The skeleton of the members of this class, the highest of the vertebrata, has the following characteristics:—

Some part of the integument at some period of life is always provided with hairs; these are epidermal structures arising from short papillae of the Malpighian layer of the epidermis, which at once grow inwards and become imbedded in pits of the dermis. Sometimes scales or spines occur, and epidermal exoskeletal structures in the form of hoofs, nails, claws and horns are also characteristic. As regards the endoskeleton, the vertebral centra have terminal epiphyses except in the Ornithodelphia and some Sirenia. In the skull the cranial region is greatly developed as compared with that in lower vertebrates, and whereas in many reptiles the true cranium is largely concealed by a false roof, in mammals the only relic of this secondary roof is found in the zygomatic arch, and postorbital bar. In the adult all the bones except the mandible, hyoid, and auditory ossicles are firmly united together. The basisphenoid is well ossified, and there is no parasphenoid. The pro-otic ossifies, and unites with the epi-otic and opisthotic before they coalesce with any other bones.

The skull articulates with the vertebral column by means of two convex occipital condyles formed mainly by the exoccipitals, and the mandible articulates with the squamosal without the intervention of the quadrate. The latter is much reduced, and is converted into the tympanic ring, while the hyomandibular of fish is represented by the auditory ossicles[108].

The teeth are always attached to the maxillae, premaxillae and mandibles, never to any of the other bones. They are nearly always implanted in distinct sockets, and are hardly ever ankylosed to the bone. The teeth of mammals are generally markedly heterodont, four forms, incisors, canines, premolars, and molars, being commonly distinguishable. Some mammals are monophyodont, having only a single set of teeth, but the great majority are diphyodont, having two sets, a deciduous or milk dentition, and a permanent dentition.

The incisors, the front teeth, are simple, one-rooted, adapted for cutting, and are nearly always borne by the premaxillae. Next come the canines, one on each side in each jaw. They are generally large teeth adapted for tearing or holding, and get their name from the fact that they are largely developed in the dog. The remaining teeth form the grinding series, the more posterior of them being the molars, which are not preceded by milk teeth[109]. Between the molars and the canines are the premolars, which do as a rule have milk or deciduous predecessors, though very frequently the first of them is without a milk predecessor.

In describing the dentition of any mammal, for the sake of brevity a formula is generally made use of. Thus, the typical mammalian dentition is expressed by the formula

i ³/₃ c ¹/₁ pm ⁴/₄ m ³/₃ = ¹¹/₁₁,

giving twenty-two teeth on each side, or forty-four altogether[110]. The incisors are represented by i, the canines by c, the premolars by p or pm, and the molars by m. The numbers above the lines represent the teeth in the upper jaw, those below the lines the teeth in the lower jaw. The milk dentition is expressed by a similar formula with d (deciduous) prefixed to the letter expressing the nature of the tooth.

The following terms are of frequent use as characterising certain forms of the grinding surfaces of teeth, and it will be well to define them at once.

Bunodont is a term applied to teeth with broad crowns raised into rounded tubercles, e.g. the grinding teeth of Pigs and Hippopotami;

Bilophodont to teeth marked by a simple pair of transverse ridges, with or without a third ridge running along the outer border of the tooth at right angles to the other two, e.g. the grinding teeth of Lophiodon, Kangaroo, Manatee, Tapir, Dinotherium;

Selenodont to teeth marked by crescentic ridges running from the anterior towards the posterior end of the tooth, e.g. the grinding teeth of the Ox and Sheep.

Teeth whose crowns are low so that their whole structure is visible from the grinding surface are called brachydont, while those with higher crowns, in which the bases of the infoldings of enamel are invisible from the grinding surface are said to be hypsodont. Bunodont teeth are brachydont, the teeth of the Horse and Ox are hypsodont.

Passing now to the appendicular skeleton—the shoulder girdle differs markedly from that of Sauropsids in the fact that the coracoid, except in the Ornithodelphia, is greatly reduced, generally forming only a small process on the scapula. In the pelvis the pubes meet in a ventral symphysis, except in some Insectivora and Chiroptera. In many mammals a fourth pelvic element, the acetabular bone, is distinguishable. The ankle joint is cruro-tarsal, or situated between the proximal tarsal bones and the tibia and fibula. Carpalia 4 and 5 are united forming the unciform; and the ulnar sesamoid bone or pisiform is generally well developed. In the proximal row of tarsal elements there are only two bones, the calcaneum and astragalus. Of these the calcaneum is the fibulare, and the astragalus is generally regarded as the tibiale and intermedium fused[111].

Subclass I. Ornithodelphia or Prototheria.

This subclass contains only a single order, the Monotremata, and the following characteristics are equally applicable to the subclass and to the order. The vertebral centra have no epiphyses, and the odontoid process remains for a long time free from the centrum of the second vertebra. With the exception of the atlas of Echidna the cervical vertebrae are without zygapophyses. The cranial walls are smooth and rounded, and the sutures between the several bones early become completely obliterated as in birds. The mandible is a very slight structure, with no ascending ramus, and with the coronoid process (see p. 398) and angle rudimentary. The auditory ossicles show a low state of development. The tubercula of the ribs articulate with the sides of the centra of the thoracic vertebrae, not with the transverse processes. Some of the cervical ribs remain for a long time separate from the vertebrae. Well ossified sternal ribs occur. No true teeth are present in the adult. The young Ornithorhynchus has functional molar teeth, but in the adult their place is taken by horny plates. In the Echidnidae neither teeth nor horny plates occur.

