With very rare exceptions, and those only of the latest geological period (Quaternary), the fossil remains of mammals consist only of bones and teeth. The evolutionary changes, so far as these are preserved, are recorded therefore in terms of dental and skeletal modifications. To render these changes intelligible, it is necessary to give some account of the mammalian skeleton and teeth, with no more use of technical language than is unavoidable; ordinary speech does not furnish a sufficient number of terms, nor are most of these sufficiently precise. With the aid of the figures, the reader may easily gain a knowledge of the skeleton which is quite adequate for the discussion of fossil series, which will follow in the subsequent chapters.

I. The Skeleton

I. The most obvious distinction of the skeletal parts is into axial and appendicular portions, the former comprising the skull, backbone or vertebral column, ribs and breast-bone or sternum, and the latter including the limb-girdles, limbs and feet. In the axial skeleton only the ribs and certain bones of the skull are paired, but in the appendicular all the bones are in pairs, for the right and left sides respectively.

The skull is a highly complex structure, made up of many parts, most of which are immovably fixed together, and performing many functions of supreme importance. In the first place, it affords secure lodgement and protection for the brain and higher organs of sense, those of smell, sight and hearing, and second, it carries the teeth and, by its movable jaws, enables these to bite, to take in and masticate food. The portion of the skull which carries the brain, eyes and ears, is called the cranium, and the portion in front of this is the face, the boundary between the two being an oblique line drawn immediately in front of the eye-socket (Fig. 7). A great deal of the endless variety in the form of the skull of different mammals depends upon the differing proportions of cranium and face. In the human skull, for example, the cranium is enormously developed and forms a great dome, while the face is shortened almost to the limit of possibility; the skull of the Horse, on the other hand, goes to nearly the opposite extreme of elongation of the facial and shortening of the cranial region. The posterior surface of the skull, or occiput, is made up of four bones, which in most adult mammals are fused into a single occipital bone. At the base of the occiput is a large opening, the foramen magnum, through which the spinal cord passes to its junction with the brain; and on each side of the opening is a large, smooth, oval prominence, the occipital condyles, by means of which the skull is articulated with the neck. The paroccipital processes are bony styles of varying length, which are given off, one on each side external to the condyles. The boundary of the occiput is marked by a ridge, the occipital crest, which varies greatly in prominence, but is very well marked in the more primitive forms and tends to disappear in the more highly specialized ones. The roof and much of the sides of the cranium are formed by two pairs of large bones, the parietals behind and the frontals in advance; along the median line of the cranial roof, where the two parietals meet, is usually another ridge, the sagittal crest, which joins the occipital crest behind. The sagittal crest also varies greatly in prominence, being in some mammals very high and in others entirely absent, and, like the occipital crest, is a primitive character; as a rule, it is longest and highest in those mammals which have the smallest brain-capacity. As pointed out by Professor Leche, the development of the sagittal crest is conditioned by the relative proportions of the brain-case and the jaws. Powerful jaws and a small brain-case necessitate the presence of the crest, in order to provide sufficient surface of attachment for the temporal muscles, which are important in mastication, while with large brain-case and weak jaws the crest is superfluous. Though the brain-case proper may be quite small, yet it may have its surface enormously increased by great thickening of the cranial bones, as is true of elephants and rhinoceroses, and in them sufficient surface for attachment is afforded to the muscles without the development of a crest.

The structure of these cranial bones, more particularly of the parietals, is subject to important changes; in most mammals they are of moderate thickness and have dense layers, or “tables,” forming the outer and inner surfaces and, between these, a layer of spongy bone. In many large mammals, however, especially those which have heavy horns or tusks, the cranial bones become enormously thick and the spongy layer is converted into a series of communicating chambers, or sinuses, the partitions between which serve as braces, thus making the bone very strong in proportion to its weight. Sinuses are very generally present in the frontals and communicate by small openings with the nasal passage, even in genera of moderate size and without horns or tusks. The frontals form the roof of the eye-sockets, or orbits, and usually there is a projection from each frontal, which marks the hinder border of the orbit and is therefore called the postorbital process. The roof of the facial region is made by the nasals, which are commonly long and narrow bones, but vary greatly in form and proportions in different mammals; in those which have a proboscis, like tapirs and elephants, or a much inflated snout, such as the Moose (Alce) or the Saiga Antelope (Saiga tatarica) the nasals are always very much shortened and otherwise modified in form.

