PRIMATES 533
INDEX 591
* * * * *
{vii}
Scheme of the Classification Adopted In This Book
Sub-Class Prototheria (p. 105).
Order. Sub-order. Family. Sub-family.
MONOTREMATA { { Echidnidae (p. 110).
(p. 106) { { Ornithorhynchidae
{ (p. 112).
?ALLOTHERIA
(p. 96).
Sub-class Eutheria (p. 116)
{ { Macropodinae { Macropodinae (p. 132).
{ { (p. 129) { Potoroinae (p. 137).
{ { { Hypsiprymnodontinae
{ { { (p. 138).
{ {
{ Diprotodontia { Phalangeridae { Phalangerinae (p. 140).
{ (p. 128) { (p. 138) { Phascolarctinae
{ { { (p. 144).
{ { { Phascolomyinae
{ { { (p. 144).
MARSUPIALIA { { { Tarsipedinae (p. 145).
(p. 122) { { Epanorthidae (p. 145).
{
{ { Dasyuridae (p. 149).
{ Polyprotodontia { Didelphyidae (p. 155).
{ (p. 149) { Peramelidae (p. 156).
{ { Notoryctidae (p. 158).
{ { Myrmecophagidae (p. 166).
{ { Bradypodidae (p. 170).
{ Xenarthra { Dasypodidae (p. 173).
EDENTATA { (p. 166) { Mylodontidae (p. 179).
(p. 161) { { Megalonychidae (p. 183).
{ { Megatheriidae (p. 183).
{ { Glyptodontidae (p. 184).
{
{ Nomarthra { Orycteropodidae (p. 187).
{ (p. 186) { Manidae (p. 188).
GANODONTA { { Stylinodontidae (p. 191).
(p. 190) { { Conoryctidae (p. 193).
{viii}
{ Condylarthra (p. 202).
{ Amblypoda (p. 205).
{ Ancylopoda (p. 211).
{ Typotheria (p. 212).
{ Toxodontia (p. 214).
{ Proboscidea { Elephantidae (p. 217).
{ (p. 216) { Dinotheriidae (p. 231).
{ Hyracoidea (p. 232).
{ { Equidae (p. 237).
{ { Lophiodontidae (p. 247).
{ Perissodactyla { Palaeotheriidae (p. 247).
{ (p. 235) { Tapiridae (p. 260).
{ { Rhinocerotidae (p. 253).
{ { Titanotheriidae (p. 264).
UNGULATA {
(p. 195) { Litopterna { Macraucheniidae (p. 267).
{ (p. 267)
{
{ { Hippopotamidae (p. 273).
{ { Suidae (p. 275).
{ { Dicotylidae (p. 278).
{ { Tragulidae (p. 282).
{ { Proceratidae (p. 284).
{ { Camelidae (p. 285).
{ { Cervidae (p. 291) { Cervinae (p. 293).
{ Artiodactyla { { Moschinae (p. 299).
{ (p. 269) { Giraffidae (p. 301).
{ { Antilocapridae (p. 306).
{ { Bovidae (p. 307).
{ { Anthracotheriidae (p. 328).
{ { Caenotheriidae (p. 329).
{ { Xiphodontidae (p. 329).
{ { Oreodontidae (p. 330).
{ { Anoplotheriidae (p. 332).
SIRENIA (p. 333).
{ Mystacoceti { Balaenopteridae
{ (p. 353) { (p. 355).
{ { Balaenidae (p. 358).
CETACEA {
(p. 339) { Odontoceti { Physeteridae { Physeterinae (p. 363).
{ (p. 362) { (p. 362) { Ziphiinae (p. 367).
{ { Delphinidae (p. 372).
{ { Platanistidae (p. 380).
{ { Squalodontidae (p. 384).
{
{ Archaeoceti { Zeuglodontidae (p. 384).
{ (p. 384)
{ { Felidae (p. 390).
{ { Machaerodontidae
{ { (p. 401).
{ {
{ { { Euplerinae (p. 403).
{ { { Galidictiinae (p. 404).
{ { Viverridae { Cryptoproctinae (p. 404).
{ { (p. 403) { Viverrinae (p. 405).
CARNIVORA { Fissipedia { { Herpestinae (p. 409).
{ix}
(p. 386) { (p. 387) { Hyaenidae (p. 411).
{ { Canidae (p. 413).
{ { Procyonidae (p. 426).
{ { { Melinae (p. 432).
{ { Mustelidae { Mustelinae (p. 433).
{ { (p. 431) { Lutrinae (p. 439).
{ { Ursidae (p. 442).
{
{ Pinnipedia { Otariidae (p. 450).
{ (p. 446) { Trichechidae (p. 451).
{ { Phocidae (p. 452).
CREODONTA
(p. 455).
{ { Anomaluridae (p. 462).
{ { Soiuridae (p. 463).
{ { Castoridae (p. 467).
{ { Haplodontidae (p. 469).
{ { Gliridae (p. 470).
{ { { Murinae (p. 471).
{ { { Phlaeomyinae (p. 473).
{ { { Hydromyinae (p. 474).
{ { { Rhynchomyinae (p. 474).
{ { { Gerbillinae (p. 475).
{ { Muridae { Otomyinae (p. 475).
{ { (p. 471) { Dendromyinae (p. 476).
{ { { Lophiomyinae (p. 476).
{ { { Microtinae (p. 477).
{ { { Sigmodontinae (p. 479).
{ Simplicidentata { { Neotominae (p. 480).
RODENTIA { (p. 462) { Bathyergidae (p. 480).
(p. 458) { { Spalacidae (p. 482).
{ { Geomyidae (p. 483).
{ { Heteromyidae (p. 484).
{ { Dipodidae (p. 484).
{ { Pedetidae (p. 486).
{ { { Octodontinae (p. 487).
{ { Octodontidae { Loncherinae (p. 488).
{ { (p. 487) { Capromyinae (p. 489).
{ { Ctenodactylidae (p. 490).
{ { Caviidae (p. 491).
{ { Dasyproctidae (p. 493).
{ { Dinomyidae (p. 495).
{ { Chinchillidae (p. 496).
{ { Cercolabidae (p. 497).
{ { Hystricidae (p. 499).
{
{ Duplicidentata { Leporidae (p. 502).
{ (p. 502) { Lagomyidae (p. 505).
TILLODONTIA
(p. 506).
{ { Erinaceidae (p. 509).
{ { Tupaiidae (p. 511).
{ { Centetidae (p. 511).
{ Insectivora { Potamogalidae (p. 513).
{ Vera { Solenodontidae (p. 513).
{ (p. 509) { Chrysochloridae (p. 514).
INSECTIVORA { { Macroscelidae (p. 515).
(p. 508) { { Talpidae (p. 516).
{ { Soricidae (p. 518).
{
{ Dermoptera { Galeopithecidae (p. 520).
{ (p. 520)
{x}
{ Megachiroptera { Pteropodidae (p. 524).
{ (p. 524)
{
{ { Rhinolophidae (p. 527).
CHIROPTERA { { Nycteridae (p. 527).
(p. 521) { Microchiroptera { Vespertilionidae (p. 528).
{ (p. 526) { Emballonuridae (p. 530).
{ Phyllostomatidae
{ (p. 531).
{ { { Indrisinae (p. 538).
{ { Lemuridae { Lemurinae (p. 540).
{ { (p. 538) { Galagininae (p. 542).
{ { { Lorisinae (p. 545).
{ Lemuroidea { Chiromyidae (p. 548).
{ (p. 534) { Tarsiidae (p. 550).
PRIMATES { { Anaptomorphidae (p. 552).
(p. 533) { { Chriacidae (p. 552).
{ { Megaladapididae (p. 554).
{
{ { Hapalidae (p. 556).
{ Anthropoidea { Cebidae (p. 557).