The coracoid (fig. 66, 3) is complete and well developed, and articulates with the sternum. A precoracoid (epicoracoid) occurs in front of the coracoid, and there is a large interclavicle (fig. 66, 6). The ridge on the scapula, corresponding to the spine of other mammals, is situated on the anterior border instead of in the middle of the outer surface. Epipubic bones are present. In the Echidnidae, but not in Ornithorhynchus[112], the central portion of the acetabulum is unossified as in birds. The humerus has a prominent deltoid crest; its ends are much expanded, and the distal end is pierced by an ent-epicondylar foramen. The fibula has a broad proximal process resembling an olecranon. The limbs and their girdles bear a striking resemblance to those of some Theromorphous reptiles.

The order Monotremata includes only two living families, the Echidnidae and Ornithorhynchidae.

Mesozoic Mammalia[113].

It will be well here to briefly refer to certain mammals of small size, the remains of which have been found in deposits of Mesozoic age. In the great majority of cases they are known only by the lower jaw, or sometimes only by isolated teeth. A large number of them are commonly grouped together as the Multituberculata, and are sometimes, partly owing to the resemblance of their teeth to those of Ornithorhynchus, placed with the Prototheria, sometimes between the Prototheria and the Metatheria. They are characterised by having a single pair of large incisors in the lower jaw, and one large with one or two smaller incisors in each premaxillae. The lower canines are very small or altogether wanting. The incisors are separated by a diastema from the grinding teeth, which are sometimes (Tritylodon) characterised by the possession of longitudinal rows of little tubercles separated by grooves, sometimes by having the premolars provided with high cutting edges, whose surfaces are obliquely grooved. Some of the Mesozoic mammals found associated with the Multituberculata, have however a dentition of an altogether different type, with at least three lower incisors, well developed canines and premolars, and numerous molars with peculiar three-cusped or tritubercular grinding surfaces. These mammals, one of the best known of which is Phascolotherium, are commonly separated from the Multituberculata, and are divided by Osborn into two groups, one allied to the Marsupials, and one to the Insectivores. The group showing Marsupial affinities is further subdivided into carnivorous, omnivorous, and herbivorous subgroups. The members of both groups commonly have four premolars, and six to eight molars in each mandibular ramus.

Subclass II. Didelphia or Metatheria.

This subclass, like the previous one, contains only a single order, viz. the Marsupialia[114]; but the forms referable to it are far more numerous than in the case of the Monotremata.

The integument is always furry, and the teeth are always differentiated into incisors, canines, premolars and molars. Except in Phascolomys, the number of incisors in the upper and lower jaws is never equal, and the number in the upper usually exceeds that in the lower jaw. There is no such regular succession and displacement of teeth as in most mammals. Sometimes the anterior teeth are diphyodont, and as a general rule the tooth commonly regarded as the last premolar is preceded by a milk tooth. The majority of the permanent teeth of most Marsupials are regarded as belonging to the milk series for two reasons, (1) they are developed from the more superficial tissues of the jaws, (2) a second set, the permanent teeth, begin to develop as outgrowths from them, but afterwards become aborted[115].

The odontoid process at an early stage becomes fused with the centrum of the second cervical vertebra, and the number of thoraco-lumbar vertebrae is always nineteen. The skull has several characteristic features. The tympanic bone remains permanently distinct, and the anterior boundary of the tympanic cavity is formed by the alisphenoid. The carotid canal perforates the basisphenoid, and the lachrymal canal opens either outside the orbit or at its margin. There are generally large vacuities in the palate. The angle of the mandible is (except in Tarsipes) more or less inflected; and as a rule the jugal furnishes part of the articular surface for the mandible. There is no precoracoid (epicoracoid) or interclavicle, and the coracoid is reduced to form a mere process of the scapula, not coming near the sternum.

Epipubic, or so-called marsupial bones[116], nearly always occur, and a fourth pelvic element, the acetabular bone, is frequently developed. The fibula is always complete at its distal end, sometimes it is fused with the tibia, but often it is not only free but is capable of a rotatory movement on the tibia. This is the case in the families Phascolomyidae, Didelphyidae, and Phalangeridae.

The Marsupialia can be subdivided into two main groups, according to the character of the teeth:—

1. Polyprotodontia.

In this group the incisors are small, subequal and numerous, not less than 4/3. The canines are larger than the incisors, and the molars have sharp cusps. The members of this group are all more or less carnivorous or insectivorous. The group includes the families Didelphyidae, Dasyuridae, Peramelidae, and Notoryctidae[117].

2. Diprotodontia.

In this group the incisors do not exceed 3/3, and are usually 3/1, occasionally 1/1. The first upper and lower incisors are large and cutting. The lower canines are always small or absent, and so in most cases are the upper canines. The molars have bluntly tuberculated, or transversely ridged crowns. The group includes the families Phascolomyidae, Phalangeridae, Macropodidae, and Epanorthidae.

