The scapula is somewhat triangular in shape, the apex being directed downwards and forwards, and being expanded to form the shallow glenoid cavity with which the head of the humerus articulates. The inner surface of the scapula is nearly flat, while the outer is drawn out into a very prominent ridge, the spine, which, arising gradually near the dorsal end, runs downwards, dividing the surface into two nearly equal parts, the prescapular and postscapular fossae, and ends in a short blunt process, the acromion. The anterior border of the scapula is somewhat curved, and is called the coracoid border; it is terminated ventrally by a slight blunt swelling, the coracoid process, which ossifies from a different centre from the rest of the scapula, and is probably the sole representative of the coracoid. The dorsal or suprascapular border of the scapula is rounded, while the posterior or glenoid border is nearly straight. The clavicle[144] or collar bone, which in a large proportion of mammals is well seen, in the dog is very imperfectly developed; it is short and broad, and is suspended in the muscles, not reaching either the scapula or sternum.

The Anterior Limb.

The anterior limb of the dog is divisible into the usual three portions, the brachium or upper arm, the antibrachium or fore-arm, and the manus or wrist and hand.

The brachium or upper arm includes only a single bone, the humerus.

The humerus is a stout elongated bone, articulating by its large proximal head (fig. 77, 1) with the glenoid cavity of the scapula, and at its distal end by the trochlea with the bones of the fore-arm. The head passes on its inner side into an area roughened for the attachment of muscles and called the lesser tuberosity (fig. 77, 2); while in front it is divided by the shallow bicipital groove from a large roughened area, the greater tuberosity (fig. 77, 3), which is continued as a slight roughened ridge, extending about one-third of the way down the outer side of the shaft. This ridge, which in many animals is much more strongly developed than it is in the dog, is called the deltoid ridge. The trochlea (fig. 77, 5) at the distal end of the bone is a pulley-like surface, elevated at the sides and grooved in the middle. It articulates with the radius and ulna of the fore-arm. On each side of it are slight roughened projections, the internal and external condyles (fig. 77, 7). In the cat and many other animals there is a foramen, the ent-epicondylar foramen above the internal condyle, but in the dog this is not developed. Passing up the shaft from the external condyle is a slight ridge, the supinator or ectocondylar ridge; this is better developed in many mammals. Immediately above the trochlea in front and behind are the deep supra-trochlear fossae, which communicate with one another through the supra-trochlear foramen (fig. 77, 8). The posterior of these, the olecranon fossa, is much the deeper, and receives the olecranon process of the ulna when the arm is extended. The head and tuberosities of the humerus ossify from one centre, the shaft from a second, and the trochlea and condyles from a third.

The fore-arm or antibrachium contains two bones, the radius and ulna; they are immovably articulated with one another, but not fused. The pre-axial bone, the radius (fig. 77, B), which lies more or less in front of the ulna, is external to the ulna at its proximal end, and at its distal end is internal to that bone. It articulates with the external portion of the trochlea, while the ulna articulates with the internal portion. It is a straight bone with its distal end slightly larger than its proximal end. The proximal end articulates with the trochlea, the distal end with the bones of the carpus.

The postaxial bone, the ulna (fig. 77, C), has the proximal end much enlarged, forming the olecranon (fig. 77, 11), and tapers gradually to the distal end. Near its proximal end the ulna is marked by a deep sigmoid notch, which bears on its inner side a concave surface (fig. 77, 12) for articulation with the trochlea. The pointed proximal end of the sigmoid notch is called the coronoid process. Somewhat in front of and below the sigmoid notch is a smaller hollow (fig. 77, 13), with which the radius articulates.

In the young animal the ends of both radius and ulna are seen to ossify from centres different from those forming the shafts. The epiphyses forming both ends of the radius, and the distal end of the ulna are large, while that at the proximal end of the ulna is small, and forms only the end of the olecranon.

The Manus is divided into

a. The carpus or wrist, formed of a group of small bones.

b. The hand, which includes firstly some elongated bones, the metacarpals, forming what corresponds to the palm of the hand, and secondly the phalanges, which form the fingers.

