Chapter II: Structure and Present Distribution of the Mammalia (2)

THE STERNUM.--All the Mammalia so far as is known possess a sternum. This is the bone, or series of bones (sternebrae), which lies upon the ventral surface of the chest, and to which the ribs are attached below. The development of the sternum has been shown to take place from the fusion of the ribs below into two lateral bands, one on each side; the approximation of these bands forms the single and unpaired sternum of most mammals. Very considerable traces, however, of the paired state of the sternal bones often exist; thus in the Sperm Whale the first piece of the sternum is divided into two by a longitudinal division, and the second piece is longitudinally grooved. The development of the sternum out of the fused ends of ribs is shown in a more complete condition in some species of _Manis_ than in many other mammals. Thus in _M. tricuspis_ the last ribs of those which are attached to the sternum are completely fused together into a single piece on each side.[17] As a general rule the last ribs which come into relation with the sternum do so only in an imperfect way, being simply firmly attached at their sides to, but not fused with, the last ribs which are definitely articulated with the sternum. Contrary to what is found in lower {32} Vertebrates, the sternum of the Mammalia consists of a series of pieces, as many as eight or nine or even sixteen in _Choloepus_, of which the first is called the manubrium sterni, and the last the ensiform cartilage, xiphisternum, or xiphoid process. The latter often remains largely cartilaginous throughout life; in fact this is generally but not universally the case with that part of the breastbone. The most extraordinary modification of the xiphoid process is seen in the African species of the genus _Manis_, where it diverges into two long cartilages, which run back to the pelvis and then, curving round, run forwards and fuse together in the middle line anteriorly. These processes serve for the attachment of certain tongue-muscles. They were looked upon by Professor Parker as the equivalents of the "abdominal ribs" of reptiles elsewhere non-existent among mammals. This view is not, however, usually held. The manubrium sterni is often keeled in the middle line below; this is so with the Bats, which thus approach the birds, and probably for the same reason, i.e. the need of an enlarged origin for the pectoral muscle, which is concerned in the movements of flight. In many forms this part of the sternum is much broader than the pieces which follow; this is so with the Viscacha. In the Pig the precise reverse is seen, the manubrium being narrower than the rest of the sternal bonelets. It will be noticed, however, that in this and similar cases there are no clavicles. Ribs are attached between the successive pieces of the sternum. When the sternum is reduced, as it is in the Cetacea and in the Sirenia, it is the intermediate part of the series of bones which becomes abbreviated or vanishes. The Sperm Whale has only a manubrium sterni and a following piece belonging to the mesosternum. It is fair to say that the xiphoid process and the rest of the sternum have disappeared, since among the Toothed Whales a progressive shortening of the sternum can be seen. In the Whalebone Whales the sternum is still further reduced; the manubrium is alone left, and to it are attached but a single pair of ribs. In _Balaena_, however, a rudimentary {33} piece, apparently comparable to a xiphoid process, has been detected.

FIG. 20.--Sternum of Rudolphi's Whale (_Balaenoptera borealis_), showing its relation to the inferior extremities of the first pair of ribs. x 1/10. (From Flower's _Osteology_.)

FIG. 21.--Sternum of a young Dugong (_Halicore indicus_). x 1/4. From a specimen in the Leyden Museum, _ps_, Presternum; _xs_, xiphisternum. (From Flower's _Osteology_).

From the instances which have been described, as well as from the mode of development of the sternum and from the number of free ribs, _i.e._ ribs which are not attached to it, it would seem that the sternum has undergone a considerable reduction in its size. This reduction may be possibly accounted for by the need for respiratory activity, which is clearly increased by a less-marked fixity of the walls of the thoracic cavity. In the case of the Whales one can hardly help coming to that conclusion. The arrangement in the Monotremata does not, however, point in the same direction; for these animals are precisely like the higher Mammalia in the reduction of the sternum and of the number of ribs which reach it.

FIG. 22.--Shoulder girdle of Ornithorhynchus. _c_^1, _c_^2, _c_^3, First, second, third ribs; _cl_, clavicle; _e.c_, epicoracoid; _es_' and _es_", interclavicle (episternum); _m.c_, metacoracoid; _m.s_, manubrium sterni; _sc_, scapula; _st_, sternebra. (From Wiedersheim's _Structure of Man_.)

FIG. 23.--Episternum of an embryo Mole. (After A. Goette.) _cl_, Clavicle; _es_', central portion of the episternum; _es_", lateral portion of the same; _r.c_, costal ribs; _st_, sternum. (The figure was constructed from two consecutive horizontal sections.) (From Wiedersheim's _Structure of Man_.)

THE EPISTERNUM.--The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large [18]-shaped bone which does not overlie the sternum as in reptiles, but is anterior to it. The relations of this bone to the clavicles seem to leave no doubt that it is the equivalent of the Lacertilian interclavicle or episternum. The Monotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is {34} provided with pieces of bone which overlie the manubrium sterni and are attached to the clavicles, and are no doubt to be regarded as the same structure. Probably in many mammals the manubrium will be found to be partly made up of corresponding rudiments. In any case, vestiges of an episternum in the shape of two minute ossicles have been discovered in Man, lying in front of the manubrium. They have been termed ossa suprasternalia. In Man and in the Mole the paired nature of the episternum is clearly apparent. It has been suggested that this structure in its entirety belongs to the clavicles, just as the sternum belongs to the ribs; _i.e._ that it formed out of the approximated and fused ends of the clavicles. Dr. Mivart[19] figured a good many years since a pair of ossicles in _Mycetes_, lying in one case between the ends of the clavicles and the manubrium sterni, and in another example anterior to the ventral ends of the clavicles. Gegenbaur has figured a {35} pair of similar bones in the Hamster.[20] It is possible that these are to be referred to the same category. It has also been suggested that these supposed episternal rudiments are the vestiges of a pair of cervical ribs.

