Chapter XXIII (2)

In INSECTIVORA the characters of the auditory ossicles are very diverse. Many forms such as shrews, moles, hedgehogs, and the Centetidae have a low type of malleus resembling that of Edentates. _Chrysochloris_ has very extraordinary auditory ossicles. The head of the malleus is drawn out into a great club-shaped process, the incus is long and narrow, and differs much from the ordinary type.

In CHIROPTERA the ossicles and especially the malleus much resemble those of shrews. The stapes is always normal in character, never becoming at all columelliform.

PRIMATES. In Man and the Anthropoid Apes the malleus has a rounded head, a short neck, and the manubrium, a processus longus and a processus brevis. The incus consists of an anvil-shaped portion from which arises a long tapering process. The stapes has diverging crura and consequently a wide canal. The crura in other monkeys do not diverge so much as in man and anthropoid apes. The New World monkeys have no neck to the malleus.

THE STERNUM[168].

In MONOTREMES and most MARSUPIALS the sternum does not present any characters of special importance. The presternum is strongly keeled in _Notoryctes_.

The sternum in EDENTATES is very variable: in the Sloths it is very long, the mesosternum of _Choloepus_ having twelve segments. In the ant-eaters and armadillos the presternum is broad and sometimes as in _Priodon_ strongly keeled. In _Manis macrura_ the xiphisternum is drawn out into a pair of cartilaginous processes about nine inches long.

In the SIRENIA the sternum is simple and elongated, and of fairly equal width throughout, in the adult it shows no sign of segmentation. Its origin from the union of two lateral portions can be well seen in _Manatus_.

Two distinct types of sternum are met with in the CETACEA. In the Odontoceti the sternum consists of a broad presternum followed by three or four mesosternal segments, but with no xiphisternum. Indications of the original median fissure can be traced, and are very evident in _Hyperoödon_. In the Mystacoceti, on the other hand, the sternum consists simply of a broad flattened presternum which is sometimes more or less heart-shaped, sometimes cross-shaped. Only a single pair of ribs are united to it.

The sternum in UNGULATA is generally long and narrow and formed of six or generally seven segments. The presternum is as a rule small and compressed, often much keeled, especially in the horse and tapir. The segments of the mesosternum gradually widen as followed back and the xiphisternum is often terminated by a cartilaginous plate.

In the RODENTIA the sternum is long and narrow and generally has a large presternum, and a xiphisternum terminated by a broad cartilaginous plate.

In the CARNIVORA, too, the sternum (fig. 76) is long and narrow and formed of eight or nine pieces, all of nearly the same size. The xiphisternum generally ends in an expanded plate of cartilage.

In INSECTIVORA the sternum is well developed but variable. The presternum is commonly large and is sometimes as in the Hedgehog (_Erinaceus_) bilobed in front, sometimes as in the Shrew (_Sorex_) trilobed. It is especially large in the Mole (_Talpa_) and is expanded laterally and keeled below.

In the CHIROPTERA the presternum is strongly keeled and so is sometimes the mesosternum.

Among PRIMATES, in Man and the Anthropoid Apes the sternum is rather broad and flattened; the mesosternum consists of four segments which are commonly fused together and the xiphisternum is imperfectly ossified.

THE RIBS.

Free ribs are borne as a rule only by the thoracic vertebrae; ribs may be found in other regions, especially the cervical and sacral, but these are almost always ankylosed to the vertebrae. As a general rule the first thoracic rib joins the presternum, while the succeeding ones are attached between the several segments of the mesosternum. Some of the posterior ribs frequently do not reach the sternum; they may then be attached by fibrous tissue to the ribs in front, or may end freely (_floating ribs_). There are generally thirteen pairs of ribs, and in no case do they have uncinate processes.

In MONOTREMES (fig. 102, B) each rib is divided not into two but into three parts, an intermediate portion being interposed between the vertebral and sternal parts. The sternal ribs are well ossified, and some are very broad and flat. The intermediate portions are unossified, those of the anterior ribs are short and narrow, but they become longer and wider further back.

In MARSUPIALS there are almost always thirteen pairs of ribs, whose sternal portions are very imperfectly ossified. _Notoryctes_ has fourteen pairs of ribs, eight of which are floating: the first rib is very stout, and is abruptly bent on itself to join the sternum. It has no distinct sternal portion. All the other ribs are slender.

Of the EDENTATES the Sloths have very numerous ribs; twenty-four pairs occur in _Choloepus_, and half of these reach the sternum. In the Armadillos there are only ten or twelve pairs of ribs, but the sternal portions are very strongly ossified. The first rib is remarkably broad and flat, and is not divisible into vertebral and sternal portions.

In the SIRENIA there are a very large number of ribs noticeable for their great thickness and solidity, but not more than three are attached to the sternum.

CETACEA. In the Whalebone whales the ribs are remarkable for their very loose connection both with the vertebral column and with the sternum. The capitula are scarcely developed, and the attachment of the tubercula to the transverse processes is loose. The first rib is the only one connected with the sternum. In the Toothed whales the anterior ribs have capitula articulating with the centra, as well as tubercula articulating with the transverse processes; in the posterior ones, however, only the tubercula remain. Seven pairs of well-ossified sternal ribs generally meet the sternum. In the Physeteridae most of the ribs are connected to the vertebrae by both capitula and tubercula.

In the UNGULATA the ribs are generally broad and flattened, and this is especially the case in the genera _Bos_ and _Bubalus_ (fig. 101, 6). The anterior ribs are short and nearly straight, and sternal ribs are well developed. The Artiodactyla have twelve to fifteen pairs of ribs, the Perissodactyla eighteen or nineteen, and _Procavia_ twenty to twenty-two. The Elephant has nineteen to twenty-one pairs, seven of which may be floating ribs.

1. premaxillae.
2. nasal.
3. orbit.
4. neural spine of first thoracic vertebra.
5. scapula.
6. rib.
7. femur.
8. patella.
9. tibia.
10. metatarsals.
11. radius.
12. metacarpals.]

In the RODENTIA there are generally thirteen pairs of ribs, which do not present any marked peculiarities.