The anterior end of the skull is formed by a pair of rather small bones, the premaxillaries, which carry the incisor teeth; they bound the sides of the nasal opening, or anterior nares, reaching to the nasals, when the latter are of ordinary length; they also form the front end of the hard or bony palate, which divides the nasal passage from the mouth. The maxillaries, or upper jaw-bones, make up nearly all of the facial region on each side and send inward to the median line from each side a bony plate which together constitute the greater part of the hard palate; the remainder of the upper teeth are implanted in the maxillaries. A varying proportion of the hinder part of the hard palate is formed by the palatines, which also enclose the posterior nares, the opening by which the nasal passage enters the back part of the mouth. The maxillary of each side extends back to the orbit, which it bounds anteriorly and in the antero-superior border of which is the usually small lachrymal. The inferior, and more or less of the anterior, border of the orbit is made by the cheek-bone (malar or jugal) which may or may not have a postorbital process extending up toward that of the frontal; when the two processes meet, the orbit is completely encircled by bone, but only in monkeys, apes and Man is there a bony plate given off from the inner side of the postorbital processes, which extends to the cranial wall and converts the orbit into a funnel-shaped cavity. For most of its length, the jugal projects freely outward from the side of the skull and extends posteriorly beneath a similar bar of bone, the zygomatic process of the squamosal. This process and the jugal together constitute the zygomatic arch, which on each side of the skull stands out more or less boldly, and the size and thickness of which are subject to great variation in different mammals, the massiveness of the arch being proportional to the power of the jaws. One of the principal muscles of mastication (the masseter) is attached to the zygomatic arch.

The squamosal itself is a large plate, which makes up a great part of the side-wall of the cranium and articulates above with the frontal and parietal; it also supports the lower jaw, the articular surface for which is called the glenoid cavity. The lower jaw is held in place by the postglenoid process, which is a projection, usually a transverse ridge, behind the cavity. Back of the postglenoid process is the entrance to the middle ear, the auditory meatus, which may be merely an irregular hole, or a more or less elongated tube. The meatus is an opening into the tympanic, a bone which at birth is a mere ring and in some mammals remains permanently in that condition, but as a rule develops into a swollen, olive-shaped auditory bulla, which sometimes reaches enormous proportions, especially in nocturnal mammals. The labyrinth of the internal ear is contained in the periotic, a very dense bone which is concealed in the interior of the cranium, but in many mammals a portion of it, the mastoid, is exposed on the surface between the squamosal and occipital.

The lower jaw-bone (inferior maxillary, or mandible) is the only freely movable element of the skull; it consists of two halves which meet anteriorly at the chin in a contact of greater or less length, called the symphysis. In nearly all young mammals and in many adult forms the two halves of the lower jaw are separate and are held together at the symphysis only by ligaments, while in others, as in Man, they are indistinguishably fused to form a single bone. Each half consists of two portions, a horizontal part or ramus and an ascending ramus or vertical part; the former supports all of the lower teeth, and its length, depth and thickness are very largely conditioned by the number and size of those teeth. The ascending ramus is a broad, rather thin plate, divided at the upper end into two portions, the hinder one of which terminates in the condyle, a rounded, usually semicylindrical projection, which fits into the glenoid cavity of the squamosal. The anterior portion of the ascending ramus ends above in the coronoid process, which serves for the insertion of the temporal muscle, the upper portion of which is attached to the walls of the cranium and thus, when the muscle is contracted, the jaws are firmly closed; the coronoid process passes inside of the zygomatic arch. The lower jaw is therefore a lever of the third order, in which the power is applied between the weight (i.e. the food, the resistance of which is to be overcome) and the fulcrum, which is the condyle. At the postero-inferior end of the ascending ramus is the angle, the form of which is characteristically modified in the various mammalian orders and is thus employed for purposes of classification.

