Chapter IX: The Order Ungulata (6)

_Digestive Organs._—The lips are flexible and prehensile. The membrane that lines them and the cheeks is quite smooth. The palate is long and narrow; its mucous surface has seventeen pairs of not very sharply defined oblique ridges, extending as far back as the last molar tooth, beyond which the velum palati extends for about 3 inches, having a soft corrugated surface, and ending posteriorly in an arched border without uvula. This embraces the base of the epiglottis, and shuts off all communication between the cavity of the mouth and the nasal passages, respiration being, under ordinary circumstances, carried on exclusively through the nostrils. Between the mucous membrane and the bone of the hard palate is a dense vascular and nervous plexus. The membrane lining the fauces is soft and corrugated. An elongated raised glandular mass, 3 inches long and 1 inch from above downwards, extending backwards from the root of the tongue along the side of the fauces, with openings on the surface leading into crypts with glandular walls, represents the tonsil. The tongue, corresponding to the general form of the mouth, is long and narrow. It consists of a compressed intermolar portion with a flat upper surface, broad behind and becoming narrower in front; and of a depressed anterior part rather shorter than the former, which is narrow behind but widens towards the evenly rounded apex. The dorsal surface generally is very soft and smooth. There are two large circumvallate papillæ near the base, rather irregular in form, about a quarter of an inch in diameter and half an inch apart. The conical papillæ are very small and close set, though longer and more filamentous on the intermolar portion. There are no fungiform papillæ on the dorsum, but a few not very conspicuous ones scattered along the sides of the organ.

Of the salivary glands the parotid is by far the largest; elongated in the vertical direction, and narrower in the middle than at either upper or lower extremity. Its upper extremity embraces the lower surface of the cartilaginous ear-conch; its lower end reaches the level of the inferior margin of the mandible, along the posterior margin of which it is placed. Its duct leaves the inferior anterior angle, at first descends a little, and runs forward under cover of the rounded inferior border of the mandibular ramus, then curves up along the anterior margin of the masseter muscle, becoming superficial, pierces the buccinator, and enters the mouth by a simple aperture opposite the middle of the crown of the third premolar tooth. It is not quite so thick as a goose-quill when distended, and nearly a foot in length.

The submaxillary gland is of very similar texture to the last, but much smaller; it is placed deeper, and lies with its main axis horizontal. It is elongated and slender, and flattened from within outwards. Its posterior end rests against the anterior surface of the transverse process of the atlas, from which it extends forwards and downwards, slightly curved, to beneath the ramus of the jaw. The duct which runs along its upper and internal border passes forwards in the usual course, lying in the inner side of the sublingual gland, to open on the outer surface of a distinct papilla, situated on the floor of the mouth, half an inch from the middle line, and midway between the lower incisor teeth and the attachment of the frænum linguæ. The sublingual is represented by a mass of glands lying just beneath the mucous membrane of the floor of the mouth on the side of the tongue, causing a distinct ridge, extending from the frænum backwards, and the numerous ducts opening separately along the summit of the ridge. The buccal glands are arranged in two rows parallel with the molar teeth. The upper ones are the largest, and are continuous anteriorly with the labial glands, the ducts of which open on the mucous membrane of the upper lip.

The stomach of the Horse is simple in its external form, with a largely developed right _cul de sac_, and is a good deal curved on itself, so that the cardiac and pyloric orifices are brought near together. The antrum pyloricum is small and not very distinctly marked off. The interior is divided by the character of the lining membrane into two very distinct portions, right and left. Over the latter the dense white smooth epithelial lining of the œsophagus is continued, terminating abruptly by a raised crenellated border. Over the right part (rather the larger portion) the mucous membrane has a grayish-red colour and a velvety appearance, and contains very numerous peptic glands, which are wanting in the cardiac portion. The œsophageal orifice is very small, and is guarded by a strong crescentic or rather horse-shoe-like band of muscular fibres, which is supposed to be the cause of the difficulty of vomiting in the Horse. The small intestine is of great length (80 to 90 feet), its mucous membrane being covered with numerous fine villi. The cæcum is of conical form, about 2 feet long and nearly a foot in diameter; its walls are sacculated, especially near the base, having four longitudinal fibrous bands; and its capacity is about twice that of the stomach. It lies with its base near the lower part of the abdomen, and its apex directed towards the thorax. The colon is about one-third the length of the small intestine, and very capacious in the greater part of its course. As usual, it may be divided into an ascending, transverse, and descending portion; but the middle or transverse portion is folded into a great loop, which descends as low as the pubis; so that the colon forms altogether four folds, generally parallel to the long axis of the body. The descending colon is much narrower than the rest, and not sacculated, and being considerably longer than the distance it has to traverse, is thrown into numerous folds.

The liver (Fig. 166) is tolerably symmetrical in its general arrangement, being divided nearly equally into segments by a well-marked umbilical fissure. Each segment is again divided by lateral fissures, which do not extend quite to the posterior border of the organ; of the central lobes thus cut off, the right is rather the larger, and has two fissures in its free border subdividing it into lobules. The extent of these varies, however, in different individuals, being not usually so marked as in the figure, which is from a fœtal specimen. The two lateral lobes are subtriangular in form. The Spigelian lobe is represented by a flat surface between the portal fissure and the posterior border, not distinctly marked off from the left lateral by a fissure of the ductus venosus, as this vessel is buried deep in the hepatic substance, but the caudate lobe is distinct and tongue-shaped, its free apex reaching nearly to the border of the right lateral lobe. In most works on the anatomy of the Horse this has been confounded with the Spigelian lobe of man. There is no gall-bladder (as in all other Perissodactyles), and the biliary duct enters the duodenum about 6 inches from the pylorus. The pancreas has two lobes or branches—a long one passing to the left and reaching the spleen, and a shorter right lobe. The principal duct enters the duodenum with the bile-duct, and there is often a second small duct which opens separately near to this.