{ (p. 554) { Cercopithecidae (p. 562).
{ { Simiidae (p. 570).
{ { Hominidae (p. 585).
* * * * *
{1}
CHAPTER I
INTRODUCTORY
The Mammalia form a group of vertebrated animals which roughly correspond
with what are termed in popular language "quadrupeds," or with the still
more vernacular terms of "beasts" or "animals." The name "Mammal" is
derived from the most salient characteristic of the group, _i.e._ the
possession of teats; but if the term were used in an absolutely strict
etymological sense, it could not include the Monotremes, which, though they
have mammary glands, have not fully-differentiated teats (see p. 16). There
are, however, as will be seen shortly, other characters which necessitate
the inclusion of these egg-laying quadrupeds within the class Mammalia.
The Mammalia are unquestionably the highest of the Vertebrata. This
statement, however, though generally acceptable, needs some explanation and
justification. "Highest" implies perfection, or, at any rate, relative
perfection. It might be said with perfect truth that a serpent is in its
way an example of perfection of structure: not incommoded with limbs it can
slip rapidly through the grass, swim like a fish, climb like a monkey, and
dart upon its prey with rapidity and accuracy. It is an example of an
extremely specialised reptile, the loss of the limbs being the most obvious
way in which it is specialised from more generalised reptilian types.
Specialisation in fact is often synonymous with degradation, and, this
being the case, implies a restricted life. On the other hand,
simplification is not always to be read as degeneration. The lower jaw, for
instance, of mammals has fewer bones in it than that of reptiles, and is
more concisely articulated to the skull; this implies greater efficiency
{2} as a biting organ. The term highest, however, includes increased
complexity as well as simplification, the two series of modifications being
interwoven to form a more efficient organism. It cannot be doubted that the
increased complexity of the brain of mammals raises them in the scale, as
does also the complex and delicately adjusted series of bonelets which form
the organ for the transmission of sound to the internal ear. The separation
of the cavity containing the lungs, and the investment of the partition so
formed with muscular fibres, renders the action of the lungs more
effective; and there are other instances among the Mammalia of greater
complexity of the various parts and organs of the body when compared with
lower forms, which help to justify the term "highest" generally applied to
these creatures.
Complexity and finish of structure are often accompanied by large size; and
the Mammalia are, on the whole, larger than any other Vertebrates, and also
contain the most colossal species. The huge Dinosaurs of the Mesozoic
epoch, though among the largest of animals, are exceeded by the Whales; and
the latter group includes the mightiest creature that exists or has ever
existed, the eighty-five-feet-long Sibbald's Rorqual. Confining ourselves
rigidly to facts, and avoiding all theorising on the possible relation
between complexity and nicety of build and the capacity for increase in
bulk, it is plain from the history of more than one group of mammals that
increase in bulk accompanies specialisation of structure. The huge
Dinocerata when compared with the ancestral _Pantolambda_ teach us this, as
do many similar examples. Within the mammalian group, as in the case of
other Vertebrates, difference of size has a certain rough correspondence
with difference of habitat. The Whales not only contain the largest of
animals, but their average size is great; so too with the equally aquatic
Sirenia and very aquatic Pinnipedia. Here the support offered by the water
and the consequent decreased need for muscular power to neutralise the
effects of gravity permit of an increase in bulk. Purely terrestrial
animals come next; and finally arboreal, and, still more, "flying" mammals
are of small size, since the maintenance of the position when moving and
feeding needs enormous muscular effort.
The Mammals are more easily to be separated from the Vertebrates lying
lower in the series than any of the latter are from each other in ascending
order. A large number of {3} characters might be used in addition to those
which will be made use of in the following brief catalogue of essential
mammalian features, were it not for the low-placed Monotremata on the one
hand and the highly specialised Whales on the other. Including those forms,
the Mammalia are to be distinguished from all other Vertebrates by the
following series of structural features, which will be expanded later into
a short disquisition upon the general structure of the Mammalia. The class
Mammalia may, in fact, be thus defined:--
Hair-clad Vertebrates, with cutaneous glands in the female, secreting milk
for the nourishment of the young. Skull without prefrontal, postfrontal,
quadrato-jugal, and some other bones, and with two occipital condyles
formed entirely by the exoccipitals. Lower jaw composed of dentary bone
only, articulating only with the squamosal. Ear bones a chain of three or
four separate bonelets. Cervical vertebrae sharply distinguished from the
dorsals, and if with free ribs, showing no transition between these and the
thoracic ribs. Brain with four optic lobes. Lungs and heart separated from
abdominal cavity by a muscular diaphragm. Heart with a single left aortic
arch. Red blood-corpuscles non-nucleate.
The following characters are also very nearly universal, and in any case
absolutely distinctive:--Cervical vertebrae, seven; vertebrae with
epiphyses. Ankle-joint "cruro-tarsal," _i.e._ between the leg and the
ankle, and not in the middle of the ankle.[1] Attachment of the pelvis to
the vertebral column pre-acetabular in position.
The Mammalia since they are hot-blooded creatures are more independent of
temperature than reptiles; they are thus found spread over a wider area of
the earth's surface. As however, though hot-blooded, they have not the
powers of locomotion possessed by birds, they are not quite so widely
distributed as are those animals. The Mammalia range up into the extreme
north, but, excepting only forms mainly aquatic, such as the Sea Lions, are
not known to occur on the Antarctic continent. With the exception of the
flying Bats, indigenous mammals are totally absent from New Zealand; and it
seems to be doubtful whether those supposed oceanic islands which have a
mammalian fauna are really {4} oceanic in origin. The continents and oceans
are peopled by rather over three thousand species of Mammalia, a number
which is considerably less than that of either birds or reptiles. It seems
clear that, so far at any rate as concerns the numbers of families and
genera, the mammalian fauna of to-day is less varied than it was during the
Mid-tertiary period, the heyday of mammalian life. It is rather remarkable
to contrast in this way the mammals and the birds. The two classes of the
animal kingdom seem to have come into being at about the same period; but
the birds either have reached their culminating point to-day, or have not
yet reached it. The Mammalia, on the other hand, multiplied to an
extraordinary extent during the Eocene and the Miocene periods, and have
since dwindled. The break is most marked at the close of the Pleistocene,
and may be in part due to the direct influence of man. At present man
exercises so enormous an effect, both directly and indirectly, that the
future history of the Mammalia is probably foreshadowed by the instances of
the White Rhinoceros and the Quagga. On the other hand, the economic
usefulness of the Mammalia is greater than that of any other animals; and
the next most important era in their history will be probably that of
domesticity and "preservation."
* * * * *
{5}
CHAPTER II
STRUCTURE AND PRESENT DISTRIBUTION OF THE MAMMALIA
EXTERNAL FORM.--It would be quite impossible for any one to confuse any
other quadrupedal animal with a mammal. The body of a reptile is, as it
were, slung between its limbs, like the body of an eighteenth century
chariot between its four wheels; in the mammal the body is raised entirely
above, and is supported by, the four limbs. The axes of these limbs too, as
a general rule, are parallel with the vertical axis of the body of their
possessor. There is thus a greater perfection of the relations of the limbs
to the trunk from the point of view of a terrestrial creature, which has to
use those limbs for rapid movement. The same perfection in these relations
is to be seen, it should be observed, in such running forms among the lower
Vertebrata as the Birds and the Dinosaurs, where the actual angulation of
the limbs is as in the purely running Mammalia. These relations are of
course absolutely lost in the aquatic Cetacea, and not marked in various
burrowing creatures. The way in which the fore- and hind-limbs are
angulated is considerably different in the two cases. In the latter, which
are most used and, as it were, push on the anterior part of the body, the
femur has its lower end directed forwards, the tibia and the fibula project
backwards at the lower end, while the ankle and foot are again inclined in
the same direction as the femur. With the fore-limbs there is not this
regular alternation. The humerus is directed backwards, the fore-arm
forwards, and the hand still more forwards. This angulation seems to
facilitate movement, inasmuch as it is seen in even the Amphibia and the
lower Reptiles, in which, however, the differences between the fore- and
hind-limbs are less marked, {6} indicating therefore a less specialised
condition of the limbs. It is an interesting fact that the angulation of
the limbs is to some extent obliterated in very bulky creatures, and almost
entirely so in the elephants (see p. 217), which seem to need strong and
straight pillars for the due support of their huge bodies.
