigrade. The entepicondylar foramen has, as in the Coryphodonts, disappeared. The os centrale of the carpus has become fused, and no longer exists as a separate bone. The fibula no longer articulates with the calcaneum, but both that bone and the ulna are well developed. The genus _Astrapotherium_ is placed among the Amblypoda by some authorities.[131]

{211}

SUB-ORDER 3. ANCYLOPODA.

The history of the discovery of the members of this order is very instructive as illustrating the dangers of laying too much classificatory importance upon detached fragments of animals. So long ago as 1825 terminal phalanges of a new creature were found in the Miocene of Eppelsheim, and sent to Cuvier. Cuvier named them "Pangolin gigantesque," deeming them, on account of their general form and cleft terminations, to pertain to the Edentata. In the same bed some seven years later were found certain teeth clearly of an Ungulate character, to which the generic name of _Chalicotherium_ was applied. It was subsequently discovered that the teeth and the claws belonged to the same animal, and, later, further remains turned up which disclosed a creature having the anomalous composition of an Ungulate with decisively Ungulate teeth, but with the feet to a large extent like those of an unguiculate animal. The same confusion of characters occurs also, it will be remembered, in the distinctly Artiodactyle _Agriochoerus_ (see p. 331). Indeed the feet of the latter when first discovered were erroneously, as it now appears, referred to the present order of Ungulates under the name of _Artionyx_. It is probable that the genus _Moropus_ of North America is a member of this group, and that it is probably congeneric with a somewhat different type of Ancylopod known as _Macrotherium_. It is also clear that _Anisodon_, _Schizotherium_, and _Ancylotherium_, if not congeneric with either of the two recognised genera, are at least very close to them.

_Chalicotherium_ has a skull which recalls that of some of the earlier Ungulates; it has, however, no incisors at all, and no canines in the upper jaw; this feature has led to the belief that the animal is related to the Edentata, and that it is in fact a link between them and the Ungulata. The molars, like those of the Perissodactyla, are of the buno-selenodont type. It also agrees with that group (to which it has been approximated by several writers) in the. tridactyl manus and pes, and in the characters of the tarsus. But although tridactyl, the axis of the limb passes through the fourth digit. _Chalicotherium_ is not mesaxonic, as are the Perissodactyles. Moreover, it has no third {212} trochanter, and the unguiculate claws have already been referred to. As to the latter, which are short, it is not the end phalanx but the first which is retracted; thus _Chalicotherium_ differs markedly from both Carnivorous and Edentate types; for in the former it is the last phalanx which is retracted, while in the Edentates the same phalanx is flexed downwards. The limbs of _Chalicotherium_ are nearly of the same size, and the animal seems to have been stout and quadrupedal.[132]

_Macrotherium_, like the last genus, seems to have been common to both New and Old Worlds. It is to be distinguished by a number of characters. It is supposed to have been "semi-arboreal and fossorial"; the fore-limbs are much longer than the hind, the relative proportions of the radius and tibia being 70 to 29. The ulna was distinct from the radius, whereas in _Chalicotherium_ the two are coalesced, or nearly so. Young specimens appear to possess a full set of incisors; whether this is the case or not with _Chalicotherium_ is not known.[133]

_Homalodontotherium_ is sometimes placed in the group.

SUB-ORDER 4. TYPOTHERIA.

It is a little difficult to be confident that the Typotheria are rightly referred to the Ungulata, since they contradict two important Ungulate rules. They have clavicles, which are elsewhere missing, and the thumb looks as if it were opposable.[134] An Ungulate is essentially a running animal, and has no need of a grasping finger. Still Typotheria are placed by most within the Ungulate series, though their undoubted likenesses to other groups, especially to the Rodentia, are admitted, and indeed emphasised. Cope places them definitely with the Toxodonts.

The Typotheria are an extinct group of smallish beasts, confined, like the Toxodontia, to South America, a region which during the Tertiary period, and into the Pleistocene, abounded with strange and varied types of Ungulate animals.