_Circulatory and Respiratory Organs._—The heart has the form of a rather elongated and pointed cone. There is one anterior vena cava, formed by the union of the two jugular and two axillary veins. The aorta gives off a large branch (the anterior aorta) very near its origin, from which arise—first, the left axillary, and afterwards the right axillary and the two carotid arteries.

Under ordinary circumstances the Horse breathes entirely by the nasal passages, the communication between the larynx and the mouth being closed by the velum palati. The nostrils are placed laterally, near the termination of the muzzle, and are large and very dilatable, being bordered by cartilages upon which several muscles act. Immediately within the opening of the nostril, the respiratory canal sends off on its upper and outer side a diverticulum or blind pouch (called “false nostril”) of a conical form, and curved, 2 to 3 inches in depth, lying in the notch formed between the nasal and premaxillary bones. It is lined by mucous membrane continuous with that of the nasal passage, but its use is not apparent. It is longer in the Ass than in the Horse. A similar structure is found in the Rhinoceros, and in a much more developed condition in the Tapir. Here may be mentioned the guttural pouches, large air sacs, diverticula from the Eustachian tubes, and lying behind the upper part of the pharynx. These are likewise found in other Perissodactyles, but their use is also still not clearly understood. The larynx has the lateral sacculi well developed, though entirely concealed within the alæ of the thyroid cartilage. The trachea divides into two bronchi, one for each lung.

_Nervous System._—The brain differs little, except in details of arrangement of convolutions, from that of other Ungulates. The cerebral hemispheres are rather elongated and subcylindrical, the olfactory lobes are large and project freely in front of the hemispheres, and the greater part of the cerebellum is uncovered. The eye is provided with a nictitating membrane or third eyelid, at the base of which the ducts of the Harderian gland open.

_Reproductive System._—The testes are situated in a distinct sessile or slightly pedunculated scrotum, into which they descend from the sixth to the tenth month after birth. The accessory generative glands are the two vesiculæ seminales, with the median third vesicle, or _uterus masculinus_, lying between them, the single bilobed prostate, and a pair of globular Cowper’s glands. The penis is large, cylindrical, with a truncated, expanded, flattened termination. When in a state of repose it is retracted by a muscle arising from the sacrum, within the prepuce, a cutaneous fold attached below the symphysis pubis.

The uterus is bicornuate. The vagina is often partially divided by a membraneous septum or hymen. The mammæ (as in other members of the suborder), are two, inguinally placed. The surface of the chorion is covered evenly with minute villi, constituting a diffuse non-deciduate placenta. The period of gestation is eleven months.

_Bibliography._—M. S. Arloing, “Organisation du pied chez le
cheval,” _Ann. Sci. Nat._ 1867, viii. pp. 55-81; H. Burmeister,
_Los caballos fosiles de la Pampa Argentina_, Buenos Ayres,
1875; Chanveau and Arloing, _Traité d’anatomie comparée des
animaux domestiques_, Paris, 1871, and English edition by G.
Fleming, 1873; E. Cuyer and E. Alix, _Le Cheval_, 1886; A.
Ecker, “Das Europäische Wildpferd und dessen Beziehungen zum
domesticirten Pferd,” _Globus_, Bd. xxxiv. Brunswick, 1878;
Forsyth-Major, “Beiträge zur Geschichte der fossilen Pferde
besonders Italiens,” _Abh. Schw. Pal. Ges._ iv. pp. 1-16, pt.
iv.; George, “Études zool. sur les Hémiones et quelques autres
espèces chevalines,” _Ann. Sci. Nat._ 1869, xii. p. 5; E. F.
Gurlt, _Anatomische Abbildungen der Haussäugethiere_, 1824,
and _Hand. der vergleich. Anat. der Haussäugethiere_, 2 vols.
1822; Huet, “Croisement des diverses espèces du genre cheval,”
_Nouv. Archives du Muséum_, 2d sér. tom. ii. p. 46, 1879;
Leisering, _Atlas der Anatomie des Pferdes_, Leipsic, 1861;
J. M’Fadyean, _The Anatomy of the Horse_, 1884; O. C. Marsh,
“Notice of New Equine Mammals from the Tertiary Formation,”
_Am. Journ. of Science and Arts_, vol. vii. March 1874; Id.
“Fossil Horses in America,” _Amer. Naturalist_, vol. viii.
May 1874; Id. “Polydactyle Horses,” _Am. Journ. of Science
and Arts_, vol. xvii. June 1879; Franz Müller, _Lehrbuch der
Anatomie des Pferdes_, Vienna, 1853; R. Owen, “Equine Remains
in Cavern of Bruniquel,” _Phil. Trans._ vol. clix. (1870), p.
535; W. Percivall, _The Anatomy of the Horse_, 1832; G. Stubbs,
_Anatomy of the Horse_, 1766. F. H. Huth’s _Bibliographical
Record of Hippology_ (1887) contains a list of nearly four
thousand works on Horses and Equitation, published in the
various languages of the civilised world.

_Family_ RHINOCEROTIDÆ.

Although the existing members of this family are readily distinguished from the other living representatives of the suborder by the simple crescentoid form assumed by the ridges of the lower cheek-teeth, yet it is exceedingly difficult to give a definition by which they can be distinguished from the _Lophiodontidæ_, from some members of which they are, indeed, probably derived. The outer columns of the upper molars (Fig. 167) are, however, so excessively flattened as to produce a continuous thick and nearly straight outer wall, which is often produced in advance of the anterior transverse ridge; both transverse ridges being but little curved, and intimately connected with the outer wall. The upper premolars are in most cases nearly or quite as complex as the molars, and the ridges of the lower cheek-teeth are crescentoid. The last lower molar has no third lobe. The height of the crowns of the cheek-teeth is variable. The skull is large, with the orbit confluent with the temporal fossa. There are either three or four digits in the manus, and three in the pes. One or more dermal horns are attached to the fronto-nasal region of the skull of existing forms, but these were wanting in some of the fossil species.

