Under this term may be included provisionally a large and rather heterogeneous group of mammals, the existing members of which form the Pecora and Belluæ of Linnæus, the Ruminantia and Pachydermata of Cuvier. A few years ago it was found convenient to restrict the order to a well-marked and distinctly circumscribed group, comprising the two sections known as Perissodactyla and Artiodactyla, and to leave out such isolated forms as the Elephant and Hyrax; but the discovery of a vast number of extinct species, which could not be brought under the definition of either perissodactyle or artiodactyle Ungulates, and yet are evidently allied to both, and to a certain extent bridge over the interval between these and the isolated groups just mentioned, makes it necessary either to introduce a number of new and ill-defined ordinal divisions, or to widen the scope of the original order so as to embrace them all.

The existing forms are all animals eminently adapted for a terrestrial life, and in the main for a vegetable diet. Though a few are more or less omnivorous, and may under some circumstances kill living creatures smaller and weaker than themselves for food, none are distinctly and habitually predaceous. Their teeth are markedly heterodont and diphyodont,—the milk set being well developed and not completely changed until the animal attains its full stature. The molars have broad crowns with tuberculated or ridged surfaces. There are no clavicles.[173] Their toes are provided with blunt, broad nails, or in the majority of cases with hoofs, more or less enclosing the ungual phalanges. The scaphoid and lunar bones of the carpus are always distinct. The humerus has no entepicondylar foramen. The number of digits varies from five to one; and the radius and ulna may be united together.

The more generalised of the fossil forms do not conform in all respects to the above-mentioned characters; clavicles being present in Typotherium, and perhaps in some of the Condylarthra, while in the latter group the humerus may have an entepicondylar foramen, and thus approximate to the corresponding bone of the Carnivora. Wide as is the gap between existing Carnivores and Ungulates, there are indeed more or less strongly marked evidences of affinity between the earlier members of the two orders, as will be again noticed under the head of the suborder Condylarthra. A departure from the normal type of foot-structure is exhibited by the extinct Macrotherium, provisionally included in the Perissodactyla, where the digits terminated in long and curved claws.

As a general rule, the cheek-teeth have distinct roots, and in those of the existing suborders a gradual increase in the height of the crowns of these teeth may be noticed in passing from the more generalised to the more specialised types. Those teeth in which the crowns are low, and their whole structure visible from the grinding surface, are termed brachydont (Fig. 122); while those with higher crowns, in which the bases of the infoldings of enamel are invisible from the grinding surface, are known as hypsodont (Fig. 123). Again, when the tubercles on the crowns of the molars are more or less cone-like in form the tooth is said to be bunodont; but when they are expanded in an antero-posterior direction and curved into a crescent shape the tooth is described as selenodont.

The whole order may be divided into the Ungulata Vera, containing the suborders Perissodactyla and Artiodactyla, and a somewhat heterogeneous assemblage of animals which may be called Subungulata or Ungulata Polydactyla. Cope has pointed out a character in the structure of the carpus by which the latter are differentiated from the former. Thus in all the Subungulata the bones of the proximal and distal row retain the primitive or more typical relation to each other (see Fig. 98); the os magnum of the second row articulating mainly with the lunar of the first, or with the cuneiform, but not with the scaphoid. But in the group to which the vast majority of modern Ungulates belong the second or distal row has been shifted altogether towards the inner side of the limb (see Fig. 99), so that the magnum is brought considerably into relation with the scaphoid, and is entirely removed from the cuneiform, as in the great majority of existing mammals.

It will be on the whole more convenient to commence our survey of the members of this suborder with the more specialised group of the Ungulata Vera, in which the Artiodactyla will be taken first.

Ungulata Vera.[174]

In the typical Ungulata the feet are never plantigrade, and the functional toes do not exceed four—the inner digit being suppressed, at all events in all forms which have existed since the Upper Eocene period.[175] The os magnum of the carpus articulates freely with the scaphoid. The allantois is largely developed, and the placenta, so far as is known, is non-deciduate; the chorionic villi being either evenly diffused or collected in groups or cotyledons (in Pecora). The testes descend into a scrotum. There is never an os penis. The uterus is bicornuate. The mammæ are usually few and inguinal, or may be numerous and abdominal (as in Suina), but are never solely pectoral. The cerebral hemispheres in existing Ungulates are well convoluted.