The Carpus or wrist. The carpus of the dog consists of seven small bones, arranged in a proximal row of three, and a distal row of four. It differs much from the simpler type met with in the newt. The largest bone of the proximal row is the scapho-lunar (fig. 80, 1), formed by the fused scaphoid (radiale), lunar (intermedium), and centrale; it has a large convex proximal surface for articulation with the radius, and articulates distally with the trapezium, trapezoid, and magnum, and internally with the cuneiform. The cuneiform (ulnare) (fig. 80, 2) has a posterior rounded surface articulating with the ulna; it articulates in front with the unciform, and internally with the pisiform (fig. 80, 7), which is a comparatively large sesamoid bone on the ulnar side of the carpus. Frequently also there is a small sesamoid bone on the radial side of the carpus. The trapezium (carpale 1), trapezoid (carpale 2), and magnum (carpale 3) (fig. 80, 5) are all small bones, and support respectively the first, second, and third metacarpals. The unciform (carpalia 4 and 5) (fig. 80, 6) is larger, and supports the fourth and fifth metacarpals.

The hand has five digits, each consisting of an elongated metacarpal, followed by phalanges, the last of which, the ungual phalanx, is pointed and curved, and bears the claw. Each of the metacarpals is seen in the young animal to have its distal end formed by a prominent epiphysis, and each of the phalanges, except those bearing the claws, has a similar epiphysis at its proximal end.

The pollex (fig. 80, A, I ) is far shorter than the other digits, and normally does not touch the ground in walking. It has only two phalanges, while each of the other digits has three. A pair of small sesamoid bones are developed on the ventral or flexor side of the metacarpo-phalangeal articulations of all the digits except the pollex. Frequently similar sesamoid bones occur also on the dorsal side of the phalangeal articulations.

The Pelvic Girdle.

The pelvic girdle consists of two halves, which lie nearly parallel to the vertebral column.

Each half is firmly united to its fellow in a ventral symphysis behind, and is in front expanded and united to the sacrum. Each half or innominate bone is seen in the young animal to consist of four distinct parts, the ilium or dorsal element, the pubis or anterior ventral element, the ischium or posterior ventral element, and a small fourth part, the acetabular or cotyloid bone, wedged in between the three others. These parts, though all distinct in the young animal, are in the adult so completely fused that their respective boundaries cannot be distinguished. At about the middle of the outer surface of the innominate bone is a very deep cavity, the acetabulum (fig. 78, A, 1) with which the head of the femur articulates; all the bones except the pubis take part in its formation.

The ilium is a rather long bone, expanded in front and contracted behind; it forms about half the acetabulum. On its inner or sacral surface (fig. 78, 4) is a large roughened patch for articulation with the sacrum; its outer or gluteal surface is concave. The posterior part of the bone is flattened below, forming the narrow iliac surface (fig. 78, A, 5).

The ischium (fig. 78, 9) is a wide flattened bone forming the posterior part of the innominate bone. It meets the pubis ventrally, but is separated from it for the greater part of its length by the large obturator or thyroid foramen (fig. 78, 2). At its posterior end externally it bears a rather prominent roughened ischial tuberosity (fig. 78, A, 10). The ischium meets its fellow in a ventral symphysis, and forms about one-third of the acetabulum.

The pubis (fig. 78, 12) is smaller than either the ischium or ilium; it does not take part in the formation of the acetabulum, and like the ischium, meets its fellow in a ventral symphysis. The acetabular bone (fig, 78, B, 14) is small and triangular, and is wedged in between the other three. It forms about one-sixth of the acetabulum.

The Posterior Limb.

The posterior limb, like the anterior, is divisible into three parts; these are the thigh, the crus or shin, and the pes.

The thigh contains only a single bone, the femur.

The femur is a long straight bone with a nearly smooth shaft and expanded ends. The proximal end bears on its inner side the large rounded head (fig. 79, A, 1) which articulates with the acetabulum. External to the head and divided from it by a deep pit is a large rough outgrowth, the great trochanter (fig. 79, 3). The deep pit is the trochanteric or digital fossa. On the inner side below the head is a smaller roughened surface, the lesser trochanter. The lower or distal end of the bone bears two prominent rounded surfaces, the condyles, which articulate with the tibia. They are separated from one another by the deep intercondylar notch, which is continued above and in front as a shallow groove, lodging a large sesamoid bone, the patella or knee-cap. At the back of the knee-joint are a pair of smaller sesamoids, the fabellae (fig. 79, 7).