The earlier forms of Typotheria may be exemplified by some {213} account of the genus _Protypotherium_. This animal was of about the size of a _Hyrax_, which indeed it resembles in several points of structure. The teeth have the primitive number of forty-four, and they are close set, leaving no diastema; the molars are rootless and grow persistently; they are simple and Rodent-like in surface pattern. The shape of the lower jaw is like that of _Hyrax_, being rounded in outline posteriorly; there is no projecting angle as in the Rodents, and this remark applies to the Typotheria in general. The aspect of the Rodent lower jaw is characteristically different from that of _Hyrax_ and the forms under consideration.

Some other characters of these early forms of Typotheria can be gathered from an inspection of other genera. In _Icochilus_ both hand and foot were five-toed, and, as in ancient Ungulates generally, the bones of the wrist and of the ankle are serially and not alternately arranged. Moreover, an os centrale is present in the carpus. Both thumb and big toe were opposable. The skull has a remarkably Rodent-like appearance, but the palate is not so narrowed as in these animals.

In the more recent forms of Typotheria the dentition has become reduced. The canines are lost, and as the incisors are reduced also, to one on each side of the upper, and two on each side of the lower jaw, the likeness to a Rodent skull is increased. There is also evidence of a modification from the more primitive forms in the loss of one premolar or even more, in the alternating bones of the carpus, in the disappearance of the centrale, and in the loss of a toe upon the hind-foot. In these more recent forms the fibula articulates with the astragalus instead of with the calcaneum. Typotheria of these more recent forms may be illustrated by the typical genus _Typotherium_. It has the reduced dental formula I 1/2 C 0/0 Pm 2/1 M 3/3; the molars are simple in pattern, and much like those of _Toxodon_. The upper incisors are powerful and curved, but are surrounded by a layer of enamel, which is not limited to the anterior face, as it practically is in Rodents. The sacrum is composed of a large number of vertebrae--some seven--a state of affairs which recalls the Edentata. The shoulder blade is not Ungulate in form. It has a strong spine, with an acromion and a well-developed metacromion. The terminal phalanges are enlarged and hoof-like.

In the genus _Pachyrucos_ there are three premolars, otherwise {214} the formula is the same as in _Typotherium_. The animal seems to have had nails rather than hoofs. The thumb was opposable. The fibula is fused below with the tibia, whereas in the last genus these two bones are quite separate from each other.

SUB-ORDER 5. TOXODONTIA

The group Toxodontia,[135] like so many others, is exceedingly hard to define. Nor are its limits any easier to mark out than many others of the groups of Ungulates. It will be best perhaps to give an account of _Toxodon_, and of a few types which seem to lie near it in the system, and then to indicate how far they resemble or depart in structure from other Ungulates. _Toxodon_ itself is known from complete skeletons. It lived in Argentina during the "Pampean" period, which seems to be of the Pleistocene age. A large number of species, however, have been described, some of which seem to go farther back in time, and to have existed during the Miocene period further south in Patagonia.

The size of this creature was about that of a large Rhinoceros; it has a bulky body and a large head, which was borne low down, on account of the bending downwards of the anterior vertebrae; in this aspect the figures of the skeletons recall _Glyptodon_ and similar Edentates. The beast was discovered by Darwin, and originally described by Owen. "During his (Mr. Darwin's) sojourn in Banda Oriental," writes the Rev. H. Hutchinson, "having heard of some 'giants' bones' at a farmhouse on the Sarandis, a small stream entering the Rio Negro, he rode there, and purchased for the sum of eighteenpence the skull which has been described by Sir R. Owen. The people at the farm-house told Mr. Darwin that the remains were exposed by a flood having washed down part of a bank of earth. When found, the head was quite perfect, but the boys knocked the teeth out with stones, and then set up the head as a mark to throw at." The whole of the Pampean area is a valley of dry bones, and the remains of _Toxodon_ are abundant there. The skull of _Toxodon_ is not unlike that of a horse in general aspect; but the orbit is not separated from the temporal fossa. The premaxillae are furnished above with a slight protuberance directed towards {215} the free end of the nasals, which may be related to the presence of a short proboscis. The zygomatic arch is strong and broad: the mandibles are provided with a long symphysis. The dental formula is I 2/3 C (0-1)/1 Pm 4/(3-4) M 3/3. The teeth are prismatic and hypselodont, growing from persistent pulps. The molar teeth are slightly arched in form, whence the name of _Toxodon_, "bow teeth." The strong chisel-shaped incisors suggest the Rodents and _Hyrax_. The cheek teeth, moreover, are by no means unlike those of Rodents in their pattern. They are at any rate not at all like those of existing Ungulates. The small size of the canine and of the first premolar produces a diastema in the tooth series. The sacrum consists of five vertebrae, and the ischium does not articulate with it.