The group is now, and has been throughout almost the whole of the Tertiary period, composed of two perfectly distinct sections, differing from each other, not only in the obvious characters of the structure of the limbs, but in so many other parts of their organisation that they must be considered as of the rank at least of suborders. The characters of these divisions, first indicated by Cuvier, were thoroughly established by Owen, by whom the names whereby they are now generally known were proposed.

Suborder Artiodactyla.

This is a well-defined group, traceable from the Eocene period, though then apparently by no means so numerous as the Perissodactyles. Some of its types, as that represented in the existing Swine, have retained to the present time much of the primitive character of the group; but others have been gradually becoming more specialised and perfected in structure, and its latest modification, the Cavicorn Ruminants or Bovidæ (Antelopes, Sheep, and Oxen), are now the dominating members of the great Ungulate order, widespread in geographical range, rich in generic and specific variation, and numerous in individuals—forming in all these respects a great contrast to such decadent types as those represented by the Tapirs and Rhinoceroses.

The principal anatomical characters by which the Artiodactyles are distinguished from the Perissodactyles are as follows. The premolar and molar teeth usually not alike, the former being single and the latter two-lobed. The last lower molar of both first and second dentition almost invariably three-lobed; and the first tooth of the upper cheek series always without a milk-predecessor. Nasal bones not expanded posteriorly. No alisphenoid canal. Dorsal and lumbar vertebræ together always nineteen, though the former may vary from twelve to fifteen. Femur without third trochanter. Third and fourth digits of both feet almost equally developed, and their ungual phalanges flattened on their inner or contiguous surfaces, so that each is not symmetrical in itself, but when the two are placed together they form a figure symmetrically disposed to a line drawn between them. Or, in other words, the axis or median line of the whole foot is a line drawn between the third and fourth digits, while in the Perissodactyles it is a line drawn down the centre of the third digit. Distal articular surface of the astragalus divided into two nearly equal facets, one for the navicular and the other for the cuboid bone. The calcaneum with an articular facet for the lower end of the fibula. Stomach almost always more or less complex. Colon convoluted. Cæcum small. Placenta diffused or cotyledonary. Mammæ few and inguinal, or numerous and abdominal.

In treating of many sections of mammals, it is only from the existing species that our characters and classification can be derived, and to these chiefly our observations upon the group must be directed, many of the extinct forms being so little known that they can only be referred to incidentally. With the Ungulata, however, it is quite otherwise. The history of the Artiodactyla throughout the Tertiary period is now well known, and throws great light upon the position and relations of the existing groups.

The principal modifications which have taken place in the type from its earliest known and most generalised manifestation have been the following:—

1. As regards the teeth. Assumption by the grinding surfaces of the molar teeth either of a bunodont or of a selenodont form. Modification of the latter from a brachydont to a hypsodont type. Loss of upper incisors. Development of canines into projecting tusks. Loss of anterior premolars.

2. As regards the limbs. Reduction of the ulna from a complete and distinct bone to a comparatively rudimentary state, in which it coalesces more or less firmly with the radius. Reduction of the fibula till nothing but its lower extremity remains. Reduction and final loss of external pair of digits (second and fifth), with coalescence of the metapodial bones of the two middle digits. Union of the navicular and cuboid, and sometimes the ectocuneiform, bones of the tarsus.

3. Change of form of the odontoid process of the axis vertebra from a cone to a hollow half-cylinder.

4. Development of horns or antlers on the frontal bones, and gradual complication of form of antlers.

5. By inference only, increasing complication of stomach with ruminating function superadded. Modification of placenta from simple diffused to cotyledonary form.