In the young animal there are three epiphyses to the shaft of the femur, one forming the head, one the great trochanter, and one the distal end.

The crus or shin contains two bones, the tibia and fibula. The tibia is a fairly thick straight bone, expanded at both ends, especially at the head or proximal end. The proximal end is triangular in cross section, and bears two facets for articulation with the condyles of the femur. The anterior surface of the proximal end of the tibia is marked by the strong cnemial crest (fig. 79, 8), which runs some way down the shaft. The distal end of the tibia articulates with the astragalus by an irregular, somewhat square surface.

The shaft of the tibia ossifies from one centre, the distal end from a second, and the proximal end from two more.

The fibula (fig. 79, C) is a distinct but very slender bone, somewhat expanded at both ends. It lies external to the tibia and articulates by its proximal end with the head of the tibia, and by its distal end with the calcaneum. Its shaft and proximal end ossify from one centre, and its distal end from a second.

The Pes.

The structure of the pes corresponds closely with that of the manus. It is divided into:—

a. The tarsus or ankle formed of a group of small bones.

b. The foot, which includes, firstly, some elongated bones, the metatarsals, forming what corresponds to the sole of the foot, and secondly the phalanges, which form the toes.

The Tarsus. The tarsus of the dog consists of seven bones arranged in two rows, of two and four respectively, with a centrale between them. The two bones of the proximal row are the astragalus and calcaneum.

The astragalus (fig. 80, 10) corresponds to the fused tibiale and intermedium of the typical tarsus. Its proximal end is much wider than its distal end, and forms a large rounded condyle articulating with the tibia, while its posterior end meets the navicular. It lies to the dorsal side of the foot.

The calcaneum (fibulare) (fig. 80, 11), the thickest bone in the pes, lies somewhat behind, and to the outer side of the astragalus. It articulates with the astragalus and fibula, and is drawn out behind into a long rounded process, which forms the heel, and is in the young animal terminated by an epiphysis. Between the proximal and distal rows of tarsals is the navicular (centrale) (fig. 80, 12), a somewhat flattened and square bone articulating with the astragalus.

The distal row of tarsals consists of four bones. The internal cuneiform (tarsale 1) is a smooth flattened bone lying to the inner side of the foot; it articulates with the first metatarsal and with the navicular. The middle cuneiform (tarsale 2) (fig. 80, 13) is a still smaller bone, lying external to the internal cuneiform. It articulates with the second metatarsal and with the navicular. The external cuneiform (tarsale 3) (fig. 80, 14) is a larger, somewhat square bone lying external to the middle cuneiform. It articulates with the third metatarsal and with the navicular. The cuboid (tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying to the outer side of the foot. It articulates with the fourth and fifth metatarsals and with the calcaneum.

The pes has sometimes five digits, sometimes four, the hallux being absent. Even when present the hallux (fig. 80, B, I) is commonly much reduced, and may be quite vestigial, and represented only by a small nodular metatarsal.

Each of the other digits consists of a long metatarsal, which in the young animal has a prominent epiphysis at its distal end, and of three phalanges. The proximal and middle phalanges have epiphyses at their proximal ends, while the distal phalanx is without epiphyses and is claw-shaped.

CHAPTER XXII.
GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA.

Sometimes as in Hippopotamus, Orycteropus and the Sirenia, the hair, though scattered over the whole surface, is extremely scanty, while in the Cetacea it is limited to a few bristles in the neighbourhood of the mouth, or may even be absent altogether in the adult. In most mammals the hairs are shed and renewed at intervals, sometimes twice a year, before and after the winter. The vibrissae or large hairs which occur in many animals upon the upper lip, and the mane and tail of Equidae are probably persistent.

In the hedgehogs, porcupines and Echidna certain of the hairs are modified and greatly enlarged, forming stiff spines. Similar spines occur in the young of Centetes, and in Acanthomys among the Muridae.