The CARNIVORA have thirteen to fifteen pairs of ribs, whose vertebral portions are slender, nearly straight and subcylindrical, while their sternal portions are long and imperfectly ossified (fig. 76, 5). There is nothing that calls for special remark about the ribs, in either INSECTIVORA or CHIROPTERA.

PRIMATES. In Man and the Orang (_Simia_) there are generally twelve pairs of ribs; in the Gorilla and Chimpanzee (_Anthropopithecus_), and Gibbons (_Hylobates_), there are thirteen, in the Cebidae twelve to fifteen, and in the Lemuroidea twelve to seventeen pairs. The first vertebral rib is shorter than the others, and the sternal ribs generally remain cartilaginous throughout life, though in man the first may ossify.

APPENDICULAR SKELETON.

THE PECTORAL GIRDLE.

By far the most primitive type of the pectoral or shoulder girdle is found in the MONOTREMATA. The scapula (fig. 102, A, 1) is long and recurved, and has only two surfaces, one corresponding to the prescapular[1] fossa, the other to the postscapular[1] and subscapular[169] fossae. The coracoid is a short bone attached above to the scapula and below to the presternum; it forms a large part of the glenoid cavity. In front of the coracoid there is a fairly large flattened epicoracoid (fig. 102, 6); there is also a large =T=-shaped interclavicle (fig. 102, 4), which is expanded behind and rests on the presternum. The clavicles rest on and are firmly united to the anterior border of the interclavicle. This shoulder girdle differs greatly from that of any other mammals, and recalls that of some Lacertilia.

1. scapula.
2. suprascapula.
3. clavicle.
4. interclavicle.
5. coracoid.
6. epicoracoid.
7. glenoid cavity.
8. presternum.
9. second sternal rib.
10. second vertebral rib.]

In MARSUPIALS, as in all mammals except the Monotremes, the shoulder girdle is much reduced; there are no epicoracoids and interclavicle, and the coracoid forms simply a small process on the scapula, ossifying from a centre separate from that giving rise to the rest of the bone. The scapula has a long acromion, and a clavicle is always present except in _Perameles_. Unossified remains of the precoracoids are found at either end of the clavicle. The scapula of _Notoryctes_ has a very high overhanging spine, and there is a second strong ridge running along the proximal part of the glenoid border.

The shoulder girdle of the EDENTATA shows some very curious variations. In _Orycteropus_ the scapula is of very normal form and the clavicle is well developed. In the Pangolins and Anteaters the scapula is very broad and rounded; there is no clavicle in the Pangolins, and generally only a vestigial one in Anteaters. In Armadillos, Sloths, and Megatheriidae, the acromion is very long and the clavicle is well developed. In the Sloths, _Megatherium_, and _Myrmecophaga_, a connection is formed between the coracoid, which is unusually large, and the coracoid border of the scapula, converting the coraco-scapula notch into a foramen. In _Bradypus_ the clavicle is very small, and is attached to the coracoid, which sometimes forms a distinct bone[170].

In the SIRENIA the scapula is somewhat narrow and curved backwards: the spine, acromion, and coracoid process are moderately developed, and there is no clavicle.

CETACEA. In nearly all the Odontoceti the scapula is broad and somewhat fan-shaped; the prescapular fossa is much reduced, and the acromion and coracoid process form flattened processes, extending forwards nearly parallel to one another. Some of the Mystacoceti, such as _Balaenoptera_, have a broad, fan-shaped scapula, with a long acromion and coracoid process, extending parallel to one another. Others, such as _Balaena_, have a higher and narrower scapula, with a smaller coracoid process.

In UNGULATA the scapula is always high and rather narrow, and neither acromion nor coracoid process is ever much developed. In no adult Ungulate except _Typotherium_ is there any trace of a clavicle, but a vestigial clavicle has been described in early embryos of sheep[171].

1. hyoid.
2. atlas vertebra.
3. seventh cervical vertebra.
4. scapula.
5. imperfectly ossified suprascapula.
6. olecranon process of ulna.
7. metacarpals.
8. ilium.
9. patella.
10. calcaneum.]

UNGULATA VERA. In the Ruminantia the suprascapular region (fig. 103, 5) is very imperfectly ossified, and when this is removed the upper border of the scapula is very straight (fig. 101, 5). The spine is prominent, and generally has a fairly well-marked acromion. In _Hippopotamus_ the acromion is fairly prominent, but in the other Suina, though the spine is prominent, the acromion is not developed. The Perissodactyla have no acromion, but while the Equidae and _Hyracotherium_ have the scapula long and slender, with the spine very slightly developed, the other living Perissodactyla have the spine prominent and strongly bent back at about the middle of its length.

SUBUNGULATA. _Typotherium_ (Toxodontia) differs from all other known Ungulates in having well-developed clavicles; its scapula has a strong backwardly-projecting process, much like that in _Rhinoceros_.

_Phenacodus_ (Condylarthra), has a curiously rounded scapula, with the coracoid and suprascapular borders passing imperceptibly into one another. The scapula resembles that of a carnivore more than does that of any existing Ungulate.

_Procavia_ has a triangular scapula with a prominent spine and no acromion; there is a large unossified suprascapular region.

The scapula in the Proboscidea has a large rounded suprascapular border and a narrow, slightly concave glenoid border. The spine is large, and has a prominent process projecting backwards from about its middle. The spine lies towards the front end of the scapula, so that the postscapular fossa is much larger than the prescapular fossa.

In RODENTIA the shoulder girdle is of a rather primitive type. The scapula is generally high and narrow, somewhat as in Ruminantia; it differs, however, from the Ruminant scapula in having a high acromion, which is often, as in the Hares and Rabbits, terminated by a long metacromion. The development of the clavicle varies, and sometimes it is altogether absent. It is frequently connected by cartilaginous bands or ligaments (fig. 104, 7 and 9), on the one hand with the scapula, and on the other with the sternum. These unossified bands are remains of the precoracoid. Epicoracoidal vestiges of the sternal ends of the coracoids (fig. 104, 11) are also often present.