The primitive Artiodactyles, with the typical number (44) of incisor, canine, and molar teeth, brachydont molars, conical odontoid process, four distinct toes on each foot, with metapodium and all carpal bones distinct, no frontal appendages, and (in all probability) simple stomach and diffused placenta, were separated at a very early period into Bunodonts and Selenodonts, although there is evidence of intermediate forms showing a complete transition from the one modification to the other. These and other fossil forms so completely connect the four groups—Suina, Tylopoda, Tragulina, and Pecora—into which the existing members of the suborder have become divided, that in a general classification embracing both living and extinct forms these divisions cannot be maintained. In the present work, however, it will be convenient to retain them, mention being made of some of the chief annectant forms in separate sections.

Suina.

The existing members of this group are characterised by their bunodont molars, and the absence of a complete fusion of the third and fourth metapodials to form a “cannon-bone.” The full Eutherian dentition is very frequently present.

Remains of very generalised swine-like animals have been abundantly found in Tertiary formations both in America and Europe. In the former continent they never (so far as present evidence indicates) underwent any great diversity of modification, but gradually dwindled away and almost died out, being only represented in the actual fauna by the two closely allied species of Peccary, among the smallest and most insignificant members of the group, which have existed almost unchanged since the Miocene age at least, if the evidence of teeth alone can be trusted. In the Old World, on the other hand, the swine have played a more important part in recent times, having become widely distributed, and throwing off some curiously specialised forms. At the present time, though not very numerous in species, they range through the greater part of the Old World, except within or near the Arctic Circle, although, in common with all the other members of the great Ungulate order, they were completely absent from the whole of the Australian region, until introduced by man in very recent times.

The existing swine-like animals may be divided naturally into three families:—I. Hippopotamidæ; II. Suidæ, or true Pigs; III. Dicotylidæ, or Peccaries.[176]

Family Hippopotamidæ.

Muzzle very broad and rounded. Feet short and broad, having four subequal toes, with short rounded hoofs, all reaching the ground in walking. Incisors not rooted, but continuously growing; those of the upper jaw curved and directed downwards; those of the lower straight and procumbent. Canines very large, curved, continuously growing; those of the upper jaw directed downwards. Stomach complex. No cæcum.

Hippopotamus.[177]—This genus may be taken to include all the known members of the family; it appears to have been always confined to the Old World. The dentition may be expressed by the formula i ²⁻³⁄₁₋₃, c ¹⁄₁, p ⁴⁄₄, m ³⁄₃. The crowns of the molars (Fig. 100) when worn present trefoil-shaped surfaces of dentine; and those of the premolars are sharp. The facial portion of the skull is much elongated, the orbits are tubular and very prominent, and the mandible has a large rounded descending flange at its angle. The ears are small, the tail is short, and the legs are likewise so short that the belly is raised but a little distance above the ground. The brain is not richly convoluted, and differs very considerably from that of the Pigs, approximating in some respects to that of the Camel and Giraffe, but on the whole standing very much by itself. The stomach of the common species is of enormous dimensions, having an axial length of 11 feet, and measuring upwards of 15 feet along the greater curvature. Its axis is longitudinal, the pylorus being situated almost in the pelvis, and it is divided into three distinct compartments, of which the third is cylindrical. The liver of the adult is of extremely simple form, elongated transversely, and narrow from above downwards. With the exception of a few tufts of hair on the lips, on the sides of the head and neck, and at the extremity of the short compressed tail, the skin of the hippopotamus, some portions of which are two inches in thickness, is entirely destitute of covering.

The common Hippopotamus (H. amphibius), widely distributed in the rivers and lakes of the African continent, is a huge bulky animal, characterised by having only two incisors on either side of each jaw; the central lower pair being very much larger than the outer ones. A male from the Upper Nile which lived for nearly thirty years in the gardens of the Zoological Society of London measured 12 feet along the back from the nose to the root of the tail.

The Hippopotamus lives in herds of from twenty to forty individuals on the banks and in the beds of rivers, in the neighbourhood of which it finds its food. This consists chiefly of grass and aquatic plants, of which it consumes enormous quantities, the stomach being capable of containing from 5 to 6 bushels. These animals feed principally by night, remaining in the water during the day, although in districts where they are undisturbed by man they are less exclusively aquatic. In such regions they put their heads boldly out of the water to blow, but when rendered suspicious by persecution, they become exceedingly cautious, only exposing their eyes and nostrils above the water, and even this they prefer doing amid the shelter of water plants. In spite of their enormous size and uncouth form, they are expert swimmers and divers, and can remain under the water from five to eight minutes. They are said to walk with considerable rapidity on the bottoms of rivers, beneath at least a foot of water. At nightfall they come on land to feed; and when, as often happens on the banks of the Nile, they reach cultivated ground, they do immense damage to growing crops, destroying by their ponderous tread even more than they devour.