The hyoid arch is a U-shaped series of small and slender bones, with an unpaired element closing the arch below; each vertical arm of the U is attached to the tympanic of its own side and the whole forms a flexible support for the tongue, but with no freely movable joint like that between the lower jaw and the squamosal.

The mammalian skull in its primitive form may be thought of as a tube divided into two parts, of which the hinder one is the brain-chamber, or cranial cavity, and the forward one the nasal chamber or passage. With the growth of the brain and consequent enlargement of the cranium, this tubular character is lost; and various modifications of the teeth, jaws and facial region, the development of horns and tusks, bring about the many changes which the skull has undergone.

This brief sketch of the skull-structure is very incomplete, several of its elements having been altogether omitted and only those parts described which are needful in working out the history and descent of the various mammalian groups.

The second portion of the axial skeleton is the backbone, or vertebral column, which is made up of a number of separate bones called vertebræ. These are so articulated together as to permit the necessary amount of flexibility and yet retain the indispensable degree of strength. The function of the backbone is a twofold one: (1) to afford a firm support to the body and give points of attachment to the limbs, and (2) to carry the spinal cord, or great central axis of the nervous system, in such a manner that it shall be protected against injury, a matter of absolutely vital necessity.

While the vertebræ differ greatly in form and appearance in the various regions of the neck, body and tail, in adaptation to the various degrees of mobility and strength which are required of them, yet they are all constituted upon the same easily recognizable plan. The principal mass of bone in each vertebra is the body, or centrum, which is typically a cylinder, or modification of that form, and the two ends of the cylinder are the faces, by which the successive vertebræ are in contact with one another. In the living animal, however, the successive centra are not in actual contact, but are separated by disks of cartilage (gristle) which greatly add to the elasticity of the column. From the upper surface of the centrum arises an arch of bone, the neural arch, enclosing with the centrum the neural canal, through which runs the spinal cord. As already mentioned, the protection of the spinal cord is essential to the life of the animal, yet this protection must be combined with a certain flexibility, both lateral and vertical. Mere contact of the centra, even though these be held in place by ligaments, would not give the column strength to endure, without dislocation, the great muscular stresses which are put upon it. Additional means of articulation between the successive vertebræ are therefore provided, and these vary in size, form and position in different regions of the backbone, in nice adjustment to the amount of motion and degree of strength needed at any particular part of the column. Of these additional means of articulation, which are called the zygapophyses, each vertebra has two pairs, an anterior and a posterior pair, placed upon the neural arch. From the summit of the arch arises the neural spine, a more or less nearly straight rod or plate of bone, which may be enormously long or extremely short, massive or slender, in accordance with the muscular attachments which must be provided for. Finally, should be mentioned the transverse processes, rod-like or plate-like projections of bone, which arise, one on each side of the vertebra, usually from the centrum, less commonly from the neural arch; these also differ greatly in form and size in the various regions of the column. Anatomists distinguish several other processes of the vertebra, but for our purpose it is not necessary to take these into consideration.

Five different regions of the backbone may be distinguished, in each of which the vertebræ are modified in a characteristic way. There is (1) the cervical region, or neck, the vertebræ of which, among mammals (with only one or two exceptions) are always seven in number, however long or short the neck may be; the immoderately long neck of the Giraffe has no more and the almost invisible neck of the Whale has no less, and thus the elongation of the neck is accomplished by lengthening the individual vertebræ and not by increasing their number. (2) Those vertebræ to which ribs are attached are named dorsal or thoracic and can always be recognized by the pits or articular facets which receive the heads of the ribs. (3) Behind the dorsal is the lumbar region, or that of the loins, made up of a number of vertebræ which carry no ribs. The dorso-lumbars are known collectively as the trunk-vertebræ and are generally quite constant in number for a given group of mammals, though often differently divided between the two regions in different members of the same group. In the Artiodactyla, for example, there are very constantly 19 trunk-vertebræ, but the Hippopotamus has 15 dorsals and 4 lumbars, the Reindeer (Rangifer) 14 D., 5 L., the Ox (Bos taurus) 13 D., 6 L., the Camel (Camelus dromedarius) 12 D. and 7 L. (4) Next follows the sacrum, which consists of a varying number of coalesced vertebræ. The number of sacral vertebræ varies from 2 to 13, but is usually from 3 to 5. (5) Finally, there are the caudal vertebræ, or those of the tail, which are extremely variable in number and size, depending upon the length and thickness of the tail.