In the CARNIVORA VERA the scapula is large, and generally has rather rounded borders. The spine and acromion are well developed, and the prescapular and postscapular fossae are nearly equal in size. The coracoid is very small, and the clavicle is never completely developed, being often absent, as in the Bears and most of their allies. In the Seals (Phocidae) the scapula is elongated and curved backwards, and has a very concave glenoid border. In the Eared Seals (Otariidae) the scapula is proportionally much larger and wider, the prescapular fossa being specially large, and being traversed by a ridge, which converges to meet the spine.

1. postscapular fossa.
2. prescapular fossa.
3. spine.
4. suprascapular border unossified.
5. coracoid process.
6. acromion.
7. cartilaginous vestige of precoracoid at scapular end of clavicle.
8. clavicle.
9. cartilaginous vestige of precoracoid at sternal end of clavicle.
10. omosternum.
11. epicoracoid.
12. presternum.
13. first segment of mesosternum.
14. xiphisternum.
15. cartilaginous termination of xiphisternum.
16. 2nd sternal rib.
17. 1st vertebral rib.]

In the INSECTIVORA the shoulder girdle is well developed and, as in Rodents, remains are met with of various parts not generally seen in mammals. In the Shrews the scapula is long and narrow, and has a well-marked spine, whose end bifurcates, forming the acromion and metacromion. The clavicle is long and slender, and is connected with the sternum and acromion by vestiges of the precoracoid. Considerable remains of the sternal end of the coracoid are also found. In _Potamogale_, however, there are no clavicles. In the Mole the shoulder girdle is greatly developed, and of very remarkable form. The scapula is high and very narrow, with the spine and acromion very little developed. The other shoulder girdle element is an irregular bone, which articulates with the humerus and presternum, and is connected by ligaments with the scapula. This bone appears to represent both the coracoid and the clavicle, being formed partly of cartilage bone, partly of membrane bone.

In the CHIROPTERA the scapula is large and oval, and has a moderately high spine and a large acromion. The coracoid process is well developed and is often forked. The clavicles are also well developed, and vestiges of the precoracoid and of the sternal end of the coracoid are often found.

In PRIMATES the clavicle and coracoid process are always well developed. In Man and the Gorilla the scapula has a long straight suprascapular border, a well-developed coracoid process and spine, and a large curved acromion. Vestiges of the precoracoid occur at each end of the clavicle. The shape of the scapula varies much in the lower Primates.

THE UPPER ARM AND FORE-ARM.

In the MONOTREMATA the humerus is short, very broad at each end and contracted in the middle. The radius and ulna are stout and of nearly equal size, while the ulna has a greatly expanded olecranon.

In the MARSUPIALIA the humerus is generally a strong bone, broad at the distal end and having well marked deltoid and supinator ridges, which are specially large in _Notoryctes_. An ent-epicondylar or supracondylar foramen (fig. 105, 5) is almost always present except in _Notoryctes_. The radius and ulna are always distinct and well developed, and a certain amount of rotation can take place between them. The ulna of _Notoryctes_ has an enormous hooked olecranon which causes the bone to be nearly twice as long as the radius.

1. head.
2. greater tuberosity.
3. lesser tuberosity.
4. deltoid ridge.
5. ent-epicondylar (supracondylar) foramen.
6. supinator ridge.
7. external condyle.
8. internal condyle.
9. articular surface for radius.
10. articular surface for ulna.]

EDENTATA. The Sloths have long slender arm bones; the humerus is nearly smooth and has a very large ent-epicondylar foramen in _Choloepus_, but not in _Bradypus_. The radius and ulna can be rotated on one another to a considerable extent. The humerus in all other Edentates is very strong and has the points for the attachment of muscles much developed, especially in the Armadillos and Megatheriidae. An ent-epicondylar foramen is found in all living forms. The radius and ulna are well developed, but are not capable of much rotation.

In the SIRENIA the humerus is well developed and of a normal character. It is expanded at each end and has a prominent internal condyle, a small olecranon fossa, and no ent-epicondylar foramen. In the Dugong and _Rhytina_ there is a bicipital groove and the tuberosities are distinct, but in the Manatee there is no bicipital groove, and the tuberosities coalesce. The radius and ulna are about equally developed and ankylosed together at both ends.

In the CETACEA the arm bones are very short and thick. The humerus has a globular head, and a distal end terminated by two equal flattened surfaces to which the radius and ulna are united. There is no bicipital groove, and the tuberosities coalesce. The radius and ulna are flat expanded bones fixed parallel to one another, but the ulna has a definite olecranon. Scarcely any movement can take place between them and the humerus, and in old animals the three bones are often ankylosed together.

In the UNGULATA VERA the humerus is stout and rather short. The great tuberosity is always large and often overhangs the bicipital groove, it is especially large in _Titanotherium_ (_Brontops_). There is never an ent-epicondylar foramen. The radius is always large at both ends, but the condition of the ulna is very variable. Sometimes, as in _Tapirus_, _Rhinoceros_, _Macrauchenia_, Suina and Tragulina, the ulna is well developed, and quite distinct from the radius; but in most forms, although complete, it is much reduced distally, and is fused to the radius. Sometimes, as in the Horse and Giraffe, it is reduced to the olecranon and to a very slender descending process which does not nearly reach the carpus. In the Tylopoda, though the ulna is complete and its distal end is often distinct, it has coalesced with the radius throughout its whole length; the olecranon is generally very large.

SUBUNGULATA. In the large Condylarthra the humerus has an ent-epicondylar foramen, and the radius and ulna are stout bones nearly equal in size.

In _Procavia_ the humerus is rather long, and has a very prominent greater tuberosity, and a large supra-trochlear fossa, but no ent-epicondylar foramen.

In the Proboscidea the humerus is marked by a greatly developed supinator ridge, and is very long, longer than the radius and ulna. The ulna has a remarkable development, having its distal end larger than that of the radius, it has also a larger articular surface for the humerus than has the radius.