A much smaller species, known as the Pigmy Hippopotamus (H. liberiensis), inhabits some of the rivers of Western Africa, and is characterised by having only a single pair of lower incisors. Mainly on this account, it has been proposed to regard this species as representing a distinct genus, under the name of Chœropsis; but since it agrees so essentially in other characters with the common form, and sometimes has two incisors on one side of the lower jaw, it appears preferable to include it in the type genus. The greater relative size of the brain-cavity as compared with the facial portion of the skull renders, indeed, the contour of the skull decidedly different from that of H. amphibius, but this is a feature generally found in young individuals of larger species, and also in the adults of allied smaller forms.

Both the existing species are now exclusively confined to Africa, but in the Pleistocene and Pliocene periods the genus was widely spread over the Old World. Thus in the Upper Pliocene of the Continent and the Pleistocene of England we meet with remains of a very large fossil Hippopotamus which cannot be specifically distinguished from H. amphibius. In the Pleistocene and Pliocene of India there are two species having three pairs of incisors in both jaws. Of these H. palæindicus has the second pair in the lower jaw very minute, and evidently just about to disappear; from which we learn that it is this pair which is missing in H. amphibius. In the still more generalised H. sivalensis the three incisors in the lower jaw are of equal size. Hexaprotodont species also occur in the Upper Tertiaries of Burma and Algeria. Small tetraprotodont species (H. pentlandi and H. minutus) have left their remains in enormous quantities in the caves and fissures of Sicily and Malta.

Family Suidæ.

An elongated mobile snout, with an expanded, truncated, nearly naked, flat, oval terminal surface in which the nostrils are placed. Feet narrow; four completely developed toes on each. Hoofs of the two middle toes with their contiguous surfaces flattened. The outer (second and fifth) digits of existing forms not reaching to the ground in the ordinary walking position. Teeth variable in number, owing to the suppression in some forms of an upper incisor and one or more premolars. Incisors rooted. Upper canines curving more or less outwards or upwards. Stomach simple, except for a more or less developed pouch near the cardiac orifice. A cæcum. Colon spirally coiled. Confined to the Old World.

The mandible has no descending flange at the angle. The crowns of the molars do not wear into such distinct trefoils as in the Hippopotamus, and are oblong in shape. The last molar of both the upper and lower jaw (Fig. 102) has an additional hinder lobe or talon, varying in size in the different species. The upper premolars are simpler than the true molars.

Sus.[178]—Dentition: i ³⁄₃, c ¹⁄₁, p ⁴⁄₄, m ³⁄₃; total 44. Upper incisors diminishing rapidly in size from the first to the third. Lower incisors long, narrow, closely approximated, and almost horizontal in position, their apices inclining towards the middle line; the second slightly larger than the first, the third much smaller. Canines strongly developed and with persistent roots and partial enamel-covering, those of the upper jaw not having the usual downward direction, but curving strongly outwards, upwards, and finally inwards, while those of the lower jaw are directed upwards and outwards with a gentle backward curve, their hinder edges working and wearing against the front edges of the upper canines[179]. They appear externally to the mouth as tusks, the form of the upper lip being modified to allow of their protrusion, but are much less developed in the females than in the males. The teeth of the molar series gradually increase in size and complexity from first to last, and are arranged in contiguous series, except that the first lower premolar is separated by an interval from the second. First and second upper premolars with compressed crowns and two roots. The third and fourth have an inner lobe developed on the crown, and an additional pair of roots. The first and second true molars have quadrate crowns, with four principal obtuse conical cusps, around which numerous accessory cusps are clustered. The length of the third molar is nearly equal (antero-posteriorly) to that of the first and second together, its crown having, in addition to the four principal cusps, a large posterior talon or heel, composed of numerous clustered conical cusps, and supported by several additional roots. The lower molar teeth resemble generally those of the upper jaw, but are narrower. Milk dentition: i ³⁄₃, c ¹⁄₁, m ³⁄₃; total 28,—the first permanent premolar having no predecessor in this series. The third incisor, in both upper and lower jaws, is large, developed before the others, and has much the size, form, and direction of the canine. Vertebræ: C 7, D 13-14, L 6, S 4, C 20-24. The hairy covering of the body varies much under different conditions of climate, but when best developed, as in the European Wild Boar, consists of long stiff bristles, mostly abundant on the back and sides, and of a close softer curling undercoat.