_Rhinoceros._[267]—Incisors variable, reduced in number, often quite rudimentary, and early deciduous. Upper canines absent. Molar series, consisting of the full number of four premolars and three molars above and below, all in contact and closely resembling each other, except the first, which is much smaller than the rest and often deciduous; and the last, in which the hinder lobe is partly aborted, so that the contour of the crown is triangular. Head large, skull elongated, elevated posteriorly into a transverse occipital crest. No postorbital processes. Nasal bones large and stout, co-ossified, and standing out freely above the premaxillæ, from which they are separated by a deep and wide fissure; the latter small, generally not meeting in the middle line in front, often quite rudimentary. Tympanics small, not forming a bulla. Brain cavity very small for the size of the skull. Vertebræ: C 7, D 19-20, L 3, S 4, C about 22. Limbs stout, and of moderate length. Three completely developed toes, with distinct broad rounded hoofs on each foot (Fig. 151, p. 368), some fossil forms having a fourth in the manus. Eyes small. Ears of moderate size, oval, erect, prominent, placed near the occiput. Skin very thick, in many species thrown into massive folds. Hairy covering scanty. When one horn is present it is situated over the conjoined nasal bones; when two, the hinder one is over the frontals. These horns, which are of a more or less conical form and usually recurved, often grow to a great length (three or even four feet), and are composed of a solid mass of hardened epidermic cells growing from a cluster of long dermal papillæ. The cells formed on each papilla constitute a distinct horny fibre, like a thick hair, and the whole are cemented together by an intermediate mass of cells which grow up from the interspaces between the papillæ. It results from this that the horn has the appearance of a mass of agglutinated hairs, which, in the newly growing part at the base, readily fray out on destruction of the softer intermediate substance; but the fibres differ from true hairs in growing from a free papilla of the derm, and not within a follicular involution of the same.

The large lower cutting teeth of the typical Rhinoceroses have been very generally regarded as incisors, but comparison with fossil allied types, in which three lower incisors and canines are present, leaves little doubt but that they are really canines. The upper molar teeth present some amount of specific variation; thus while one type (Fig. 168, _A_) has only a simple “crotchet” projecting from the posterior transverse ridge into the median valley, in others (Fig. 168, _B_) this crotchet joins a “crista,” or “combing-plate,” projecting from the outer wall to cut off a distinct fossette from the median valley. Occasionally, however (as in Fig. 167), the crotchet and combing-plate do not completely join, although the fossette is distinctly indicated. The first upper premolar may occasionally be preceded by a milk-tooth. The Rhinoceroses differ from the Horses and agree with the Tapirs in the direction of the cæcum.

The living species of _Rhinoceros_ are all animals of large size, but of little intelligence, generally timid indisposition, though ferocious when attacked and brought to bay, using the nasal horns as weapons, by which they strike and toss their assailant. Their sight is dull, but their hearing and scent are remarkably acute. They feed on herbage, shrubs, and leaves of trees, and, like so many other large animals which inhabit hot countries, sleep the greater part of the day, being most active in the cool of the evening or even during the night. They are fond of bathing and wallowing in water or mud. None of the species have been domesticated. Animals of the group have existed in both the Old and New Worlds since the latter part of the Eocene period. In America they all became extinct before the end of the Pliocene period. In the Old World their distribution has become greatly restricted, and they are no longer found in Europe and North Asia, but only in Africa and portions of the Indian and Indo-Malayan region.

_Existing Species._—The existing (as well as many of the extinct) species of Rhinoceroses naturally divide into three groups, which are regarded by some zoologists as of generic value.

_Rhinocerotic, or Typical Group._—The adults with a single large compressed incisor above on each side, and occasionally a small lateral one; below, a very small incisor and a very large, procumbent, pointed canine. Nasal bones pointed in front. A single nasal horn. Skin very thick, and raised into strong, definitely arranged ridges or folds.

There are two well-marked species of one-horned Rhinoceroses. (1) The Indian Rhinoceros, _R. unicornis_ (Fig. 169) of Linnæus,[268] the largest and best known, from being the most frequently exhibited alive in England, is at present only met with in a wild state in the terai region of Nipal and Bhutan, and in the upper valley of the Brahmaputra or province of Assam, though it formerly had a wider range. The first Rhinoceros seen alive in Europe since the time when these animals, in common with nearly all the large remarkable beasts of both Africa and Asia, were exhibited in the Roman shows, was of this species. It was sent from India to Emmanuel, King of Portugal, in 1513; and from a sketch of it, taken in Lisbon, Albert Dürer composed his celebrated but rather fanciful engraving, which was reproduced in so many old books on natural history. Both in this and the following species the post-glenoid and post-tympanic processes of the squamosal bone of the skull unite below so as to completely surround the external auditory meatus. The molar teeth are hypsodont, and have a horizontal plane of wear; those of the upper jaw (Fig. 168, _b_) being characterised by the presence of a combing-plate joining the crotchet, and the absence of a distinct buttress at the antero-external angle. The stomach departs from the ordinary Perissodactyle type. The small intestine is beset over most of its surface with long and fine villi; and the Spigelian lobe of the liver is well developed. There is a gland behind the foot. Teeth from the Pleistocene of the Narbada valley in India apparently indicate the existence of the Indian Rhinoceros at that epoch. (2) The Javan Rhinoceros (_R. sondaicus_, Fig. 170) is a smaller form, readily distinguished by dental and internal characters, as well as by the different arrangement of the plications of the skin (as seen in the figures); the horn in the female appears to be very little developed, if not altogether absent. This species has a more extensive geographical range, being found in the Bengal Sunderbans near Calcutta, Burma, the Malay Peninsula, Java, Sumatra, and probably Borneo. The molar teeth have shorter crowns than in the preceding species, and wear into ridges; those of the upper jaw (Fig. 168, _a_) having no combing-plate, and a strongly marked buttress at the antero-external angle (not distinctly shown in the figure). The visceral anatomy, according to Beddard,[269] does not differ materially from that of the next species. In respect to its dentition and anatomical characters this species is indeed more nearly allied to the Sumatran than to the Indian Rhinoceros; and thereby indicates that the division of the existing Rhinoceroses into separate genera is not advisable.