FIG. 24.--Episternal vestiges in Man. _cl_, Clavicle, sawn through; _es_, "episternum" (sternoclavicular cartilage); _l_', interclavicular ligament; _l_", costoclavicular ligament; _m.s_, manubrium sterni; _o.s_, ossa suprasternalia; _r.c_, first rib; _st_, sternum. (From Wiedersheim's _Structure of Man_.)

THE PECTORAL GIRDLE.--The skeleton by which the fore-limb is connected with the trunk is known as the Pectoral Girdle. The main part of this girdle is formed by the large scapula, or blade-bone as it is often termed. The coracoidal elements will be dealt with later. The scapula is not firmly connected with the backbone; it is attached merely by muscles, thus presenting a great difference from the corresponding pelvic girdle. The reason for this difference is not easy to understand. On the one hand it may be pointed out that in all running animals at any rate there is a greater need for the fixation in a particularly firm way of the hind-limbs; but, again, in the climbing creatures both limbs would, one might suppose, be bettered by a firm fixation. It must be remembered, however, that in the latter case the same result is at least partly brought about by a well-developed clavicle, which fixes the girdle to the sternum and so to the vertebral column by means of the ribs.

Broadly speaking, too, the fore-limbs require a greater freedom and variety of movement than the hind-limbs, which are supports {36} for or serve to push along the rapidly-moving body. Stronger fixation is therefore a greater necessity posteriorly than anteriorly. In any case, whatever the explanation, this important difference exists.

FIG. 25.--Right scapula of Dog (_Canis familiaris_). x 1/4. _a_, Acromion; _af_, prescapular fossa; _c_, coracoid; _cb_, coracoid or anterior border; _css_, indicates the position of the coraco-scapular suture, obliterated in adult animals by the complete ankylosis of the two bones; _gb_, glenoid or posterior border; _gc_, glenoid cavity; _pf_, postscapular fossa; _s_, spine; _ss_, suprascapular border. (From Flower's _Osteology_.)

FIG. 26.--Right scapula of Red Deer (Cervus elaphus). x 1/4. _a_, Acromion; _af_, anterior or prescapular fossa; _c_, coracoid; _gc_, glenoid cavity; _pf_, postscapular fossa; _ss_, partially ossified suprascapular border. (From Flower's _Osteology_.)

The shoulder-blade of mammals is as a rule a much-flattened bone with a ridge on the outer surface known as the spine; this ridge ends in a freely-projecting process, the acromion, from which a branch often arises known as the metacromion. This gives a bifurcate appearance to the end of the ridge. The spine is less developed and the scapula is narrower in such animals as the Dog and the Deer which simply run, and whose fore-limbs therefore are not endowed with the complexity of movement seen, for instance, in the Apes.

{37}

FIG. 27.--Right scapula of Dolphin (_Tursiops tursio_). x 1/4. _a_, Acromion; _af_, prescapular fossa; _c_, coracoid; _gc_, glenoid cavity; _pf_, postscapular fossa. (From Flower's _Osteology_.)

FIG. 28.--Side view of right half of shoulder girdle of a young Echidna (_Echidna hystrix_). x 2/3. _a_, Acromion; _c_, coracoid; _cb_, coracoid border; _cl_, clavicle; _css_, coraco-scapular suture; _ec_, epicoracoid; gb, glenoid border; _gc_, glenoid cavity; _ic_, interclavicle; _pf_, postscapular fossa; _ps_, presternum; _s_, spine; _ss_, suprascapular epiphysis; _ssf_, subscapular fossa. (From Flower's _Osteology_.)

It has been pointed out that the area which lies in front of the spine, the prescapular lamina, is most extensively developed in such animals as perform complex movements with the fore-limbs. The Sea Lion and the Great Anteater are cited by Professor G. B. Howes as examples of this preponderance of the anterior portion of the scapula over that which lies behind the spine. The general shape of the scapula varies considerably among the different orders of mammals; but it always presents the characters mentioned, which are nowhere seen among the Sauropsida except among certain Anomodonts, which will be duly referred to (see p. 90). The most conspicuous divergences from the normal are to be found in the Cetacea and the Monotremata. In the former the acromion is approximated so nearly to the anterior border of the blade-bone that the prescapular fossa is reduced to a very small area; and in _Platanista_ the acromion actually coincides with the anterior border, so that that fossa actually disappears. In the Whales, too, the scapula is as a rule very broad, especially above; it has frequently a fan-like contour. In the Monotremata the acromion also coincides with the anterior border of the scapula; but the sameness of appearance which it thus presents (in this feature) to the Cetacean scapula is {38} apparently not due to real resemblance. What has happened in the Monotremata is, that the prescapular fossa is so enormously expanded that it occupies the whole of the inner side of the blade-bone, while the subscapular fossa which, so to speak, should occupy that situation, has been thus pushed round to the front, where it is divided from the postscapular fossa by a slight ridge only.

The clavicle is a bone which varies much in mammals. It is sometimes indeed, as in the Ungulata, entirely absent; in other forms it shows varying degrees of retrocession in importance; it is only in climbing, burrowing, digging, and flying mammals that it is really well developed.

FIG. 29.--Shoulder girdle, with upper end of sternum (inner surface) of Shrew (_Sorex_), after Parker, x 7. _a_, Acromion; _c_, coracoid; _cl_, clavicle; _ec_, partially ossified "epicoracoid" of Parker, or rudiment of the sternal extremity of the coracoid; ''ma'', metacromial process; _mss_, ossified "mesoscapular segment"; _ost_, omosternum; _pc_, rudiment of precoracoid (Parker); _ps_, presternum; _sr_^1, first sternal rib; _sr_^2, second sternal rib. (From Flower's _Osteology_.)