The alertness and general intellectual superiority of mammals to all
animals lying below them in the series (with the exception of the birds,
which are in their way almost on a level with the Mammalia) are seen by
their active and continuous movements. The lengthy periods of absolute
motionlessness, so familiar to everybody in such a creature as the
Crocodile, are unknown among the more typical Mammalia except indeed during
sleep. This mental condition is clearly shown by the proportionate
development of the external parts of all the organs of the higher senses.
The Mammalia as a rule have well-developed, often extremely large, flaps of
skin surrounding the entrance to the organ of hearing, often called "ears,"
but better termed "pinnae." These are provided with special muscles, and
can be often moved and in many directions. The nose is always, or nearly
always, very conspicuous by its naked character; by the large surface,
often moist, which surrounds the nostrils; and again by the muscles, which
enable this tract of the integument to be moved at will. The eyes, perhaps,
are less marked in their predominance over the eyes of lower Vertebrates
than are the ears and nose; but they are provided as a rule with upper and
lower eyelids, as well as by a nictitating membrane as in lower
Vertebrates. The apparent predominance of the senses of smell and hearing
over that of sight appears to be marked in the Mammalia, and may account
for their diversity of voice as well as of odour, and for the general
sameness of coloration which distinguishes this group from the
brilliantly-coloured birds and reptiles. The head, too, which bears these
organs of special sense, is more obviously marked out from the neck and
body than is the case with the duller creatures occupying the lower
branches of the Vertebrate stem.
[Illustration]
FIG. 1.--A, Section of human skin. _Co_, Dermis; _D_, sebaceous glands;
_F_, fat in dermis; _G_, vessels in dermis; _GP_, vascular papillae; _H_,
hair; _N_, nerves in dermis; _NP_, nervous papillae; _Sc_, horny layer of
epidermis; _SD_, sweat gland; _SD_^1, duct of sweat gland; _SM_, Malpighian
layer. B, Longitudinal section through a hair (diagrammatic). _Ap_, Band of
muscular fibres inserted into the hair-follicle; _Co_, corium (dermis);
_F_, external longitudinal; _F_^1, internal circular, fibrous layer of
follicle; _Ft_, fatty tissue in the dermis; _GH_, hyaline membrane between
the root-sheath and the follicle; _HBD_, sebaceous gland; _HP_,
hair-papilla with vessels in its interior; _M_, medullary substance (pith)
of the hair; _O_, cuticle of root-sheath; _R_, cortical layer; _Sc_, horny
layer of epidermis; _Sch_, Hair shaft; _SM_, Malpighian layer of epidermis;
_WS_, _WS_^1, outer and inner layers of root-sheath. (From Wiedersheim's
_Comparative Anatomy_.)
[Illustration]
THE HAIR.--The Mammalia are absolutely distinguished from all other
Vertebrates (or, for the matter of that, Invertebrates) by the possession
of hair. To define a mammal as a Vertebrate with hair would be an entirely
exclusive definition; even in the smooth Whales a few hairs at least are
present, which may be {7} reduced to as few as two bristles on the lips.
The term "hair," however, is apt to be somewhat loosely applied; it has
been made use of to describe, for example, the slender processes of the
chitinous skin of the Crustacea. It will be necessary, therefore, to enter
into the microscopical structure and development of the mammalian hair.
Hair is found in every mammal. The first appearance of a hair is a slight
thickening of the stratum Malpighii of the epidermis, the cells taking part
in this being {8} elongated and converging slightly above and below. Dr.
Maurer has called attention to the remarkable likeness between the
embryonic hair when at this stage and the simple sense-organs of lower
Vertebrates. Later there is formed below this a denser aggregation of the
corium, which ultimately becomes the papilla of the hair. This is the
apparent homologue of the first formed part of a feather, which projects as
a papilla before the epidermis has undergone any modification. Hence there
is from the very first a difference between feathers and hairs--a
difference which must be carefully borne in mind, especially when we
consider the strong superficial resemblance between hairs and the simple
barbless feathers. Still later the knob of epidermic cells becomes
depressed into a tubular structure, which is lined with cells also derived
from the stratum Malpighii, but is filled with a continuation of the more
superficial cells of the epidermis. This is the hair-follicle, and from the
epidermic cells arises the hair by direct metamorphosis of those cells;
there is no excretion of the hair by the cells, but the cells become the
hair. From the hair-follicle also grows out a pair of sebaceous glands,
which serve to keep the fully-formed hair moist.
[Illustration]
FIG. 2.--Four diagrams of stages in the development of a hair. A, Earliest
stage in one of those mammals in which the dermal papilla appears first; B,
C, D, three stages in the development of the hair in the human embryo.
_blb_, Hair-bulb; _crn_, horny layer of the epidermis; _foll_,
hair-follicle; _grm_, hair-germ; _h_, hair, in D, projecting on the
surface; _muc_, Malpighian layer of epidermis; _pp_, dermal papilla; _seb_,
developing sebaceous glands; _sh.1_, _sh.2_, inner and outer root-sheaths.
(After Hertwig.)
Dr. Meijerle[2] has lately described in some detail the {9} particular
arrangement of the individual hairs among mammals; they are not by any
manner of means scattered without order, but show a definite and regular
arrangement, which varies with the animal. For instance, in an American
Monkey (_Midas_), the hairs arise in threes--three hairs of equal size
springing from the epidermis close together; in the Paca (_Coelogenys_)
there are in each group three stout hairs alternating with three slender
hairs. In some forms a number of hairs spring from a common point: in the
Jerboa (_Dipus_) twelve or thirteen arise from a single hole; in _Ursus
arctos_ there is the same general plan, but there is one stout hair and
four or five slender ones. There are numerous other complications and
modifications, but the facts, although interesting, do not appear to throw
any light upon the mutual affinities of the animals. Allied forms may have
a very different arrangement, while in forms which have no near
relationship the plan may be very similar, as is shown by the examples
cited from Dr. Meijerle's paper. The groups of hairs, moreover, have
themselves a definite placing, which the same anatomist has compared with
the disposition of the bundles of hairs behind and between the scales of
the Armadillo, and which has led him to the view that the ancestors of
mammals were scaly creatures--a view also supported by Professor Max
Weber,[3]and not in itself unreasonable when we consider the numerous
points of affinity between the primitive Mammalia and certain extinct forms
of reptiles.[4]
The hairs are greatly modified in form in different mammals and in
different parts of their bodies. It is very commonly the case that a soft
under-fur can be distinguished from the longer and coarser hairs, which to
some extent hide the latter. Thus the "sealskin" of commerce is the
under-fur of the _Otaria ursina_ of the North. The coarser hairs may be
further differentiated into bristles; these again into spines, such as
those of the Hedgehog and of the Porcupine. Again, the flattening and
agglutination of hairs seems to be responsible for the scales of the
_Manis_ {10} and for the horns of the _Rhinoceros_. It is a matter of
common knowledge that upon the head of various animals, _e.g._ the Domestic
Cat, long and sensitive hairs are developed, which are connected with the
terminations of nerves, and perform a sensory, probably tactile function.
These occur on the snout, above the eyes, and in the neighbourhood of the
ears. It is an interesting fact that a tuft of quite similar hairs occurs
on the hand of many mammals close to the wrist, which, at least in the case
of _Bassaricyon_, are connected with a strong branch from the arm-nerve.