Subclass III. Monodelphia or Eutheria.

This great group includes all the Mammalia except the orders Monotremata and Marsupialia. Coming to their general characteristics—as in the Didelphia the odontoid process and cervical ribs early become fused with the centra which bear them, while the coracoid is reduced so as to form a mere process on the scapula, and there is no precoracoid (epicoracoid), such as is found in Ornithodelphia. Clavicles may be present or absent; when fully developed they articulate with the sternum, usually directly, but occasionally, as in some Rodents and Insectivores, through the remains of the sternal end of the precoracoid. There is never any interclavicle in the adult, though sometimes traces of it occur during development. In the pelvis the acetabula are imperforate; and well-developed epipubic bones are never found in the adult, though traces of them occur in some Carnivores and foetal Ungulates.

Order 1. Edentata[118].

Teeth are not, as the name of the order seems to imply, always wanting; and sometimes they are very numerous. They are, however, always imperfect, and, with very few exceptions, are homodont and monophyodont. They have persistent pulps, and so grow indefinitely and are never rooted. In all living forms they are without enamel, consisting merely of dentine and cement, and are never found in the front part of the mouth in the situation occupied by the incisors of other mammals. These characters derived from the teeth are the only ones common to the various members of the order, which includes the living sloths, ant-eaters, armadillos, pangolins and aard varks, together with various extinct forms, chiefly found in beds of late tertiary age in both North and South America, the best known being the Megatheridae and Glyptodonts.

Order 2. Sirenia[119].

The skeleton of these animals has a general fish-like form, in correlation with their purely aquatic habits. The fore limbs have the form of paddles, but the number of phalanges is not increased beyond the normal. There are no external traces of hind limbs.

The whole skeleton and especially the skull and ribs is remarkably massive and heavy. The dentition varies; in the two living genera Manatus and Halicore, incisor and molar teeth are present, in one extinct genus, Rhytina, teeth are entirely absent, while in another, Halitherium, the dentition is more decidedly heterodont than in living forms. In the two living genera the dentition is monophyodont, but in Halitherium the anterior grinding teeth are preceded by milk teeth. The tongue and anterior part of the palate and lower jaw are covered with roughened horny plates. The skull is noticeable for the size and backward position of the anterior nares, also for the absence or small size of the nasal bones. There is no union of certain of the vertebrae to form a sacrum, and in living forms the centra are not terminated by well-formed epiphyses[120].

The cervical vertebrae are much compressed, but they are never ankylosed together. In Manatus there are only six cervical vertebrae. The caudal vertebrae have well-developed chevron bones. The humerus is distinctly articulated to the radius and ulna, and these two bones are about equally developed, and are often fused together. There are no clavicles, and the pelvis is vestigial, consisting of a pair of somewhat cylindrical bones suspended at some distance from the vertebral column. In living forms there is no trace of a posterior limb, but in Halitherium there is a vestigial femur connected with each half of the pelvis.

Order 3. Cetacea[121].

In these mammals the general form is more fish-like than is the case even in the Sirenia. The skin is generally almost completely naked, but hairs are sometimes present in the neighbourhood of the mouth, especially in the foetus. In some Odontoceti vestiges of dermal ossicles have been described, and in Zeuglodon the back was probably protected by dermal plates. The anterior limbs have the form of flattened paddles, showing no trace of nails, the posterior limb bones are quite vestigial or absent, and there is never any external sign of the limb. Teeth are always present at some period of the life history, but in the whalebone whales they are only present during foetal life, their place in the adult animal being taken by horny plates of baleen. In all living forms the teeth are simple and uniform structures without enamel; they have single roots, and the alveoli in which they are imbedded are often incompletely separated from one another. As in some forms traces of a replacing dentition have been described, it has been concluded that the functional teeth of Cetacea belong to the milk dentition.

The texture of the bones is spongy. The cervical vertebrae are very short, and though originally seven in number, are in many forms completely fused, forming one solid mass (fig. 67). The odontoid process of the axis is short and blunt, or may be completely wanting. The lumbar and caudal vertebrae are large and numerous, and as zygapophyses are absent, are very freely movable on one another; zygapophyses are also absent from the posterior thoracic vertebrae. The lumbar vertebrae are sometimes more numerous than the thoracic. The epiphyses are very distinct, and do not unite with the centra till the animal is quite adult. None of the vertebrae are united to form a sacrum, but the caudal vertebrae have large chevron bones.

The skull is peculiarly modified; the bones forming the occipital segment show a specially strong development, and the cranial cavity is short, high, and almost spherical. The supra-occipital is very large and rises up to meet the frontals, thus with the interparietal completely separating the parietals from one another.

The frontals are expanded, forming large bony plates, which roof over the orbits. The zygomatic process of the squamosal is extremely large and extends forwards to meet the supra-orbital process of the frontal; the zygomatic process of the jugal is on the contrary very slender. The face is drawn out into a long rostrum, formed of the maxillae and premaxillae surrounding the vomer and the mesethmoid cartilage. The maxillae are specially large, and extend backwards so as to partially overlap the frontals. The nasals are always small, and the anterior nares open upwards between the cranium and rostrum. The periotics are loosely connected with the other bones of the skull and the tympanics are commonly large and dense. The mandible has hardly any coronoid process, and the condyles are at its posterior end.