Several other forms of epidermal exoskeleton are met with in mammals, including:—

(a) Scales. These overlie the bony scutes of armadillos and occur covering the tail in several groups of mammals, such as beavers and rats. In the Manidae the body is covered by flat scales which overlap.

(b) The horns of Bovine Ruminants. These, which must on no account be confused with antlers, are hollow cases of hardened epidermis fitting on to bony outgrowths of the frontals. In almost every case they are unbranched structures growing continuously throughout life, and are very rarely shed entire. In the Prongbuck Antilocapra however they are bifurcated and are periodically shed. Horns are nearly always limited to a single pair, but the four-horned antelope Tetraceros has two pairs, the anterior pair being the smaller.

(c) The horns of Rhinoceroses. These are conical structures composed of a solid mass of hardened epidermal cells growing from a cluster of long dermal papillae. From each papilla there grows a fibre which resembles a thick hair, and cementing the whole together are cells which grow from the interspaces between the papillae. These fibres differ from true hairs in not being developed in pits in the dermis. Rhinoceros horns may be either one or two in number, and are borne on the fronto-nasal region of the skull. They vary much in length, the longest recorded having the enormous length of fifty-seven inches.

(d) Nails, hoofs and claws. In almost all mammals except the Cetacea, these are found terminating the digits of both limbs. Nails are more or less flattened structures, claws are pointed and somewhat curved. In most mammals the nails tend to surround the ends of the digits much more than they do in man. Sometimes the nail of one digit differs from that of all the others; thus the second digit of the pes in the Hyracoidea and Lemuroidea is terminated by a long claw, the other digits having flat nails. In the Felidae the claws are retractile, the ungual phalanx with claw attached folding back when the animal is at rest into a sheath, above, or by the side of the middle phalanx. In the Sloths and Bats enormously developed claws occur, forming hooks by which the animals suspend themselves. In Notoryctes the third and fourth digits of the manus bear claws of great size; similar claws occur in Chrysochloris, being correlated in each case with fossorial habits. The nail at its maximum development entirely surrounds the terminal phalanx of the digit to which it is attached, and is then called a hoof. Hoofs are specially characteristic of the Ungulata.

(e) Spurs and beaks are structures which are hardly represented among mammals, while so characteristic of birds. They are however both found in the Monotremata. In both Echidna and Ornithorhynchus the male has a peculiar hollow horny spur borne on a sesamoid bone articulated to the tibia. The jaws in Ornithorhynchus are cased in horny beaks similar to those of birds, and are provided with horny pads which act as teeth.

(f) Horny plates of a ridged or roughened character occur upon the anterior portion of the palate, and of the mandibular symphysis in all three genera of recent Sirenia; also upon the toothless anterior portion of the palate in Ruminants.

(g) The baleen of whales also belongs to the epidermal exoskeleton. It consists of a number of flattened horny plates arranged in a double series along the palate. The plates are somewhat triangular in form and have their bases attached to the palate at right angles to its long axis, while their apices hang downwards into the mouth cavity. The outer edge of each plate is hard and smooth, while the inner edge and apex fray out into long fibres which look like hair. At the inner edge of each principal plate are subsidiary smaller plates. The plates are formed of a number of fibres each developed round a dermal papilla in the same way as are the fibres forming the horns of Rhinoceros. Baleen and Rhinoceros horn likewise agree in that the fibres are bound together by less hardened epithelial cells, which readily wear away and allow the harder fibres to fray out. The greatest development of baleen occurs in the Northern Right whale, Balaena mysticetus, in which the plates number three hundred and eighty or more on each side, and reach a length of ten or twelve feet near the middle of the series.

Dermal Exoskeleton.

Mammals show two principal kinds of exoskeletal structures which are entirely or partially dermal in origin, viz. the bony scutes of armadillos, and teeth.