_Ceratorhine Group._—The adults with a moderate-sized compressed incisor above, and a laterally placed, pointed, procumbent canine below, which is sometimes lost in old animals. Nasal bones narrow and pointed anteriorly. A well-developed nasal, and a small frontal horn separated by an interval. The skin thrown into folds, but these not so strongly marked as in the former group. The smallest living member of the family, the Sumatran Rhinoceros, _R. sumatrensis_, Cuvier, now represents this group. Its geographical range is nearly the same as that of the Javan species, though not extending into Bengal; but it has been found in Assam, Chittagong, Burma, the Malay Peninsula, Sumatra, and Borneo. So far as can be determined during the life of the type specimen, it appears that the hairy form from Chittagong, described as _R. lasiotis_, is only a variety of this species.[270] The molar teeth of the Sumatran Rhinoceros are almost indistinguishable from those of the Javan species, and reference has already been made to the resemblance between the visceral anatomy of these species.[271] The form of the stomach is very similar to that of the Horse. The liver (Fig. 171) has a comparatively large caudate lobe, but is chiefly remarkable for the peculiar shape of the Spigelian lobe, which mainly consists of a thin strip of tissue, 8 inches long, ¾ inch wide, and ¼ inch deep. The small intestine, in place of the villi of _R. unicornis_, has throughout the greater part of its length a uniform series of thin and nearly or quite continuous transverse foldings, like the valvulæ conniventes of the human small intestine. There is no gland behind the foot. The post-glenoid and post-tympanic processes of the squamosal do not unite below the auditory meatus. The presence of a lateral nasal diverticulum, like that of the Horses and Tapirs, has been verified only in this species, although it doubtless occurs in the others.

_Atelodine Group._—In the adults the incisors and canines quite rudimentary or entirely wanting. Nasal bones thick, rounded and truncated in front. Well-developed anterior and posterior horns in close contact. Skin without any definite permanent folds.

The two well-marked existing species are peculiar to the African continent.

The common Two-horned Rhinoceros, _R. bicornis_, is the smaller of the two, with a pointed prehensile upper lip, and a narrow compressed deep symphysis of the lower jaw. It ranges through the wooded and watered districts of Africa, from Abyssinia in the north to the Cape Colony, but its numbers are yearly diminishing, owing to the inroads of European civilisation, and especially of English sportsmen. It feeds exclusively upon leaves and branches of bushes and small trees, and chiefly frequents the sides of wood-clad rugged hills. Specimens in which the posterior horn has attained a length as great as, or greater than, the anterior have been separated under the name of _R. keitloa_, but the characters of these appendages are too variable to found specific distinctions upon. The Common African Rhinoceros is far more rarely seen in menageries in Europe than either of the three Oriental species, but one has lived in the gardens of the London Zoological Society since 1868. The molar teeth of this species are of the general type of those of _R. sondaicus_, having no combing-plate to join the crotchet in those of the upper jaw. The conch of the ear is much rounded at its extremity, and edged by a fringe of short hairs; while the nostrils are somewhat rounded. The eye is placed immediately below the posterior horn.[272] Both in this and the following species the post-glenoid and post-tympanic processes of the squamosal do not unite below the auditory meatus. Nothing is known of the anatomy of the soft parts of either of them.

Burchell’s or the Square-mouthed Rhinoceros (_R. simus_), sometimes called the White Rhinoceros, though the colour (dark slate) is not materially different from that of the last species, is the largest of the whole group, and differs from all the others in having a square truncated upper lip and a wide, shallow, spatulate symphysis to the lower jaw. In conformity with the structure of the mouth, this species lives entirely by browsing on grass, and is therefore more partial to open countries or districts where there are broad grassy valleys between the tracts of bush. It is only found in Africa south of the Zambesi, and of late years has become extremely scarce, owing to the persecutions of sportsmen; indeed, the time of its complete extinction cannot be far off. No specimen of this species has ever been brought alive to Europe. Mr. F. C. Selous[273] gives the following description of its habits from extensive personal observation:—