There are no clavicles, but the scapula and humerus are well developed. The humerus moves freely in the glenoid cavity, but all the other articulations of the anterior limb are imperfect; the various bones have flattened ends, and are connected with one another by fibrous tissue, which allows of hardly any movement. Frequently the carpus is imperfectly ossified.

The number of digits in the manus is generally five, sometimes four, and when there are four digits it is the third and not the first that is suppressed. The number of phalanges in the second and third digits almost always exceeds that which is normal in mammals, and the phalanges are also remarkable for having epiphyses at both ends. The pelvis is represented by two small bones which lie suspended horizontally at some distance below the vertebral column; in some cases vestiges of the skeleton of the hind limb are attached to them.

The Cetacea are divided into three suborders.

Suborder (1). Archaeoceti.

The members of this group are extinct; they differ from all living Cetacea in having the dentition heterodont and in the fact that the back was probably protected by dermal plates. The skull is elongated and depressed, and the brain cavity is very small. The temporal fossae are large, and there is a strong sagittal crest. The nasals and premaxillae are a good deal larger than they are in living Cetacea, and the anterior nares are usually far forward. The cervical vertebrae are not fused with one another, and the lumbar vertebrae are unusually elongated.

The limbs are very imperfectly known, but while the humerus is much longer than in modern Cetaceans, it is nevertheless flattened distally, indicating that the limb was paddle-like, and that there was scarcely any free movement between the fore-arm and upper arm.

The best known genus is Zeuglodon, which is found in beds of Eocene age in various parts of Europe, and in Alabama.

Suborder (2). Mystacoceti or Balaenoidea.

These are the Whalebone Whales or True Whales.

Calcified teeth representing the milk dentition occur in the foetus, but the teeth are never functional, and always disappear before the close of foetal life. There is a definite though small olfactory fossa. The palate is provided with plates of baleen or whalebone. The skull is symmetrical, and is extremely large in proportion to the body. The nasals are moderately well developed, and the maxillae do not overlap the orbital processes of the frontals. The lachrymals are small and distinct from the jugals. The tympanics are ankylosed to the periotics, and the rami of the mandible do not meet in a true symphysis. The ribs articulate only with the transverse processes, and the capitula are absent or imperfectly developed. Only one pair of ribs meets the sternum, which is composed of a single piece.

The group includes among others the Right whale (Balaena), Humpbacked whale (Megaptera), and Rorqual (Balaenoptera).

Suborder (3). Odontoceti.

Teeth always exist after birth and baleen is never present. The teeth are generally numerous, but are sometimes few and deciduous; the dentition is homodont (except in Squalodon). The dorsal surface of the skull is somewhat asymmetrical, there is no trace of an olfactory fossa, the nasals are quite rudimentary, and the hind ends of the maxillae cover part of the frontals; in all these respects the skull differs from that of the Mystacoceti. The lachrymal may either be united to the jugal or may be large and distinct. The tympanic is not ankylosed to the periotic. The rami of the mandible are nearly straight and become united in a long symphysis. Some of the ribs have well developed capitula articulating with the vertebral centra. The sternum is almost always composed of several pieces as in other mammals, and several pairs of ribs are connected with it. There are always five digits to the manus, though the first and fifth are usually very little developed.

The suborder includes the Sperm Whale (Physeter), Narwhal (Monodon), Dolphin (Delphinus), Porpoise (Phocoena), and many other living forms as well as the extinct Squalodon which differs from the other members of the suborder in its heterodont dentition.

Order 4. Ungulata.

This order includes a great and somewhat heterogeneous group of animals, a large proportion of which are extinct. They all (except certain extinct forms) agree in having the ends of the digits either encased in hoofs or provided with broad flat nails. The teeth are markedly heterodont and diphyodont, and the molars have broad crowns with tuberculated or ridged surfaces. Clavicles are never present in the adult except in a few generalised extinct forms such as Typotherium, and it is only recently that vestigial clavicles have been discovered in the embryo[122]. The scaphoid and lunar are always distinct.

The order Ungulata may be subdivided into two main groups, Ungulata vera and Subungulata.

Section I. Ungulata vera[123].

The cervical vertebrae except the atlas are generally opisthocoelous. The feet are never plantigrade[124]. In all the living and the great majority of the extinct forms the digits do not exceed four, the first being suppressed. In the carpus the os magnum articulates freely with the scaphoid, and is separated from the cuneiform by the lunar and unciform. In the tarsus the cuboid articulates with the astragalus as well as with the calcaneum, and the proximal surface of the astragalus is marked by a pulley-like groove. All the bones of the carpus and tarsus strongly interlock. These characters with regard to the carpus and tarsus do not hold in Macrauchenia and its allies. The humerus never has an ent-epicondylar foramen.

The group is divided into two very distinct suborders:—

Suborder (1). Artiodactyla.