The bony scutes of armadillos are quadrate or polygonal in shape and are in general aggregated together, forming several shields protecting various regions of the body. The head is generally protected by a cephalic shield, the anterior part of the body by a scapular, and the posterior by a pelvic shield. The tail is also generally encased in bony rings, and scutes are irregularly scattered over the surface of the limbs. The mid-body region is protected by a varying number of bands of scutes united by soft skin, so as to allow of movement. Corresponding to each dermal scute is an epidermal plate. In Chlamydophorus the scutes are mainly confined to the posterior region where they form a strong vertically-placed shield which coalesces with the pelvis. The anterior part of the body is mainly covered by horny epidermal plates with very little ossification beneath. In the gigantic extinct Glyptodonts the body is covered with a solid carapace formed by the union of an immense number of plates, and there are no movable rings. The top of the head is defended by a similar plate, the tail is generally encased in an unjointed bony tube, and there is commonly a ventral plastron.

In Phocaena phocaenoides the occurrence of vestigial dermal ossicles has been described, and in Zeuglodon the back was probably protected by dermal plates.

Teeth[145].

Teeth are well developed in the vast majority of mammalia, and are of the greatest morphological and systematic importance, many extinct forms being known only by their teeth. Mammalian teeth differ from those of lower animals in various well-marked respects. (1) They are attached only to the maxillae, premaxillae and mandible, never to the palatines, pterygoids or other bones. (2) They frequently have more than one root. (3) They are always, except in some Odontoceti, placed in distinct sockets. (4) They are hardly ever ankylosed to the bone. (5) They are in most cases markedly heterodont. (6) They are commonly developed in two sets, the milk dentition and permanent dentition.

It sometimes happens that teeth after being formed are reabsorbed without ever cutting the gum. This is the case, for instance, with the upper incisors of Ruminants.

The form of mammalian teeth varies much, some are simple conical structures comparable to those of most reptiles, and these may either have persistent pulps, as in the case of the upper canines of the Walrus and the tusks of Elephants, or may be rooted as in most canine teeth. Some teeth have chisel-shaped edges, and this may be their original form, as in the human incisors, or may, as in those of Rodents, be brought about by the more rapid wearing away of the posterior edge, the anterior edge being hardened by a layer of enamel. Then, again, the crown may, as in the majority of grinding teeth, be more or less flattened. The various terms used in describing some of the forms of the surface of grinding teeth are defined on page 345.

The teeth of the Aard Varks are compound, and differ completely from those of all other mammals (see p. 425).

As a rule, the higher the general organisation of an animal the better are its milk teeth developed, and the more do they form a reproduction on a small scale of the permanent set. This fact is well seen in the Primates, Carnivora and Ungulata. The method of notation by which the dentition of any mammal can be briefly expressed as a formula has been already described. The regular mammalian arrangement of teeth for each side is expressed by the formula

i ³/₃ c ¹/₁ pm ⁴/₄ m ³/₃ × 2; total, 44.

Monotremata. In Echidna teeth are quite absent. In the young Ornithorhynchus[146] functional molar teeth of a multi-tubercular type resembling those of some Mesozoic mammalia are present, but in the adult they disappear, their office being discharged by horny plates.

Marsupialia[147] have a heterodont dentition, which has generally been regarded as almost monophyodont, the only tooth which has an obvious deciduous predecessor being the last premolar. The researches of Röse[148] and Kükenthal[149] tend to show that the teeth of Marsupials are developed in the same way as in other mammals, and are diphyodont. In the case of the premolars, teeth which are homologous with the permanent teeth of other mammals begin to develop as lateral outgrowths from the milk teeth, but afterwards become absorbed, so that the teeth which actually persist belong to the milk series. The last premolar, however, does as a rule develop and replace its milk predecessor; sometimes, however, as in Didelphys, it takes its place among the milk molars without replacing one of them.

The types of dentition characteristic of the different groups of placental mammals may mostly be paralleled among the Marsupials. Thus among the polyprotodont forms the Didelphyidae or opossums, and some of the Dasyuridae, such as Sarcophilus and Thylacinus, have a typical carnivorous dentition with small incisors, large canines, and molars with pointed compressed crowns. The dental formula of Thylacinus, is i ⁴/₃ c ¹/₁ pm ³/₃ m ⁴/₄, total 46.

