CLASS MAMMALIA.
The skeleton of the members of this class, the highest of the
vertebrata, has the following characteristics:--
Some part of the integument at some period of life is always provided
with hairs; these are epidermal structures arising from short papillae
of the Malpighian layer of the epidermis, which at once grow inwards
and become imbedded in pits of the dermis. Sometimes scales or spines
occur, and epidermal exoskeletal structures in the form of hoofs,
nails, claws and horns are also characteristic. As regards the
endoskeleton, the vertebral centra have terminal epiphyses except in
the Ornithodelphia and some Sirenia. In the skull the cranial region
is greatly developed as compared with that in lower vertebrates, and
whereas in many reptiles the true cranium is largely concealed by a
false roof, in mammals the only relic of this secondary roof is found
in the zygomatic arch, and postorbital bar. In the adult all the bones
except the mandible, hyoid, and auditory ossicles are firmly united
together. The basisphenoid is well ossified, and there is no
parasphenoid. The pro-otic ossifies, and unites with the epi-otic and
opisthotic before they coalesce with any other bones.
The skull articulates with the vertebral column by means of two convex
occipital condyles formed mainly by the exoccipitals, and the mandible
articulates with the squamosal without the intervention of the
quadrate. The latter is much reduced, and is converted into the
tympanic ring, while the hyomandibular of fish is represented by the
auditory ossicles[108].
The teeth are always attached to the maxillae, premaxillae and
mandibles, never to any of the other bones. They are nearly always
implanted in distinct sockets, and are hardly ever ankylosed to the
bone. The teeth of mammals are generally markedly heterodont, four
forms, incisors, canines, premolars, and molars, being commonly
distinguishable. Some mammals are _monophyodont_, having only a single
set of teeth, but the great majority are _diphyodont_, having two
sets, a deciduous or milk dentition, and a permanent dentition.
The _incisors_, the front teeth, are simple, one-rooted, adapted for
cutting, and are nearly always borne by the premaxillae. Next come the
_canines_, one on each side in each jaw. They are generally large
teeth adapted for tearing or holding, and get their name from the fact
that they are largely developed in the dog. The remaining teeth form
the grinding series, the more posterior of them being the _molars_,
which are not preceded by milk teeth[109]. Between the molars and the
canines are the _premolars_, which do as a rule have milk or deciduous
predecessors, though very frequently the first of them is without a
milk predecessor.
In describing the dentition of any mammal, for the sake of brevity a
formula is generally made use of. Thus, the typical mammalian
dentition is expressed by the formula
_i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 = 11/11,
giving twenty-two teeth on each side, or forty-four altogether[110].
The incisors are represented by _i_, the canines by _c_, the premolars
by _p_ or _pm_, and the molars by _m_. The numbers above the lines
represent the teeth in the upper jaw, those below the lines the teeth
in the lower jaw. The milk dentition is expressed by a similar formula
with _d_ (deciduous) prefixed to the letter expressing the nature of
the tooth.
The following terms are of frequent use as characterising certain
forms of the grinding surfaces of teeth, and it will be well to define
them at once.
_Bunodont_ is a term applied to teeth with broad crowns raised into
rounded tubercles, e.g. the grinding teeth of Pigs and Hippopotami;
_Bilophodont_ to teeth marked by a simple pair of transverse ridges,
with or without a third ridge running along the outer border of the
tooth at right angles to the other two, e.g. the grinding teeth of
_Lophiodon_, Kangaroo, Manatee, Tapir, _Dinotherium_;
_Selenodont_ to teeth marked by crescentic ridges running from the
anterior towards the posterior end of the tooth, e.g. the grinding
teeth of the Ox and Sheep.
Teeth whose crowns are low so that their whole structure is visible
from the grinding surface are called _brachydont_, while those with
higher crowns, in which the bases of the infoldings of enamel are
invisible from the grinding surface are said to be _hypsodont_.
Bunodont teeth are brachydont, the teeth of the Horse and Ox are
hypsodont.
Passing now to the appendicular skeleton--the shoulder girdle differs
markedly from that of Sauropsids in the fact that the coracoid, except
in the Ornithodelphia, is greatly reduced, generally forming only a
small process on the scapula. In the pelvis the pubes meet in a
ventral symphysis, except in some Insectivora and Chiroptera. In many
mammals a fourth pelvic element, the _acetabular bone_, is
distinguishable. The ankle joint is _cruro-tarsal_, or situated
between the proximal tarsal bones and the tibia and fibula. Carpalia 4
and 5 are united forming the _unciform_; and the ulnar sesamoid bone
or _pisiform_ is generally well developed. In the proximal row of
tarsal elements there are only two bones, the calcaneum and
astragalus. Of these the calcaneum is the fibulare, and the
astragalus is generally regarded as the tibiale and intermedium
fused[111].
_Subclass I._ ORNITHODELPHIA OR PROTOTHERIA.
This subclass contains only a single order, the Monotremata, and the
following characteristics are equally applicable to the subclass and
to the order. The vertebral centra have no epiphyses, and the odontoid
process remains for a long time free from the centrum of the second
vertebra. With the exception of the atlas of _Echidna_ the cervical
vertebrae are without zygapophyses. The cranial walls are smooth and
rounded, and the sutures between the several bones early become
completely obliterated as in birds. The mandible is a very slight
structure, with no ascending ramus, and with the coronoid process (see
p. 398) and angle rudimentary. The auditory ossicles show a low state
of development. The tubercula of the ribs articulate with the sides of
the centra of the thoracic vertebrae, not with the transverse
processes. Some of the cervical ribs remain for a long time separate
from the vertebrae. Well ossified sternal ribs occur. No true teeth
are present in the adult. The young _Ornithorhynchus_ has functional
molar teeth, but in the adult their place is taken by horny plates. In
the Echidnidae neither teeth nor horny plates occur.