In RODENTIA the humerus varies much in its development according to the animal's mode of life. In the Hares it is long and straight, with a small distal end, and a slight deltoid ridge. In the Beaver on the other hand the deltoid and supinator ridges are considerably developed. There is generally a large supra-trochlear fossa, but no ent-epicondylar foramen.

CARNIVORA. In the Carnivora vera the humerus has large tuberosities, a prominent deltoid ridge and a deep olecranon fossa. The shaft is generally curved, and an ent-epicondylar foramen is often found, though not in the Canidae, Hyaenidae, and Ursidae. The radius and ulna are never united. The radius (fig. 77, B) has a very similar development throughout its whole length, while the ulna has a large olecranon (fig. 77, C, 11) and a shaft tapering somewhat towards the distal end.

In the Pinnipedia the arm bones are very strongly developed. The humerus has a very prominent deltoid ridge, and the proximal end of the ulna and distal end of the radius are much expanded.

In the INSECTIVORA the arm bones are well developed, and the radius and ulna, though sometimes united, are generally distinct; as a rule there is an ent-epicondylar foramen, but this is absent in the Hedgehog. The Mole has an extraordinary humerus, very short and curved, and much flattened and expanded at both ends. It articulates both with the scapula and coraco-clavicle. The ulna has a greatly developed olecranon.

In the CHIROPTERA both humerus and radius are exceedingly long and slender; the ulna is reduced to little more than the proximal end and is fused to the radius. There is no ent-epicondylar foramen.

All PRIMATES have the power of pronation and supination of the fore-arm, by the rotation of the distal end of the radius round that of the ulna.

In Man and the Anthropoid Apes the humerus is long and straight, and has a globular head; neither of the tuberosities, nor the deltoid nor supinator ridges are much developed. The olecranon fossa is deep and there is no ent-epicondylar foramen. The radius is curved and has a narrow proximal, and expanded distal end, the ulna is straighter than the radius and has the distal end much smaller than the proximal; the olecranon is not much developed.

In the lower Primates, although the radius and ulna are always quite separate, the power of pronation and supination is not nearly so great as in the higher forms. In most of the Cebidae and Lemurs an ent-epicondylar foramen occurs.

THE MANUS.

The Manus is divisible into two parts, viz. the carpus or wrist, and the hand which is composed of the metacarpals and phalanges. The carpal bones are always modified from their primitive arrangement, sometimes more, sometimes less. One modification however is always found in mammals, viz. the union of carpalia, 4 and 5 to form the _unciform_ bone. Two sesamoid bones are commonly developed, one on each side of the carpus, the _pisiform_ or one on the ulnar side being much the larger and more constant: it has been suggested that these represent respectively vestiges of a prepollex and a post-minimus digit[172].

One or more of the five digits commonly present may be lost, and sometimes all are lost except the third. The terminal or ungual phalanges of the digits are commonly specially modified to support nails, claws, or hoofs. There are as a rule two small sesamoid bones developed on the ventral or flexor side of the metacarpo-phalangeal articulations, and sometimes similar bones occur on the dorsal or extensor side.

MONOTREMATA. In _Echidna_ the carpus is broad, the scaphoid and lunar are united and there is no centrale. The pisiform is large and several other sesamoid bones occur. Each of the five digits is terminated by a large ungual phalanx. In _Ornithorhynchus_ the manus is more slender, but the general arrangement is the same as in _Echidna_.

MARSUPIALIA. The carpus has no centrale and the lunar is generally small or absent. Five digits are almost always present. In _Choeropus_ however the only two functional digits are the second and third, which have very long closely apposed metacarpals; the fourth digit is vestigial, but has the normal number of phalanges, while the first and fifth are absent. The manus in _Notoryctes_ is extraordinarily modified, the scaphoid and all the distal carpalia are apparently fused, the first, second, and fifth digits are very small, the third and fourth, though having only one phalanx apiece, bear each an enormous claw. Lying on and obscuring the ventral surface of the manus is a large bone, probably a sesamoid.

Among the EDENTATA there is a great diversity in the structure of the manus, the centrale is however always wanting, and except in _Manis_ the scaphoid and lunar are distinct. In the Sloths the manus is very long, narrow, and curved, and terminated by two or three long hooked claws, borne by the second and third, or the second, third and fourth digits. The fifth digit is absent, and the fourth is represented only by a small metacarpal. In the Anteaters the third digit is greatly developed and bears a long hooked claw. In _Myrmecophaga_ all five digits are fairly well though irregularly developed, in _Cycloturus_ the first, fourth, and fifth, are vestigial. In the Armadillos the manus is broad, and has strongly developed ungual phalanges. The digits, though almost always five in number, vary much in their relative arrangement. In _Dasypus_ they are regular, but are remarkably irregular in Priodon. The pollex is absent in Glyptodonts and in _Megatherium._ In _Megatherium_ the fifth digit is clawless while the second, third, and fourth bear enormous claws. In the Manidae the scaphoid and lunar are united; five digits are present, the third and fourth being very large, and all being terminated by deeply cleft ungual phalanges. In _Orycteropus_ the pollex is absent, while the other digits are terminated by pointed ungual phalanges.

In SIRENIA the general structure of the manus is quite of the ordinary mammalian type. In _Manatus_ most of the bones of the carpus are distinct, but in _Halicore_ many, especially those of the distal row, have coalesced. The digits are always five in number and have the normal number of flattened phalanges.

In the CETACEA, on the other hand, the manus is much modified by the fact that the number of phalanges may be greatly increased above the normal number of three, thirteen or fourteen sometimes occurring in each digit. These are believed to be duplicated epiphyses. In the Mystacoceti the manus remains largely cartilaginous, in the Odontoceti it is better ossified, and the phalanges commonly have epiphyses at both ends. In _Physeter_ the carpal bones also have epiphyses. The carpus generally consists of six bones arranged in two rows of three each. Five digits are generally present, but sometimes as in _Balaenoptera musculus_, there are four, the third being suppressed. Their relative development varies much. The Sperm Whale which till recently was placed in the entrance hall of the Natural History Museum at South Kensington has one phalanx to the first digit, four to the second, five to the third, four to the fourth, and three to the fifth. Generally the manus is short and broad, but sometimes, as in _Globicephalus_, it is much elongated owing to the great development of the second and third digits.