In the higher Mammalia the coracoid[21] is present, but does not reach the sternum as in the Monotremata. It is known to human anatomists as the coracoid process of the scapula. It has been found, however, by Professor Howes[22] and others, that this process really consists of two separate centres of ossification, forming two separate bonelets, which in the adult become firmly ankylosed to each other and to the scapula. These two separate bones have been met with in the embryo of _Lepus, Sciurus_, and the young of various other mammals belonging to very diverse orders, such as Edentates and Primates. The separation even occasionally persists in the adult. The question is, What is the relation of these bonelets to the coracoid of the Monotremata and to the corresponding regions of reptiles? Professor Howes terms the lower patch of bone the metacoracoid and the upper the epicoracoid; {39} the former is alone concerned with the glenoid cavity. It must therefore, one would suppose, correspond to the "coracoid" of the Monotremata, while the upper piece of bone is the epicoracoid process of that mammal. The Mammalia, therefore, higher as well as lower, differ from the reptiles in that the coracoid is formed of two bones, the exceptions being, among some other extinct forms, certain of the Anomodontia, a group which it will be recollected is the nearest of all reptiles to the mammals.

FIG. 30.--Distal extremity of the humerus to show Epicondylar Foramina. A, In _Hatteria_; B, in a Lizard (_Lacerta ocellata_); C, in the Domestic Cat; D, in Man. _c.e_, External condyle; _c.i_, internal condyle. In A the two foramina are developed (at _i_, the entepicondylar; at _ii_, the ectepicondylar). The only canal (+) present in the Lizard (B) is on the external ulnar side, in the cartilaginous distal extremity. In Man (D) an entepicondylar process (_pr_) is sometimes developed and continued as a fibrous band. (From Wiedersheim's _Anatomy of Man_.)

THE FORE-LIMB.--The humerus is of varying length among mammals. A feature which it sometimes shares with the humerus of lower forms is the presence of an entepicondylar foramen, a defect of ossification situated above the inner condyle of that bone which transmits a nerve. The same foramen and an additional ectepicondylar foramen are found in the ancient reptilian type _Hatteria_ (_Sphenodon_); it occurs also in the Anomodont reptiles. It is as a rule only the lower forms among mammals which show this foramen; thus it is present in the Mole and absent in the {40} Horse. The fact that it is occasionally met with in Man is an additional proof of the, in many respects, ancient structure of the highest type of Primate.

The radius and the ulna, which together constitute the fore-arm, are both present in a large number of mammals, but the ulna tends to vanish in the purely walking and digitigrade Ungulates, being present, however, in the more ancient forms of these Ungulates. In Man and in many other mammals the radius can be moved from its normal position and crossed over the ulna; this movement of pronation has been permanently fixed in the Elephant, where the bones are crossed but cannot be altered in position by the contractions of any muscles. Other types agree with the Elephant in this fixation of the two bones.

FIG. 31.--Bones of fore-arm and manus of Mole (_Talpa europaea_). x 2. _c_, Cuneiform; _ce_, centrale; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _rs_, radial sesamoid (falciform); _s_, scaphoid; _td_, trapezoid; _tm_, trapezium; _U_, ulna; _u_, unciform; _I-V_, the digits. (From Flower's _Osteology_.)

The bones of the wrist show great variation among mammals. The greatest number present are to be seen in such a type as the Mole. Here we have a proximal row, consisting of the scaphoid, lunar, cuneiform, and pisiform, which are arranged in their proper order, beginning with that on the radial side of the limb, that side which bears the first digit. A second row articulates proximally with these bonelets and distally with the metacarpals; the bones composing it are, mentioning them in the same order, trapezium, trapezoid, centrale, magnum, unciform.

The centrale does not, however, really belong to the distal carpal row, and is as a rule situated in the middle of the carpus away from articulation with the metacarpals. It is a bone which is not commonly present in the mammalian hand, but is present in various lower forms, such as the Beaver and Hyrax. It also occurs in such high types as the majority of Monkeys; it is to be found in the Human foetal carpus. Many extinct forms possessed a separate centrale. Its importance in the formation of the interlocking condition of the Ungulate foot is referred to later, {41} on p. 196. The only mammal which appears to have the proper five bones in the distal row of the carpus corresponding to the five metacarpals is _Hyperoodon_, where this state of affairs at least occasionally occurs. The final bone of that series, the unciform, seems to represent two bones fused. Very often the carpus is reduced by the fusion of certain of the carpal bones; thus among the Carnivora it is usual for the scaphoid and the lunar to be fused. It is interestingly significant that these bones retain their distinctness in the ancestral Creodonts. In many Ungulates the trapezium vanishes. The reduction of the toes in fact implies a reduction of the separate elements of the carpus.

As to the digits of the mammalian hand, the greatest number is five, the various supplementary bonelets known as prepollex and postminimus being, it is now generally held, merely supplementary ossifications not representing the rudiments of pre-existing fingers. They may, however, bear claws.[23] The number of phalanges which follow upon the metacarpals is almost constantly three in the mammals, excepting for the thumb, which has only two. This is highly characteristic of the group as opposed to reptiles and birds, and the increase in the number of these bones in the Whales and to a very faint degree in the Sirenia is a special reduplication, which will be mentioned when those animals are treated of.