In Myrmecobius five or six molar teeth occur on each side, and the total number of teeth reaches fifty-two or fifty-six. The teeth bear rows of tubercles, and resemble those of the Multituberculate mesozoic Mammals[150], more than do those of any other living form. Calcified teeth have recently been described[151] in Myrmecobius earlier than the functional or milk set. This would relegate the milk teeth of mammals in general to a second series, and the permanent teeth to a third. In Notoryctes the dental formula[152] is given as i ³/₂ c ¹/₁ pm ²/₃ m ⁴/₄, total 40. The canines are small, and the anterior molars have strongly developed cusps, and much resemble those of Chrysochloris (Insectivora).

Among the diprotodont types the Phascolomyidae, or Wombats, have a dentition recalling that of the Rodents. All the teeth grow from persistent pulps, and the incisors have enamel only on the anterior surface as in Rodents. The dental formula is i ¹/₁ c ⁰/₀ pm ¹/₁ m ⁴/₄, total 24. There are indications of a vestigial second pair of incisors.

The Macropodidae, or Kangaroos, have a herbivorous dentition with the formula i ³/₁ c ^(0—1)/₀ pm ²/₂ m ⁴/₄. The incisors are sharp and cutting, and are separated by a long diastema or gap from the molars, which have their crowns marked by ridges or cusps. There are indications of several vestigial incisors.

Coenolestes, a remarkable form recently described from America, belongs to the diprotodont section, and is the only living member of the section known outside the Australian region[153]. An exceptional dentition is seen in the case of the extinct Thylacoleo, in which the functional teeth are reduced to two pairs; one pair of large cutting incisors and one of compressed sharp-edged premolars.

Edentata. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as far as is known, absolutely toothless at all stages of their existence; being the only mammals except Echidna in which no tooth germs have been discovered; others, viz. the Manidae, though showing foetal tooth germs, are quite toothless in post-foetal life; others, viz. some of the armadillos, have the largest number of teeth met with in land mammals. The teeth are homodont except in the Aard Varks, and grow from persistent pulps. In the sloths (Bradypodidae) and the Megatheriidae, there are five pairs of teeth in the upper and four in the lower jaw. The teeth of sloths consist of a central axis of vasodentine, surrounded firstly by a thin coating of hard dentine, and secondly by a thick coating of cement.

In no living Edentate have the teeth any enamel; it has, however, been described as occurring in certain early Megatheroid forms from S. America[154], and an enamel organ has also been discovered in an embryo Dasypus[155]. In the Armadillos (Dasypodidae) the number of teeth varies from ⁸/₈ or ⁷/₇ in Tatusia, to upwards of ²⁵/₂₅ in Priodon, which therefore may have upwards of a hundred teeth, the largest number met with in any land mammal. In Tatusia all the teeth except the last are preceded by two-rooted milk teeth. The Aard Varks are diphyodont, and milk teeth are also known in a species of Dasypus, but with these exceptions Edentates are, as far as is known, monophyodont. In Glyptodon the teeth are almost divided into three lobes by two deep grooves on each side.

The Aard Varks (Orycteropodidae) are quite exceptional as regards their teeth, which are cylindrical in shape, and are made up of a number of elongated denticles fused together. Each denticle contains a pulp cavity from which a number of minute tubes radiate outwards. These teeth are diphyodont and somewhat heterodont, eight to ten pairs occur in the upper jaw and eight in the lower, but they are not all in place at one time. The last three teeth in each jaw are not preceded by milk teeth[156].

Sirenia. The teeth of Sirenia show several very distinct types, the least modified being that of the extinct Halitheriidae, which have large incisors in the upper jaw, and five or six pairs of tuberculated grinding teeth in each jaw, the anterior ones being preceded by milk teeth.

In both the living genera the dentition is monophyodont. In Manatus the dentition is i ²/₂ pm and m ¹¹/₁₁. The incisors are vestigial, and disappear before maturity. The grinding teeth have square enamelled crowns marked by transverse tuberculated ridges. They are not all present in the jaw at the same time. In Halicore the upper jaw bears a pair of straight tusklike incisors; in the male these have persistent pulps and project out of the mouth; in the female they soon cease to grow and are never cut. They are separated by a long diastema from the grinding teeth which have tuberculated crowns and are ⁵/₅ or ⁶/₆ in number, but are not all in place at once. Several other pairs of slender teeth occur in the young animal, but are absorbed or fall out before maturity. In Rhytina teeth are altogether absent.