UNGULATA[173]. The manus of the members of this great order is of very great classificatory and morphological importance. All the members agree in having the scaphoid and lunar distinct, and in almost every case the ends of the digits are either encased in hoofs or provided with broad flat nails. It is by means of characters derived from the manus and pes that the group is subdivided into the Ungulata vera and the Subungulata.

In the UNGULATA VERA the manus is never plantigrade, and there are not more than four digits, the pollex being almost always completely suppressed: in _Cotylops_ among extinct Artiodactyla however a vestigial pollex is found. The centrale is absent, and the magnum articulates freely with the scaphoid, and is separated from the cuneiform by the unciform and lunar. All the bones of the carpus interlock strongly, and the axis of the third digit passes through the magnum and between the scaphoid and lunar.

There is a very strong distinction between the manus of the suborders Artiodactyla and Perissodactyla. In the Artiodactyla the axis of the manus passes between the third and fourth digits, which are almost equally developed and, except in the Hippopotami and some extinct forms such as _Anoplotherium_, have their ungual phalanges flattened on their contiguous surfaces.

In all _ARTIODACTYLA_ the third and fourth digits are large, but a gradual reduction in the second and fifth can be well traced. Thus in the Suina the second and fifth digits, though smaller than the third and fourth, are well developed and all four metacarpals are distinct. In the Tragulina too all four metacarpals are developed, and in _Dorcatherium_ the third and fourth commonly remain distinct as in the Suina. In the other Artiodactyla however the third and fourth metacarpals are almost always united, though indications of their separate origin remain. In some Ruminantia, such as many Deer, the second and fifth digits are reduced to minute splint bones attached to the proximal end of the fused third and fourth metacarpals, and to small hoof-bearing phalanges, sometimes attached to splint-like distal vestiges of the metacarpals, sometimes altogether unconnected with any other skeletal structures. In some other Ruminants, such as the Sheep and Oxen, the only remnants of the second and fifth digits are nodules of bone supporting the hoofs, and in others, such as the Giraffe, _Anoplotherium commune_, some Antelopes and the Tylopoda, all traces of these digits have disappeared. The Camels differ from all living Ungulata vera in not having the distal phalanges completely encased in hoofs, and from all except the Hippopotami in placing a considerable amount of the manus on the ground in walking.

_A._ LEFT MANUS OF _Tapirus_. (After VON ZITTEL.)

_B._ RIGHT MANUS OF _Titanotherium_. (After MARSH.)

_C._ LEFT MANUS OF _Chalicotherium gigantium_. (After GERVAIS.)

1. scaphoid.
2. lunar.
3. cuneiform.
4. trapezoid.
5. magnum.
6. unciform.
7. trapezium.
II, III, IV, V. second, third, fourth and fifth digits.]

While the manus of the Artiodactyla is symmetrical about a line drawn between the third and fourth digits, that of the _PERISSODACTYLA_ is symmetrical about a line drawn through the middle of the third digit, which is larger than the others and has its ungual phalanx evenly rounded and symmetrical in itself. The most reduced manus in the whole of the mammalia is found in the Horse and its allies, in which the third digit, terminated by a very wide ungual phalanx, is the only one functional. Small splint bones representing the second and fourth metacarpals are attached to the upper part of the third metacarpal. In _Hipparion_[174] and other early horse-like animals the second and fourth digits, though very small and functionless, are complete and are terminated by small hoofs. In _Rhinoceros_ the second and fourth digits are equally developed and nearly as large as the third, and reach the ground in walking, a vestige of the fifth is also present. In the Tapir (fig. 106, A) and _Hyracotherium_ the fifth digit is fully developed but is scarcely functional. In _Titanotherium_ (_Brontops_) (fig. 106, B) it is nearly as well developed as any of the others, and there is little or no difference between the relative development of the third and fourth digits.

The Chalicotheriidae[175], though distinctly Perissodactyles in various respects such as their cervical vertebrae and teeth, differ not only from all other Perissodactyles, but from almost all other Ungulates, in the very abnormal character of their manus. For while the carpus and metacarpus are like those of ordinary Perissodactyles, the phalanges resemble those of Edentates, each second phalanx having a strongly developed trochlea, and each distal one being curved, pointed and deeply cleft at its termination (fig. 106, C).

The Macraucheniidae, while agreeing with Perissodactyles in having only three digits, with the limb symmetrical about a line drawn through the middle of the third, have a carpus which approaches closely to the subungulate condition, the magnum articulating regularly with the lunar, and only to a slight extent with the scaphoid.

In the SUBUNGULATA the manus sometimes has five functional digits, and a considerable part of it rests on the ground in walking. The bones of the carpus retain their primitive relation to one another, the magnum articulating with the lunar, but not with the scaphoid. This character does not however hold in the Toxodontia, for in most of the animals belonging to this group the magnum does articulate with the scaphoid. The corner of the scaphoid just reaches the magnum also in Amblypoda.

As far as is known the _TOXODONTIA_ generally have three, sometimes five digits to the manus, and the third is symmetrical in itself--a Perissodactyloid feature.

In _Phenacodus_ (fig. 107, B) (_CONDYLARTHRA_) all five digits are well developed, the pollex being the smallest. The carpal bones retain their primitive arrangement, the magnum articulating with the lunar and not with the scaphoid. There is no separate centrale.

_A. Coryphodon hamatus._ (After MARSH.) × 1/5.

_B. Phenacodus primaevus._ (After COPE.) × 1/3.

_C. Procavia (Dendrohyrax) arboreus._ (After VON ZITTEL.) × 6/7.

1. scaphoid.
2. lunar.
3. cuneiform.
4. trapezium.
5. trapezoid.
6. magnum.
7. unciform.
8. centrale.
9. pisiform.
I, II, III, IV, V. first, second, third, fourth and fifth digits
respectively.]

In the _HYRACOIDEA_ (fig. 107, C) the manus is very similar to that in _Phenacodus_, but a centrale is present and the pollex is much reduced.