We must next consider briefly some of the structural features which characterize the vertebræ of the different regions. (1) The length of the neck varies greatly in different mammals and, up to a certain point, flexibility increases with length, but, as the number of 7 cervicals is almost universally constant among mammals and the lengthening of the neck is accomplished by an elongation of the individual vertebræ, a point is eventually reached, where greater length is accompanied by a diminution of mobility. For instance, in the Giraffe the movements of the neck are rather stiff and awkward, in striking contrast to the graceful flexibility of the Swan’s neck, which has 23 vertebræ, more than three times as many.

The first two cervical vertebræ are especially and peculiarly modified, in order to support the skull and give to it the necessary degree of mobility upon the neck. The first vertebra, or atlas, is hardly more than a ring of bone with a pair of oval, cuplike depressions (anterior cotyles) upon the anterior face (superior in Man) into which are fitted the occipital condyles of the skull. By the rolling of the condyles upon the atlas is effected the nodding movement of the head, upward and downward, but not from side to side; this latter movement is accomplished by the partial rotation of skull and atlas together upon the second vertebra in a manner presently to be explained. On the hinder aspect are two articular surfaces (posterior cotyles) in shape like the anterior pair, but very much less concave, which are in contact with corresponding surfaces on the second vertebra. The neural arch of the atlas is broad and low and the neural canal is apparently much too large for the spinal cord, but, in fact, only a part of the circular opening belongs to the neural canal. In life, the opening is divided by a transverse ligament into an upper portion, the true neural canal, and a lower portion, which lodges a projection from the second vertebra. The atlas usually has no neural spine and never a prominent one; the transverse processes are broad, wing-like plates and each is perforated by a small canal, which transmits the vertebral artery.

The second vertebra, or axis, is a little more like the ordinary vertebra, having a definite and usually elongate centrum, on the anterior end of which are the two articular surfaces for the atlas. Between these is a prominent projection, the odontoid process, which fits into the ring of the atlas and has a special articulation with the lower bar of that ring. In most mammals the odontoid process is a bluntly conical peg, varying merely in length and thickness, but in many long-necked forms the peg is converted into a semicylindrical spout, convex on the lower side and concave above. The neural spine of the axis is almost always a relatively large, hatchet-shaped plate, which is most developed in the carnivorous forms, and the transverse processes are commonly slender rods.

The five succeeding cervical vertebræ are much alike, though each one has a certain individuality, by which its place in the series may readily be determined. The centrum has a convex anterior and concave posterior face, which in long-necked animals form regular “ball and socket” joints; neural spines are frequently wanting and, when present, are almost always short and slender; the zygapophyses are very prominent and are carried on projections which extend before and behind the neural arch; the transverse processes are long, thin plates and, except in the seventh cervical, are usually pierced by the canal for the vertebral artery, but in a few forms (e.g. the camels) this canal pierces the neural arch.

(2) The dorsal or thoracic vertebræ have more or less cylindrical centra, with nearly flat faces, and on the centra, for the most part at their ends, are the concave facets for the rib-heads. The transverse processes are short and rod-like and most of them articulate with the tubercles of the ribs. The zygapophyses are smaller than in the cervical region, less prominent and less oblique; the anterior pair, on the front of the neural arch, face upward and the posterior pair downward. The neural spines are very much longer than those of the neck and those of the anterior dorsals are often of relatively enormous length, diminishing toward the hinder part of the region.

(3) The lumbar vertebræ are almost always heavier and larger than those of the dorsal region; they carry no ribs and their neural spines and transverse processes are broad and plate-like and the latter are far larger and more prominent than those of the dorsals. As an especial degree of strength is frequently called for in the loins, together with a greater flexibility than is needed in the dorsal region, the modes of articulation between the successive vertebræ are more complex, sometimes, as in the Edentata, most elaborately so. Taking the dorso-lumbars, or trunk-vertebræ, as a single series, we may note that in a few mammals (e.g. the elephants) all the neural spines have a backward slope, but in the great majority of forms this backward inclination ceases near the hinder end of the dorsal region, where there is one vertebra with erect spine, while behind this point the spines slope forward.