Cetacea.

Archaeoceti. Zeuglodon has the following dentition, i ³/₃ c ¹/₁ pm and m ⁵/₅, total 36. The incisors and canines are simple and conical; the cheek teeth are compressed and have serrated cutting edges like those in some seals.

In the Mystacoceti, or whalebone whales, calcified tooth germs probably belonging to the milk dentition are present in the embryo, but they are never functional, and are altogether absent in the adult. The anterior of these germs are simple, the posterior ones are originally complex, but subsequently split up into simple teeth like those of the anterior part of the jaw. Hence according to Kükenthal, who described these structures, the Cetacean dentition was originally heterodont.

In the living Odontoceti the dentition is homodont and monophyodont. In some cases traces occur of a replacing dentition which never comes to maturity, and renders it probable that the functional teeth of the Odontoceti are really homologous with the milk teeth of other mammals. Some of the dolphins afford the apparently simplest type of mammalian dentition known. The teeth are all simple, conical, slightly recurved structures, with simple tapering roots and without enamel. The dentition is typically piscivorous, being adapted for seizing active slippery animals such as fish. The prey is then swallowed entire without mastication. Sometimes the teeth are excessively numerous, reaching two hundred or more (fifty to sixty on each side of each jaw) in Pontoporia. This multiplication of teeth is regarded by Kükenthal as due to the division into three parts of numbers of trilobed teeth similar to those of some seals.

In the Sperm whale, Physeter, the lower jaw bears a series of twenty to twenty-five stout conical recurved teeth, while in the upper jaw the teeth are vestigial and remain imbedded in the gum. An extinct form, Physodon, from the Pliocene of Europe and Patagonia is allied to the Sperm whale, but has teeth in both jaws. In the Killer Orca, the teeth number about ¹²/₁₂, and are very large and strong. In some forms the teeth are very much reduced in number; thus in Mesoplodon the dentition consists simply of a pair of conical teeth borne in the mandible. In the Narwhal Monodon the dentition is practically reduced to a single pair of teeth, which lie horizontally in the maxillae, and in the female normally remain permanently in the alveoli. In the male the right tooth remains rudimentary, while the left is developed into an enormous cylindrical tusk marked by a spiral groove. Occasionally both teeth develop into tusks, and there is reason for thinking that two-tusked individuals are generally or always female. In the extinct Squalodon the dentition is decidedly heterodont, and the molars have two roots. The dental formula is

i ³/₃ c ¹/₁ pm ⁴/₄ m ⁷/₇, total 60.

It is probable that the homodont condition of modern Odontoceti is not primitive, but due to retrogressive evolution.

Ungulata.

Just as in the Cetacea a piscivorous dentition is most typically developed, so the Ungulata are, as a group, the most characteristic representatives of a herbivorous dentition in its various forms.

Ungulata vera.

Artiodactyla. As regards the living forms, the Artiodactyla can be readily divided into two groups, namely those with bunodont and those with selenodont teeth. It has, however, been shown that selenodont teeth always pass through an embryonic bunodont stage[157]. The bunodont type is best seen in Pigs and Hippopotami and such extinct forms as Hyotherium. In Hippopotamus the dental formula is i ^(2-3)/_(1-3) c ¹/₁ pm ⁴/₄ m ³/₃.

The incisors and canines of Hippopotamus are very large and grow continuously. The genus Sus, which affords a good instance of an omnivorous type of dentition, has the regular unmodified Mammalian dental formula i ³/₃ c ¹/₁ pm ⁴/₄ m ³/₃, total 44. The canines, specially in the male, are large and have persistent pulps, and the upper canines do not have the usual downward direction but pass outwards and upwards. In the Wart Hog, Phacochaerus, they are enormously large, but a still more extraordinary development of teeth is found in Babirussa. In the male Babirussa the canines, which are without enamel, are long, curved and grow continuously. Those of the upper jaw never enter the mouth, but pierce the skin of the face and curve backwards over the forehead. The dental formula of Babirussa is i ²/₃ c ¹/₁ pm ²/₂ m ³/₃, total 34.