These tufts also occur in Lemurs, in the Cat, various Rodents and
Marsupials, and are probably quite general in mammals who "feel" with their
fore-limbs;--in which, in fact, the fore-limbs are not exclusively running
organs. That the last remaining hairs of the Cetacea are found upon the
muzzle, is perhaps significant of the importance of these sensory bristles.
The entire absence of hairs is quite common in this order, although traces
of them are sometimes found in the embryo. The Sirenia, too, are
comparatively hairless, as are also many Ungulates. Whether the presence of
blubber in the former case and the existence of a very thick skin in the
latter animals are facts which have had anything to do with the
disappearance of hair or not, is a matter for further inquiry.
The intimate structure of the hair varies considerably. The variations
concern the form of the hair, which may be round in transverse section, or
so oval as to appear quite flat when the hair is examined in its entirety.
The substance of the hair is made up of a central medulla or pith with a
peripheral cortex; the latter is scaled, and the scales are often
imbricated and with prominent edges. The amount of the two constituents
also differs, and the cortex may be reduced to a series of bands
surrounding only tracts of the enclosed pith. In the hair is contained the
pigment to which the colour of mammals is chiefly due. Tracts of
brightly-coloured skin may exist, as in the Apes of certain genera; but
such structures are not general. The pigment of the hair seems to consist
of those pigmentary substances known as melanins. It is remarkable to find
such a uniform cause of coloration, when we consider the great variety of
feather-pigments found in birds. The variations of colour of the hair of
mammals are due to the unequal distribution of these brown pigments. There
are very few mammals which can {11} be called brightly coloured. The Bats
of the genus _Kerivoula_ have been compared to large butterflies, and some
of the Flying Squirrels have strongly-marked contrasts of reddish brown,
white, and yellow. The same may be said of the spines of certain
Porcupines. But we find in the hair no bright blues, greens, and reds such
as are common among birds.
There are certain general facts about the coloration of mammals which
require some notice here. Next to the usually sombre hues of these animals
the general absence of secondary sexual coloration is noteworthy. In but a
few cases among the Lemurs and Bats do we find any marked divergences in
hues between males and females. Secondary sexual characters in mammals are,
it is true, often exhibited by the great length of certain hair-tracts in
the male, such as the mane of the Lion, the throat- and leg-tufts of the
Barbary Sheep, and so forth; but apart from these, the secondary sexual
characters of mammals are chiefly shown in size, _e.g._ the Gorilla, or in
the presence of tusks, _e.g._ various Boars, or of horns, as in the Deer,
etc. The coloration of mammals frequently exhibits conspicuous patterns of
marking. These are in the form of longitudinal stripes, of cross-stripes,
or of spots; the latter may be "solid" spots, or broken up, as in the
Leopard and Jaguar, into groups of smaller spots arranged in a
rosette-fashion. We never find in mammals the complicated "eyes" and other
markings which occur in so many birds and in other lower Vertebrates. It is
important to note that in the Mammalia whose sense of sight is quite keen
there should be a practical absence of secondary sexual colours. As to the
relationship of the various forms of marking that do occur, it seems clear
that there has been a progression from a striped or spotted condition to
uniform coloration. For we find that many Deer have spotted young; that the
young Tapir of the New World is spotted, while its parents are uniform
blackish brown; the strongly-marked spotting of the young Puma contrasts
with the uniform brown of the adult; and the Lion cub, as every one knows,
is also spotted, the adult lioness showing considerable traces of the
spots.
The seasonal change in the colours of certain mammals is a subject upon
which much has been written. The extreme of this is seen in those
creatures, such as the Polar Hare and the Arctic Fox, which become entirely
blanched in the winter, recovering {12} their darker coat in the spring.
This is, however, only an extreme case of a change which is general. Most
animals get a thicker fur in winter and exchange it for a lighter one in
summer. And the hues of the coat change in correspondence.
GLANDS OF THE SKIN.--The great variety of integumental glands possessed by
the Mammalia distinguishes them from any group of lower Vertebrates. This
variability, however, only concerns the anatomical structure of the glands
in question. Histologically they are all of them apparently to be referred
to one of two types, the sudoriparous or sweat gland and the sebaceous
gland. Simple sweat and sebaceous glands are abundant in mammals, with but
a few exceptions. The structures that we are now concerned with are
agglomerations of these glands. The mammary glands will be treated of in
connexion with the marsupium; they are either masses of sweat glands, or of
sebaceous glands whose secretion has been converted into milk.
Many Carnivora possess glands opening to the exterior, near the anus, by a
large orifice. These secrete various odoriferous substances, of which the
well-known "civet" is an example. Other odoriferous glands are the musk
glands of the Musk-deer and of the Beaver; the suborbital gland of many
Antelopes; the dorsal gland of the Peccary, which has given the name of
_Dicotyles_ to the genus on account of its resemblance in form to a navel.
This gland may be seen to secrete a clear watery fluid. The Elephant has a
gland situated on the temple, which is said to secrete during certain
periods only, and to be a warning to leave the animal alone. Very
remarkable are the foot glands of certain species of _Rhinoceros_; they are
not universally present in those animals, and are therefore useful as
specific distinctions. On the back of the root of the tail in many Dogs are
similar glands. The Gentle Lemur (_Hapalemur_) has a peculiar gland upon
the arm, about the size of an almond, which in the male underlies a patch
of spiny outgrowths. In _Lemur varius_ is a hard patch of black skin which
may be the remnants of such a gland. It is thought that the callosities on
the legs of Horses and Asses are remnants of glands.
One of the most complex of these structures which has been examined
microscopically exists in the Marsupial _Myrmecobius_.[5] On the skin of
the anterior part of the chest, just in front of the {13} sternum, is a
naked patch of skin which is seen to be perforated by numerous pores.
Besides the ordinary sebaceous and sweat glands there are a series of
masses of glands, opening by larger orifices, which present the appearance
of groups of sebaceous glands, and are of a racemose character; but the
existence of muscular fibres in their coats seems to show that they should
be referred rather to the sudoriparous series. Beneath the integument is a
large compound tubular gland quite half an inch in diameter.
In _Didelphys dimidiata_ there is a precisely similar glandular area and
large underlying gland, the correspondence being remarkable in two
Marsupials so distant in geographical position and affinities. Even among
the Diprotodont genera there is something of the kind; for in _Dorcopsis
luctuosa_ and _D. muelleri_ is a collection of four unusually large
sebaceous follicles upon the throat, and in the Tree Kangaroo (_Dendrolagus
bennettii_) there is the same collection of enlarged hair-follicles, though
they are apparently somewhat reduced as compared with those of _Dorcopsis_.
These are of course a few examples out of many.
It seems to be possible that the functions of these various glands is at
least twofold. In the first place, they may serve, where predominant in one
sex, to attract the sexes together. In the second place, the glands may be
useful to enable a strayed animal of a gregarious species to regain the
herd. It is perfectly conceivable too that in other cases the glands may be
a protection, as they most undoubtedly are in the Skunk, from attacks. In
connexion with the first, and more especially the second, of the possible
uses of these glands, it is interesting to note that in purely terrestrial
creatures, such as the Rhinoceros, the glands are situated on the feet, and
would therefore taint the grass and herbage as the animal passed, and thus
leave a track for the benefit of its mate. The same may be said of the
rudimentary glands of Horses if they are really glands. The secretion of
the "crumen" of Antelopes is sometimes deposited deliberately by
_Oreotragus_ upon surrounding objects, a proceeding which would attain the
same end. One may even perhaps detect "mimicry" in the similar odours of
certain animals. Prey may be lured to their destruction, or enemies
frightened away. The defenceless Musk-deer may escape its foes by the
suggestion of the musky odour of a crocodile. It is at any rate perfectly
conceivable that the variety of odours among mammals may play a very {14}
important part in their life, and it is perhaps worthy of note that birds
with highly-variegated plumage are provided only with the uropygial gland,
while mammals with usually dull and similar coloration have a great variety
of skin glands. Scent is no doubt a sense of higher importance in mammals
than in birds. The subject is one which will bear further study.