The coracoid (fig. 66, 3) is complete and well developed, and
articulates with the sternum. A precoracoid (epicoracoid) occurs in
front of the coracoid, and there is a large interclavicle (fig. 66,
6). The ridge on the scapula, corresponding to the spine of other
mammals, is situated on the anterior border instead of in the middle
of the outer surface. Epipubic bones are present. In the Echidnidae,
but not in _Ornithorhynchus_[112], the central portion of the
acetabulum is unossified as in birds. The humerus has a prominent
deltoid crest; its ends are much expanded, and the distal end is
pierced by an ent-epicondylar foramen. The fibula has a broad proximal
process resembling an olecranon. The limbs and their girdles bear a
striking resemblance to those of some Theromorphous reptiles.
[Illustration FIG. 66. VENTRAL VIEW OF THE SHOULDER-GIRDLE AND STERNUM
OF A DUCKBILL (_Ornithorhynchus paradoxus_) × 3/4 (after PARKER).
1 and 2. scapula.
3. coracoid.
4. precoracoid (epicoracoid).
5. glenoid cavity.
6. interclavicle.
7. clavicle.
8. presternum.
9. third segment of mesosternum.
10. sternal rib.
11. intermediate rib.
12. vertebral rib.]
The order Monotremata includes only two living families, the
Echidnidae and Ornithorhynchidae.
MESOZOIC MAMMALIA[113].
It will be well here to briefly refer to certain mammals of small
size, the remains of which have been found in deposits of Mesozoic
age. In the great majority of cases they are known only by the lower
jaw, or sometimes only by isolated teeth. A large number of them are
commonly grouped together as the Multituberculata, and are sometimes,
partly owing to the resemblance of their teeth to those of
_Ornithorhynchus_, placed with the Prototheria, sometimes between the
Prototheria and the Metatheria. They are characterised by having a
single pair of large incisors in the lower jaw, and one large with one
or two smaller incisors in each premaxillae. The lower canines are very
small or altogether wanting. The incisors are separated by a diastema
from the grinding teeth, which are sometimes (_Tritylodon_)
characterised by the possession of longitudinal rows of little
tubercles separated by grooves, sometimes by having the premolars
provided with high cutting edges, whose surfaces are obliquely
grooved. Some of the Mesozoic mammals found associated with the
Multituberculata, have however a dentition of an altogether different
type, with at least three lower incisors, well developed canines and
premolars, and numerous molars with peculiar three-cusped or
tritubercular grinding surfaces. These mammals, one of the best known
of which is _Phascolotherium_, are commonly separated from the
Multituberculata, and are divided by Osborn into two groups, one
allied to the Marsupials, and one to the Insectivores. The group
showing Marsupial affinities is further subdivided into carnivorous,
omnivorous, and herbivorous subgroups. The members of both groups
commonly have four premolars, and six to eight molars in each
mandibular ramus.
_Subclass II._ DIDELPHIA OR METATHERIA.
This subclass, like the previous one, contains only a single order,
viz. the Marsupialia[114]; but the forms referable to it are far more
numerous than in the case of the Monotremata.
The integument is always furry, and the teeth are always
differentiated into incisors, canines, premolars and molars. Except in
_Phascolomys_, the number of incisors in the upper and lower jaws is
never equal, and the number in the upper usually exceeds that in the
lower jaw. There is no such regular succession and displacement of
teeth as in most mammals. Sometimes the anterior teeth are diphyodont,
and as a general rule the tooth commonly regarded as the last premolar
is preceded by a milk tooth. The majority of the permanent teeth of
most Marsupials are regarded as belonging to the milk series for two
reasons, (1) they are developed from the more superficial tissues of
the jaws, (2) a second set, the permanent teeth, begin to develop as
outgrowths from them, but afterwards become aborted[115].
The odontoid process at an early stage becomes fused with the centrum
of the second cervical vertebra, and the number of thoraco-lumbar
vertebrae is always nineteen. The skull has several characteristic
features. The tympanic bone remains permanently distinct, and the
anterior boundary of the tympanic cavity is formed by the alisphenoid.
The carotid canal perforates the basisphenoid, and the lachrymal canal
opens either outside the orbit or at its margin. There are generally
large vacuities in the palate. The angle of the mandible is (except in
_Tarsipes_) more or less inflected; and as a rule the jugal furnishes
part of the articular surface for the mandible. There is no
precoracoid (epicoracoid) or interclavicle, and the coracoid is
reduced to form a mere process of the scapula, not coming near the
sternum.
Epipubic, or so-called marsupial bones[116], nearly always occur, and
a fourth pelvic element, the acetabular bone, is frequently developed.
The fibula is always complete at its distal end, sometimes it is fused
with the tibia, but often it is not only free but is capable of a
rotatory movement on the tibia. This is the case in the families
Phascolomyidae, Didelphyidae, and Phalangeridae.
The Marsupialia can be subdivided into two main groups, according to
the character of the teeth:--
1. POLYPROTODONTIA.
In this group the incisors are small, subequal and numerous, not less
than 4/3. The canines are larger than the incisors, and the molars
have sharp cusps. The members of this group are all more or less
carnivorous or insectivorous. The group includes the families
Didelphyidae, Dasyuridae, Peramelidae, and Notoryctidae[117].
2. DIPROTODONTIA.
In this group the incisors do not exceed 3/3, and are usually 3/1,
occasionally 1/1. The first upper and lower incisors are large and
cutting. The lower canines are always small or absent, and so in most
cases are the upper canines. The molars have bluntly tuberculated, or
transversely ridged crowns. The group includes the families
Phascolomyidae, Phalangeridae, Macropodidae, and Epanorthidae.
_Subclass III._ MONODELPHIA OR EUTHERIA.
This great group includes all the Mammalia except the orders
Monotremata and Marsupialia. Coming to their general characteristics--
as in the Didelphia the odontoid process and cervical ribs early
become fused with the centra which bear them, while the coracoid is
reduced so as to form a mere process on the scapula, and there is no
precoracoid (epicoracoid), such as is found in Ornithodelphia.
Clavicles may be present or absent; when fully developed they
articulate with the sternum, usually directly, but occasionally, as in
some Rodents and Insectivores, through the remains of the sternal end
of the precoracoid. There is never any interclavicle in the adult,
though sometimes traces of it occur during development. In the pelvis
the acetabula are imperforate; and well-developed epipubic bones are
never found in the adult, though traces of them occur in some
Carnivores and foetal Ungulates.