“The square-mouthed rhinoceros is a huge ungainly-looking beast, with a disproportionately large head, a large male standing 6 feet 6 inches at the shoulder. Like elephants and buffaloes they lie asleep during the heat of the day, and feed during the night and in the cool hours of early morning and evening. Their sight is very bad; but they are quick of hearing, and their scent is very keen; they are, too, often accompanied by rhinoceros birds, which, by running about their heads, flapping their wings, and screeching at the same time, frequently give them notice of the approach of danger. When disturbed they go off at a swift trot, which soon leaves all pursuit from a man on foot far behind; but if chased by a horseman they break into a gallop, which they can keep up for some distance. However, although they run very swiftly, when their size and heavy build is considered, they are no match for an average good horse. They are, as a rule, very easy to shoot on horseback, as, if one gallops a little in front of and on one side of them, they will hold their course, and come sailing past, offering a magnificent broadside shot, while under similar circumstances a prehensile-lipped rhinoceros will usually swerve away in such a manner as only to present his hind-quarters for a shot. When either walking or running, the square-mouthed rhinoceros holds its head very low, its nose nearly touching the ground. When a small calf accompanies its mother it always runs in front, and she appears to guide it by holding the point of her horn upon the little animal’s rump; and it is perfectly wonderful to note how in all sudden changes of pace, from a trot to a gallop or _vice versâ_, the same position is always exactly maintained. During the autumn and winter months (_i.e._ from March to August) the square-mouthed rhinoceros is usually very fat; and its meat is then most excellent, being something like beef, but yet having a peculiar flavour of its own. The part in greatest favour among hunters is the hump, which, if cut off whole and roasted just as it is in the skin, in a hole dug in the ground, would, I think, be difficult to match either for juiciness or flavour.”

The molar dentition is of the type obtaining in _R. unicornis_, so that in this respect _R. simus_ has the same relation to _R. bicornis_ as is presented by _R. unicornis_ to _R. sondaicus_. The ear-conch of the Square-mouthed Rhinoceros is very large, elongated, and pointed at its extremity, which bears only a slight tuft of hair; it is much expanded in the middle, and the lower portion has its edges united to form a short tube. The nostrils have a long slit-like aperture; and the eye is situated behind the posterior horn.

_Extinct Species._—Using the generic term _Rhinoceros_ in its widest signification, a very large number of fossil forms may be referred to it, the earliest of which date from the Upper Eocene (Oligocene) Phosphorites of Central France. Only a few of the more important of these types can, however, be even mentioned in this place.

In the Pliocene Siwaliks of India _R. sivalensis_ appears to have been the direct ancestor of _R. sondaicus_; while _R. palæindicus_ was probably nearly related to _R. unicornis_, although the upper molars had not developed a combing-plate.

_R. schleirmacheri_, of the Lower Pliocene of Europe, falls into the Ceratorhine group, although differing from _R. sumatrensis_ by the union of the post-glenoid and post-tympanic processes of the squamosal beneath the auditory meatus. The Middle Miocene _R. sansaniensis_ was a closely allied if not identical form.

The Atelodine group was very widely spread in past epochs. Thus the huge _R. platyrhinus_ of the Indian Pliocene, and the equally large _R. antiquitatis_ of the Pleistocene of Europe, were specialised forms with a dentition resembling that of _R. simus_, to which they were probably allied. An upper molar of _R. antiquitatis_—the so-called Tichorine, or Woolly Rhinoceros—is shown in the woodcut on p. 402. Of this species nearly whole carcases, with the thick woolly external covering, have been discovered associated with those of the Mammoth, preserved in the frozen soil of the north of Siberia. In common with some other extinct species it had a solid median wall of bone supporting the nasals, from which it is inferred that the horns were of a size and weight surpassing that of the modern species. In the Lower Pliocene of Attica _R. pachygnathus_ appears to have been closely allied to _R. bicornis_. Several species such as _R. leptorhinus_ (Fig. 173), _R. megarhinus_, and _R. etruscus_, occur in the European Pleistocene which do not present a marked relationship to any of the living forms. This group is also represented in the Pleistocene of Southern India by the small _R. deccanensis_ and _R. karnuliensis_.

In the Upper Miocene, or Lower Pliocene, of North America numerous Rhinoceroses with incisor teeth occur which have no nasal horn, although in those forms of which the limbs are known the fore feet resembled those of existing species in having only three digits. These species have been generically separated as _Aphelops_, but so closely do they resemble existing Rhinoceroses that at one time Professor Cope proposed to refer the hornless female of _R. sondaicus_ (described by Lesson as _R. inermis_) to the same genus. If these American types be included in _Rhinoceros_ there seems no valid reason for separating the European Lower Pliocene and Miocene forms described as _Aceratherium_, at least some of which have four digits in the manus. This group is represented in the Upper Eocene Phosphorites of France, and also by a very large species in the Pliocene of India. Lastly, _R. minutus_, of the Lower Miocene of France, and an allied North American species are distinguished by carrying a pair of very small horns placed transversely across the nasals, from which feature it has been proposed that they should be separated genetically as _Diceratherium_.

_Extinct Generic Types._—The Tertiary deposits of different parts of the world have yielded remains of many extinct forms more or less closely related to the Rhinoceroses, and some of which should certainly be included in the same family; although others perhaps form the types of one or more distinct families. One of the most remarkable of these extinct types is the huge _Elasmotherium_, from the Pleistocene of Siberia, in which the dentition was reduced to two premolars and three molars on either side of each jaw. The structure of the skeleton is essentially rhinocerotic, the skull having an ossified nasal septum, and a huge frontal prominence for the support of a very large horn. The teeth are extremely hypsodont, with the enamel plicated to a remarkable degree, and unlike those of _Rhinoceros_. The genus is evidently a very specialised one.

The other genera we have to notice are more generalised types. Of these the North American _Hyracodon_, with the full typical number of teeth, and without nasal horn, appears to connect the Rhinoceroses with the Lophiodont _Hyrachyus_. The genera _Amynodon_ and _Metamynodon_ (Fig. 174), from the American Tertiaries, are forms allied to the Rhinoceroses, with the full number of incisors and canines, and the hinder lobe of the last upper molar not aborted. The lower canines are either upright, or less proclivous than in the Rhinoceroses; in _Metamynodon_ the premolars are reduced to ³⁄₂. Molar teeth from the Phosphorites of Central France, described under the name of _Cadurcotherium_, are constructed on the general plan of those of the Rhinoceroses, although distinguished by their extreme narrowness; this type of tooth being very similar to that found in _Homalodontotherium_ from Tertiary deposits in Patagonia. The latter has the full number of teeth, without any diastema in the series. Until we have some knowledge of the skeleton of these remarkable forms nothing definite can be said as to their serial position.