The Artiodactyla have a number of well marked characters, one of the most obvious being the fact that many of the most characteristic forms have large paired outgrowths on the frontal bones. These may be (1) solid deciduous bony antlers, or (2) more or less hollow bony outgrowths which are sheathed with permanently growing horn.

The premolar and molar teeth are usually dissimilar, the premolars being one-lobed and the molars two-lobed; the last lower molar of both the milk and permanent dentitions is almost always three-lobed.

The grinding surfaces of the molar teeth have a tendency to assume one of two forms. In the Pigs and their allies the crowns are bunodont[1], while in the more highly specialised Ruminants the crowns are selenodont[125]. The nasals are not expanded posteriorly, and there is no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always nineteen. The symphysis of the ischia and pubes is very elongated, and the femur has no third trochanter. The limbs never have more than four digits, and are symmetrical about a line drawn between the third and fourth digits; the digits, on the other hand, are never symmetrical in themselves. The astragalus has pulley-like surfaces both proximally and distally, and articulates with the navicular and cuboid by two nearly equal facets. The calcaneum articulates with the lower end of the fibula when that bone is fully developed.

In the Artiodactyla are included the following living groups:—

a. Suina. Pigs and Hippopotami.

b. Tylopoda. Camels and Llamas.

c. Tragulina. Chevrotains.

d. Ruminantia or Pecora. Deer, giraffes, oxen, sheep and antelopes.

Suborder (2). Perissodactyla[127].

In this group there are never any bony outgrowths from the frontals. The grinding teeth form a continuous series, the posterior premolars resembling the molars in complexity, and the last lower molar generally has no third lobe. The cervical vertebrae with the exception of the atlas almost always have markedly opisthocoelous centra, but in Macrauchenia they are flat. The nasals are expanded posteriorly, and an alisphenoid canal is present. The thoraco-lumbar vertebrae are never less than twenty-two in number and are usually twenty-three. The femur has a third trochanter (except in Chalicotherium). The third digit of the manus and pes is symmetrical in itself, and larger than the others, and in some cases the other digits are quite vestigial. The number of the digits of the pes is always odd. The astragalus is abruptly truncated distally, and the facet by which it articulates with the cuboid, is much smaller than that by which it articulates with the navicular. The calcaneum does not articulate with the fibula, except in Macrauchenia. The group includes many extinct forms, and the living families of the Tapirs, Horses and Asses, and Rhinoceroses.

Section II. Subungulata.

In this group is placed a heterogeneous collection of animals, the great majority of which are extinct. There is really no characteristic which is common to them all, and which serves to distinguish them as a group from the Ungulata vera. But the most distinctive character common to the greatest number of them is to be found in the carpus, whose bones in most cases retain their primitive relation to one another, the os magnum articulating with the lunar and sometimes just meeting the cuneiform, but in living forms at any rate not articulating with the scaphoid. The feet frequently have five functional digits, and may be plantigrade. The proximal surface of the astragalus is generally flattened instead of being pulley-like as in Ungulata vera.

Suborder (1). Toxodontia.

This suborder includes some very aberrant extinct South American ungulates, which have characters recalling the Proboscidea, both groups of Ungulata vera, and the Rodentia. The limbs are subplantigrade or digitigrade, and the digits are three, rarely five, in number, the third being most developed. The carpus resembles that of the Ungulata vera, in that the bones interlock and the magnum articulates with the scaphoid. In the tarsus, however, the bones do not interlock. The astragalus has a pulley-like proximal surface (except in Astrapotherium, in which it is flat), and articulates only with the navicular, not meeting the cuboid. The calcaneum has a large facet for articulation with the fibula, as in Artiodactyla. There is no alisphenoid canal, and the orbit is confluent with the temporal fossa. Some of the forms (e.g. Nesodon) referred to this group have the typical mammalian series of forty-four teeth, but in others the canines are undeveloped. In Toxodon all the cheek teeth have persistent pulps, while in Nesodon and Astrapotherium they are rooted. A clavicle is sometimes present (Typotherium), and the femur sometimes has a third trochanter (Typotherium and Astrapotherium), sometimes is without one (Toxodon).

The remains of these curious Ungulates have been found in beds of late Tertiary age in South America.

Suborder (2). Condylarthra[128].

This group includes some comparatively small extinct ungulates, which are best known from the Lower Eocene of Wyoming, though their remains have also been found in deposits of similar age in France and Switzerland. Their characters are little specialised, and they show relationship on the one hand to the Ungulata vera and on the other to the Hyracoidea. They also have characters allying them to the Carnivora. They generally have the typical mammalian series of forty-four teeth, the molars being brachydont and generally bunodont. The premolars are more simple than the molars. The limbs are plantigrade, and have five digits with rather pointed ungual phalanges. The os magnum, as in living Subungulates, articulates with the lunar, not reaching the scaphoid. The astragalus has an elongated neck, a pulley-like proximal and a convex distal articular surface, and does not articulate with the cuboid. The humerus has an ent-epicondylar foramen, and the femur has a third trochanter. The best known genus is Phenacodus; it is perhaps the most primitive ungulate whose skeleton is thoroughly well known, and is of special interest from the fact that it is regarded as the lowest stage in the evolutionary series of the horse. Its remains are found in the Lower Eocene of Wyoming.