THE PELVIC GIRDLE.--The pelvic girdle or hip girdle is the combined set of bones which are attached on the one hand to the sacrum and on the other articulate with the hind-limb. Four distinct elements are to be recognised in each "os innominatum," the name given to the conjoined bones of each half of the entire pelvis. These are:--the ilium, which articulates with the sacrum; the ischium, which is posterior; the pubis, which is anterior; and finally, a small element, the cotyloid, which lies within the acetabular cavity where the femur articulates. The epipubes of the Monotreme and the Marsupial are dealt with elsewhere (see p. 116) as they are peculiar to those groups.

Professor Huxley pointed out many years since that while the Eutherian Mammalia differ from the reptiles in the fact that the axis of the ilium lies at a less angle with that of the sacrum, {42} _Ornithorhynchus_ comes nearest to the reptile in the fact that this axis is nearly at right angles to that of the sacrum. It is particularly interesting to find that this peculiarity of _Ornithorhynchus_ is only acquired later in life, and that the pelvis of the foetus conforms in these angles to the adults of other mammalian groups. In any case, the backward rotation of the pelvis is a mammalian characteristic, and it is most nearly approached among reptiles by the extinct Anomodontia, whose affinities to mammals will be dealt with on a later page (p. 90). Another peculiarity of the mammalian pelvis appears to be the cotyloid bone already referred to. In the Rabbit this bone completely shuts out the pubis from any share in the acetabular cavity; later it ankyloses with that bone. In _Ornithorhynchus_ the cotyloid or os acetabuli is a larger element of the girdle than is the pubis. In other mammals, therefore, it seems to be a rudimentary structure. But it seems to be a bone peculiar to and thus distinctive of the mammals as compared with other vertebrates. The acetabular cavity is perforated in _Echidna_ as in birds; but in certain Rodents the same region is very thin and only closed by membrane, as in _Circolabes villosus_.

The number and the arrangement of the bones in the HIND-LIMB correspond exactly to those of the fore-limb. The femur, which corresponds to the humerus, shows some diversities of form. The neck, which follows upon the almost globular head, the surface of articulation to the acetabular cavity of the pelvis, has two roughened areas or tuberosities for the insertions of muscles. A third such area, known as the third trochanter, is present or absent as the case may be, and its presence or absence is of systematic import. As a general rule the thigh-bones of the ancient types of mammals are smoother and less roughened by the presence of these three trochanters than in their modern representatives. The radius and the ulna are represented in the hind-leg by the tibia and the fibula. These bones are not crossed, and do not allow of rotation as is the case with the radius and the ulna. In Ungulate animals there is the same tendency to the shortening and rudimentary character of the fibula that occurs in the case of the ulna, but it is more marked. It has been shown in tracing the history of fossil Ungulates that the hind-limbs in their degree of degeneration are as a rule ahead of the fore-limbs. This is natural when we reflect that {43} the hind-limbs must have preceded the fore-limbs in their thorough adaptation to the cursorial mode of progression. In the Mammalia the ankle-joint is always what is termed cruro-tarsal, _i.e._ between the ends of the limb-bones and the proximal row of tarsals; not in the middle of the tarsus as in some Sauropsida (reptiles and birds). The bones of the ankle are much like those of the hand; but there are never more than two bones in the proximal row, which are the astragalus and the calcaneum. The former is perhaps to be looked upon as the equivalent of the cuneiform and lunar together. But the views as to the homologies of the tarsal bones differ widely. Below these is the navicular, regarded as a centrale. The distal row of the tarsus has four bones, three cuneiforms and a cuboid. Reduction is effected by the soldering together of two cuneiforms as in the Horse, by the fusion of the navicular and cuboid as in the Deer. No mammal has more than five toes, and the number tends to become reduced in cursorial animals (Rodents, Ungulates, Kangaroos).

FIG. 32.--Anterior aspect of right femur of Rhinoceros (_Rhinoceros indicus_). x 1/2. _h_, Head; _t_, great trochanter; _t_', third trochanter. (From Flower's _Osteology_.)

TEETH.--The teeth of the Mammalia[24] differ from those of other vertebrated animals in a number of important points. These, however, entirely concern the form of the adult teeth, their position in the mouth, and the succession of the series of teeth. Developmentally and histologically there are no fundamental divergences from the teeth of vertebrates lower in the scale.

In mammals, as for example in the Dog, the teeth consist of three kinds of tissue--the enamel, the dentine, and the cement. The enamel is derived from the epidermis of the mouth cavity, and the two remaining constituents from the underlying dermis. The teeth originate quite independently of the jaws, with which they are later so intimately connected; the independence of origin being one of the facts upon which the current theory {44} of the nature of teeth is founded. It has been pointed out that the scales of the Elasmobranch fishes consist of a cap of enamel upon a base of dentine, the former being derived from the epidermis and modelled upon a papilla of the dermis whose cells secrete the dentine. The fact that similar structures arise within the mouth (_i.e._ the teeth) is explicable when it is remembered that the mouth itself is a late invagination from the outside of the body, and that therefore the retention by its tissues of the capacity to produce such structures is not remarkable.

FIG. 33.--Diagrammatic sections of various forms of teeth. _I_, Incisor or tusk of Elephant, with pulp cavity persistently open at base; _II_, Human incisor during development, with root imperfectly formed, and pulp cavity widely open at base; _III_, completely formed Human incisor, with pulp cavity opening by a contracted aperture at base of root; _IV_, Human molar with broad crown and two roots; _V_, molar of the Ox, with the enamel covering the crown deeply folded, and the depressions filled up with cement; the surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white; the dentine represented by horizontal lines, and the cement by dots. (After Flower and Lydekker.)