_Order 1._ EDENTATA[118].
Teeth are not, as the name of the order seems to imply, always
wanting; and sometimes they are very numerous. They are, however,
always imperfect, and, with very few exceptions, are homodont and
monophyodont. They have persistent pulps, and so grow indefinitely and
are never rooted. In all living forms they are without enamel,
consisting merely of dentine and cement, and are never found in the
front part of the mouth in the situation occupied by the incisors of
other mammals. These characters derived from the teeth are the only
ones common to the various members of the order, which includes the
living sloths, ant-eaters, armadillos, pangolins and aard-varks,
together with various extinct forms, chiefly found in beds of late
tertiary age in both North and South America, the best known being the
Megatheridae and Glyptodonts.
_Order 2._ SIRENIA[119].
The skeleton of these animals has a general fish-like form, in
correlation with their purely aquatic habits. The fore limbs have the
form of paddles, but the number of phalanges is not increased beyond
the normal. There are no external traces of hind limbs.
The whole skeleton and especially the skull and ribs is remarkably
massive and heavy. The dentition varies; in the two living genera
_Manatus_ and _Halicore_, incisor and molar teeth are present, in one
extinct genus, _Rhytina_, teeth are entirely absent, while in another,
_Halitherium_, the dentition is more decidedly heterodont than in
living forms. In the two living genera the dentition is monophyodont,
but in _Halitherium_ the anterior grinding teeth are preceded by milk
teeth. The tongue and anterior part of the palate and lower jaw are
covered with roughened horny plates. The skull is noticeable for the
size and backward position of the anterior nares, also for the absence
or small size of the nasal bones. There is no union of certain of the
vertebrae to form a sacrum, and in living forms the centra are not
terminated by well-formed epiphyses[120].
The cervical vertebrae are much compressed, but they are never
ankylosed together. In _Manatus_ there are only six cervical
vertebrae. The caudal vertebrae have well-developed chevron bones. The
humerus is distinctly articulated to the radius and ulna, and these
two bones are about equally developed, and are often fused together.
There are no clavicles, and the pelvis is vestigial, consisting of a
pair of somewhat cylindrical bones suspended at some distance from the
vertebral column. In living forms there is no trace of a posterior
limb, but in _Halitherium_ there is a vestigial femur connected with
each half of the pelvis.
_Order 3._ CETACEA[121].
In these mammals the general form is more fish-like than is the case
even in the Sirenia. The skin is generally almost completely naked,
but hairs are sometimes present in the neighbourhood of the mouth,
especially in the foetus. In some Odontoceti vestiges of dermal
ossicles have been described, and in _Zeuglodon_ the back was probably
protected by dermal plates. The anterior limbs have the form of
flattened paddles, showing no trace of nails, the posterior limb bones
are quite vestigial or absent, and there is never any external sign of
the limb. Teeth are always present at some period of the life history,
but in the whalebone whales they are only present during foetal life,
their place in the adult animal being taken by horny plates of baleen.
In all living forms the teeth are simple and uniform structures
without enamel; they have single roots, and the alveoli in which they
are imbedded are often incompletely separated from one another. As in
some forms traces of a replacing dentition have been described, it has
been concluded that the functional teeth of Cetacea belong to the milk
dentition.
The texture of the bones is spongy. The cervical vertebrae are very
short, and though originally seven in number, are in many forms
completely fused, forming one solid mass (fig. 67). The odontoid
process of the axis is short and blunt, or may be completely wanting.
The lumbar and caudal vertebrae are large and numerous, and as
zygapophyses are absent, are very freely movable on one another;
zygapophyses are also absent from the posterior thoracic vertebrae.
The lumbar vertebrae are sometimes more numerous than the thoracic.
The epiphyses are very distinct, and do not unite with the centra till
the animal is quite adult. None of the vertebrae are united to form a
sacrum, but the caudal vertebrae have large chevron bones.
[Illustration FIG. 67. CERVICAL VERTEBRAE OF A CA'ING WHALE
(_Globicephalus melas_) × 1/4. (Camb. Mus.)
1. centrum of seventh cervical vertebra.
2. neural arch of seventh cervical vertebra.
3. transverse process of atlas.
4. foramen for exit of first spinal nerve.
5. transverse process of axis.
6. fused neural spines of atlas and axis.]
The skull is peculiarly modified; the bones forming the occipital
segment show a specially strong development, and the cranial cavity is
short, high, and almost spherical. The supra-occipital is very large
and rises up to meet the frontals, thus with the interparietal
completely separating the parietals from one another.
The frontals are expanded, forming large bony plates, which roof over
the orbits. The zygomatic process of the squamosal is extremely large
and extends forwards to meet the supra-orbital process of the frontal;
the zygomatic process of the jugal is on the contrary very slender.
The face is drawn out into a long rostrum, formed of the maxillae and
premaxillae surrounding the vomer and the mesethmoid cartilage. The
maxillae are specially large, and extend backwards so as to partially
overlap the frontals. The nasals are always small, and the anterior
nares open upwards between the cranium and rostrum. The periotics are
loosely connected with the other bones of the skull and the tympanics
are commonly large and dense. The mandible has hardly any coronoid
process, and the condyles are at its posterior end.
There are no clavicles, but the scapula and humerus are well
developed. The humerus moves freely in the glenoid cavity, but all the
other articulations of the anterior limb are imperfect; the various
bones have flattened ends, and are connected with one another by
fibrous tissue, which allows of hardly any movement. Frequently the
carpus is imperfectly ossified.
The number of digits in the manus is generally five, sometimes four,
and when there are four digits it is the third and not the first that
is suppressed. The number of phalanges in the second and third digits
almost always exceeds that which is normal in mammals, and the
phalanges are also remarkable for having epiphyses at both ends. The
pelvis is represented by two small bones which lie suspended
horizontally at some distance below the vertebral column; in some
cases vestiges of the skeleton of the hind limb are attached to them.
The Cetacea are divided into three suborders.
_Suborder_ (1). ARCHAEOCETI.