_Families_ LAMBDOTHERIIDÆ, CHALICOTHERIIDÆ, AND TITANOTHERIIDÆ.

These families contain a large number of more or less nearly related extinct types from Tertiary beds of both the Old and New Worlds, some of which present most remarkable deviations from the ordinary Ungulate structure. All are characterised by their brachydont molars, which depart widely from the normal lophodont type. The upper molars consist of four columns, of which the two external ones are expanded to form an outer wall; the posterior pair being connected in some cases by an oblique transverse ridge, while there may be traces of an anterior ridge. The premolars are simpler.

_Lambdotheriidæ._—This family is confined to the Upper Eocene and Miocene of North America, where it is represented by _Lambdotherium_, _Palæosyops_, and _Limnosyops_; it presents the normal type of foot structure, and all the genera except the first have the full complement of teeth. There were four digits in the manus. The last lower molar has a third lobe. _Limnosyops_ differs from _Palæosyops_ in having two inner columns to the last upper molar.

_Chalicotheriidæ._—The genus _Chalicotherium_, which is found in the Tertiaries of Europe, Asia, and North America, differs so remarkably in the structure of the feet from all other Ungulates that it has been proposed to regard it as the representative of a distinct order, Ancylopoda. The molars are, however, almost indistinguishable from those of the preceding and following families; while the cervical vertebræ and portions of the limbs are of a Perissodactyle type. On the other hand, the femur has lost its third trochanter; while the phalanges are strangely modified, the terminal ones forming long curved claws, while the others (Fig. 175) have strong ginglymoid distal articulations. These phalanges were, indeed, long regarded as referable to Edentates, being described in Europe as _Macrotherium_, and in the United States as _Morotherium_ and _Moropus_. _Ancylotherium_, of the Grecian Pikermi beds, is founded upon phalanges which indicate an allied genus. The Indian species of _Chalicotherium_ is distinguished by the loss of the incisors and the upper canine; while all the species want the first premolar.

_Titanotheriidæ._—This exclusively North American family includes gigantic forms closely allied to the _Lambdotheriidæ_, but with the last upper premolar as complex as the molars, and frequently with large bony protuberances in the nasal region. The best known genus, _Titanotherium_ (_Menodus_,[274] _Brontotherium_, _Symborodon_, _Allops_, etc.), may either have the full complement of teeth, or the incisors may be reduced to ²⁄₀. The canines and incisors are small, and there is no diastema when the full dental series is developed. The skull is very like that of the Rhinoceroses; but has a transverse pair of large bony prominences on the nasal region, varying considerably in shape and size in the different species, which in the living animal were probably covered with horny sheaths. The third trochanter of the femur was aborted. These huge animals—inferior in size only to the Elephant—appear to have been abundant in the United States during the Miocene period.

_Family_ MACRAUCHENIIDÆ.

This extinct South American family is best known by the genus _Macrauchenia_, as represented by _M. patachonica_ and _M. boliviensis_, which are apparently from Pleistocene formations. They are very singular and specialised forms, quite out of the line of descent of any of the existing Perissodactyles, and the steps by which they are connected with the rest of the group have not yet been discovered. Of the larger species, _M. patachonica_, the skeleton is completely known. It had the full number of forty-four teeth, forming an almost uninterrupted series. The cervical vertebræ resemble those of the Camels in the position of the vertebrarterial canal, but the ends of the centra are flat, and not opisthocœlous as in the allied forms. In some of the limb characters it resembles the _Equidæ_, but in the articulation of the fibula with the calcaneum it agrees with the Artiodactyles. The structure of the feet is, however, distinctly Perissodactylate, there being three toes on each. The teeth approximate to a Rhinocerotine structure; and the incisors have an infolding of the enamel of their crowns, as in those of the Horses. The nares open on the top of the skull, and it is probable that the muzzle was produced into a short proboscis. Several other South American forms have been referred to this family, some of which have received distinct generic names, but further evidence is required before many of them can be accepted. Possibly _Homalodontotherium_ should be placed here.

_Family_ PROTEROTHERIIDÆ.

_Proterotherium._—Here may be noticed certain very remarkable Perissodactyles from the South American Tertiaries, for which the name _Proterotherium_ has been proposed. The cheek-teeth are so like those of _Anchitherium_ that they have been described under that name. The upper jaw has one pair of canine-like incisors and no canines, while the lower jaw carries two pairs of incisors. In the skull the orbits were completely closed, as in the Horses. The feet were tridactyle, like those of _Hipparion_, but the tarsus was constructed on an Artiodactyle type.

SUBUNGULATA.

By far the greater number of the Subungulata are extinct, and of many of those whose former existence has been revealed, chiefly by the labours of the American palæontologists, our knowledge is at present necessarily imperfect, though daily extending. It will only be possible here to give details of some of the more interesting or best-known forms.

The characters by which the skeleton of the feet of the Subungulata are distinguished from those of the Ungulata Vera have been already mentioned on p. 275. In addition to these it may be observed that the feet frequently have five functional digits, and may be plantigrade; while the upper surface of the astragalus is generally flattened, instead of presenting the strongly-marked pulley-like ridges and groove so characteristic of the Ungulata Vera.

_Suborder_ HYRACOIDEA.

_Family_ HYRACIDÆ.