The skull of the Pigs (Figs. 103-105) has the axis of the face bent down upon the basicranial axis, as is also the case with the Sheep. Its most striking feature is the elevation and backward slope of the occipital crest formed by the union of the supraoccipital and parietals. The broad and flat frontals have small postorbital processes, which do not join the zygomata, so that the orbits are open behind. The nasals are very long and narrow; and the premaxillæ send up long nasal processes, stopping short of the frontals. A peculiar prenasal bone is developed at the anterior extremity of the mesethmoid, which serves to strengthen the cartilaginous snout. The palate is long and narrow, and extends behind the last molar tooth. In most species the occipital crest is more nearly vertical than in the skull represented in Fig. 104.

This genus occurs at present under three principal modifications or subgenera.

A.—Sus proper comprises a number of animals found in a wild state throughout the greater part of Europe (except where exterminated by human agency), the north of Africa, southern continental Asia, and the great islands of the Malayan archipelago, Formosa, and Japan. The following among others have been admitted by many zoologists as distinct species:—Sus scrofa, the Wild Boar of Europe, Asia Minor, and North Africa, once common throughout the British Isles; S. sennaarensis, North-East Africa; S. cristatus, India; S. vittatus, Java, Borneo, Amboyna, Batchian; S. papuensis, New Guinea; S. timorensis, Timor and Rotti; S. andamanensis, Andaman Islands; S. taëvanus, Formosa; S. leucomystax, Japan; S. verrucosus, Java, Borneo, Ceram; S. barbatus, Borneo; S. celebensis, Celebes, Philippines, and Moluccas; S. longirostris, Borneo and Java. The last four species form an allied group in which the facial portion of the skull may be greatly elongated; S. barbatus, and S. celebensis being characterised by the small size and simple structure of the talon of the third molars. The skull of S. longirostris is shown in Figs. 104 and 105. The small S. andamanensis also has very simple third molars. S. vittatus, S. leucomystax, S. cristatus, S. taëvanus, and S. papuensis form another group, in which the third molar is generally of very complex structure, more or less closely allied to the Wild Boar; and Dr. Nehring is inclined to think that the whole five might be included under a single specific name. This list will give some idea of the geographical distribution of wild Pigs, but it must be borne in mind that through the whole of this region, and in fact now throughout the greater part of the habitable world, Pigs are kept by man in a domesticated state, and it is still an open question whether some of the wild Pigs of the islands named above may not be local races derived originally from, or crossed with, imported domestic specimens. In New Zealand a wild or rather “feral” race is already established, the origin of which is of course quite recent, since it is well ascertained that no animal of the kind ever lived upon the island until after its settlement by Europeans. Whether the various breeds of domestic Pigs have been derived from one or several sources is still unknown. As in so many similar cases, there is no historic evidence upon the subject, and the researches of naturalists, as Nathusius, Rütimeyer, Rolleston, Nehring, and others, who have endeavoured to settle the question on anatomical evidence, have not led to any satisfactory conclusions. It is, however, tolerably certain that all the species or forms of wild Pigs enumerated above and all the domestic races are closely allied, and it is probable (though of this there has been no opportunity of proof) will breed freely together. It is a curious circumstance that the young of all the wild kinds of Pigs (so far as yet is known) present a uniform coloration, being dark brown with longitudinal stripes of a paler colour, a character which completely disappears after the first few months. On the other hand, this peculiar marking is rarely seen in domestic Pigs in any part of the world, although it has been occasionally observed. It is stated by Darwin that the Pigs which have run wild in Jamaica and the semiferal Pigs of New Granada have resumed this aboriginal character, and produce longitudinally striped young; these must of course be the descendants of domestic animals introduced from Europe since the Spanish conquest, as before that time there were no true Pigs in the New World. Another character by which the European domestic Pig differs from any of the wild species is the concave outline of the frontal region of the skull, a form still retained by the feral Pigs in New Zealand.