The members of this group are extinct; they differ from all living
Cetacea in having the dentition heterodont and in the fact that the
back was probably protected by dermal plates. The skull is elongated
and depressed, and the brain cavity is very small. The temporal fossae
are large, and there is a strong sagittal crest. The nasals and
premaxillae are a good deal larger than they are in living Cetacea, and
the anterior nares are usually far forward. The cervical vertebrae are
not fused with one another, and the lumbar vertebrae are unusually
elongated.
The limbs are very imperfectly known, but while the humerus is much
longer than in modern Cetaceans, it is nevertheless flattened
distally, indicating that the limb was paddle-like, and that there was
scarcely any free movement between the fore-arm and upper arm.
The best known genus is _Zeuglodon_, which is found in beds of Eocene
age in various parts of Europe, and in Alabama.
_Suborder_ (2). MYSTACOCETI OR BALAENOIDEA.
These are the Whalebone Whales or True Whales.
Calcified teeth representing the milk dentition occur in the foetus,
but the teeth are never functional, and always disappear before the
close of foetal life. There is a definite though small olfactory
fossa. The palate is provided with plates of baleen or whalebone. The
skull is symmetrical, and is extremely large in proportion to the
body. The nasals are moderately well developed, and the maxillae do not
overlap the orbital processes of the frontals. The lachrymals are
small and distinct from the jugals. The tympanics are ankylosed to the
periotics, and the rami of the mandible do not meet in a true
symphysis. The ribs articulate only with the transverse processes, and
the capitula are absent or imperfectly developed. Only one pair of
ribs meets the sternum, which is composed of a single piece.
The group includes among others the Right whale (_Balaena_),
Humpbacked whale (_Megaptera_), and Rorqual (_Balaenoptera_).
_Suborder_ (3). ODONTOCETI.
Teeth always exist after birth and baleen is never present. The teeth
are generally numerous, but are sometimes few and deciduous; the
dentition is homodont (except in _Squalodon_). The dorsal surface of
the skull is somewhat asymmetrical, there is no trace of an olfactory
fossa, the nasals are quite rudimentary, and the hind ends of the
maxillae cover part of the frontals; in all these respects the skull
differs from that of the Mystacoceti. The lachrymal may either be
united to the jugal or may be large and distinct. The tympanic is not
ankylosed to the periotic. The rami of the mandible are nearly
straight and become united in a long symphysis. Some of the ribs have
well developed capitula articulating with the vertebral centra. The
sternum is almost always composed of several pieces as in other
mammals, and several pairs of ribs are connected with it. There are
always five digits to the manus, though the first and fifth are
usually very little developed.
The suborder includes the Sperm Whale (_Physeter_), Narwhal
(_Monodon_), Dolphin (_Delphinus_), Porpoise (_Phocoena_), and many
other living forms as well as the extinct _Squalodon_ which differs
from the other members of the suborder in its heterodont dentition.
_Order 4._ UNGULATA.
This order includes a great and somewhat heterogeneous group of
animals, a large proportion of which are extinct. They all (except
certain extinct forms) agree in having the ends of the digits either
encased in hoofs or provided with broad flat nails. The teeth are
markedly heterodont and diphyodont, and the molars have broad crowns
with tuberculated or ridged surfaces. Clavicles are never present in
the adult except in a few generalised extinct forms such as
_Typotherium_, and it is only recently that vestigial clavicles have
been discovered in the embryo[122]. The scaphoid and lunar are always
distinct.
The order Ungulata may be subdivided into two main groups, Ungulata
vera and Subungulata.
_Section I._ UNGULATA VERA[123].
The cervical vertebrae except the atlas are generally opisthocoelous.
The feet are never _plantigrade_[124]. In all the living and the great
majority of the extinct forms the digits do not exceed four, the first
being suppressed. In the carpus the os magnum articulates freely with
the scaphoid, and is separated from the cuneiform by the lunar and
unciform. In the tarsus the cuboid articulates with the astragalus as
well as with the calcaneum, and the proximal surface of the astragalus
is marked by a pulley-like groove. All the bones of the carpus and
tarsus strongly interlock. These characters with regard to the carpus
and tarsus do not hold in _Macrauchenia_ and its allies. The humerus
never has an ent-epicondylar foramen.
The group is divided into two very distinct suborders:--
_Suborder_ (1). ARTIODACTYLA.
The Artiodactyla have a number of well marked characters, one of the
most obvious being the fact that many of the most characteristic forms
have large paired outgrowths on the frontal bones. These may be (1)
solid deciduous bony _antlers_, or (2) more or less hollow bony
outgrowths which are sheathed with permanently growing horn.
The premolar and molar teeth are usually dissimilar, the premolars
being one-lobed and the molars two-lobed; the last lower molar of both
the milk and permanent dentitions is almost always three-lobed.
The grinding surfaces of the molar teeth have a tendency to assume one
of two forms. In the Pigs and their allies the crowns are bunodont[1],
while in the more highly specialised Ruminants the crowns are
selenodont[125]. The nasals are not expanded posteriorly, and there is
no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always
nineteen. The symphysis of the ischia and pubes is very elongated, and
the femur has no third trochanter. The limbs never have more than four
digits, and are symmetrical about a line drawn between the third and
fourth digits; the digits, on the other hand, are never symmetrical in
themselves. The astragalus has pulley-like surfaces both proximally
and distally, and articulates with the navicular and cuboid by two
nearly equal facets. The calcaneum articulates with the lower end of
the fibula when that bone is fully developed.
In the Artiodactyla are included the following living groups:--
_a._ Suina. Pigs and Hippopotami.
_b._ Tylopoda. Camels and Llamas.
_c._ Tragulina. Chevrotains.
_d._ Ruminantia or Pecora. Deer, giraffes, oxen, sheep and antelopes.
_Suborder_ (2). PERISSODACTYLA[127].
In this group there are never any bony outgrowths from the frontals.