This division is constituted to receive a single family of mammals, the affinities of which have long constituted a puzzle to zoologists. They were first placed among the Rodents, to which animals their small size and general appearance and habits give them much superficial resemblance. Cuvier’s investigations into their anatomical structure, and especially their dental characters, led him to place them among the Ungulates, near the genus _Rhinoceros_, a position long accepted by many zoologists. Further knowledge of their organisation and mode of development caused Milne-Edwards, Huxley, and others to disassociate them from this connection, and, failing to find any agreement with any other known forms, to place them in an order entirely apart. Palæontology has thrown no light upon the affinities of this anomalous and isolated group, as no extinct animals possessing their distinctive characters have as yet been discovered.

The dentition, according to the usual interpretation, consists only of incisors and molars, the formula in all known species being _i_ ¹⁄₂, _c_ ⁰⁄₀, _p_ ⁴⁄₄, _m_ ³⁄₃. The upper incisors have persistent pulps, and are curved longitudinally, forming a semicircle as in Rodents. They are, however, not flattened from before backwards as in that order, but prismatic, with an antero-external, an antero-internal, and a posterior surface, the first two only being covered with enamel; their apices are consequently not chisel-shaped, but sharp pointed. They are preceded by functional, rooted milk-teeth. The outer lower incisors, which should perhaps be regarded rather as canines, have long tapering roots, but not of persistent growth. They are straight, procumbent, with awl-shaped, trilobed crowns. Behind the incisors is a considerable diastema. The molars and premolars are all contiguous, and formed almost exactly on the pattern of some of the Perissodactyle Ungulates. The hyoid arch is unlike that of any known mammal. The dorsal and lumbar vertebræ are very numerous, 28 to 30, of which 21 or 22 bear ribs. The tail is extremely short. There are no clavicles. In the fore foot the three middle toes are subequally developed, the fifth is present, but smaller, and the hallux is rudimentary, although, in one species at least, all its normal bones are present. The ungual phalanges of the four outer digits are small, somewhat conical, and flattened in form. The carpus has a distinct os centrale. There is a slight ridge on the femur in the place of a third trochanter. The fibula is complete, thickest at its upper end, where it generally ankyloses with the tibia. The articulation between the tibia and astragalus is more complex than in other mammals, the end of the malleolus entering into it. The hind foot is very like that of _Rhinoceros_, having three well-developed toes. There is no trace of a hallux, and the fifth metatarsal is represented only by a small nodule. The ungual phalanx of the inner (or second) digit is deeply cleft, and has a peculiar long curved claw, the others have short broad nails. The stomach is formed upon much the same principle as that of the Horse or Rhinoceros, but is more elongated transversely and divided by a constriction into two cavities—a large left _cul de sac_, lined by a very dense white epithelium and a right pyloric cavity, with a very thick, soft, vascular lining. The intestinal canal (Fig. 178) is long, and has an arrangement perfectly unique among mammals, indeed among vertebrated animals, for, in addition to the ordinary short, but capacious and sacculated cæcum (_cm_) at the commencement of the colon, there is, lower down, an additional pair of large, conical, pointed, supplemental cæca (_c_). The liver is much subdivided, and there is no gall-bladder. The brain resembles that of the typical Ungulates far more than the Rodents. The testes are permanently abdominal. The ureters open into the fundus of the bladder, as in some Rodents. The female has six teats, of which four are inguinal and two axillary; and the placenta is zonary, as in the Elephant and Carnivora.

There are two distinct forms of Hyrax, differing both in structure and habits, which may be accorded generic rank.

_Hyrax._[275]—Molar teeth having the same pattern as those of _Rhinoceros_. Interval between upper incisors less than the width of the teeth. Lower incisors slightly notched at the cutting edge. Vertebræ: C 7, D 22, L 8, S 6, C 6. Of this form the earliest known species, _H. capensis_ (Fig. 176) is the type. There are several other species, as _H. habessinicus_ and _syriacus_, from Eastern Africa and Syria. They inhabit mountainous and rocky regions, and live on the ground.

_Dendrohyrax._[276]—Molar teeth having the same pattern as _Palæotherium_ (except that the third lower molar has but two lobes). Interval between upper incisors exceeding the width of the teeth. Lower incisors with very distinctly trilobed crowns. Vertebræ: C 7, D 21, L 7, S 5, C 10. The members of this section frequent the trunks and large branches of trees, sleeping in holes. There are several species, not distinctly defined, from western and south Africa, as _D. arboreus_ and _D. dorsalis_. The members of both groups appear to have a power like that possessed by the Lizards called Geckos of clinging to vertical surfaces of rocks and trees by the soles of their feet.

It should be added that some writers separate three of the African species usually included in _Hyrax_ (viz. _H. bocagei_, _H. bakeri_, and _H. blainvillei_) under the designation of _Heterohyrax_.[277]

_Suborder_ PROBOSCIDEA.

This name has been appropriated to a well-marked group of animals, presenting some very anomalous characters, allied in many respects to the typical Ungulata, but belonging neither to the Artiodactyle nor Perissodactyle type of that order. It has been thought that they possess some, though certainly not very close, affinities with the Rodentia, and also with the Sirenia. It is certain, however, that the two species of Elephant, which are the sole living representatives of the group, stand quite alone among existing mammals, differing widely from all others in many points of their structure. In some respects, as the skull, proboscis, and dentition, they are highly specialised; but in others, as in the presence of two anterior venæ cavæ and in the structure of the limbs, they retain a low or generalised condition. A considerable series of extinct forms, extending back through the Pliocene and Miocene epochs, show the same type under different modifications, and in still more generalised outlines; and certain forms from the Eocene of North America, if their affinities are rightly interpreted, appear to link the true Proboscidea to some unknown primitive type of Ungulata.