B.—The diminutive Pig of the Nipal, Terai, and Bhutan, Sus salvanius, has been separated from the rest by Hodgson under the generic name of Porcula, but all the alleged distinctive characters prove on more careful investigation to have little real value. Owing to its retired habits and power of concealment under bushes and long grass in the depths of the great Sal Forest, which is its principal home, very little has been known of this curious little animal, scarcely larger than a hare. The acquisition of living specimens in the London Zoological Gardens has, however, afforded opportunities for careful anatomical observation.[180]

C.—Two well-marked species of African Swine have been with more reason separated under the name of Potamochœrus. The dentition differs from that of the true Sus, inasmuch as the anterior premolars have a tendency to disappear; sometimes in adult specimens the first upper premolar is retained, but it is usually absent, as well as the first and often the second lower premolars. The molar teeth are also less complex: the last especially having a much less developed talon. There are likewise characteristic cranial differences. The two species are very distinct in outward appearance and coloration. One is S. africanus, the South African River-Hog, or Bosch-Vark, of a gray colour, and the other S. porcus, the West African Red River-Hog (Fig. 107), remarkable for its vivid colouring and long pencilled ears. It should be noted that the young of both these species, as well as of the pigmy S. salvanius, present the striped character of the true Sus, a strong indication of close affinities, whereas in all the following forms this is absent.

The genus Sus, in the above extended sense, is well represented in the Tertiaries of the Old World from the period of the Lower Pliocene upwards. In the Pliocene and Pleistocene of India S. falconeri and S. karnuliensis are characterised by the extremely complex structure of the molars, in which they show decided signs of approximation to the Wart-Hogs; the same feature being exhibited by S. phacochœroides of the Algerian Pliocene. S. titan and S. giganteus, of the Indian Pliocene, together with S. antiquus and S. erymanthius, of the corresponding European deposits, are very large species characterised by their comparatively simple molars; S. titan being fully as large as a Tapir. S. hysudricus of the Pliocene of India, and S. palæochœrus of that of Europe, are smaller allied species not improbably related to S. andamanensis, with which they agree in molar structure. S. arvernensis, of the Upper Pliocene of France, appears to be allied to S. africanus; while in the diminutive S. punjabiensis of the Pliocene of North-Western India we probably have the direct ancestor of S. salvanius.

Babirusa.[181]—Dentition: i ²⁄₃, c ¹⁄₁, p ²⁄₂, m ³⁄₃; total 34. The total number of teeth is therefore considerably reduced, the outer upper incisor and the two anterior premolars of both jaws being absent. The molars, especially the last, are smaller and simpler than in Sus; but the great peculiarity of this genus is the extraordinary development of the canines of the male. These teeth (Fig. 108) are ever-growing, long, slender, and curved, and entirely without enamel covering. Those of the upper jaw are directed upwards from their base, so that they never enter the mouth, but piercing the skin of the face, resemble horns rather than teeth, and curve backwards, downwards, and finally often forwards again, almost or quite touching the skin of the forehead. Vertebra: C 7, D 13, L 16, S 4. There is but one species (B. alfurus), found only in the islands of Celebes and Buru. Its external surface is almost entirely devoid of hair. With regard to the curiously modified dentition, Wallace (Malay Archipelago, vol. i. p. 435) makes the following observations:—“It is difficult to understand what can be the use of these horn-like teeth. Some of the old writers supposed that they served as hooks by which the creature could rest its head on a branch. But the way in which they usually diverge just over and in front of the eye has suggested the more probable idea, that they serve to guard these organs from thorns and spines while hunting for fallen fruits among the tangled thickets of rattans and other spiny plants. Even this, however, is not satisfactory, for the female, who must seek her food in the same way, does not possess them. I should be inclined to believe rather that these tusks were once useful, and were then worn down as fast as they grew, but that changed conditions of life have rendered them unnecessary, and they now develop into a monstrous form, just as the incisors of the Beaver and Rabbit will go on growing if the opposite teeth do not wear them away. In old animals they reach an enormous size, and are generally broken off as if by fighting.”