The grinding teeth form a continuous series, the posterior premolars
resembling the molars in complexity, and the last lower molar
generally has no third lobe. The cervical vertebrae with the
exception of the atlas almost always have markedly opisthocoelous
centra, but in _Macrauchenia_ they are flat. The nasals are expanded
posteriorly, and an alisphenoid canal is present. The thoraco-lumbar
vertebrae are never less than twenty-two in number and are usually
twenty-three. The femur has a third trochanter (except in
_Chalicotherium_). The third digit of the manus and pes is symmetrical
in itself, and larger than the others, and in some cases the other
digits are quite vestigial. The number of the digits of the pes is
always odd. The astragalus is abruptly truncated distally, and the
facet by which it articulates with the cuboid, is much smaller than
that by which it articulates with the navicular. The calcaneum does
not articulate with the fibula, except in _Macrauchenia_. The group
includes many extinct forms, and the living families of the Tapirs,
Horses and Asses, and Rhinoceroses.
_Section II._ SUBUNGULATA.
In this group is placed a heterogeneous collection of animals, the
great majority of which are extinct. There is really no characteristic
which is common to them all, and which serves to distinguish them as a
group from the Ungulata vera. But the most distinctive character
common to the greatest number of them is to be found in the carpus,
whose bones in most cases retain their primitive relation to one
another, the os magnum articulating with the lunar and sometimes just
meeting the cuneiform, but in living forms at any rate not
articulating with the scaphoid. The feet frequently have five
functional digits, and may be plantigrade. The proximal surface of the
astragalus is generally flattened instead of being pulley-like as in
Ungulata vera.
_Suborder_ (1). TOXODONTIA.
This suborder includes some very aberrant extinct South American
ungulates, which have characters recalling the Proboscidea, both
groups of Ungulata vera, and the Rodentia. The limbs are
subplantigrade or digitigrade, and the digits are three, rarely five,
in number, the third being most developed. The carpus resembles that
of the Ungulata vera, in that the bones interlock and the magnum
articulates with the scaphoid. In the tarsus, however, the bones do
not interlock. The astragalus has a pulley-like proximal surface
(except in _Astrapotherium_, in which it is flat), and articulates
only with the navicular, not meeting the cuboid. The calcaneum has a
large facet for articulation with the fibula, as in Artiodactyla.
There is no alisphenoid canal, and the orbit is confluent with the
temporal fossa. Some of the forms (e.g. _Nesodon_) referred to this
group have the typical mammalian series of forty-four teeth, but in
others the canines are undeveloped. In _Toxodon_ all the cheek teeth
have persistent pulps, while in _Nesodon_ and _Astrapotherium_ they
are rooted. A clavicle is sometimes present (_Typotherium_), and the
femur sometimes has a third trochanter (_Typotherium_ and
_Astrapotherium_), sometimes is without one (_Toxodon_).
The remains of these curious Ungulates have been found in beds of late
Tertiary age in South America.
_Suborder_ (2). CONDYLARTHRA[128].
This group includes some comparatively small extinct ungulates, which
are best known from the Lower Eocene of Wyoming, though their remains
have also been found in deposits of similar age in France and
Switzerland. Their characters are little specialised, and they show
relationship on the one hand to the Ungulata vera and on the other to
the Hyracoidea. They also have characters allying them to the
Carnivora. They generally have the typical mammalian series of
forty-four teeth, the molars being brachydont and generally bunodont.
The premolars are more simple than the molars. The limbs are
plantigrade, and have five digits with rather pointed ungual
phalanges. The os magnum, as in living Subungulates, articulates with
the lunar, not reaching the scaphoid. The astragalus has an elongated
neck, a pulley-like proximal and a convex distal articular surface,
and does not articulate with the cuboid. The humerus has an
ent-epicondylar foramen, and the femur has a third trochanter. The
best known genus is _Phenacodus_; it is perhaps the most primitive
ungulate whose skeleton is thoroughly well known, and is of special
interest from the fact that it is regarded as the lowest stage in the
evolutionary series of the horse. Its remains are found in the Lower
Eocene of Wyoming.
_Suborder_ (3). HYRACOIDEA[129].
This group of animals is very isolated, having no very close allies,
either living or extinct. The digits are provided with flat nails,
except the second digit of the pes, which is clawed. Canine teeth are
absent, and the dental formula is usually given as _i_ 1/2, _c_ 0/0,
_pm_ 4/4, _m_ 3/3. The upper incisors are long and curved, and have
persistent pulps as in Rodents; their terminations are, however,
pointed, not chisel-shaped, as in Rodents. The lower incisors have
pectinated edges. The grinding teeth have a pattern much like that in
_Rhinoceros_. In the skull (fig. 83) the postorbital processes of the
frontal and jugal almost or quite meet. The jugal forms part of the
glenoid cavity for articulation with the mandible, and also extends
forwards so as to meet the lachrymal. There is an alisphenoid canal.
There are as many as twenty-one or twenty-two thoracic vertebrae, and
the number of thoraco-lumbar vertebrae reaches twenty-eight or thirty.
There are no clavicles, and the scapula has no acromion; the coracoid
process is, however, well developed. The ulna is complete. In the
manus the second, third and fourth digits are approximately equal in
size, the fifth is smaller, and the first is vestigial. The femur has
a slight ridge representing the third trochanter. The fibula is
complete, but is generally fused with the tibia proximally. There is a
complicated articulation between the tibia and astragalus, which has a
pulley-like proximal surface. In the pes the three middle digits are
well developed, but there is no trace of a hallux, and the fifth digit
is represented only by a vestigial metatarsal.
The only representatives of the suborder are some small animals
belonging to the genus _Procavia_ (_Hyrax_), which is found in Africa
and Syria; some of the species are by many authors placed in a
distinct genus _Dendrohyrax_.
_Suborder_ (4). AMBLYPODA[130].
This suborder includes a number of primitive extinct Ungulates, many
of which are of great size. Their most distinguishing characteristics
are afforded by the extremities. In the carpus the bones interlock a
little more than is the case in most Subungulata, and the corner of
the os magnum reaches the scaphoid, while the lunar articulates
partially with both magnum and unciform, instead of only with the
magnum. In the tarsus the cuboid articulates with both the calcaneum
and the astragalus, which is remarkably flat. The manus and pes are
short, nearly or quite plantigrade, and have the full number of
digits. The cranial cavity is singularly small. Canine teeth are
present in both jaws, and the grinding teeth have short crowns, marked
by =V=-shaped ridges. The pelvis is large, the ilia are placed
vertically, and the ischia do not take part in the ventral symphysis.