The following are the principal characters common to existing, and, by inference, to the extinct, Proboscidea. The nose extended into a long, muscular, very flexible and prehensile proboscis, at the end of which the nostrils are situated, and from which the name given to the group is derived. The teeth consisting of ever-growing incisors of very great size, but never exceeding one pair in each jaw, and often present in one jaw only; no canines; large and transversely ridged molars. No clavicles. Limbs strong, the upper segment, especially in the hind limb, the longer. Radius and ulna distinct, the latter articulating extensively with the carpus. Fibula and tibia distinct. Astragalus very flat on both surfaces. Manus and pes short, broad, and massive, each with five toes, though the outer pair may be more or less rudimentary, all encased in a common integument, though with distinct, broad, short hoofs. Third digit the largest. Two anterior venæ cavæ entering the right auricle. Stomach simple. A capacious cæcum. Testes permanently abdominal. Uterus bicornuate. Placenta non-deciduate and zonary. Mammæ two, pectoral.

With regard to the teeth, the incisors,[278] which project largely out of the mouth, and are commonly called “tusks,” are of an elongated conical form, and generally curved. They are composed mainly of solid dentine, the fine elastic quality and large mass of which renders it invaluable as “ivory” for commerce and the arts. A peculiarity of the dentine of most Proboscidea is that it shows, in transverse fractures or sections, striæ proceeding in the arc of a circle from the centre to the circumference in opposite directions, and forming by their decussations curvilinear lozenges, as in the “engine-turning” of the case of a watch. The enamel-covering in existing species is confined to the extreme apex, and very soon wears off, but in some extinct species it forms persistent longitudinal bands of limited breadth. The tusks have small milk-predecessors, shed at an early age.

The molar teeth present a remarkable series of modifications, from the comparatively simple form in _Dinotherium_, with two or three strongly pronounced transverse ridges and a normal mode of succession, to the extremely complex structure and anomalous mode of replacement found in the true Elephants. The intermediate conditions occur in the various species of _Mastodon_. In this genus the enamel-covered transverse ridges of each tooth are generally more numerous than in _Dinotherium_, and often complicated by notches dividing their edge or by accessory columns attached to them, but in the unworn tooth they stand out freely on the surface of the crown, with deep valleys between (Fig. 179, I). In the Elephants the ridges are still further increased in number, and consequently narrower from before backwards, and are greatly extended in vertical height, so that, in order to give solidity to what would otherwise be a laminated or pectinated tooth, it becomes necessary to envelop and unite the whole in a large mass of cement, which completely fills up the valleys, and gives a general smooth appearance to the organ when unworn; but as the wear consequent upon the masticating process proceeds, the alternate layers of tissue of different hardness—cement, dentine, and enamel—which are disclosed upon the surface form a fine and very efficient triturating instrument. The modification of the tooth of a Mastodon into that of an Elephant is therefore precisely the same in principle as that of the molar of a Palæotherium into that of a Horse, or of the corresponding tooth of one of the primitive Artiodactyles into that of an Ox. The intermediate stages, moreover, even in the present state of our knowledge, are so numerous that it is not possible to draw a definite line between the two types of tooth structure (see Fig. 179, I, II, III, IV).

As regards the mode of succession, that of modern Elephants is, as before mentioned, very peculiar. During the complete lifetime of the animal there are but six molar teeth on either side of each jaw, with occasionally a rudimentary one in front, completing the typical number of seven. The last three represent the true molars of ordinary mammals; those in front appear to be milk-molars, which are never replaced by permanent successors, but the whole series gradually moves forwards in the jaw, and the teeth become worn away and their remnants cast out in front, while development of others proceeds behind. The individual teeth are so large, and the processes of growth and destruction by wear take place so slowly, that not more than one, or portions of two, teeth are ever in place and in use on either side of each jaw at one time, and the whole series of changes coincides with the usual duration of the animal’s life. On the other hand, the Dinotherium, the opposite extreme of the Proboscidean series, has the whole of the molar teeth in place and use at one time, and the milk-molars are vertically displaced by premolars in the ordinary fashion. Among Mastodons transitional forms occur in the mode of succession as well as in structure, many species showing a vertical displacement of one or more of the milk-molars, and the same has been observed in one extinct species of Elephant (_E. planifrons_) as regards the posterior of these teeth.

All known Proboscideans are animals of comparatively large dimensions, and some are the most colossal of land mammals. The head is of great proportionate size; and, as the brain case increases but little in bulk during growth, while the exterior wall of the skull is required to be of great superficial extent to support the trunk and the huge and ponderous tusks, and to afford space for the attachment of muscles of sufficient size and strength to wield the skull thus heavily weighted, an extraordinary development of air-cells takes place in the cancellous tissue of nearly all the bones of the cranium (Fig. 180). These cells are not only formed in the walls of the cranium proper, but are also largely developed in the nasal bones and upper part of the premaxillæ and maxillæ, the bones forming the palate and the basicranial axis, and even extend into the interior of the ossified mesethmoid and vomer. Where two originally distinct bones come into contact, the cells pass freely from one to the other, and almost all the sutures become obliterated in old animals. The intercellular lamellæ in the great mass which surrounds the brain cavity superiorly and laterally mostly radiate from the inner to the outer table, but in the other bones their direction is more irregular. Like the similar but less developed air-cells in the skulls of many other mammals, they all communicate with the nasal passages, and they are entirely secondary to the original growth of the bones, their development having scarcely commenced in the new-born animal, and they gradually enlarge as the growth of the creature proceeds towards maturity. The nasal bones are very short, and the anterior narial aperture is situated high in the face. The zygomatic arch is slender and straight, the jugal bone being small, and forming only the middle part of the arch, the anterior part of which (unlike that of typical Ungulates) is formed only by the maxilla. The maxillo-turbinals are but rudimentary, the elongated proboscis supplying their place functionally in warming and clearing from dust the inspired air.