The shoulder blade has a strong spine, but only a rudimentary acromion; nor is the coracoid well developed. The radius crosses the ulna, as in the Elephant; the whole fore-limb is shorter than the hind-limb, which must have exaggerated the hang-dog expression of the creature when alive. The elements of the carpus interlock in the modern fashion. Those of the tarsus, however, are primitive in lying below each other without alternation. The carpus has a centrale. The fibula articulates with the calcaneum. The femur has no third trochanter. There are three toes to all the limbs. It is clear that this assemblage of characters will not allow the placing of _Toxodon_ in any living Ungulate order. If the middle toes appear by their slight pre-eminence to approach the Perissodactyle form, the peculiar surface contour of the molar teeth, letting alone the absence of a third trochanter on the femur, will not permit this classification.

Allied to _Toxodon_ is the genus _Nesodon_. It was so named from an "island lobe" on the inner side of the upper molars. This creature, smaller than _Toxodon_, also differs from it in the fact that the dentition is complete, and in the pattern of the molars, which is rather more complex. There is still the slight projection upon the premaxillary bones, but the nostril is directed more forwards than in _Toxodon_. The zygoma, too, is massive. The second pair of incisors in the upper jaw and the outer (third) pair in the lower jaw form biggish tusks in the adult. These and the molar teeth are, however, finally rooted, and do not grow, as in _Toxodon_, from persistent pulps. The genus is from the older Tertiary of Patagonia. Five or six species have been described. Some are as large as a Rhinoceros, others as small as a Sheep. {216}

There is no doubt about the close alliance of the two genera just referred to. It is more doubtful whether _Homalodontotherium_ and its allies should be placed, as they often are, in the neighbourhood of the Toxodonts. _Homalodontotherium_ owes its name to its even row of teeth without a diastema. It was a creature of equally large size with _Toxodon_, and also came from the Tertiary strata of Patagonia. The teeth are the typical forty-four, and the molars like those of a Rhinoceros; they are, however, brachyodont and not hypselodont as in _Toxodon_. This genus, however, shows an important difference from the Rhinocerotidae and from the other Toxodontia in the fact that it was five-toed, and that the bones of the carpus and tarsus are set in relation to each other in the linear serial fashion.

Undoubtedly a near relative of _Homalodontotherium_ is _Astrapotherium_. This creature was of equal bulk, and was also Patagonian in range. The teeth are reduced in number, but the animal was provided, like a Wild Boar, with great tusks, which were, however, formed by the incisors. This animal is very imperfectly known; it is the form of the molars and the large size of the incisors which have led to its association with the Toxodontia. As to the resemblance of the teeth of this genus and of _Homalodontotherium_ to those of _Rhinoceros_, it is difficult to regard it as evidence of near affinity. The likeness is probably to be looked upon as a case of parallelism in development. Exactly the same explanation is possibly to be given to the likeness which the teeth of _Toxodon_ and _Nesodon_ show to Rodents, or even to Edentates. As to their affinities Zittel observes:--

"The entirety of their osteological characters argues for the Toxodon a separate position in the neighbourhood of the Perissodactyla, Proboscidea, Typotheria, and Hyracoidea. The relations to the Rodentia rest mainly upon the converging development of the teeth, not upon true relationship."