The best known animals belonging to this suborder are the
Uintatheriidae (Dinocerata)[131], found in the Upper Eocene of
Wyoming. They are as large as elephants, and are characterised by the
long narrow skull drawn out into three pairs of rounded protuberances,
by the strong occipital crest, and by the very large upper canines.
_Suborder_ (5). PROBOSCIDEA.
This suborder includes the largest of land mammals, the Elephants, and
certain of their extinct allies. The limbs are strong, and are
vertically placed; the proximal segment is the longest, and the manus
and pes are pentedactylate and subplantigrade. The digits are all
enclosed in a common integument, and each is provided with a broad
hoof. The vertebral centra are much flattened and compressed,
especially in the cervical region. The number of thoracic vertebrae is
very great, reaching twenty. The skull (figs. 96 and 97) is extremely
large, this being due to the great development of air cells, which
takes place in nearly all the bones of the adult skull. In the young
skull there are hardly any air cells, and the growth of the cranial
cavity does not by any means keep pace with the growth of the skull in
general. The supra-occipital is very large, and forms a considerable
part of the roof of the skull. The nasals and jugals are short, and
the premaxillae very large. The rami of the mandible meet in a long
symphysis, and the ascending portion is very high. Canine teeth are
absent, and the incisors have the form of ever-growing tusks composed
mainly of dentine; in living forms they are present in the upper jaw
only. The grinding teeth are large, and in living forms have a very
complex structure and mode of succession. In some of the extinct
forms, such as _Mastodon_ and especially _Dinotherium_, the teeth are
much more simple. In every case the teeth have the same general
structure, consisting of a series of ridges of dentine, coated with
enamel. In the more specialised forms the valleys between the ridges
are filled up with cement. The acromion of the scapula has a recurved
process, similar to that often found in rodents. Clavicles are absent.
The radius and ulna are not ankylosed, but are incapable of any
rotatory movement. All the bones of the extremities are very short and
thick; the scaphoid articulates regularly with the trapezoid and the
lunar with the magnum. The ilia are vertically placed, and are very
much expanded; the ischia and pubes are small, and form a short
symphysis. The femur has no third trochanter, and the tibia and fibula
are distinct. The fibula articulates with the calcaneum, and the
astragalus is very flat.
* * * * *
Here brief reference may be made to the TILLODONTIA[132], a group of
extinct mammals found in the Eocene beds of both Europe and North
America. They seem to connect together the Ungulata, Rodentia, and
Carnivora.
The skull resembles that of bears, but the grinding teeth are of
Ungulate type, while the second incisors resemble those of rodents,
and have persistent pulps. The femur has a third trochanter, and the
feet resemble those of bears in being plantigrade and having pointed
ungual phalanges, differing, however, in having the scaphoid and lunar
distinct.
_Order 5._ RODENTIA.
The Rodents form a very large and well-defined group of mammals easily
distinguishable by their peculiar dentition. Canines are absent, and
the incisors are very large and curved, growing from persistent pulps.
They are rectangular in section and are much more thickly coated with
enamel on their anterior face than elsewhere; consequently, as they
wear down they acquire and retain a chisel-shaped (scalpriform) edge.
There is never more than one pair of incisors in the mandible, and
except in the Hares and Rabbits, there is similarly only a single pair
in the upper jaw. These animals are, too, the only rodents which have
well developed deciduous incisors. There is always a long diastema
separating the incisors from the grinding teeth. The grinding teeth,
which are arranged in a continuous series, vary in number from two to
six in the upper jaw, and from two to five in the lower jaw. The
number of premolars is always below the normal, often they are
altogether wanting, but generally they are 1/1. Sometimes the grinding
teeth form roots, sometimes they grow persistently.
The premaxillae are always large, and the orbits always communicate
freely with the temporal fossae. The condyle of the mandible is
elongated from before backwards, and owing to the absence of a
postglenoid process to the squamosal, a backward and forward motion of
the jaw can take place. The zygomatic arch is complete, but the jugal
is short and only forms the middle of it. The palate is small, being
sometimes, as in the hares, narrowed from before backwards, sometimes
as in the mole-rats (Bathyerginae) narrowed transversely.
The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles
are generally present, and the acromion of the scapula is commonly
very long. The feet are as a rule plantigrade, and provided with five
clawed digits.
There are two main groups of Rodentia; the Duplicidentata, or Hares
and Rabbits, which have two pairs of upper incisors, whose enamel
extends round to the posterior surface; and the Simplicidentata, in
which there is only a single pair of upper incisors, whose enamel is
confined to the anterior surface. This group includes all the Rodents
except the Hares and Rabbits.
_Order 6._ CARNIVORA.
The living Carnivora form a natural and well-marked group, but as is
the case with so many other groups of animals, when their extinct
allies are included, it becomes impossible to readily define them.
The manus and pes never have less than four well-developed digits, and
these are nearly always provided with more or less pointed nails,
generally with definite claws. The hallux and pollex are never
opposable. The dentition is diphyodont and markedly heterodont. The
teeth are always rooted, except in the case of the canines of the
Walrus. The incisors are generally 3/3, and are comparatively small,
while the canines are large, pointed, and slightly recurved. The cheek
teeth are variable, and are generally more or less compressed and
pointed; sometimes their crowns are flattened and tuberculated, but
they are never divided into lobes by deep infoldings of enamel. The
squamosal is drawn out into a postglenoid process, and the mandible
has a large coronoid process. The condyle of the mandible is
transversely elongated, and the glenoid fossa is very deep; in
consequence of this arrangement the mandible can perform an up and
down movement only, any rotatory or back and fore movement being
impossible. The jugal is large, and the zygomatic arch generally
strong, while the orbit and temporal fossa are in most cases
completely confluent. The scapula has a large spine. The clavicle is
never complete and is often absent, this forming an important
distinction between the skeleton of a Carnivore and of any Insectivore
except _Potamogale_. The humerus often has an ent-epicondylar foramen,
and the radius and ulna, tibia and fibula are always separate. The
manus is often capable of the movements of pronation and supination,
and the scaphoid, lunar and centrale are in living forms always united
together.
The order Carnivora includes three suborders.