Phacochœrus.[182]—The Wart-Hogs, so called from the large cutaneous lobes projecting from each side of the face, have the teeth still more remarkably modified than in Babirusa. The milk-dentition, and even the early condition of the permanent dentition, is formed on the same general type as that of Sus, except that certain of the typical teeth are absent, the formula being i ¹⁄₃, c ¹⁄₁, p ³⁄₂, m ³⁄₃, total 34; but as age advances all the teeth have a tendency to disappear, except the canines and the posterior molars, which in some cases are the only teeth left in the jaws, and attain an extraordinary development. The upper canines especially are of great size, and curve outwards, forwards, and upwards. Their enamel covering is confined to the apex, and soon wears away. The lower canines are much more slender, but follow the same curve: except on the posterior surface, their crowns are covered with enamel. Unlike those of the Babirusa, the canines of the Wart-Hog are large in both sexes. The third molar tooth of both jaws is of great size, and presents a structure at first sight unlike that of any other mammal, being composed of numerous (22-25) parallel cylinders or columns, each with pulp-cavity, dentine, and enamel covering, and packed together with cement. Careful examination will, however, show that a similar modification to that which has transformed the comparatively simple molar tooth of the Mastodon into the extremely complex grinder of the Indian Elephant has served to change the tooth of the common Pig into that of Phacochœrus, and, as already mentioned, some of the fossil Indian and African species of Sus indicate the mode in which this transition came about. The tubercles which cluster over the surface of the crown of the molars of the common Pig are elongated and drawn out into columns in the Wart-Hog, as the low transverse ridges of the Mastodon’s tooth become the leaf-like plates of the Elephant’s.

Two species of this genus are commonly but rather doubtfully distinguished:—P. africanus, Ælian’s Wart-Hog, widely distributed over the continent; and P. æthiopicus, Pallas’s Wart-Hog, confined to South-Eastern Africa. In specimens attributed to the latter species the dentition reaches its most complete reduction, as in adult animals the upper incisors are absent and the lower ones worn down to the roots.

Family Dicotylidæ.

Snout as in Suidæ. Dentition: i ²⁄₃, c ¹⁄₁, p ³⁄₃, m ³⁄₃; total 38. Incisors rooted; upper canines directed downwards, with sharp cutting hinder edges. Toes, four on the fore feet and three on the hind feet (the fifth wanting). Stomach complex. A cæcum. Confined to the New World.

Dicotyles.[183]—The teeth of the Peccaries (Dicotyles) differ from those of the true Pigs (Sus) numerically in wanting the upper outer incisor and the anterior premolar on either side of each jaw, and also in the circumstance that the last premolar is nearly as complex as the molars. The upper canines have their points directed downwards, not outwards or upwards as in the Boars, and are very sharp, with cutting hinder edges, and completely covered with enamel until worn. The lower canines are large, directed upwards and outwards, and slightly curved backwards. The premolar and molar teeth form a continuous series, gradually increasing in size from the first to the last. The true molars have square quadricuspidate crowns. The stomach is much more complex than in the true Pigs, almost approaching that of the ruminants. In the feet the two middle (third and fourth) metapodial bones, which are completely separate in the Pigs, are united at their upper ends, as in the ruminants. On the fore foot the two (second and fifth) outer toes are equally developed as in Pigs, but on the hind foot, although the inner (or second) is present, the outer (or fifth) toe is entirely wanting, giving an unsymmetrical appearance of the member, very unusual in Artiodactyles. Vertebræ: C 7, D 14, L 5, S 4, C 7. As in the Pigs, the snout is truncated, and the nostrils are situated in its flat, expanded, disc-like termination. The ears are rather small, ovate, and erect; and there is no external appearance of a tail. The surface of the body is well covered with thick bristly hair, and rather behind the middle of the back is a large and peculiar gland, which secretes an oleaginous substance with a powerful musky odour. This was mistaken by the old travellers for a second navel, a popular error which suggested to Cuvier the name of Dicotyles. When the animal is killed for food, it is necessary speedily to remove this gland, otherwise it will taint the whole flesh so as to render it uneatable.