NAILS AND CLAWS.--Except for the Cetacea (where rudiments have been found
in the foetus), the extremities of the fingers and of the toes of mammals
are covered by, or encased in, horny epidermic plates, known as nails,
claws, and hoofs.
The variety in the shape and development of these corneous sheaths to the
digits is highly characteristic of mammals as opposed to lower Vertebrates.
If we take extreme cases, such as the nail of the thumb in Man, the hoof of
a Horse, and the claw of a Cat, it is easy to distinguish the three kinds
of phalangeal horny coverings. But the differences become extinguished as
we pass from these to related types. The nail of the little finger in Man
approaches the claw-like form; and the hoofs of the Lama are almost claws
in the sharpness of their extremities. On the whole it may be said that
claws and hoofs embrace the bone which they cover, while nails lie only
upon its dorsal surface. The form of the distal phalanx which bears the
nail shows, however, two kinds of modification which do not support such a
classification. When those phalanges are clad with hoofs or covered by a
nail they end in a rounded and flattened termination. On the other hand,
when they bear a claw they are themselves sharpened at the extremity and
often grooved above.
THE MARSUPIUM.--It may appear to be unnecessary at this juncture to speak
of the marsupial pouch, which is so usually believed to be a characteristic
of the group Marsupialia. Rudiments of this structure have, however, been
recently discovered in the higher mammals, and, as Dr. Klaatsch[6] has
remarked, all researches into the "history of the mammals culminate in the
question whether the placental mammals pass through a marsupial stage or
not." We cannot, therefore, look upon the marsupial pouch as a matter
affecting only the Marsupials, though it is true that this organ is at
present functional only in them and in the Monotremata. {15}
[Illustration] FIG. 3.--_Echidna hystrix._ A, Lower surface of brooding
female; B; dissection showing a dorsal view of the pouch and mammary
glands; ++, the two tufts of hair in the lateral folds of the mammary pouch
from which the secretion flows, _b.m_, Pouch; _cl_, cloaca; _g.m_, groups
of mammary glands. (From Wiedersheim's _Comparative Anatomy_, after W.
Haacke.)
In the Marsupials the pouch shelters the young, which are born in an
exceedingly imperfect state, minute, nude, and blind, with a "larval" mouth
fitted only to grasp in a permanent fashion the teat, upon which they are
carefully fixed by the parent. But even later the pouch is made use of as a
temporary harbour of refuge: from the pouch of female Kangaroos at the
Zoological Gardens may frequently be observed to protrude the tail and
hind-legs of a young Kangaroo as big as a Cat, and perfectly well able to
take care of itself.
In the Monotremata (in _Echidna_) there is a deep fold of the skin which
lodges the unhatched egg, and into which the mammary glands open, one on
either side. This structure is only periodically developed, and arises from
two rudiments, one corresponding to each mammary area; but in the female
with eggs or young there is but a single deep depression, which occupies
the same region of the body as the marsupial pouch of the {16}
Marsupials.[7] It is usually held that this structure is not of precisely
the same morphological value as the pouch of the Marsupial; and the
difference is expressed by terming the one (that of _Echidna_) the mammary
pouch, and the other the marsupium. At first sight it may appear to be an
unnecessary refinement to separate two structures which have so many and
such obvious likenesses. It is not quite certain, however, that the
difference is not even more profound than later opinions seem to indicate.
The Monotremata not only have no teats, as has already been pointed out,
but the mammary glands themselves are of a perfectly different nature to
those of the higher mammals, including the Marsupials. There is therefore
no _a priori_ objection to the view that the accessory parts developed in
connexion with the mammary glands should also be different. The teat of the
higher Mammalia grows up round the area upon which the ducts of the mammary
glands open; it is a fold of skin which eventually assumes the cylindrical
form of the adult teat, and which includes the ducts of the milk glands. It
has been suggested that the two folds of skin which form the mammary pouch
of _Echidna_ are to be looked upon as the equivalent of the commencing teat
of the higher mammal.[8] In this case it is clear that the marsupial folds
of the Marsupial cannot correspond accurately with the apparently similar
folds of _Echidna_, because there are teats as well. It is the teats which
correspond to the marsupial folds of _Echidna_. This view is in apparent
contradiction to an interesting discovery in a specimen of a Phalanger by
Dr. Klaatsch.[9] This Marsupial, like most others, has a well-developed
marsupial pouch, in which the young are lodged at birth; but round two of
the teats is another distinct fold on either side, the outer wall of which
forms the general wall of the pouch. Dr. Klaatsch thinks that these smaller
and included pouches are the equivalents of the mammary pouches of
_Echidna_. They contain teats, but this comparison does not do away with
the validity of Gegenbaur's suggestion already referred to, because the
teats are (see above) {17} secondary. If this fact be fairly to be
interpreted in the sense which Dr. Klaatsch attaches to it, we have an
interesting case of the growth of a new organ out of and partly replacing
an old organ. In the Monotremes there is a pouch which facilitates or
performs both nutritive and protective functions; in the Phalanger these
two functions are carried on in separate pouches; finally, in other
Marsupials, there is a return to the undifferentiated state of affairs
found in the Monotremata, but with the help of a new organ not found in
them.
[Illustration]
FIG. 4.--Diagram of the development of the nipple (in vertical section). A,
Indifferent stage, glandular area flat; B, elevation of the glandular area
with the nipple; C, elevation of the periphery of the glandular area into
the false teat, _a_, Periphery of the glandular area; _b_, glandular area;
_gl_, glands. (From Gegenbaur.)
Though so characteristic of Marsupials, the marsupial pouch is not always
developed in them. It is present in all the Kangaroos, Wallabies, and
Wombats, in fact in the Diprotodonts. It is also present in a number of the
carnivorous Polyprotodont Marsupials; but in _Phascologale_ it is only
present in rudiment, and in _Myrmecobius_ it is entirely obsolete. In the
American Opossums the state of the pouch is variable. "Generally absent,
sometimes merely composed of two lateral folds of skin separate at each
end, rarely complete," is Mr. Thomas' summary in his definition of the
family Didelphyidae.[10] Another curious feature of the pouch in the
Marsupials is the variability in the position of the mouth of the pouch: in
all the Diprotodonts it looks forward; but in many Polyprotodonts it looks
backward. This, however, has some connexion with the habitual attitude of
the possessor: in the Kangaroo, leaping along on its hind-legs, it is
requisite that the pouch should open forwards; but in the dog-like
Thylacine, going on all fours, the fact that the pouch {18} opens backwards
is less disadvantageous to the contained young.
The male Thylacine has a pouch which is quite or very nearly as well formed
as in the female. There are also rudiments of a pouch in the male foetuses
of many Marsupials, especially of those belonging to the Polyprotodont
section of the order, though these rudiments are by no means confined to
that subdivision. Up to so late a period as the age of four months (length
19.8 cm.) the male _Dasyurus ursinus_ has a pouch.