(4) The sacral vertebræ, varying from 2 to 13 in number, are fused together solidly into one piece, the combined centra forming a heavy mass and the neural canals a continuous tube, while the neural spines are united into a ridge. As a rule, only the first two vertebræ of the sacrum are in contact with the hip-bones, to support which they have developed special processes, the remainder of the mass projecting freely backward.

(5) The caudal vertebræ vary greatly, in accordance with the length and thickness of the tail. In an animal with well-developed tail several of the anterior caudals have the parts and processes of a typical vertebra, centrum, neural arch and spine, zygapophyses and transverse processes. Posteriorly, these gradually diminish, until only the centrum is left, with low knobs or ridges, which are the remnants of the various processes. A varying number of long, cylindrical centra, diminishing backward in length and diameter, complete the caudal region and the vertebral column. In some mammals, chevron bones are attached to the under side of the anterior and middle caudals; these are forked, Y-shaped bones, which form a canal for the transmission of the great blood-vessels of the tail.

The ribs, which are movably attached to the backbone, together with the dorsal vertebræ and breast-bone, compose the thorax, or chest. The articulation with the vertebræ is by means of a rounded head; in most cases the head has two distinct facets, the pit being formed half on the hinder border of one dorsal vertebra and half on the front border of the next succeeding one, but posteriorly the pit is often shifted, so as to be on a single vertebra. A second articulation is by means of the tubercle, a smooth projecting facet on the convexity of the rib’s curvature and near the head; the tubercle articulates with the transverse process of its vertebra. The ribs, in general, are curved bars of bone, which in small mammals generally and in the clawed orders are slender and rod-like, while in the hoofed mammals they are broader, thinner and more plate-like, especially the anterior ones. The number of pairs of ribs is most commonly 13, but ranges among existing mammals from 9 in certain whales to 24 in the Two-toed Sloth (Cholœpus didactylus). The complex curvature of the ribs, outward and backward, is such that, when they are drawn forward (in Man upward) by muscular action, the cavity of the thorax is enlarged and air is drawn into the lungs, and when they are allowed to fall back, the cavity is diminished and the air expelled.

Below, a varying number of the ribs are connected by the cartilages in which they terminate with the breast-bone (sternum); sometimes these cartilages are ossified and then form the sternal ribs, but there is always a flexible joint between the latter and the true ribs. In certain edentates, notably the anteaters and the extinct †ground-sloths, these sternal ribs, at their lower ends, are provided with head and tubercle, for articulation with the sternum.

The sternum, or breast-bone, is made up of a number of distinct segments, usually broad and flat, but often cylindrical, which may unite, but far more commonly remain separate throughout life. The number, size and form of these segments often give useful characters in classification. The first segment, or manubrium, has quite a different shape from the succeeding ones and is considerably longer.

II. The appendicular skeleton consists of the limb-girdles and the bones of the limbs and feet. The limb-girdles are the means of attaching the movable limbs to the body, so as to combine the necessary mobility with strength. The anterior, or shoulder-girdle, has no direct articulation with the vertebral column, but is held in place by muscles; it is made up of the shoulder-blade and collar-bone, though very many mammals have lost the latter.

The shoulder-blade, or scapula, is a broad, thin, plate-like bone, which contracts below to a much narrower neck, ending in a concave articular surface, the glenoid cavity, for the head of the upper arm-bone, the two together making the shoulder-joint. On the outer side the blade is divided into two parts by a prominent ridge, the spine, which typically ends below in a more or less conspicuous projection, the acromion, which may, however, be absent, its prominence being, generally speaking, correlated with the presence of the collar bone. A hook-like process, the coracoid, rises from the antero-internal side of the glenoid cavity and varies greatly in size in the different groups of mammals; though it usually appears to be merely a process of the scapula, with which it is indistinguishably fused, yet its development shows it to be a separate element and in the lowest mammals (Prototheria), as in the reptiles and lower vertebrates generally, it is a large and important part of the shoulder-girdle and articulates with the sternum.