Suborder (3). Hyracoidea[129].

This group of animals is very isolated, having no very close allies, either living or extinct. The digits are provided with flat nails, except the second digit of the pes, which is clawed. Canine teeth are absent, and the dental formula is usually given as i ½, c ⁰/₀, pm ⁴/₄, m ³/₃. The upper incisors are long and curved, and have persistent pulps as in Rodents; their terminations are, however, pointed, not chisel-shaped, as in Rodents. The lower incisors have pectinated edges. The grinding teeth have a pattern much like that in Rhinoceros. In the skull (fig. 83) the postorbital processes of the frontal and jugal almost or quite meet. The jugal forms part of the glenoid cavity for articulation with the mandible, and also extends forwards so as to meet the lachrymal. There is an alisphenoid canal. There are as many as twenty-one or twenty-two thoracic vertebrae, and the number of thoraco-lumbar vertebrae reaches twenty-eight or thirty. There are no clavicles, and the scapula has no acromion; the coracoid process is, however, well developed. The ulna is complete. In the manus the second, third and fourth digits are approximately equal in size, the fifth is smaller, and the first is vestigial. The femur has a slight ridge representing the third trochanter. The fibula is complete, but is generally fused with the tibia proximally. There is a complicated articulation between the tibia and astragalus, which has a pulley-like proximal surface. In the pes the three middle digits are well developed, but there is no trace of a hallux, and the fifth digit is represented only by a vestigial metatarsal.

The only representatives of the suborder are some small animals belonging to the genus Procavia (Hyrax), which is found in Africa and Syria; some of the species are by many authors placed in a distinct genus Dendrohyrax.

Suborder (4). Amblypoda[130].

This suborder includes a number of primitive extinct Ungulates, many of which are of great size. Their most distinguishing characteristics are afforded by the extremities. In the carpus the bones interlock a little more than is the case in most Subungulata, and the corner of the os magnum reaches the scaphoid, while the lunar articulates partially with both magnum and unciform, instead of only with the magnum. In the tarsus the cuboid articulates with both the calcaneum and the astragalus, which is remarkably flat. The manus and pes are short, nearly or quite plantigrade, and have the full number of digits. The cranial cavity is singularly small. Canine teeth are present in both jaws, and the grinding teeth have short crowns, marked by V-shaped ridges. The pelvis is large, the ilia are placed vertically, and the ischia do not take part in the ventral symphysis.

The best known animals belonging to this suborder are the Uintatheriidae (Dinocerata)[131], found in the Upper Eocene of Wyoming. They are as large as elephants, and are characterised by the long narrow skull drawn out into three pairs of rounded protuberances, by the strong occipital crest, and by the very large upper canines.

Suborder (5). Proboscidea.

This suborder includes the largest of land mammals, the Elephants, and certain of their extinct allies. The limbs are strong, and are vertically placed; the proximal segment is the longest, and the manus and pes are pentedactylate and subplantigrade. The digits are all enclosed in a common integument, and each is provided with a broad hoof. The vertebral centra are much flattened and compressed, especially in the cervical region. The number of thoracic vertebrae is very great, reaching twenty. The skull (figs. 96 and 97) is extremely large, this being due to the great development of air cells, which takes place in nearly all the bones of the adult skull. In the young skull there are hardly any air cells, and the growth of the cranial cavity does not by any means keep pace with the growth of the skull in general. The supra-occipital is very large, and forms a considerable part of the roof of the skull. The nasals and jugals are short, and the premaxillae very large. The rami of the mandible meet in a long symphysis, and the ascending portion is very high. Canine teeth are absent, and the incisors have the form of ever-growing tusks composed mainly of dentine; in living forms they are present in the upper jaw only. The grinding teeth are large, and in living forms have a very complex structure and mode of succession. In some of the extinct forms, such as Mastodon and especially Dinotherium, the teeth are much more simple. In every case the teeth have the same general structure, consisting of a series of ridges of dentine, coated with enamel. In the more specialised forms the valleys between the ridges are filled up with cement. The acromion of the scapula has a recurved process, similar to that often found in rodents. Clavicles are absent. The radius and ulna are not ankylosed, but are incapable of any rotatory movement. All the bones of the extremities are very short and thick; the scaphoid articulates regularly with the trapezoid and the lunar with the magnum. The ilia are vertically placed, and are very much expanded; the ischia and pubes are small, and form a short symphysis. The femur has no third trochanter, and the tibia and fibula are distinct. The fibula articulates with the calcaneum, and the astragalus is very flat.

Here brief reference may be made to the Tillodontia[132], a group of extinct mammals found in the Eocene beds of both Europe and North America. They seem to connect together the Ungulata, Rodentia, and Carnivora.

The skull resembles that of bears, but the grinding teeth are of Ungulate type, while the second incisors resemble those of rodents, and have persistent pulps. The femur has a third trochanter, and the feet resemble those of bears in being plantigrade and having pointed ungual phalanges, differing, however, in having the scaphoid and lunar distinct.

Order 5. Rodentia.