The manus of the _AMBLYPODA_, such as _Coryphodon_ (fig. 107, A) and _Uintatherium_, is short and broad, with five well developed digits and large carpal bones. The carpals however interlock to a slight extent, and the corner of the magnum reaches the scaphoid.

In the _PROBOSCIDEA_ the manus is very short and broad, with large somewhat cubical carpals which articulate by very flat surfaces and do not interlock at all. All five digits are present, and none of them are much reduced in size. The manus in Proboscidea and in _Coryphodon_ is subplantigrade.

In the Tillodontia the manus is plantigrade and has pointed ungual phalanges, in this respect approaching the Carnivora. It differs however from that of all living Carnivora in having the scaphoid and lunar distinct.

In RODENTIA the manus nearly always has five digits with the normal number of phalanges: the pollex may however be very small as in the Rabbit, or absent as sometimes in the Capybara. The scaphoid and lunar are generally united, and a centrale may be present or absent. In _Pedetes caffer_ the radial sesamoid is double and the distal bone bears a nail-like horny covering. In _Bathyergus_ the pisiform is double. It is upon these facts that the contention for the former existence of prehallux and post-minimus digits has partly been based.

In living CARNIVORA the scaphoid, lunar and centrale are always united, forming a single bone. All five digits are present, but as a rule in Carnivora vera the pollex is small, and in _Hyaena_ is represented only by a small metacarpal. Sometimes, as in Cats and Dogs, the manus is digitigrade, sometimes, as in Bears, plantigrade. The ungual phalanges are large and pointed, and in forms like the Cats, whose claws are retractile, they can be folded back into a deep hollow on the ulnar side of the middle phalanx; a small radial sesamoid is often present.

In Pinnipedia the manus is large and flat and the digits are terminated by ungual phalanges which are blunt (sea lions and walrus), or slightly curved and pointed (seals). The pollex is nearly or quite as long as the second digit, and as a rule the digits then successively diminish in size.

The Creodonta differ from living Carnivora in the fact that the scaphoid and lunar are usually separate.

In INSECTIVORA the scaphoid and lunar are sometimes united, sometimes separate, and a separate centrale is usually present. There are generally five digits, but sometimes the pollex is absent. In the Mole the manus is greatly developed and considerably modified. It is very wide, its breadth being increased by the great development of the radial sesamoid which is very large and sickle-shaped. The ungual phalanges are also large and are cleft at their extremities.

In the CHIROPTERA the manus is greatly modified for the purpose of flight. The pollex is short and is armed with a rather large curved claw, the other digits are enormously elongated, the elongation in the case of the Insectivorous bats being mainly due to the metacarpals, and in the Frugivorous bats to the phalanges. In the Frugivorous bats the second digit is clawed as well as the pollex, in other bats this claw is always absent, and so is often the ungual phalanx, the middle phalanx then tapering gradually to its termination.

In PRIMATES as a rule the manus is moderately short and wide. The carpus has the scaphoid and lunar distinct, and generally also the centrale; sometimes however, as in Man, the Gorilla, Chimpanzee, and some Lemurs, the centrale has apparently fused with the scaphoid. There are almost always five well-developed digits, but in the genera _Colobus_ and _Ateles_ the pollex is vestigial.

The magnum in man is the largest bone of the carpus. The pisiform also is well developed, but there is no radial sesamoid. In Man, the Gorilla, Chimpanzee, and Orang, the carpus articulates only with the radius, in most Primates it articulates also with the ulna. The third digit of the Aye-Aye (_Chiromys_) is remarkable for its extreme slenderness.

THE PELVIC GIRDLE.

The pelvic girdle in all mammals except the Sirenia and Cetacea consists of two halves, usually united with one another at the symphysis in the mid-ventral line, and connected near their upper ends, with the sacral vertebrae. Each half forms one of the _innominate_ bones, and includes at least three separate elements, a dorsal bone, the ilium, and two ventral bones, the ischium and pubis. Very often a fourth pelvic element, the acetabular or cotyloid bone, occurs.

In the MONOTREMATA the pelvis is short and broad, and the pubes and ischia meet in a long symphysis. The acetabulum is perforated in _Echidna_ as in birds, but not in _Ornithorhynchus_. A pair of elongated slender bones project forwards from the edge of the pubes near the symphysis; these are sesamoid bones formed by ossifications in the tendons of the external oblique abdominal muscles, and are generally called _marsupial bones_.

In the MARSUPIALIA the ilia are generally very simple, straight, and narrow, while the pubes and ischia are well developed and meet in a long symphysis. Marsupial bones are nearly always prominent, but are not developed in _Thylacinus_ or _Notoryctes_. The ischium often has a well-marked tuberosity and in Kangaroos the pubis bears a prominent pectineal process on its anterior border close to the acetabulum. The pelvis in _Notoryctes_ differs much from that in all other Marsupials, the ilium and ischium being ankylosed with six vertebrae in a manner comparable to that of many Edentates.

In the EDENTATA the pelvis is generally well developed, but the symphysis is very short. In the Sloths the pelvis is rather weak and slender, the obturator foramina are very large and the ischia do not meet in a symphysis. In the Megatheriidae the pelvis is exceedingly wide and massive, and is firmly ankylosed with a number of vertebrae. In the Armadillos, Glyptodonts, Anteaters, and Pangolins it is much developed and firmly united to the vertebral column by both the ilia and the ischia. In _Orycteropus_ however the ischium does not become united to the vertebral column, and the pubis generally has a strongly developed pectineal process.

In the SIRENIA the pelvis is quite vestigial. In the Dugong it consists on each side of two slender bones, one of which represents the ilium and the other the ischium and pubis; the two bones are placed end to end and are commonly fused together. The ilium is attached by ligament to the transverse process of one of the vertebrae. In the Manatee each half of the pelvis is represented by a triangular bone connected by ligaments with its fellow and with the vertebral column. In neither Manatee nor Dugong is there any trace of an acetabulum but one can be made out in _Halitherium_.

In the CETACEA the pelvis is even more vestigial than in the Sirenia, consisting simply of a pair of small straight bones which probably represent the ischia, and lie parallel to and below the vertebral column at the point where the development of chevron bones commences.