There are two species,[184] so nearly allied that they will breed together freely in captivity. Unlike the true Pigs, they never appear to produce more than two young ones at a birth. The Collared Peccary (D. tajacu, Linn., torquatus, Cuvier), Fig. 109, ranges from the Red River of Arkansas through the forest districts of Central and South America as far as the Rio Negro of Patagonia. Generally it is found singly or in pairs, or at most in small herds of from eight to ten, and is a comparatively harmless creature, not being inclined to attack other animals or human beings. Its colour is dark gray, with a white or whitish band passing across the chest from shoulder to shoulder. The length of the head and body is about 36 inches. The White-lipped Peccary or Warree (D. labiatus, Cuvier) is rather larger, being about 40 inches in length, of a blackish colour, with the lips and lower jaw white. Its range is less extensive, since it is not found farther north than British Honduras or south of Paraguay. It is generally met with in large herds of from fifty to a hundred or more individuals, and is of a more pugnacious disposition than the former species, and capable of inflicting severe wounds with its sharp tusks. A hunter who encounters a herd of them in a forest has often to climb a tree as his only chance of safety. Both species are omnivorous, living on roots, fallen fruits, worms, and carrion; and when they approach the neighbourhood of villages and cultivated lands they often inflict great devastation upon the crops of the inhabitants.

Remains of the two existing species of Peccary, as well as of one much larger extinct form, are found in the cavern-deposits of Brazil; while large Peccaries also occur in the Pleistocene of the United States, which, although they have been referred to a distinct genus, Platygonus, on account of their relatively smaller incisors and somewhat simpler premolars, may well be included in Dicotyles.

Allied Extinct Genera.—In the Tertiary deposits of both the Old and New World occur remains of Pig-like animals which, so far as we can judge, appear to connect the Peccaries so closely with the true Pigs as to render the Dicotylidæ really inseparable from the Suidæ. Of these the American genus Chænohyus has the lower canine with a triangular cross section and received into a notch in the upper jaw, as in the Peccaries, but the fourth upper premolar is simpler than the molars, as in the under-mentioned genus Hyotherium. The typical forms have only three premolars, but in others, which it has been proposed to separate generically as Bothriolabis, there are four of these teeth. Hyotherium, of the Pliocene and Miocene of the Old World, is a generalised form allied both to Sus and Dicotyles as well as to certain extinct genera. The upper molars (Fig. 110) are characterised by their square crowns, the last having no distinct third lobe, and coming into use before the first is much worn, while the last premolar is simpler than the true molars. The canines, which have an oval section and are scarcely larger than the incisors, are not received into a notch in the upper jaw. In the Pliocene of India there occurs an apparently allied genus known as Hippohyus, in which the crowns of the molars are much taller, and have lateral infoldings of the enamel, producing a very complex pattern on the worn crowns. The European Miocene genus Listriodon, with the dental formula i ³⁄₃, c ¹⁄₁, p ³⁄₃, m ³⁄₃, differs from all the preceding in having the anterior and posterior pairs of tubercles of the molars united into ridges running across their crowns, so that these teeth resemble the lower molars of the Tapir. The genus is also found in the Lower Pliocene of India.

Extinct Transitional Artiodactyles.

In this place it will be convenient to notice briefly a few of the extinct types of Tertiary Artiodactyles which connect the existing bunodont Suina with the more specialised selenodont groups mentioned below so closely as to show that in a strictly palæontological classification such groups cannot be maintained. It should be mentioned that while some of these extinct forms were in all probability actual ancestral links between the bunodonts and selenodonts, others, like the Anoplotheres, died out entirely without giving rise to any more specialised descendants.