The relations of the three constituents of the tooth in its simplest form is shown in the accompanying diagram, where the intimate structure of the enamel, dentine, and cement (or crusta petrosa as it is sometimes called) is not indicated. The latter has the closest resemblance to bone. The dentine is traversed by fine canals which run parallel to each other and anastomose here and there. The enamel is formed of long prismatic fibres, and is excessively hard in structure, containing less animal matter than the other tooth tissues. To this fact is frequently {45} due the complicated patterns upon the grinding teeth of Ungulates, which are produced by the wearing away of the dentine and the cement, and the resistance of the enamel.

The centre of the tooth papilla remains soft and forms the pulp of the tooth, which is continuous with the underlying tissues of the gum by a fine canal or a wide cavity as the case may be. In teeth which persistently grow throughout the lifetime of the animal, as for example the incisors of the Rodents, there is a wide intercommunication between the cavity of the tooth and the tissues of the gum; only a narrow canal exists in, for instance, the teeth of Man, and in fact in the vast majority of cases. The three constituents of the typical teeth are not, however, found in all mammals; the layer which is sometimes wanting is the enamel. This is the case with most Edentates; but the interesting discovery has been made (by Tomes) that in the Armadillo there is a downgrowth of the epidermis similar to that which forms the enamel in other mammals, a rudimentary "enamel organ."

Teeth are present in nearly all the Mammalia; and where they are absent there is frequently some evidence to show that the loss is a recent one. The Whalebone Whales, the Monotremata, _Manis_, and the American Anteaters among the Edentata are devoid of teeth in the adult state. In several of these instances, however, more or less rudimentary teeth have been found, which either never cut the gums or else become lost early in life. The latter is the case with _Ornithorhynchus_, where there are teeth up to maturity (see p. 113). Kuekenthal has found germs of teeth in Whales, and Roese in the Oriental _Manis_. The loss of the teeth in these cases seems to have some relation to the nature of the food. In ant-eating mammals, as in the Anteaters and _Echidna_, the ants are licked up by the long and viscid tongue, and require no mastication. Yet it must be remembered that _Orycteropus_ is also an anteater, like the Marsupial _Myrmecobius_, both of which genera have teeth.

The first of the essential peculiarities of the mammalian teeth as compared with those of other vertebrates concerns the position of the teeth in the mouth. There is no undoubted mammal extinct or living in which the teeth are attached to any bones other than the dentary, the maxilla, and the {46} premaxilla. There are no vomerine, palatine, or pterygoid teeth, such as are met with in Amphibia and Reptilia.

The other peculiarities of the mammalian teeth, though true of the great majority of cases, are none of them absolutely universal.

But it is necessary to go into the subject at some length on account of the great importance which has been laid upon the teeth in deciding questions of relationship; moreover, largely no doubt on account of their hardness and imperishability, our knowledge of certain extinct forms of Mammalia is entirely based upon a few scattered teeth; while of some others, notably of the Triassic and Jurassic genera, there is not a great deal of evidence except that which is furnished by the teeth. Indeed the important place which odontography holds in comparative anatomy is from many points of view to be regretted, though inevitable. "In hardly any other system of organs of vertebrated animals," remarks Dr. Leche, "is there so much danger of confounding the results of convergence of development with true homologies, for scarcely any other set of organs is less conservative and more completely subservient to the lightest impulse from without." Affinities as indicated by the teeth are sometimes in direct contradiction to those afforded by other organs; or, as in the case of the simple Toothed Whales, no evidence of any kind is forthcoming. Dr. Leche has pointed out that, judged merely from its teeth, _Arctictis_ would be referred to the Raccoons, though it is really a Viverrid; while _Bassariscus_, which Sir W. Flower showed to be a Raccoon, is in its teeth a Viverrid. Mr. Bateson has been obliged to hamper the subject with another difficulty.

In dealing with the variations of teeth,[25] Mr. Bateson has brought together an immense number of facts, which tend to prove that the variability of these structures is much greater than had been previously recognised; that this variability is often symmetrical; and that in some animals, as in "_Canis cancrivorus_, a South American fox, the majority showed some abnormality." When we learn from Mr. Bateson that "of _Felis fontanieri_, an aberrant leopard, two skulls only are known, both showing dental abnormalities," it seems dangerous to rear too lofty a superstructure upon a single fossil jaw. It must be noted too that, {47} contrary to the prevailing superstition, it is not domestic animals which show the greatest amount of tooth variation. As to special homologies between tooth and tooth, with which we shall deal on a later page, Mr. Bateson has urged almost insuperable difficulties.

FIG. 34.--Skull of Dasyurus (lateral view). _al.sph_, Alisphenoid; _ang_, angular process of mandible; _fr_, frontal; _ju_, jugal; _lcr_, lachrymal; _max_, maxilla; _nas_, nasal; _oc.cond_, occipital condyle; _par_, parietal; _par.oc_, paroccipital process; _p.max_, premaxilla; _s.oc_, supraoccipital; _sq_, squamosal; _sq_', zygomatic process of squamosal. (From Parker and Haswell's _Zoology_.)

FIG. 35.--Upper and lower teeth of one side of the mouth of a Dolphin (_Lagenorhynchus_), illustrating the homodont type of dentition in a mammal. (After Flower and Lydekker.)

The teeth of the Mammalia are almost without exception "heterodont," _i.e._ they show differences of structure in different parts of the mouth. As a general rule, teeth can be grouped into cutting incisors, sharp conical canines, and molars, with a surface which is in the majority of cases suited for grinding. In this they contrast with the majority of the lower vertebrates, where the teeth are "homodont" (or, better, _homoeodont_), _i.e._ all more or less similar and not fitted by change of form to perform different duties. But there are exceptions on both sides. In {48} the Toothed Whales the teeth are homodont, as they are in the frog and in most reptiles; on the other hand, some of the remarkable reptiles belonging to Professor Huxley's order of the Anomodontia have distinct canines, and show other differentiations in their teeth.