The Rodents form a very large and well-defined group of mammals easily distinguishable by their peculiar dentition. Canines are absent, and the incisors are very large and curved, growing from persistent pulps. They are rectangular in section and are much more thickly coated with enamel on their anterior face than elsewhere; consequently, as they wear down they acquire and retain a chisel-shaped (scalpriform) edge. There is never more than one pair of incisors in the mandible, and except in the Hares and Rabbits, there is similarly only a single pair in the upper jaw. These animals are, too, the only rodents which have well developed deciduous incisors. There is always a long diastema separating the incisors from the grinding teeth. The grinding teeth, which are arranged in a continuous series, vary in number from two to six in the upper jaw, and from two to five in the lower jaw. The number of premolars is always below the normal, often they are altogether wanting, but generally they are ¹/₁. Sometimes the grinding teeth form roots, sometimes they grow persistently.

The premaxillae are always large, and the orbits always communicate freely with the temporal fossae. The condyle of the mandible is elongated from before backwards, and owing to the absence of a postglenoid process to the squamosal, a backward and forward motion of the jaw can take place. The zygomatic arch is complete, but the jugal is short and only forms the middle of it. The palate is small, being sometimes, as in the hares, narrowed from before backwards, sometimes as in the mole-rats (Bathyerginae) narrowed transversely.

The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles are generally present, and the acromion of the scapula is commonly very long. The feet are as a rule plantigrade, and provided with five clawed digits.

There are two main groups of Rodentia; the Duplicidentata, or Hares and Rabbits, which have two pairs of upper incisors, whose enamel extends round to the posterior surface; and the Simplicidentata, in which there is only a single pair of upper incisors, whose enamel is confined to the anterior surface. This group includes all the Rodents except the Hares and Rabbits.

Order 6. Carnivora.

The living Carnivora form a natural and well-marked group, but as is the case with so many other groups of animals, when their extinct allies are included, it becomes impossible to readily define them.

The manus and pes never have less than four well-developed digits, and these are nearly always provided with more or less pointed nails, generally with definite claws. The hallux and pollex are never opposable. The dentition is diphyodont and markedly heterodont. The teeth are always rooted, except in the case of the canines of the Walrus. The incisors are generally ³/₃, and are comparatively small, while the canines are large, pointed, and slightly recurved. The cheek teeth are variable, and are generally more or less compressed and pointed; sometimes their crowns are flattened and tuberculated, but they are never divided into lobes by deep infoldings of enamel. The squamosal is drawn out into a postglenoid process, and the mandible has a large coronoid process. The condyle of the mandible is transversely elongated, and the glenoid fossa is very deep; in consequence of this arrangement the mandible can perform an up and down movement only, any rotatory or back and fore movement being impossible. The jugal is large, and the zygomatic arch generally strong, while the orbit and temporal fossa are in most cases completely confluent. The scapula has a large spine. The clavicle is never complete and is often absent, this forming an important distinction between the skeleton of a Carnivore and of any Insectivore except Potamogale. The humerus often has an ent-epicondylar foramen, and the radius and ulna, tibia and fibula are always separate. The manus is often capable of the movements of pronation and supination, and the scaphoid, lunar and centrale are in living forms always united together.

The order Carnivora includes three suborders.

Suborder (1). Creodonta[133].

This suborder contains a number of extinct Carnivora, which present very generalised characters.

The cranial cavity is very small; and the fourth upper premolar and first lower molar are not differentiated as carnassial teeth[134], as they are in modern Carnivora. The Creodonta also differ from modern Carnivora in the fact that the scaphoid and lunar are usually separate, and that the femur has a third trochanter. The feet are plantigrade.

They resemble the Condylarthra, another very generalised group, in having an ent-epicondylar foramen.

They occurred throughout the Tertiary period in both Europe and North America, and have also been found in India. One of the best known genera is Hyaenodon.

Suborder (2). Carnivora vera or Fissipedia.

The skeleton is mainly adapted for a terrestrial mode of life, and the hind limbs have the normal mammalian position. In almost every case the number of incisors is ³/₃. Each jaw always has one specially modified carnassial or sectorial tooth which bites like a scissors blade against a corresponding tooth in the other jaw. In front of it the teeth are always more or less pointed, while behind it they are more or less broadened and tuberculated. In the manus the first digit, and in the pes the first and fifth digits are never longer than the rest, and the digits of both limbs are almost invariably clawed. Some forms are plantigrade, some digitigrade, some subplantigrade. The group includes all the ordinary terrestrial Carnivora, and is divided into three sections:—

Æluroidea[135], including the cats, civets, hyaenas, and allied forms.

Cynoidea, including the dog tribe.

Arctoidea, including the bears, raccoons, weasels, and allied forms.

Suborder (3). Pinnipedia[136].

In this suborder the limbs are greatly modified and adapted for a more or less purely aquatic life, the proximal and middle segments of the limbs are shortened, while the distal segment, especially in the leg, is much elongated and expanded. There are always five well-developed digits to each limb, and in the pes the first and fifth digits are generally larger than the others. The digits generally bear straight nails instead of claws, but even nails are sometimes absent. There is no carnassial tooth, and the teeth in other ways differ considerably from those of Carnivora vera. The incisors are always fewer than ³/₃; while the cheek teeth generally consist of four premolars and one molar, all of very uniform character, being compressed with conical crowns, and never more than two roots.