We have now to consider the interesting series of facts relative to the
permanence--in a rudimentary condition it is true--of the mammary pouch in
the higher Mammalia, facts which seem to be an additional proof that they
have been derived from an ancestor in which the pouch was an organ of
functional importance. The first definite proof of the occurrence of a
pouch in any mammal not a Marsupial or a Monotreme was made by Malkmus, who
found this structure in a Sheep. It seems, however, that the structures
found in the higher mammals are not always comparable to the marsupium of
the Marsupials, but sometimes to the mammary pouch of the Monotreme. That
the Marsupials are a side line, and not involved in the ancestry of the
Eutheria, is an opinion which is at present widely held. At the same time
it is reasonable to suppose that the original stock lying between the
Prototheria and the Metatheria, whence the latter and the Eutheria have
arisen, preserved both the mammary pouch of the lower mammal and the
marsupium of the further-developed stage, as does _Phalangista_
occasionally at the present day. Hence to find remnants of both structures
in existing mammals would not so incredible. This is what Dr. Klaatsch
believes to be the case. In certain Ungulates, including two species of
Antelope, Dr. Klaatsch found very considerable rudiments of folds provided
with unstriated muscular fibre; there were in the adult _Cervicapra
isabellina_ a pair of pouches, one on each side, and a rudiment of a second
on either side; possibly this multiplication of the pouches has relation to
the number of young. That there is more than one pouch makes a comparison
with the mammary pouch rather than with the marsupium probable. The
Ungulate teat, it must be remembered (see p. 16), is a secondary teat;
hence there is no difficulty in the comparison from this point of view. A
pouch containing a primary {19} teat would of course be absolutely
incomparable with a mammary pouch, because in that case the wall of the
teat itself would be the pouch.
Mammals belonging to quite different Orders show traces more or less marked
of a marsupium. In young Dogs the teats are borne upon an area where the
skin is thinner, the covering of hair less dense than elsewhere--all points
of resemblance to the inside of the pouch of a Marsupial; in addition to
this there are traces of the sphincter marsupii muscle. In other Carnivora
there are similar vestiges. In _Lemur catta_ a more complete rudiment of a
marsupial pouch is to be met with. In this Lemur the teats are both
inguinal and pectoral; the skin in these regions is thin and but slightly
hairy, and extends forwards as two bands of the same thinness and
smoothness on each side of the densely hairy skin covering the sternum.
This area is sharply separated from the rest of the integument by a fold
which runs parallel to the longitudinal axis of the body, and can be
comparable with nothing save the rudiment of the marsupial fold.
One is tempted to wonder how far the habit which certain Lemurs have of
carrying their young across the abdomen with the tail wrapped round the
body of the mother is a reminiscence of a marsupial pouch.
SKELETON.
The skeleton of the Mammalia consists almost solely of the endoskeleton. It
is only among the Edentata that an exoskeleton of bony plates in the skin
is met with. As in other Vertebrates, the skeleton is divisible into an
axial portion, the skull and vertebral column, and an appendicular
skeleton, that of the limbs. The bones of mammals are well ossified, and in
the adult there are but few and small tracts of cartilage left.
VERTEBRAL COLUMN.--The vertebral column of the mammals, like that of the
higher Vertebrata, consists of a number of separate and fully-ossified
vertebrae.
The constitution of a vertebra upon which all the usual processes are
marked is as follows:--There is first of all the body or centrum of the
vertebra, a massive piece of bone shaped like a disc or a cylinder. The
centra of contiguous vertebrae {20} are separated by a certain amount of
fibrous tissue forming the intervertebral disc, and the apposed surfaces of
the centra are as a rule nearly flat. In this last feature, and in the
important fact that the centra are ossified from three distinct centres,
the anterior and posterior pieces ("epiphyses") remaining distinct for a
time, even for a long time (as in the Whales), the centra in the mammals
differ from those of reptiles and birds. The epiphyses are not found
throughout the vertebral column of the lowly-organised Monotremata, and
they do not appear to exist in the Sirenia.
[Illustration]
[Illustration]
FIG. 5.--Anterior surface of Human thoracic vertebra (fourth), x 2/3. _az_,
Anterior zygapophysis; _c_, body or centrum; _l_, lamina, and _p_, pedicle,
of the neural arch; _nc_, neural canal; _t_, transverse process. (From
Flower's _Osteology of the Mammalia_.)
FIG. 6.--Side view of first lumbar vertebra of Dog (_Canis familiaris_). x
3/4. _a_, Anapophysis; _az_, anterior zygapophysis; _m_, metapophysis;
_pz_, posterior zygapophysis; _s_, spinous process; _t_, transverse
process. (From Flower's _Osteology_.)
From each side of the centrum on the dorsal side arises a process of bone
which meets its fellow in the middle line above, and is from there often
prolonged into a spine. A canal is thus formed which lodges the spinal
cord. This arch of bone is known as the neural arch, and the dorsal process
of the same as the spinous process. The sides of the neural arch bear oval
facets, by which successive vertebrae articulate with one another: those
situated anteriorly are the anterior zygapophyses, while those on the
posterior aspect of the arch are the posterior zygapophyses; these
articular facets do not exist in the tail-region of many mammals, _e.g._
Whales.
In addition to the dorsal median spinous process of the {21} vertebra there
may be a ventral median process, arising of course from the centrum, termed
the hypapophysis.
From the sides of the neural arch, or from the centrum itself, there is
commonly a longer or shorter process on each side, known as the transverse
process. This is sometimes formed of two distinct processes, one above the
other; in such cases the upper part is called a diapophysis, the lower a
parapophysis.
The neural arch may also bear other lateral processes, of which one
directed forwards is the metapophysis, the other directed backwards the
anapophysis.
The series of bones which constitute the vertebral column can be divided
into regions. It is possible to recognise cervical, dorsal, lumbar, sacral,
and caudal vertebrae. In the case of animals with only rudimentary
hind-limbs, such as the Whales, there is no recognisable sacral region. The
neck or cervical vertebrae are nearly always seven in number. The
well-known exceptions are the Manatee, where there are six, and certain
Sloths, where there are six, eight, or nine. These rare exceptions only
accentuate the very remarkable constancy in number, which is very
distinctive of the mammals as compared with lower Vertebrata. There are of
course abnormalities, the last cervical, and sometimes the last two,
assuming the characters of the ensuing dorsals, by developing a more or
less complete rib. There are also recorded examples of _Bradypus_, in which
the number of cervicals is increased to ten. The characteristics, then, of
the cervical vertebrae are, in the first place, that they do not normally
bear free ribs, and that there is a break as a rule between the last
cervical and the first dorsal on this account. In birds, for example, the
cervicals, differing in number in different families and genera, gradually
approach the dorsals by the gradually lengthening ribs. The transverse
processes of the vertebrae are commonly perforated by a canal for the
vertebral artery, and are bifid at their extremities. In some Ungulates
these vertebrae, moreover, approximate to the vertebrae of lower Vertebrata
in the fact that there are ball and socket joints between the centra,
instead of only the fibrous discs of the remaining vertebrae.
The first two vertebrae of the series are always very different from those
which follow. The first is termed the {22} atlas, and articulates with the
skull. The most remarkable fact about this bone (shared, however, by lower
Vertebrates) is that its centrum is detached from it and attached to the
next vertebra, in connexion with which it will be referred to immediately.
The whole bone thus gets a ring-like form, and the salient processes of
other vertebrae are but little developed, with the exception of the
transverse processes, which are wide and wing-like. In many Marsupials,
such as the Wombat and Kangaroo, the arch of the atlas is open below, there
being no centre of ossification. In others, such as _Thylacinus_, there is
a distinct nodule of bone in this situation not concrescent with the rest
of the arch.
[Illustration]
[Illustration]
FIG. 7.--Human atlas (young), showing development. x 3/4. _as_, Articular
surface for occiput; _g_, groove for first spinal nerve and vertebral
artery; _i a_, inferior arch; _t_, transverse process. (From Flower's
_Osteology_.)
FIG. 8.--Inferior surface of atlas of Dog. x 1/2. _sn_, Foramen for first
spinal nerve; _v_, vertebrarterial canal. (From Flower's _Osteology_.)
[Illustration]
FIG. 9.--Atlas of Kangaroo.... (From Parker and Haswell's _Zoology_.)