A second characteristic of the mammalian dentition is the limited number of the teeth, which rarely exceeds fifty-four. Here again the Toothed Whales are an exception, the number of their teeth being as great as in many reptiles. In the Mammalia the number of the teeth is fixed (excepting of course for abnormalities), while in reptiles there is frequently no precise normal. Two regions may be distinguished in every tooth--the crown and the root; the latter, as its name denotes, is imbedded in the gum, while the crown is the freely-projecting summit of the tooth. The varying proportions of these two regions of the tooth enables us to divide teeth into two series--the brachyodont and the hypselodont; in the latter the crown is developed at the expense of the root, which is small; the hypselodont tooth is one that grows from a persistent pulp or, at any rate, one that is long open. Brachyodont teeth on the contrary have narrow canals running into the dentine. The primitive form of the tooth seems undoubtedly to be a conical single-rooted tooth, such as is now preserved in the Toothed Whales and in the canine teeth of nearly all animals. The development of the teeth, that is, the simple bell-shaped form of the enamel organ, seems to go some way towards proving this; but it is quite another question whether we can fairly regard the Whales as having retained this early form of tooth. In their case the simplification, as is so often the case where organs are simplified, seems to be rather degeneration than retention of primitive characters. But this is a matter which must be deferred for the present.

The incisor teeth are generally of simple structure and nearly always single rooted. In the Rodents, in the extinct Tillodontia and in Diprotodont Marsupials, they have grown large, and, as has been already stated, they increase in size continuously from the growing pulp. These teeth have a layer of enamel only on the anterior face, which keeps a sharp chisel-like edge upon them by reason of the fact that the harder enamel is worn away more slowly than the comparatively soft dentine. The {49} "horn" of the Narwhal is another modification of an incisor, as are the tusks of Elephants. Among the Lemurs the incisors are denticulate, and serve to clean the fur in a comb-like fashion. This is markedly the case in _Galeopithecus_. The incisors are sometimes totally absent, as in the Sloths, sometimes partially absent, as in many Artiodactyles, where the lower incisors bite against a callous pad in the upper jaw, in which no trace of incisors has been found.

Canine teeth are present in the majority of mammals, but are absent without a single exception from the jaws of the Rodentia. The canine tooth of the upper jaw is that tooth which comes immediately after the suture dividing the premaxillary from the maxillary bone. The canines are as a rule simple conical teeth, with but a single root; indeed they resemble what we may presume to have been the first kind of tooth developed in mammals. In this they resemble also as a general rule the foregoing incisors. But instances are known where the canines are implanted by two roots. This is to be seen in _Triconodon_, in the pig _Hyotherium_, in the Mole and some other Insectivores, and in _Galeopithecus_, where the incisors also may be thus implanted in the jaw. Furthermore, the simple condition of the crown of the tooth may be departed from. This is the case with a Fruit Bat belonging to the genus _Pteralopex_. In the more primitive Mammalia it is common to find no great difference between the canines and incisors; such is the case with the early Ungulate types of Eocene times, such as _Xiphodon_. In modern mammals, however, especially among the Carnivora, the canines tend to become larger and stronger than the incisors, and in some of the Cats and in the Walrus these teeth are represented by enormous offensive tusks. It is not rare for the canines of male animals to be larger than those of their mates. There are also cases such as the Musk-deer and the Kanchil where the male alone possesses these teeth, but only in the upper jaw. The teeth which follow the canines are known as the grinders or cheek teeth, or more technically as premolars and molars. These two latter terms separate teeth which arise at different periods, and their use will be explained later. In the meantime it may be pointed out that the cheek teeth are the teeth which show the greatest amount of variation in their structure; this is shown by the number and variety of the cusps in which {50} the biting surface ends. The grinding teeth vary from simple one-cusped teeth, precisely like canines, to teeth with an enormous number of separate tubercles. In the former case it is hard to distinguish between incisors, canines, and cheek teeth in the lower jaw, where no suture separates the bone. Moreover it is quite common for the first cheek tooth in the lower jaw to have the characters of a canine, while the true canine approximates in its form to the antecedent incisors. This is so, for instance, with the Lemurs, where the first premolar is caniniform, and the canine shares in the curious procumbent attitude which distinguishes the lower incisors of many of those animals.

A variable number of the anterior cheek teeth may be little more than simple conical teeth; but the rest of the set are commonly more complicated. No definite laws can be laid down as to the complication of the posterior as compared with the anterior set. Broadly speaking, it is purely herbivorous creatures in which the least difference can be detected at the two extremities, and which are at the same time the most elaborately decorated with tubercles and ridges. The converse is true that in purely carnivorous animals, including insect- and fish-eating forms, there is the greatest difference between the anterior set of grinding teeth and those which follow. In these two respects such animals as a Lemur and a Rhinoceros occupy the extremes. Furthermore, it may be said that omnivorous creatures lie, as their diet would suggest, in an intermediate position. Generally speaking, when there is a marked difference between the first premolar and molars at the end of the series, there is a gradual approximation in structure of a progressive kind. The tubercles become more numerous in successive teeth; but the corollary which is apparently deducible from this, _i.e._ that the last molar is the most elaborate of the series, is by no means always true. The last cheek tooth indeed is often degenerate. On the other hand, it is very markedly the largest of the series in such diverse types as the Elephant, the hog _Phacochoerus_, and the Rodent _Hydrochoerus_. It is a rule that the cheek teeth of the upper jaw are more complicated than the corresponding teeth of the lower jaw.