The collar-bone, or clavicle, is a complexly curved bar, which, when present and fully developed, extends from the forward end of the sternum to the acromion, the projecting lower end of the scapular spine, supporting and strengthening the shoulder-joint. In many mammalian orders, notably all existing hoofed animals, the clavicle has become superfluous and is lost more or less completely; it may be said, in general, that the clavicle is developed in proportion to the freedom of motion of the shoulder-joint and to the power of rotation of the hand upon the arm. In arboreal animals, such as monkeys, in which the hand rotates freely and the arm moves in any direction on the shoulder, the clavicle is large and fully developed, as it also is in Man. Many burrowing mammals (e.g. the moles) have very stout clavicles.

The posterior, or pelvic, girdle is composed on each side of a very large, irregularly shaped bone, which is firmly attached to one or more of the coalesced vertebræ which form the sacrum and thus affords a solid support to the hind leg. Each half of the pelvis, or hip-bone, is made up of three elements, called respectively the ilium, ischium and pubis, which are separate in the very young animal, indistinguishably fused in the adult. The three elements unite in a deep, hemispherical pit, the acetabulum, which receives the head of the thigh-bone, a perfect example of the “ball and socket joint.” In the inferior median line the two pubes meet and may become coalesced, in a symphysis, the length of which differs in various mammals. The pelvis and sacrum together form a short, wide tube, the diameter of which is normally greater in the female skeleton than in the male.

The limbs are each divided into three segments, which in the anterior extremity are the arm, fore-arm and hand (or fore foot) and in the posterior extremity are the thigh, leg and foot (or hind foot), and there is a general correspondence between the structure of these segments in the fore and hind legs, however great the superficial difference. The bones of the limbs, as distinguished from those of the feet, are the long bones and, except in a few very large and heavy mammals, are essentially hollow cylinders, thus affording the maximum strength for a given weight of bone; the cavity of a long bone contains the marrow and hence is called the medullary cavity. In the young mammal each of the long bones consists of three parts, the shaft, which makes up much the greater part of the length, and at each end a bony cap, the epiphysis. Growth takes place by the intercalation of new material between the shaft and the epiphyses; when the three parts unite, growth ceases and the animal is adult.

The superior segment of the fore limb has a single bone, the humerus, the upper end of which is the rounded, convex head, which fits into the glenoid cavity of the shoulder-blade, forming the joint of the shoulder; in front of the head are two prominent and sometimes very large projections for muscular attachment, the external and internal tuberosities, separated by a groove, in which play the two tendons of the biceps muscle and is therefore called the bicipital groove. In a few mammals, such as the Horse, Camel and Giraffe, the groove is divided into two by a median tubercle or ridge. From the external tuberosity there generally passes down the front face of the shaft a rough and sometimes very prominent ridge, the deltoid crest, to which is attached the powerful deltoid muscle. At the lower end of the humerus is the trochlea, an irregular half-cylinder, for articulation with the two bones of the fore-arm and varying in form according to the relative sizes of those bones. On each side of the trochlea is frequently a rough prominence, the epicondyle, and above the inner one is, in many mammals, a perforation, the epicondylar foramen, for the passage of a nerve. Extending up the shaft from the outer epicondyle is a rough crest, the supinator ridge, to which is attached one of the muscles that rotate the hand and is conspicuously developed in those mammals which have the power of more or less free rotation and especially in burrowers. On the posterior face of the humerus, just above the trochlea, is a large, deep pit, the anconeal fossa.

The two bones of the fore-arm, the radius and ulna, are, in most mammals, entirely separate from each other, but in certain of the more highly specialized hoofed animals are immovably coössified. Primitively, the two bones were of nearly equal size, but in most of the mammalian orders there is a more or less well-defined tendency for the radius to enlarge at the expense of the ulna. These bones are normally crossed, the radius being external at the upper end and passing in front of the ulna to the inner side of the arm. The radius varies considerably in form in accordance with the uses to which the hand is put; if the capacity of rotation is retained, the upper end, or head, of the radius is small, circular or disk-like, covering little of the humeral trochlea, but when the head of the radius is broadened to cover the whole width of the humerus, then all power of rotation is lost. (Cf. Figs. 28 and 29.) As a rule, the radius broadens downward and covers two-thirds or more of the breadth of the wrist-bones.