The Wart Hog has a very anomalous dentition, for as age advances all the teeth except the canines and last molars show signs of disappearing; both pairs of persisting teeth are however very large.

Various extinct Ungulata such as Anoplotherium have teeth which are intermediate in character between the bunodont and selenodont types. Anoplotherium has the regular mammalian series of forty-four teeth. The crowns of all the teeth are equal in height, and there is no diastema—an arrangement found in no living mammal but man.

We come now to the selenodont Artiodactyla.

The Tylopoda—camels (Camelidae) and Llamas (Aucheniidae) when young have the full number of incisors, but in the adult the two upper middle ones are lost. The molars are typically selenodont and hypsodont[158]. In the Camel the dental formula is i ¹/₃ c ¹/₁ pm ³/₂ m ³/₃, total 34. The upper incisors, canines and first premolars of the Camel are very small teeth, and the first premolar is separated by a long diastema from the others.

The Tragulina or Chevrotains have no upper incisors, while the canines are largely developed, especially in the male.

The Ruminantia or Pecora are very uniform as regards their dentition. The upper incisors are always absent, for though their germs are developed they are reabsorbed without ever becoming visible, and as a rule the upper canines are absent too, while the lower canines are incisiform. The grinding teeth are typically selenodont, and in the lower jaw form a continuous series separated by a wide diastema from the canines. The dental formula is usually

i ⁰/₃ c 0-¹/₁ pm ³/₃ m ³/₃.

The canines are largely developed in the male Muskdeer (Moschus) and in Hydropotes.

Perissodactyla. The premolars and molars have a very similar structure and form a continuous series of large square teeth with complex crowns. The crowns are always constructed on some modification of the bilophodont[159] plan, as is easily seen in the case of the forms with brachydont teeth, but in animals like the Horse, in which the teeth are very hypsodont, this arrangement is hard to trace. All four premolars in the upper jaw are preceded by milk teeth, while in Artiodactyla the first has no milk predecessors.

In the Tapiridae the grinding teeth are brachydont and the lower ones are typically bilophodont. The last two upper molars have the transverse ridges united by an outer longitudinal ridge. The dentition is i ³/₃ c ¹/₁ pm ⁴/₃ m ³/₃, total 42.

In some of the extinct Perissodactyles such as Lophiodon[160], the dentition is brachydont and bilophodont, the grinding teeth in general resembling the posterior upper molars of the Tapir. The same type of brachydont tooth is seen in Palaeotherium but the transverse ridges are crescentic instead of straight, and are separated from one another by shallow valleys without cement. Some of the Palaeotheridae have the regular series of forty-four teeth.

A complete series of forms is known showing how from the simple brachydont teeth of the Palaeotheridae, were derived the complicated hypsodont teeth of the Equidae. The increase in depth of the tooth was accompanied by increase in the depth and complexity of the enamel infoldings, and of the cement filling them.

Both upper and lower grinding teeth of the Equidae are much complicated by enamel infoldings, but their derivation from the bilophodont type can still be recognised. The diastema in front of the premolars is longer in the living Equidae than in their extinct allies. In the adult horse the dental formula is i ³/₃ c ¹/₁ pm ³/₃ m ³/₃, total 40, with often a vestigial first upper premolar (fig. 82, pm 1). The last molar is not more complex than the others, and in the female the canine is quite vestigial. The incisors are large and adapted for cutting and have the enamel curiously folded in forming a deep pit. The milk dentition is di ³/₃ dc ⁰/₀ dpm ³/₃, total 24. The last milk premolar is not more complex than the premolar that succeeds it. The horse affords an excellent instance of a typically herbivorous type of dentition, the cutting incisors, reduced canines and series of large square flat-crowned grinding teeth being most characteristic.

In Rhinoceros the grinding teeth are much like those of Lophiodon, having an outer longitudinal ridge from which two crescentic transverse ridges diverge. The upper premolars are as complex as the molars, and there are no canines; in some species incisors also are absent. The dental formula is

i ^(0—2)/_(0—1) c ⁰/_(0—1) pm ⁴/₄ m ³/₃.