_Suborder_ (1). CREODONTA[133].
This suborder contains a number of extinct Carnivora, which present
very generalised characters.
The cranial cavity is very small; and the fourth upper premolar and
first lower molar are not differentiated as carnassial teeth[134], as
they are in modern Carnivora. The Creodonta also differ from modern
Carnivora in the fact that the scaphoid and lunar are usually
separate, and that the femur has a third trochanter. The feet are
plantigrade.
They resemble the Condylarthra, another very generalised group, in
having an ent-epicondylar foramen.
They occurred throughout the Tertiary period in both Europe and North
America, and have also been found in India. One of the best known
genera is _Hyaenodon_.
_Suborder_ (2). CARNIVORA VERA or FISSIPEDIA.
The skeleton is mainly adapted for a terrestrial mode of life, and the
hind limbs have the normal mammalian position. In almost every case
the number of incisors is 3/3. Each jaw always has one specially
modified _carnassial_ or sectorial tooth which bites like a scissors
blade against a corresponding tooth in the other jaw. In front of it
the teeth are always more or less pointed, while behind it they are
more or less broadened and tuberculated. In the manus the first digit,
and in the pes the first and fifth digits are never longer than the
rest, and the digits of both limbs are almost invariably clawed. Some
forms are plantigrade, some digitigrade, some subplantigrade. The
group includes all the ordinary terrestrial Carnivora, and is divided
into three sections:--
Æluroidea[135], including the cats, civets, hyaenas, and allied forms.
Cynoidea, including the dog tribe.
Arctoidea, including the bears, raccoons, weasels, and allied forms.
_Suborder_ (3). PINNIPEDIA[136].
In this suborder the limbs are greatly modified and adapted for a more
or less purely aquatic life, the proximal and middle segments of the
limbs are shortened, while the distal segment, especially in the leg,
is much elongated and expanded. There are always five well-developed
digits to each limb, and in the pes the first and fifth digits are
generally larger than the others. The digits generally bear straight
nails instead of claws, but even nails are sometimes absent. There is
no carnassial tooth, and the teeth in other ways differ considerably
from those of Carnivora vera. The incisors are always fewer than 3/3;
while the cheek teeth generally consist of four premolars and one
molar, all of very uniform character, being compressed with conical
crowns, and never more than two roots.
The suborder includes three families--Otariidae (Eared Seals),
Trichechidae (Walrus), and Phocidae (Seals).
_Order 7._ INSECTIVORA[137].
This order contains a large number of small generally terrestrial
mammals. The limbs are plantigrade or subplantigrade, and are
generally pentedactylate. All the digits are armed with claws, and the
pollex and hallux are not opposable. The teeth are diphyodont,
heterodont, and rooted. The cheek teeth have tuberculated crowns, and
there are never less than two pairs of incisors in the mandible; often
the incisors, canines, and premolars are not clearly differentiated
from one another, and special carnassial teeth are never found. The
cranial cavity is small, and the facial part of the skull is generally
much developed; often the zygomatic arch is incomplete. Clavicles are
well developed (except in _Potamogale_), and the humerus generally has
an ent-epicondylar foramen. The femur frequently has a ridge
representing the third trochanter. There are two suborders:
_Suborder_ (1). DERMOPTERA.
This suborder includes only a very aberrant arboreal genus
_Galeopithecus_, remarkable for its greatly elongated limb bones, and
peculiar dentition. The incisors of the lower jaw are deeply
pectinated or divided by several vertical fissures, the canines and
outer upper incisors have two roots. Ossified inter centra occur in
the thoraco-lumbar region of the vertebral column.
_Suborder_ (2). INSECTIVORA VERA.
This suborder includes all the ordinary Insectivora, such as moles,
shrews and hedgehogs. The upper and lower incisors are conical, not
pectinated.
_Order 8._ CHIROPTERA[138].
This order is perhaps the best marked and most easily defined of all
the orders of mammals. The anterior limbs form true wings and the
whole skeleton is modified in relation to flight.
The anterior limbs are vastly larger than the posterior; for all the
bones except the carpals are much elongated, and this applies
specially to the phalanges of all the digits except the pollex.
The pollex is clawed and so is sometimes the second digit; the other
digits of the manus are without nails or claws. The teeth are
divisible into the four usual types and the series never exceeds _i_
2/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 × 2, total 38. The milk teeth are quite
unlike the permanent teeth. The orbit is not divided by bone from the
temporal fossa. The vertebral column is short, and in old animals the
trunk vertebrae have a tendency to become partially fused together.
The cervical vertebrae are remarkably wide, and the development of
spinous processes is everywhere slight. The presternum has a prominent
keel for the attachment of the pectoral muscles. The clavicles are
very long and strong, and the scapula has a long spine and coracoid
process. The ulna is vestigial, consisting only of a proximal end
ankylosed to the radius. All the carpals of the proximal row--the
scaphoid, lunar and cuneiform--are united, forming a single bone. The
pelvis is very weak and narrow, and only in the Rhinolophidae do the
pubes meet in a symphysis. The anterior caudal vertebrae are
frequently united to the ischia. The fibula is generally vestigial,
and the knee joint is directed backwards instead of forwards. The pes
has five slender clawed digits, and the calcaneum is often drawn out
into a spur which helps to support the membrane connecting the hind
limbs with the tail.
There are two suborders of Chiroptera:
1. The MEGACHIROPTERA or Flying foxes, which almost always have smooth
crowns to the molar teeth, and the second digit of the manus clawed.
2. The MICROCHIROPTERA including all the ordinary bats which have
cusped molar teeth, and the second digit of the manus clawless.
_Order 9._ PRIMATES.
The dentition is diphyodont and heterodont, the incisors generally
number 2/2, and the molars, except in the Hapalidae (Marmosets), are
3/3. The cheek teeth are adapted for grinding, and the molars are more
complex than the premolars. A process from the jugal meets the
postorbital process of the frontal completing the postorbital bar.
The clavicle is well developed, and the radius and ulna are never
united. The scaphoid and lunar of the carpus, and commonly also the
centrale, remain distinct from one another. As a rule both manus and
pes have five digits, but the pollex may be vestigial. The pollex is
opposable to the other digits, and so is the hallux except in Man; the
digits are almost always provided with flat nails. The humerus has no
ent-epicondylar foramen and the femur has no third trochanter.