The ulna is longer than the radius, its upper end being extended into a heavy process, the olecranon, or anconeal process, into which is inserted the tendon of the great triceps muscle, the contraction of which straightens the arm; this process is the bony projection at the back of the elbow-joint. Below the olecranon is a semicircular articular concavity, which embraces the humeral trochlea and its upper angle fits into the anconeal fossa of the humerus. The ulna contracts and grows more slender downwards and its lower end covers but one of the wrist-bones. While in the more primitive mammals, and in those which retain the power of rotating the hand, the ulna has nearly or quite the same thickness as the radius, it is often much more slender and in the more highly specialized of the hoofed animals, such as the horses, camels and true ruminants, the radius carries the entire weight and the ulna has become very slender, more or less of its middle portion is lost and the two ends are coössified with the radius, so that the fore-arm appears to have but a single bone. The reverse process of enlarging the ulna and reducing the radius is very rare and practically confined to the elephant tribe.

The fore foot, or hand, for which the term manus may be conveniently employed, is divisible into three parts, corresponding in ourselves to the wrist, back and palm of the hand, and the fingers. The bones of the wrist constitute the carpus, those of the back and palm the metacarpus, and those of the fingers the phalanges.

The carpus consists primitively of nine distinct bones, though one of these, as will be shown later, is not a true carpal. These bones are of a rounded, subangular shape, closely appressed together, with very little movement between them, and are arranged in two transverse rows. The upper row contains four bones, which enumerating from the inner side are the scaphoid, lunar, pyramidal (or cuneiform) and pisiform. The scaphoid and lunar support the radius, while the ulna rests upon the pyramidal. The pisiform, though very constantly present, is not a true carpal, but an ossification in the tendon of one of the flexor muscles, which close the fingers; it projects more or less prominently backward and articulates with the ulna and pyramidal. The second row is also made up of four bones, which, from within outward, are the trapezium, trapezoid, magnum and unciform. The relations of the two rows vary much in different mammals and the arrangement may be serial or alternating; thus, the scaphoid rests upon the trapezium and trapezoid and usually covers part of the magnum; the lunar may rest upon the magnum only, but very much more frequently is equally supported by the magnum and unciform and the pyramidal by the latter only. The ninth carpal is the central, which, when present and distinct, is a small bone, wedged in between the two rows. Few existing mammals have a separate central, which, though present in the embryo, has coalesced with the scaphoid in the great majority of forms. In the more advanced and differentiated mammals the number of carpals may be considerably reduced by the coössification of certain elements or the complete suppression and loss of others. In all existing Carnivora and a few other mammals the scaphoid and lunar are united in a compound element, the scapho-lunar (or, more accurately, the scapho-lunar-central); hoofed animals with a diminished number of toes generally lose the trapezium, and other combinations occur. The second row of carpals carries the metacarpals, and primitively the trapezium, trapezoid and magnum are attached each to one metacarpal and the unciform has two.

The metacarpus consists typically of five members, a number which is never exceeded in any normal terrestrial mammal; the members are numbered from the inner side, beginning with the thumb or pollex, from I to V. Many mammals have fewer than five metacarpals, which may number four, three, two or only one; the third is never lost, but any or all of the others may be suppressed, and functionless rudiments of them may long persist as splints or nodules. The metacarpals are elongate, relatively slender and of more or less cylindrical shape; but the form varies considerably in different groups, according to the way in which the hand is used. When employed for grasping, as in many arboreal animals and pre-eminently in Man, the pollex is frequently opposable to the other fingers and enjoys much freedom of motion. In the camels and true ruminants the third and fourth metacarpals are coössified to form a cannon-bone (see Fig. 43, p. 91), but the marrow cavities and the joints for the phalanges remain separate.

The phalanges in land mammals never exceed three in each digit, except the pollex, which, when present and fully developed, has but two. The phalanges are usually slender in proportion to their length, but in very heavy hoofed animals they are short and massive. The terminal joint is the ungual phalanx, which carries the nail, claw, or hoof, its shape varying accordingly.