In UNGULATA VERA the pelvis is generally rather long and narrow. The ilium is flattened and expanded in front (fig. 103, 8), but becomes much narrower and more cylindrical before reaching the acetabulum. Both pubis and ischium contribute to the symphysis which is often very long. The ischia are large and have prominent tuberosities, especially in Artiodactyles. In most Ruminantia there is a deep depression, the supra-acetabular fossa above the acetabulum, but this is not found in the Suina or Tylopoda.

SUBUNGULATA. In _Procavia_ the pelvis is long and narrow, and bears resemblance to that in Artiodactyles.

The Proboscidea have a very large pelvis set nearly at right angles to the vertebral column; the ilium is very wide, having expanded iliac[1] and gluteal[1] surfaces, and a narrow sacral[176] surface. The pubes and ischia are rather small, but both meet their fellows in the symphysis. _Uintatherium_ (suborder Amblypoda) also has a large and vertically placed pelvis (fig. 108) with a much expanded ilium. The pelvis however differs from that of the Proboscidea in the fact that the ischia do not meet in a ventral symphysis.

In many RODENTIA the ilia have their gluteal, iliac, and sacral surfaces of nearly equal extent; in the Hares, however, the gluteal and iliac surfaces are confluent. The pubes and ischia are always well developed and sometimes, as in the Hares, the acetabular bone also. In these animals the pubis does not take part in the formation of the acetabulum, and the ischium bears on its outer side a well-marked ischial tuberosity.

In the CARNIVORA the pelvis is long and narrow. The iliac surfaces (fig. 78, A, 5) are very small and the sacral large; the crest or supra-iliac border is formed by the union of the sacral and gluteal surfaces. The symphysis is long and includes part of both pubis and ischium. The ischial tuberosity (fig. 78, A, 10) is often well marked, and sometimes as in _Viverra_ the acetabular bone is distinct. In the Pinnipedia the pelvic symphysis is little developed, or sometimes not developed at all, and the obturator foramina are remarkably large.

In some INSECTIVORA such as _Galeopithecus_, there is a long pelvic symphysis, in others such as _Erinaceus_ and _Centetes_, it is very short, in others again such as _Talpa_ and _Sorex_, there is no pelvic symphysis. The acetabular bone is exceptionally large in _Talpa_ and _Sorex_.

In CHIROPTERA the pelvis is small and narrow, and in the great majority of cases the two halves do not meet in a ventral symphysis. The pubis has a strongly developed pectineal process, which occasionally unites with a process from the ilium enclosing a large pre-acetabular foramen.

PRIMATES. In Man and the Anthropoid Apes the pelvis is very large and wide, and the ilium has much expanded iliac and gluteal surfaces. The symphysis is rather short and formed by the pubis alone. The acetabulum is deep and the obturator foramen large, and there is frequently a well-marked ischial tuberosity. In the lower Anthropoidea the ilium is long and narrow and has a small iliac surface. The ischial tuberosities are large in the old world monkeys.

1. ilium.
2. head of femur.
3. great trochanter.
4. patella.
5. fibula.
6. tibia.
7. second digit of pes.
8. ungual phalanx of fifth digit of pes.
9. calcaneum.
10. postscapular fossa.
11. prescapular fossa.
12. coracoid process.
13. humerus.
14. radius.
15. ulna.
17. unciform.
18. cuneiform.
20. lunar.
21. first metacarpal.
22. fifth metacarpal.]

THE THIGH AND SHIN.

In the MONOTREMATA the femur is short, rather narrow in the middle, and expanded at each end. The great and lesser trochanters are large and about equally developed, but there is no third trochanter. The fibula is very large and is expanded at its proximal end, forming a flattened plate much resembling an olecranon. The patella is well developed.

In the MARSUPIALIA there is no third trochanter to the femur, the fibula is well developed but not the patella as a general rule. _Notoryctes_ has a femur with a prominent ridge extending some little way down the shaft from the great trochanter; the tibia has a remarkably developed crest, and the fibula has its proximal end much expanded and perforated; there is an irregularly shaped patella closely connected with the proximal end of the tibia.

EDENTATA. In the Sloths the leg bones are all long and slender. The femur has no third trochanter, and the fibula is complete and nearly equal in size to the tibia. In the Megatheriidae the leg bones are extraordinarily massive, the circumference of the shaft of the femur in _Megatherium_ equalling or exceeding the length of the bone. There is no third trochanter in _Megatherium_. In most of the remaining Edentata the leg bones are strongly developed. The femur in the Armadillos and Aard Varks has a strong third trochanter, and the tibia and fibula are both large and are commonly ankylosed together at either end. The limb bones are very massive also in the Glyptodonts.

SIRENIA. In no living Sirenian is there any trace of a hind limb, but in _Halitherium_ a vestigial femur is found, which articulates with the pelvis by a definite acetabulum.

1. head.
2. great trochanter.
3. lesser trochanter.
4. third trochanter.
5. shaft.
6. condyles.]

In the Mystacoceti among the CETACEA small nodules of bone or cartilage occur connected with the vestigial pelvis, and may represent the femur and tibia. No trace of the skeleton of the hind limb is known in the Odontoceti.

In the UNGULATA VERA the femur is noticeable for the size of the great trochanter (fig. 109, 2); there is no definitely constricted neck separating the head from the rest of the bone, and the lesser trochanter (fig. 109, 3) is not very prominent. All Perissodactyles except the Chalicotheriidae show a strongly marked third trochanter, but this is absent in all known Artiodactyles. The development of the fibula in general corresponds to that of the ulna. In _Rhinoceros_, _Macrauchenia_, _Tapirus_ and the Suina it is distinct and fairly well developed; in the Tragulina on the other hand it is vestigial, being reduced to the proximal end only. In the Ruminantia and Tylopoda also, it is much reduced forming merely a small bone attached to the distal end of the tibia, sometimes, as in the Red deer a slender vestige of the proximal end also is preserved quite detached from the distal portion; in the Horse this proximal portion is all that there is found of the fibula. The progressive diminution of the fibula can be well seen in the series of forms that are regarded as the ancestors of the Horse. The patella of the Ungulata vera is well ossified, but fabellae[177] are not usually found.