The second vertebra, which is known as the axis or epistropheus, is a
compound structure, the anterior "odontoid process," which fits into the
ring of the atlas, being in reality the detached centrum of that
vertebra.[11] It is a curious fact about that process that it has
independently become spoon-shaped in two divisions of Ungulates; that it
has become so seems to be shown by the fact that in the earlier types of
both it has the simple peg-like form, which is the prevailing form. The
cervical {23} vertebrae are occasionally wholly (Right Whales) or partially
(many Whales, Jerboa, certain Edentates) welded into a combined mass.
Indications of this have even been recorded in the human subject.
[Illustration]
[Illustration]
FIG. 10.--Side view of axis of Dog. x 2/3. _o_, Odontoid process; _pz_,
posterior zygapophysis; _s_, spinous process; _t_, transverse process; _v_,
vertebrarterial canal. (From Flower's _Osteology_.)
FIG. 11.--Anterior surface of axis of Red Deer. x 2/3. _o_, Odontoid
process; _pz_, posterior zygapophysis; _sn_, foramen for second spinal
nerve. (From Flower's _Osteology_.)
The dorsal vertebrae vary greatly in number: nine (_Hyperoodon_) seems to
be the lowest number existing normally; while there may be as many as
nineteen, as in _Centetes_, or twenty-two, as in _Hyrax_. These vertebrae
are to be defined by the fact that they carry ribs, and the first one or
two lumbars are often "converted into" dorsals by the appearance of a small
supernumerary rib. The spinous processes of these vertebrae are commonly
long, and sometimes very long. It is only among the Glyptodons that any of
these vertebrae are fused together into a mass.
The lumbar vertebrae, which follow the dorsal, vary greatly in number.
There are as few as two in the whale _Neobalaena_, as many as seventeen in
_Tursiops_; this group, the Cetacea, contains the extremes. Nine lumbars
are found in the Lemurs _Indris_ and _Loris_. As a rule the number of
lumbars is to some extent dependent upon that of the dorsals. It often
happens that the number of thoraco-lumbar vertebrae is constant for a given
group. Thus the Artiodactyles have nineteen of these vertebrae, and the
Perissodactyles as a rule twenty-three. A greater number of dorsals implies
a smaller number of lumbars, and of course _vice versa_. The existence of a
sacral region formed of a {24} number of vertebrae fused together and
supported by the pelvic girdle is characteristic of the mammals, but is not
found in the Cetacea and the Sirenia, where functional hind-limbs are
wanting. Strictly speaking, the sacrum is limited to the two or three
vertebrae whose expanded transverse processes meet the ilia. But to these
are or may be added a variable number of vertebrae withdrawn from both the
lumbar and the caudal series, which unite with each other to form the
massive piece of bone which constitutes the sacrum of the adult.
[Illustration]
[Illustration]
FIG. 12.--_Lepus cuniculus_. Innominate bones and sacrum, ventral aspect.
_acet_, Acetabulum; _il_, ilium; _isch_, ischium; _obt_, obturator foramen;
pub, pubis; _sacr_, sacrum; _sy_, symphysis. (From Parker and Haswell's
_Zoology_.)
FIG. 13.--Anterior surface of fourth caudal vertebra of Porpoise (_Phocoena
communis_), x 1/2. _h_, Chevron bone; _m_, metapophysis; _s_, spinous
process; _t_, transverse process. (From Flower's _Osteology_.)
The caudal vertebrae complete the series. They begin in as fully developed
a condition as the lumbars, with well-marked transverse processes, etc.;
but they end as no more than centra, from which sometimes tiny outgrowths
represent in a rudimentary way the neural arches, etc. Very often the
caudal vertebrae are furnished with ventral, generally [12]-shaped,
appendages, the chevron bones or intercentra.[13] These are {25}
particularly conspicuous in the Whales and in the Edentates. In the former
group the occurrence of the first intercentrum serves to mark the
separation of the caudal from the lumbar series. The number of caudals
varies from three in Man--and those quite rudimentary--to nearly fifty in
_Manis macrura_ and _Microgale longicaudata_.
[Illustration]
FIG. 14.--Lateral view of skull of a Dog. _C.occ_, Occipital condyle; _F_,
frontal; _F.inf_, infra-orbital foramen; _Jg_, jugal; _Jm_, premaxilla;
_L_, lachrymal; _M_, maxilla; _Maud_, external auditory meatus; _Md_,
mandible; _N_, nasal; _P_, parietal; _Pal_, palatine; _Pjt_, process of
squamosal; _Pt_, pterygoid; _Sph_, alisphenoid; _Sq_, squamosal; _Sq.occ_,
supraoccipital; _T_, tympanic. (From Wiedersheim's _Comparative Anatomy_.)
THE SKULL.--The skull in the Mammalia differs from that of the lower
Vertebrata in a number of important features, which will be enumerated in
the following brief sketch of its structure. In the first place, the skull
is a more consolidated whole than in reptiles; the number of elements
entering into its formation is less, and they are on the whole more firmly
welded together than in Vertebrates standing below the Mammalia in the
series. Thus in the cranial region the post- and pre-frontals, the
post-orbitals and the supra-orbitals have disappeared, though now and again
we are reminded of their occurrence in the ancestors of the Mammalia by a
separate ossification corresponding to some of the bones. Nowhere is this
consolidation seen with greater clearness than in the lower jaw. That bone,
or rather each half of it, is in mammals formed of one bone, the dentary
(to which occasionally, as it appears, a separate mento-Meckelian {26}
ossification may be added). The angular, splenial, and all the other
elements of the reptilian jaw have vanished, though the numerous points
from which the mammalian dentary ossifies is a reminiscence of a former
state of affairs; and here again an occasional continuance of the
separation is preserved, as the case observed by Professor Albrecht of a
separate supra-angular bone in a Rorqual attests. Among other reptilian
bones that are not to be found in the mammalian skull are the
basipterygoids, quadrato-jugal, and supratemporal. A few of these bones,
however, though no longer traceable in the adult skull save in cases of
what we term abnormalities, do find their representatives in the foetal
skull. Professor Parker, for example, has described a supra-orbital in the
embryo Hedgehog; a supratemporal also appears to be occasionally
independent.
[Illustration]
FIG. 15.--Head of a Human embryo of the fourth month. Dissected to show the
auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid
and thyroid apparatus. All these parts are delineated on a larger scale
than the rest of the skull. _an_, Tympanic ring; _b.hy_, basihyal element;
_hy_, so-called hyoid bone; _in_, incus; _md_, bony mandible; _ml_,
malleus; _st_, stapes; _tp_, tympanum; _tr_, trachea; I. (_mk_), first
skeletal (mandibular) arch (Meckel's cartilage); II. second skeletal
(hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth
arches (thyroid cartilage). (From Wiedersheim's _Structure of Man_.)