SUB-ORDER 6. PROBOSCIDEA.

Large vegetable-feeding animals, usually scantily covered with hair, and with the nostrils and upper lip drawn out into a long proboscis. Digits five on both limbs. Femur and humerus not bent upon lower leg and fore-arm in a position of rest. Skull {217} with abundant air cavities in the roofing and other bones. The incisors are developed into long tusks, which exist in the upper jaw alone, in the lower jaw alone, or in both jaws. There are no canines. The molars are lophodont. The clavicle is absent. The femur has no third trochanter. The bones of the carpus are serially arranged and do not interlock. The stomach is simple. The brain has much convoluted cerebral hemispheres, but the cerebellum is completely uncovered by them. The intestine is provided with a wide caecum. The testes are abdominal. The teats are pectoral in position. The placenta is non-deciduate and zonary. There are two venae cavae superiores.

The position of the limbs in the Elephant tribe is unique among living animals: their straightness that is to say, and the absence or very slight development of angulation at the joints of the limb bones. This same feature has been observed in the extinct Dinocerata and in the Titanotheria. It must not, however, be assumed from the resemblance to these ancient forms that there is much affinity between them and the Proboscidea, or that the latter have retained an ancient feature of organisation. The oldest Ungulates for the most part, and the Creodonts to which they are undoubtedly related, have much bent limbs. It must be considered, therefore, that the arrangement obtaining in the Elephants is purely secondary. Professor Osborn has put forward the reasonable view[136] that the vertical limbs of all these colossal creatures are due to "an adaptation designed to transmit the increasing weight" of these animals. The huge bulk of the body is better borne by vertical pillars than by an angulated limb. Other points, however, such as the exposure of the cerebellum, the two venae cavae, the five digits, and the absence of a third trochanter, argue a low position for the Proboscidea in the Eutherian group.

The group can be readily divided into two families, the Elephantidae and the Dinotheriidae. We will commence with the former.

The Elephants proper, ELEPHANTIDAE, differ from the Dinotheriidae in, and are characterised by, a number of anatomical features. They possess long tusks (incisors) either in both jaws, or, if only in one jaw, in the upper. The molar teeth are very large--so large that only a few of them are simultaneously in use. There are but three definable genera of Elephantidae, of which {218} _Elephas_ alone survives. This genus also includes many extinct forms, both American and European, as well as Asiatic and African. The entirely extinct genera are _Stegodon_ and _Mastodon_. The group is clearly one dwindling towards extermination. From the Middle Miocene downwards these great "pachyderms" have existed; and from the Miocene up to Pleistocene times they were almost world-wide in range and numerous in species.

The genus _Elephas_ comprises usually large, but occasionally (the pygmy Elephant of Malta) quite small forms. The external features of the genus differ slightly in different species, and will therefore be described in relation to those species which we shall notice here. The vertebral formula is C 7, D 19-20, L 3-5, Sa 4-5, Ca 24-30, or even more.

The bodies of the vertebrae are remarkable for their shortness and for the very flattened articular surfaces.

FIG. 116.--A section of the cranium of a full-grown African Elephant, taken to the left of the middle line, and including the vomer (_Vo_) and the mesethmoid (_ME_); _an_, anterior, and _pn_, posterior narial aperture. x 1/12. (From Flower's _Osteology_.)