The structure of the cheek teeth is very diverse among the Mammalia. Broadly, two types are to be recognised. There are {51} teeth in which the grinding surface is raised into a series of two, to many, tubercles sharper or blunter as the case may be;--sharper and fewer at the same time in carnivorous and especially in insectivorous types, more abundant in omnivorous animals. To this form of tooth the term "bunodont" is applied. There is no doubt that this is the earliest type of tooth; but whether the fewer or the more cusped condition is the primitive one is a question that is reserved for consideration at the end of the present chapter. The other type of grinding tooth is known as "lophodont." This is exemplified by such types as the Perissodactyla and Ungulates generally, and by the Rodents. The tooth is traversed by ridges which have generally a transverse direction to the long axis of the jaw in which the tooth lies. The ridges may be regarded as having been developed between tubercles which they connect and whose distinctness as tubercles is thereby destroyed. Lophodont teeth are only found in vegetable-feeding animals.

FIG. 36.--Molar teeth of _Aceratherium platycephalum_. x 1/2. _m.1-m.3_., Molars; _mh_, metaloph; _p.1-p.4_, premolars; _ph_, protoloph; _ps.f_, parastyle fossa; _te_, tetartocone. (After Osborn.)

The special characteristics of the teeth of various groups of animals will be considered further under the accounts of the several orders of recent and fossil Mammalia.

FIG. 37.--Two stages in the development of the teeth of a Mammal (diagrammatic sections). _alv_, Bone of alveolus; _dent_, dentine; _dent.s_, dental sac; _en_, enamel; _en.m_, enamel membrane; _en.m_^2, enamel membrane of permanent tooth; _en.plp_, enamel pulp; _gr_, dental groove; _lam_, dental lamina; _lam_', part of dental lamina which grows downwards below the tooth germ; _n_, neck connecting germs of milk and permanent tooth; _pap_, dental papilla; _pap_^2, dental papilla of permanent tooth. (After O. Hertwig.)

A very general feature of the teeth of the Mammalia is what is usually termed the diphyodont dentition. In the majority of cases there are two sets of teeth developed, of which the first lasts for a comparatively short time, and is termed on account of its usual time of appearance the "milk dentition"; this is replaced later by the permanent dentition. In lower vertebrates the teeth are replaced as worn away. There is not, however, so great an antithesis in this matter between the Mammalia {52} and other vertebrates as was at one time assumed. But in order to explain this very important part of the subject it will be necessary to give some account of the development of the teeth. The type selected is the Hedgehog, which has been recently and carefully described by Dr. Leche of Stockholm, which type has furthermore the advantage of being a "central" type of mammal. The first step in the formation of the teeth is a continuous invagination of the epithelium covering the jaw to form a deepish wall of tissue running in the thickness of the jaw; this is perfectly continuous from end to end of the lower jaw. From this "common enamel germ" (_Schmelzleiste_ of the Germans[26]) "special enamel germs" (_Schmelzorgane_, enamel organs) are developed here and there as thickenings in the form of buds {53} which arise on the outer side of the fold of epithelium and some way above its lower termination. These ultimately acquire a bell-like form, and are as it were moulded on to a thickened concentration of the dermis beneath; they then become separate from the downgrowth of the epithelium whence they have arisen. Finally, each of the eight germs becomes one of the milk teeth of the animal. The lower end of the sheet of invaginated epithelium, the common enamel germ, is the seat of the formation of the second set of teeth, of which, however, in the animal under consideration, there are only two in each jaw. But corresponding to each of the enamel germs of the milk dentition, with the exception of the first two molars, there is a slight thickening of the end of the common enamel germ, which at a certain stage is indistinguishable from the thickening which will become one of the permanent teeth. We have thus the diphyodont arrangement. But this does not exhaust the series of rudimentary teeth, though no more come to maturity than those whose development has already been touched upon. In the upper jaw a small outgrowth of the common enamel germ arises above and to the outer side of the enamel germ of the third milk incisor; this does not develop any further, but its resemblance to the commencing germ of a tooth seems to indicate that it is the remnant of a tooth series antecedent to the milk series. Furthermore, there are indications in the fourth premolar of a fourth series of teeth posterior in appearance to the permanent dentition. We arrive therefore at the important conclusion that although here as elsewhere there are only two sets of calcified teeth ever developed, there are feeble though unmistakable remains of two other series, one antecedent to and the other posterior to the diphyodont dentition. The gap therefore which separates the mammalian dentition from that of reptiles is less than has hitherto appeared. Dr. Leche also carefully studied the tooth development of _Iguana_; he found that in this lizard there are four series of teeth which come to maturity, and a rudimentary series antecedent to these which never produces fully formed teeth.

In a few mammals there is a kind of dentition known as the monophyodont, in which only one series of teeth reaches maturity; where in fact there is no replacement of a milk series by a permanent dentition. Of the monophyodont dentition Whales form an example. The Marsupials are very nearly an instance of the {54} same phenomenon; for Sir W. Flower showed, and Mr. Thomas confirmed his discovery, that only one tooth, according to Mr. Thomas the fourth premolar, is replaced in that group. But even the purely monophyodont dentition of the Toothed Whales is a more apparent than real contrast to the diphyodont dentition elsewhere prevalent. An investigation of the embryos of various Toothed Whales by Dr. Kuekenthal and by Dr. Leche has brought to light the highly important fact that two dentitions are present, but that one only comes to maturity; from this fact obviously follows the interesting question:--To which of the two dentitions of more normal Mammalia does the monophyodont dentition of the Whales and Marsupials belong? To this question a clear answer is fortunately possible. As has been pointed out in the foregoing sketch of tooth development, and has been illustrated in the figures, the milk teeth develop as lateral outgrowths of the common enamel germ, while the permanent teeth arise from the end of the same band of tissue. This fact enables it to be stated apparently beyond a doubt that in the Whales and in the Marsupials it is the milk dentition which is the only one to arrive at maturity. Thus the earlier theoretical conclusion that the Marsupial dentition "is a secondary dentition with only one tooth of the primary set left," is proved on embryological grounds to be untrue. But there are other monophyodont animals than those already mentioned.[27] _Orycteropus_, the Cape Anteater, is an example. Mr. Thomas has lately discovered that in this Edentate there is a set of minute though calcified milk teeth which probably never cut the gum; here we have a different sort of monophyodontism, in which the teeth belong to the second and not to the first set. Between the latter condition and the diphyodont state are intermediate stages. Thus in the Sea Lions the milk teeth are developed but disappear early, probably before the animal is born.