In the mode of the articulation of the lower jaw to the skull the Mammalia
apparently, perhaps really, differ from other Vertebrates. In the Amphibia
and Reptilia, with which groups alone any comparisons are profitable, the
lower jaw articulates by means of a quadrate bone, which may be movably or
firmly attached to the skull. In the mammals the articulation of the lower
jaw is with the squamosal. The nature of this articulation is one of the
most debated points in comparative anatomy. Seeing that Professor
Kingsley[14] in the most recent contribution to the subject quotes no less
than fifty-two different views, many of which are more or less convergent,
it will be obvious that in a work like the present the matter cannot be
treated exhaustively. As, however, Professor Kingsley justly says that "no
single bone occupies a more important position in the discussion of the
origin of the Mammalia than does the quadrate," and with equal justice adds
that "upon the answer given as to its fate in this group depends, in large
measure, the broader problem of the phylogeny of the Mammalia," it becomes,
or indeed has long been, a matter which cannot be ignored in any work
dealing with the mammals. A simple view, due to the late Dr. Baur and to
Professor Dollo, commends itself at first sight as meeting the case. The
last-named author holds, or held, that in all the higher Vertebrates it is
at least on _a priori_ grounds likely that two such characteristically
vertebrate features as the lower jaw and the chain of bones bringing the
outer world {27} into communication with the internal organ of hearing
would be homologous throughout the series. He believed, therefore, that the
entire chain of ossicula auditus in the mammal is equal to the columella of
the reptile, since their relations are the same to the tympanum on the one
hand and to the foramen ovale on the other; and that the lower jaw
articulates in the same way in both. It follows, therefore, that the
glenoid part of the squamosal must be the quadrate which has become
ankylosed with it after the fashion of concentration in the mammalian skull
that has already been referred to. The fact that occasionally the glenoid
part of the squamosal is a separate bone[15] appeared to confirm this way
of looking at the {28} matter. But the hall-mark of truth is not always
simplicity; indeed the converse appears to be frequently the case. And on
the whole this view does not commend itself to zoologists at present. For
it must be borne in mind that the lower jaw of the mammal is not the
precise equivalent of that of the reptiles. Apart from the membrane bones,
which may be collectively the equivalents of the dentary of the mammal,
there is the cartilaginous articular bone to be considered, which forms the
connexion between the rest of the jaw and the quadrate in reptiles. Even in
the Anomodontia, whose relations to the Mammalia are considered elsewhere,
there is this bone. But in these reptiles the articular bone articulates
not only with the quadrate, but also to a large extent with the squamosal,
the quadrate shrinking in size and developing processes which give to it
very much the look of either the incus or the malleus of the mammalian ear.
In fact it seems on the whole to fit in with the views of the majority, as
well as with a fair interpretation of the facts of embryology, to consider
that the chain of ear bones in the mammal is not the equivalent of the
columella of the reptile, but that the stapes of the mammal is the
columella, and that the articulare is represented by the malleus and the
quadrate by the incus. It is very interesting to note this entire change of
function in the bones in question. Bones which in the reptile serve as a
means of attachment of the lower jaw to the skull are used in the mammal to
convey the waves of sound from the tympanum of the ear to the internal
organ of hearing.
Another important and diagnostic feature in the mammalian skull is that the
first vertebra of the vertebral column always articulates with two separate
occipital condyles, which are borne by the exoccipital bones and formed
mainly though not entirely by them. Certain Anomodontia form the nearest
approach to the mammals in this particular. The two condyles of Amphibia
are purely exoccipital in origin.
In the Mammalia, unlike what is found in lower Vertebrates (but here again
the Anomodontia form at least a partial exception), the jugal arch does not
connect the face with the quadrate, for, as already said, that bone does
not exist, in the Sauropsidan form, in mammals. This arch passes from the
squamosal to the maxillary, and has but one separate bone in addition to
those two, viz. the jugal or malar. {29}
[Illustration]
FIG. 16.--Under surface of the cranium of a Dog. x 1/2. _apf_, Anterior
palatine foramen; _as_, posterior opening of alisphenoid canal; _AS_,
alisphenoid; _BO_, basioccipital; _BS_, basisphenoid; _cf_, condylar
foramen; _eam_, external auditory meatus; _Ex.O_, exoccipital; _flm_,
foramen lacerum medium; _flp_, foramen lacerum posterius; _fm_, foramen
magnum; _fo_, foramen ovale; _fr_, foramen rotundum; _Fr_, frontal; _gf_,
glenoid fossa; _gp_, post-glenoid process; _Ma_, malar; _Mx_, maxilla;
_oc_, occipital condyle; _op_, optic foramen; _Per_, mastoid portion of
periotic; _pgf_, post-glenoid fossa; _Pl_, palatine; _PMx_, premaxilla;
_pp_, paroccipital process; _ppf_, posterior palatine foramen; _PS_,
presphenoid; _Pt_, pterygoid; _sf_, sphenoidal fissure or foramen lacerum
anterius; _sm_, stylomastoid foramen; _SO_, supraoccipital; _Sq_, zygomatic
process of squamosal; _Ty_, tympanic bulla; _Vo_, vomer. (From Flower's
_Osteology_.)
In connexion with the elaboration of the chain of auditory ossicles it is
very usual for mammals to possess a thin inflated bone, sometimes partly or
entirely formed out of the tympanic bone, and known as the tympanic bulla.
Whether this structure is thin and inflated or thick and depressed in form
it is characteristic of the mammals, and does not occur below them in the
series. But it is not present in all mammals. It is absent, for example, in
the Monotremes. When it is present it is sometimes formed from other bones,
as, for instance, from the alisphenoids. The tympanic ring has been held to
be the equivalent of the quadrate. It is more probably the
quadrato-jugal.[16]
[Illustration]
FIG. 17.--A, First thoracic skeletal segment for comparison with B, fifth
cervical vertebra (Man), _b.v._ Body of vertebra; _c_, first thoracic rib;
_c_', cervical rib (which has become united with the transverse process,
_tr_), the two enclosing the costo-transverse foramen (_f.c.t_); _st_,
sternum; _zy_, articular process of the arch (zygapophysis). (From
Wiedersheim's _Structure of Man_.)
RIBS.--All mammals are furnished with ribs, of which the number of pairs
differs considerably from group to group, or it may be even from species to
species. The ribs are attached as a rule by two heads, of which one, the
capitulum, arises as a rule between two centra of successive vertebrae. The
other, the tuberculum, springs from the transverse process. Only in the
Monotremes {30} are there ribs with but one, the capitular, head. In the
posterior part of the series the two heads often gradually coalesce, so
that there comes to be but one, the capitular, head. The Whales also, at
least the Whalebone Whales, are exceptional in possessing but one head to
the ribs, which is the capitular. The first rib joins the sternum below,
and a variable number after this have the same attachment. There are always
a number of ribs, sometimes called floating ribs, which have no sternal
attachment. In the Whalebone Whales it is the first rib alone which is so
attached. As a rule, to which the Whales mentioned are again an exception,
the rib is divided into at least two regions--the vertebral portion which
is always ossified, and the sternal moiety which is usually cartilaginous.
This is, however, often very short in the first rib. They are, however,
ossified in the Armadillos and in some other animals. Between the vertebral
and sternal portions an intermediate tract is separated off and ossified in
the Monotremata. The ribs of existing mammals belong only to the dorsal
region of the vertebral column, but there are traces of lumbar ribs and
also of cervical ribs. In the Monotremata, indeed, these latter {31} are
persistently free for a very long period, and in some cases never become
ankylosed with their vertebrae. But it should be noted that in this group
there is no approximation to the state of affairs which exists in many
lower Vertebrates, where there is a gradual transition between the ribs of
the cervical and those of the dorsal region of the vertebral column; for
that of the seventh ribs in Monotremes is smaller than those which precede
it.
[Illustration]
[Illustration]
FIG. 18.--Sternum and sternal ribs of the Common Mole (_Talpa europaea_),
with the clavicles (_cl_) and humeri (_H_); _M_, manubrium sterni. Nat.
size. (From Flower's _Osteology_.)
FIG. 19.--Sternum of the Pig (_Sus scrofa_). x 1/4. _ms_, Mesosternum;
_ps_, presternum; _xs_, xiphisternum. (From Flower's _Osteology_.)
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.
[Illustration]
[Illustration]
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.
[Illustration]
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_.)
[Illustration]
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.
[Illustration]
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.
[Illustration]
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_.)
[Illustration]
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} [Illustration]
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_.)
[Illustration]
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.
[Illustration]
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.
[Illustration]
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.
[Illustration]
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).
[Illustration]
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.
[Illustration]
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.
[Illustration]
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_.)
[Illustration]
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.
[Illustration]
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.
[Illustration]
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.
[Illustration]