SUBUNGULATA. Of the Toxodontia, _Toxodon_ has no third trochanter while _Typotherium_ and _Astrapotherium_ have one. In the Condylarthra the femur has well-marked lesser and third trochanters, and the fibula and patella are well developed. In the Hyracoidea there is a slight ridge on the femur in the place of the third trochanter, the fibula is complete, but is generally fused to the tibia at its proximal end.

Of the Amblypoda, _Coryphodon_ has a third trochanter, but _Uintatherium_ has none; in this respect, in the vertical position and general appearance (fig. 108) of the limb, and in the articulation of the fibula with the calcaneum, the leg of _Uintatherium_ closely approaches that of the Proboscidea.

In the Proboscidea the femur is very long and straight, the development of trochanters is slight, and the fibula though slender is complete and articulates with the calcaneum.

A third trochanter is found in the Tillodontia.

In RODENTIA the femur is variable, the great trochanter is generally large and so sometimes is the third as in the Hares. In most Rodents as in the Beaver the fibula is distinct, sometimes as in the Hares it is united distally with the tibia. The patella is well developed, and so too are the fabellae as a general rule.

CARNIVORA. In the Carnivora vera the femur (fig. 79, A) is generally rather straight and slender, and has a very distinct head. The fibula (fig. 79, C) is always distinct and there is generally a considerable interval between it and the tibia. Fabellae (fig. 79, 7) are commonly present.

In the Pinnipedia the femur is short, broad and flattened, having a prominent great trochanter. The fibula is nearly as large as the tibia, and the two bones are generally ankylosed together at their proximal ends.

The Creodonta differ from all living Carnivores in having a femur with a third trochanter.

In the INSECTIVORA a third trochanter is sometimes developed. The fibula is sometimes distinct, sometimes fused distally with the tibia, thus differing from that of a Carnivore.

In CHIROPTERA the femur is straight, slender and rather short, with a small but well-developed head. The fibula may be well developed or quite vestigial or absent. Owing to the connection of the hind limb with the wing membrane the knee joint is directed backwards.

In PRIMATES the femur is rather long and slender, having a nearly spherical head and large great trochanter. The tibia and fibula are always distinct and well developed. Fabellae are not found in the highest forms but are generally present in the others.

THE PES.

The skeleton of the pes is in most respects a counterpart of that of the manus. Just as in the manus if one digit is absent it is the pollex, so in the pes it is the hallux. But while in the manus the third digit is always well developed, however much the limb may be modified, in the pes any of the digits may be lost. In all mammals the tibiale and intermedium fuse to form the _astragalus_, and the fourth and fifth tarsalia to form the _cuboid_. Sesamoid bones are considerably developed. In almost every case the phalanges and first metatarsal have epiphyses only on their proximal ends, while the remaining four metatarsals have epiphyses only on their distal ends.

In the MONOTREMATA all the usual tarsal bones are distinct, and the five digits have the normal number of phalanges. Several sesamoid bones are developed, the most important one, found only in the male, being articulated to the tibia and bearing the curious horny spur. The ungual phalanges of the pes like those of the manus, are deeply cleft at their extremities. In the Echidnidae the pes is turned outwards and backwards in walking.

In the MARSUPIALIA the pes is subject to great modifications, but in every case the seven usual tarsal bones are distinct. In the Didelphyidae the foot is broad, all five digits are well developed, and the hallux is opposable to the others. In the Dasyuridae the foot is narrow, and the hallux may be very small, or as in _Thylacinus_ completely absent. In _Notoryctes_ the pes is much less abnormal than the manus, and all five digits have the usual number of phalanges. The fifth metatarsal has a curious projecting process, and there is a large sesamoid above the hallux. In the Wombats (Phascolomyidae) the foot is short and broad, the digits are all distinct, and the hallux is divaricated from the others.

In the remaining marsupials the second and third metacarpals and digits are very slender, and are enclosed within a common integument. This condition is known as _syndactylism_, and its effect is to produce the appearance of one toe with two claws. In the Kangaroos (Macropodidae) the pes is very long and narrow, owing to the elongation of the metacarpals. The fourth digit is greatly developed, the fifth moderately so, while the hallux is absent, and the second and third digits are very small. The Peramelidae have the foot constructed on the same plan as in the Kangaroos, and in one genus _Choeropus_ the same type of foot is carried to a greater extreme than even in the Kangaroos. Thus the fourth digit is enormously developed, the second and third are small, and the fifth smaller still, while the hallux is absent. In the Phalangers and Koalas though the second and third toes are very slender, the hallux is well developed and opposable.

EDENTATA. In the Sloths the pes much resembles the manus, being long and narrow, but in both genera the second, third and fourth digits are well developed. Most of the other Edentates have a but little modified pes with the normal number of tarsal bones and the complete series of digits. In _Cycloturus_ however the hallux is vestigial and it is absent in Glyptodonts. _Megatherium_ has a greatly modified pes, the hallux is absent, and the second digit vestigial, while the third is very large, having an enormous ungual phalanx. The calcaneum too is abnormally large.

No trace of the pes occurs in either SIRENIA or CETACEA.

In the UNGULATA the pes like the manus is subject to much variation and is of great morphological importance.

In the UNGULATA VERA the pes is never plantigrade and never has more than four digits, the hallux being absent. The cuboid always articulates with the astragalus, and the tarsal bones strongly interlock. As was the case also with the manus, the pes is formed on two well-marked types characteristic respectively of the Artiodactyla and Perissodactyla.

_ARTIODACTYLA._ Just as in the manus, the third and fourth digits are well and subequally developed; their ungual phalanges have the contiguous sides flat, and the axis of the limb passes between them, and between the cuboid and navicular. The astragalus has both the proximal and distal surfaces pulley-like, and articulates with the navicular and cuboid by two facets of nearly equal size. The calcaneum articulates with the lower end of the fibula if that bone is fully developed.