The suborder includes three families—Otariidae (Eared Seals), Trichechidae (Walrus), and Phocidae (Seals).

Order 7. Insectivora[137].

This order contains a large number of small generally terrestrial mammals. The limbs are plantigrade or subplantigrade, and are generally pentedactylate. All the digits are armed with claws, and the pollex and hallux are not opposable. The teeth are diphyodont, heterodont, and rooted. The cheek teeth have tuberculated crowns, and there are never less than two pairs of incisors in the mandible; often the incisors, canines, and premolars are not clearly differentiated from one another, and special carnassial teeth are never found. The cranial cavity is small, and the facial part of the skull is generally much developed; often the zygomatic arch is incomplete. Clavicles are well developed (except in Potamogale), and the humerus generally has an ent-epicondylar foramen. The femur frequently has a ridge representing the third trochanter. There are two suborders:

Suborder (1). Dermoptera.

This suborder includes only a very aberrant arboreal genus Galeopithecus, remarkable for its greatly elongated limb bones, and peculiar dentition. The incisors of the lower jaw are deeply pectinated or divided by several vertical fissures, the canines and outer upper incisors have two roots. Ossified inter centra occur in the thoraco-lumbar region of the vertebral column.

Suborder (2). Insectivora vera.

This suborder includes all the ordinary Insectivora, such as moles, shrews and hedgehogs. The upper and lower incisors are conical, not pectinated.

Order 8. Chiroptera[138].

This order is perhaps the best marked and most easily defined of all the orders of mammals. The anterior limbs form true wings and the whole skeleton is modified in relation to flight.

The anterior limbs are vastly larger than the posterior; for all the bones except the carpals are much elongated, and this applies specially to the phalanges of all the digits except the pollex.

The pollex is clawed and so is sometimes the second digit; the other digits of the manus are without nails or claws. The teeth are divisible into the four usual types and the series never exceeds i ²/₃ c ¹/₁ pm ³/₃ m ³/₃ × 2, total 38. The milk teeth are quite unlike the permanent teeth. The orbit is not divided by bone from the temporal fossa. The vertebral column is short, and in old animals the trunk vertebrae have a tendency to become partially fused together. The cervical vertebrae are remarkably wide, and the development of spinous processes is everywhere slight. The presternum has a prominent keel for the attachment of the pectoral muscles. The clavicles are very long and strong, and the scapula has a long spine and coracoid process. The ulna is vestigial, consisting only of a proximal end ankylosed to the radius. All the carpals of the proximal row—the scaphoid, lunar and cuneiform—are united, forming a single bone. The pelvis is very weak and narrow, and only in the Rhinolophidae do the pubes meet in a symphysis. The anterior caudal vertebrae are frequently united to the ischia. The fibula is generally vestigial, and the knee joint is directed backwards instead of forwards. The pes has five slender clawed digits, and the calcaneum is often drawn out into a spur which helps to support the membrane connecting the hind limbs with the tail.

There are two suborders of Chiroptera:

1. The Megachiroptera or Flying foxes, which almost always have smooth crowns to the molar teeth, and the second digit of the manus clawed.

2. The Microchiroptera including all the ordinary bats which have cusped molar teeth, and the second digit of the manus clawless.

Order 9. Primates.

The dentition is diphyodont and heterodont, the incisors generally number ²/₂, and the molars, except in the Hapalidae (Marmosets), are ³/₃. The cheek teeth are adapted for grinding, and the molars are more complex than the premolars. A process from the jugal meets the postorbital process of the frontal completing the postorbital bar.

The clavicle is well developed, and the radius and ulna are never united. The scaphoid and lunar of the carpus, and commonly also the centrale, remain distinct from one another. As a rule both manus and pes have five digits, but the pollex may be vestigial. The pollex is opposable to the other digits, and so is the hallux except in Man; the digits are almost always provided with flat nails. The humerus has no ent-epicondylar foramen and the femur has no third trochanter.

The order Primates is divisible into two suborders:

Suborder (1). Lemuroidea.

The skull has the orbit communicating freely with the temporal fossa beneath the postorbital bar (except in Tarsius). The lachrymal foramen is external to the margin of the orbit. Both pollex and hallux are well developed. In the pes the second digit is terminated by a long pointed claw, and so is also the third in Tarsius. The lumbar region of the vertebral column is long, sometimes including as many as nine vertebrae. Besides the Lemurs the group includes the aberrant Tarsius and Chiromys.

Suborder (2). Anthropoidea.

The skull has the orbit almost completely shut off from the temporal fossa, and the lachrymal foramen is situated within the orbit. The pollex is sometimes vestigial or absent. The second digit of the pes has a flattened nail except in the Hapalidae, in which all the digits of the pes except the hallux are clawed.

The Anthropoidea are divided into five families:

1. Hapalidae or Marmosets.

2. Cebidae or American Monkeys.

3. Cercopithecidae or Old World Monkeys.

4. Simiidae or Anthropoid Apes.

5. Hominidae or Men.