The skull is large and massive. Its large and heavy character is, as has been stated in the definition of the sub-order, due to the immense development of air cavities in the diploe; the diameter of the wall of the skull is actually greater than that of the cranial cavity. These cavities are not obvious in the young animal. They are most conspicuous in the roofing bones of the skull, but are seen elsewhere, and thicken the basis cranii, {219} the maxillae, and so forth. This state of affairs, together with the presence of the huge tusks, has, as it were, pushed back the nasal orifices to near the top of the skull in a very Whale-like fashion. As in the Cetacea, the nasal bones are limited in size, and the premaxillae send up processes to join the frontals and the nasals. There is a straight and somewhat slender zygomatic arch, but the orbit is not separated from the temporal fossa. The malar bone is small, and, as in Rodents, forms the middle part of the zygoma. This is not the case with most Ungulata. The symphysis of the mandibles forms a spout-like rim. The scapula has a narrow prescapular, but a very wide postscapular region. The spine has a strong process projecting backwards from near its middle; this is a point of likeness to certain Rodents. No Elephant has a clavicle. The most remarkable feature about the fore-limb is the separation and crossing of the radius and ulna. The arms of these animals are permanently fixed in the position of pronation. The foot is short, and the bones of the carpus are serially arranged. There are, however, traces of a commencing interlocking of these bones in many forms. The hind-feet are somewhat smaller than the fore-feet, and the tibia and fibula are both developed.

As to the teeth, this genus is to be distinguished from allied forms by the presence of tusks in the upper jaw only. These tusks have no bands of enamel such as characterise those of _Mastodon_. They are incisors. There is, however, a trace of the former enamelling in the shape of a patch at the tip, which soon wears away. The molar teeth of _Elephas_ are so large that the jaws cannot accommodate more than at the most two and a part of a third at a time. These are gradually replaced by others to the number of three, the replacement of teeth suggesting that of the Manatee. Each molar is deeply ridged, the interstices between the ridges being filled up with cement. As the tooth wears away, therefore, the surface continues to be flat. Each ridge consists of a core of dentine surrounded by a coat of enamel. The number of these ridges varies greatly from species to species. The Indian Elephant is one of those which have the greatest number of plates in a single tooth, as many as twenty-seven.[137] Of the six molars which {220} eventually appear, the first three are considered to correspond to premolars. But successional teeth are rare in the genus; that is to say as far as concerns the molars, for the tusks have their milk forerunners. As to the molars it is apparently only _E. planifrons_ which certainly shows a milk dentition. In _Mastodon_ and older types a milk dentition is commoner.

The viscera of the Elephant have been examined by many zoologists. The latest paper, dealing chiefly with the African species, but containing facts about its Indian congener also, is quoted below.[138] The Elephant is remarkable in possessing, in addition to the three usual pairs of salivary glands present in mammals, a fourth, situated in the molar region, and opening on to the cheek by many pores. This gland is especially well developed in Rodents. There is a gland which may be mentioned in this connexion, though it opens externally between the eye and ear, known as the temporal gland; its use does not seem clear. The thoracic cavity of the Elephant, as may be inferred from the large number of ribs, is very large as compared with the abdominal.

The stomach is simple in form, and the epithelium of the oesophagus does not extend into it as is the case with the Horse and Rhinoceros. A gland or a collection of smaller glands occurs in the stomach, and recalls the "cardiac gland" of the Wombat and the Beaver, also that of the Giraffe. The large intestine is long, rather more than half the length of the small intestine. The caecum is well developed in these animals. The liver has a very simple form, being but slightly lobulated. It is actually only bilobed, but it is important to notice that this division does not correspond to the two halves of the liver. As shown by the attachment of the suspensory ligament, one half consists of the left lateral lobe alone, the other half embracing the remaining primary lobes. The simplicity of the liver looks like an archaic character. No Elephant has a gall-bladder. The lungs again are simple in form through their slight lobulation. Each half in fact is without subdivisions, and is of a triangular form. In this the Elephants resemble the Whales, as in the simple liver. In both cases probably the likeness is due to the permanence of primitive features of organisation. The brain[139] of the Elephant {221} has hemispheres which are extremely well convoluted; but they leave the cerebellum entirely uncovered. This suggests a brain which is a great specialisation of a low type. The brain has been particularly compared with that of the Carnivora, with which group the Elephants agree in the characters of the placenta. It is, however, always a matter of the very greatest difficulty to compare the brains of mammals belonging to different orders.

There are but two living species of Elephant, of both of which we shall now proceed to give some account. Only a few of the rather numerous fossil forms can be touched upon here.