In the typical diphyodont dentition, such as is exhibited for example in Man and the vast majority of mammals, the milk teeth eventually completely disappear and are entirely replaced by the permanent set of teeth, with the exception, of course, of the molars, which though they are developed late belong to the milk series. {55}

Their correspondence with the milk series is shown in an interesting way by the close resemblance which the last milk premolar often bears to the first molar. These two extremes of dentition, _i.e._ purely monophyodont and, excepting for the molars, purely diphyodont, are however connected by an intermediate state of affairs, which is represented by more than one stage. In _Borhyaena_ (probably a Sparassodont) the incisors and the canines and two out of the four premolars belong to the permanent dentition, while the two remaining premolars and of course the three molars are of the milk series. _Prothylacinus_, a genus belonging to the same group, has a dentition which is a step or two further advanced in the direction of the recent Marsupials. We find, according to Ameghino,[28] whose conclusions are accepted by Mr. Lydekker, that the incisors, canines, and two premolars belong to the milk series, while the permanent series is represented only by the two remaining premolars. We can tabulate this series as follows:--

(1) Purely monophyodont, with teeth only of the first set--Toothed Whales.

(2) Incompletely monophyodont, as in the Marsupials, where there is a milk dentition with only one tooth replaced.[29]

(3) Incompletely diphyodont, with the dentition made up partly of milk, partly of permanent teeth, as in _Borhyaena_.

(4) Diphyodont, where all the teeth except the molars are of the second set; this characterises nearly all the mammals.

As we pass from older forms to their more recent representatives there is as a rule a progressive development of the form of the teeth. This is especially marked among the Ungulata. The extremely complicated type of tooth found in such a form as the existing Horse can be traced back through a series of stages to a tooth in which the crown is marked by a few separated tubercles or cusps. Arrived at this point, the differences between the teeth of ancestral Horses and ancestral Rhinoceroses and Tapirs are hard to distinguish with accuracy; and the same difficulty is experienced in attempting to give a definition of other large orders by the characters of the teeth, such as will apply to the Eocene or {56} even earlier representatives of these families. Fig. 36 (p. 51) illustrating a series of mammalian teeth will illustrate the above remarks. That there is such a convergence in tooth structure shows that it is, theoretically at least, possible to determine the ancestral form of the mammalian tooth. Practically, however, the difficulties which beset such theorising are great; that there are such divergent and such strongly-held antithetical views is sufficient proof of this. Two main views hold the field: one, which has found most favour in America, and is due chiefly to the labours and persuasiveness of Professors Cope, Scott, Osborn, and others, is known as "trituberculy."[30] The alternative view, as urged by Forsyth Major, Woodward, and Goodrich, attempts to show that the dentition of the original mammal included grinding teeth which were multicuspidate or "multitubercular." There is much to be said for both views, and something to be said against both.

FIG. 38.--Molar teeth of A, _Phenacodus_, and B, the Creodont _Palaeonictis_. _End_, endoconid; _hld_, hypoconulid; _hyd_, hypoconid; _med_, metaconid; _prd_, protoconid. (After Osborn and Wortman.)

This question is, however, wrapped up in a wider one. Its solution depends upon the ancestry of mammals. If the Mammalia are to be derived from reptiles with simple conical teeth, then the first stage in the development of trituberculy is proved. On the other hand, however, the evidence is gradually growing that the Theromorpha represent more nearly than any non-mammalian group with which we are acquainted the probable ancestral form of the mammals. These animals offer some support to both the leading views. _Cynognathus_ had triconodont teeth which, as will be pointed out later, are a theoretically intermediate stage in the evolution of tritubercular teeth; on the other hand, the teeth of _Diademodon_ and some others are multituberculate, and have been very properly compared to the multitubercular teeth of such primitive mammalia as the Ornithorhynchus. Professor Osborn is no doubt correct in italicising a remark of an anonymous writer in _Science_ to the effect that in _Diademodon_ the teeth, though multitubercular, show the prevalence of three cusps arranged in the tritubercular fashion. {57} But this may be only a proof that the multitubercular antedates the tritubercular. It may be, indeed, that the mammalian tooth was already differentiated among the mammal-like Saurians and that from such a form as _Cynognathus_ the Eutheria and other forms in which a tritubercular arrangement can be detected were evolved, and from such form as _Tritylodon_ the Monotrematous branch of the mammals. This way of looking at the matter harmonises a much-disputed question, but involves a diphyletic origin of the mammals--an origin which for other reasons is not without its supporters.

We shall now attempt to give a general idea of the facts and arguments which support or tend to support "trituberculy." As a matter of fact the name is inaccurate; for the holders of this view do not derive the mammalian molar from a trituberculate condition, but in the first place from a simple cone such as that of a crocodile!