The order Primates is divisible into two suborders:
_Suborder_ (1). LEMUROIDEA.
The skull has the orbit communicating freely with the temporal fossa
beneath the postorbital bar (except in _Tarsius_). The lachrymal
foramen is external to the margin of the orbit. Both pollex and hallux
are well developed. In the pes the second digit is terminated by a
long pointed claw, and so is also the third in _Tarsius_. The lumbar
region of the vertebral column is long, sometimes including as many as
nine vertebrae. Besides the Lemurs the group includes the aberrant
_Tarsius_ and _Chiromys_.
_Suborder_ (2). ANTHROPOIDEA.
The skull has the orbit almost completely shut off from the temporal
fossa, and the lachrymal foramen is situated within the orbit. The
pollex is sometimes vestigial or absent. The second digit of the pes
has a flattened nail except in the Hapalidae, in which all the digits
of the pes except the hallux are clawed.
The Anthropoidea are divided into five families:
1. Hapalidae or Marmosets.
2. Cebidae or American Monkeys.
3. Cercopithecidae or Old World Monkeys.
4. Simiidae or Anthropoid Apes.
5. Hominidae or Men.
FOOTNOTES:
[108] This is Gadow's view; according to Huxley the quadrate forms the
malleus; according to Baur it forms the zygomatic process of the
squamosal, and according to Broom the interarticular mandibular
cartilage.
[109] According to Leche, _Morphol. Jahrb._ XIX. p. 502, the molar
teeth belong morphologically to the first series, i.e. they are milk
teeth without vertical successors.
[110] The researches of Bateson, _P.Z.S._ 1892, p. 102, have shown
that cases of individual variation in the number of teeth are common.
[111] Baur, however, suggests (_Anat. Anz._ vol. IV. 1889), that a
tibial sesamoid found in _Procavia_, many rodents, edentates and
_Ornithorhynchus_ is a vestigial tibiale, and that the astragalus is
the intermedium.
[112] This perforation of the acetabulum in _Echidna_ is a secondary
character occurring late in development, and consequently is not of
phylogenetic importance.
[113] See R. Owen, "Monograph of the Fossil Mammalia of the Mesozoic
Formation," _Pal. Soc. Mon._ 1871.
H.F. Osborn, "Structure and Affinities of Mesozoic Mammals," _J. of
Philad. Acad._ 1888, vol. IX.
O.C. Marsh, "Jurassic Mammals," _Amer. J. Sci._ 1878 _et seq._
[114] See Oldfield Thomas, _Brit. Mus. Cat. of Marsupialia and
Monotremata_ (1888).
[115] W. Kükenthal, _Anat. Anz._ VI. p. 364, 1891. C. Röse, _Anat.
Anz._ VII. p. 639.
[116] These bones however have no connection with the marsupium, being
nearly equally developed in both male and female. They are simply
sesamoid bones forming ossifications in the inner tendon of the
external oblique muscle, and are developed as supports for the
abdominal wall. Very similar structures have been independently
developed in various Amphibians, Reptiles and monodelphian Mammals.
See W. Leche, _Biol. Fören._ III. p. 120.
[117] See H. Gadow, _P.Z.S._ 1892, p. 361.
[118] See W.H. Flower, "On the Mutual Affinities of the Animals
composing the order Edentata," _P.Z.S._ 1882, p. 358. For the fossil
Edentates of N. America see E. Cope, _Amer. Natural._ 1889; for those
of S. America see various papers by F. Ameghino, H. Burmeister and R.
Owen. Also T.H. Huxley, "On the Osteology of Glyptodon," _Phil.
Trans._ 1865.
[119] See J.F. Brandt, _Symbolae Sirenologicae_, St Petersburg, 1846,
1861, 1868.
[120] Epiphyses are fully developed in _Halitherium_, and traces occur
in _Manatus_.
[121] See P.J. van Beneden and P. Gervais, _Ostéographie des Cétacés_,
1869-80.
[122] H. Wincza, _Morphol. Jahrb._ XVI., p. 647.
[123] See M. Pavlow, "Études sur l'histoire paléontologique des
Ongulés." _Bull. Soc. Moscou_, 1887--1890.
[124] In a _plantigrade_ animal the whole of the foot is placed on the
ground in walking. A _digitigrade_ animal places only its toes on the
ground. An intermediate condition is distinguished by the term
_subplantigrade_.
[125] See p. 345.
[126] See p. 401.
[127] See E.D. Cope, "The Perissodactyla," _Amer. Natural._, 1887.
[128] See E.D. Cope, "The Condylarthra," _Amer. Natural._, 1884, and
"Synopsis of the Vertebrates of the Puerco series," _Tr. Amer. Phil.
Soc._, 1888. O.C. Marsh, "A new order of extinct Eocene Mammals
(Mesodactyla)," _Amer. J. Sci._, 1892.
[129] See O. Thomas, "On the species of Hyracoidea," _P.Z.S._, 1892,
p. 50.
[130] See E.D. Cope, "The Amblypoda," _Amer. Natural._, 1884 and 1885.
[131] See O.C. Marsh, "The Dinocerata," _U.S. Geol. Survey_, 1884,
vol. X.
[132] See O.C. Marsh, _Amer. J. Sci._, 1875 and 1876.
[133] E.D. Cope, "The Creodonta," _Amer. Natural._, 1884. W.B. Scott,
"Revision of the N. American Creodonta," _P. Ac. Philad._, 1892.
[134] See next paragraph.
[135] St G. Mivart, _The Cat_, London, 1881.
[136] St G. Mivart, _P.Z.S._, 1885.
[137] St G. Mivart, "On the Osteology of Insectivora," _J. Anat.
Physiol. norm. path._, 1867 and 1868, and _P.Z.S._, 1871. G.E. Dobson,
_Monograph of the Insectivora_, London, 1882--90.
[138] See G.E. Dobson, _Brit. Mus. Catalogue of Chiroptera_, 1878. See
also other papers by the same author and by Oldfield Thomas.