THE EOCENE PERIOD.
Before commencing the study of the subdivisions of the Kainozoic
series, there are some general considerations to be noted. In
the first place, there is in the Old World a complete and entire
physical break between the rocks of the Mesozoic and Kainozoic
periods. In no instance in Europe are Tertiary strata to be found
resting conformably upon any Secondary rock. The Chalk has invariably
suffered much erosion and denudation before the lowest Tertiary
strata were deposited upon it. This is shown by the fact that the
actually eroded surface of the Chalk can often be seen; or, failing
this, that we can point to the presence of the chalk-flints in the
Tertiary strata. This last, of course, affords unquestionable proof
that the Chalk must have been subjected to enormous denudation
prior to the formation of the Tertiary beds, all the chalk itself
having been removed, and nothing left but the flints, while these
are all rolled and rounded. In the continent of North America,
on the other hand, the lowest Tertiary strata have been shown
to graduate downwards conformably with the highest Cretaceous
beds, it being a matter of difficulty to draw a precise line
of demarcation between the two formations.
In the second place, there is a marked break in the _life_ of
the Mesozoic and Kainozoic periods. With the exception of a few
_Foraminifera_, and one _Brachiopod_ (the latter doubtful), no
Cretaceous species is known to have survived the Cretaceous period;
while several characteristic _families_, such as the _Ammonitidoe,
Belemnitidoe_, and _Hippuritidoe_, died out entirely with the
close of the Cretaceous rocks. In the Tertiary rocks, on the
other hand, not only are all the animals and plants more or less
like existing types, but we meet with a constantly-increasing
number of _living species_ as we pass from the bottom of the
Kainozoic series to the top. Upon this last fact is founded the
modern classification of the Kainozoic rocks, propounded by Sil
Charles Lyell.
The absence in strata of Tertiary age of the chambered Cephalopods,
the Belemnites, the _Hippurites_, the _Inocerami_, and the
diversified types of Reptiles which form such conspicuous features
in the Cretaceous fauna, render the palæontological break between
the Chalk and the Eocene one far too serious to be overlooked. At
the same time, it is to be remembered that the evidence afforded
by the explorations carried out of late years as to the animal
life of the deep sea, renders it certain that the extinction
of marine forms of life at the close of the Cretaceous period
was far less extensive than had been previously assumed. It is
tolerably certain, in fact, that we may look upon some of the
inhabitants of the depths of our existing oceans as the direct,
if modified, descendants of animals which were in existence when
the Chalk was deposited.
It follows from the general want of conformity between the Cretaceous
and Tertiary rocks, and still more from the great difference in
life, that the Cretaceous and Tertiary periods are separated, in
the Old World at any rate, by an enormous lapse of unrepresented
time. How long this interval may have been, we have no means of
judging exactly, but it very possibly was as long as the whole
Kainozoic epoch itself. Some day we shall doubtless find, at some
part of the earth's surface, marine strata which were deposited
during this period, and which will contain fossils intermediate
in character between the organic remains which respectively
characterise the Secondary and Tertiary periods. At present, we
have only slight traces of such deposits--as, for instance, the
Maestricht beds, the Faxöe Limestone, and the Pisolitic Limestone
of France.
CLASSIFICATION OF THE TERTIARY ROCKS.--The classification of the
Tertiary rocks is a matter of unusual difficulty, in consequence
of their occurring in disconnected basins, forming a series of
detached areas, which hold no relations of superposition to one
another. The order, therefore, of the Tertiaries in point of
time, can only be determined by an appeal to fossils; and in
such determination Sir Charles Lyell proposed to take as the
basis of classification the _proportion of living or existing
species of Mollusca which occurs in each stratum or group of
strata_. Acting upon this principle, Sir Charles Lyell divides
the Tertiary series into four groups:--
I. The _Eocene_ formation (Gr. _eos_, dawn; _kainos_, new),
containing the smallest proportion of existing species, and being,
therefore, the oldest division. In this classification, only
the _Mollusca_ are taken into account; and it was found that of
these about three and a half per cent were identical with existing
species.
II. The _Miocene_ formation (Gr. _meion_, less; _kainos_, new),
with more recent species than the Eocene, but _less_ than the
succeeding formation, and less than one-half the total number
in the formation. As before, only the _Mollusca_ are taken into
account, and about 17 per cent of these agree with existing species.
III. The _Pliocene_ formation (Gr. _pleion_, more; _kainos_, new),
with generally _more_ than half the species of shells identical
with existing species--the proportion of these varying from 35
to 50 per cent in the lower beds of this division, up to 90 or
95 per cent in its higher portion.
IV. The _Post-Tertiary Formations_, in which all _the shells
belong to existing species_. This, in turn, is divided into two
minor groups--the _Post-Pliocene_ and _Recent Formations_. In
the _Post-Pliocene_ formations, while all the _Mollusca_ belong
to existing species, most of the _Mammals_ belong to extinct
species. In the Recent period, the quadrupeds, as well as the
shells, belong to living species.
The above, with some modifications, was the original classification
proposed by Sir Charles Lyell for the Tertiary rocks, and now
universally accepted. More recent researches, it is true, have
somewhat altered the proportions of existing species to extinct,
as stated above. The general principle, however, of an increase
in the number of living species, still holds good; and this is as
yet the only satisfactory basis upon which it has been proposed
to arrange the Tertiary deposits.
EOCENE FORMATION.
The Eocene rocks are the lowest of the Tertiary series, and comprise
all those Tertiary deposits in which there is only a small proportion
of existing _Mollusca_--from three and a half to five per cent.
The Eocene rocks occur in several basins in Britain, France,
the Netherlands, and other parts of Europe, and in the United
States. The subdivisions which have been established are extremely
numerous, and it is often impossible to parallel those of one
basin with those of another. It will be sufficient, therefore,
to accept the division of the Eocene formation into three great
groups--Lower, Middle, and Upper Eocene--and to consider some of
the more important beds comprised under these heads in Europe
and in North America.
I. EOCENE OF BRITAIN. (1.) LOWER EOCENE.--The base of the Eocene
series in Britain is constituted by about 90 feet of light-coloured,
sometimes argillaceous sands (_Thanet Sands_), which are of marine
origin. Above these, or forming the base of the formation where these
are wanting, come mottled clays and sands with lignite (_Woolwich
and Reading series_), which are estuarine or fluvio-marine in
origin. The highest member of the Lower Eocene of Britain is the
"London Clay," consisting of a great mass of dark-brown or blue
clay, sometimes with sandy beds, or with layers of "septaria,"
the whole attaining a thickness of from 200 to as much as 500
feet. The London Clay is a purely marine deposit, containing
many marine fossils, with the remains of terrestrial animals and
plants; all of which indicate a high temperature of the sea and
tropical or sub-tropical conditions of the land.
(2.) MIDDLE EOCENE.--The inferior portion of the Middle Eocene
of Britain consists of marine beds, chiefly consisting of sand,
clays, and gravels, and attaining a very considerable thickness
(_Bag-shot and Bracklesham beds_). The superior portion of the
Middle Eocene of Britain, on the other hand, consists of deposits
which are almost exclusively fresh-water or brackish-water in
origin (_Headon and Osborne series_).
The chief Continental formations of Middle Eocene age are the
"Calcaire grossier" of the Paris basin, and the "Nummulitic
Limestone" of the Alps.
(3.) UPPER EOCENE.--If the Headon and Osborne beds of the Isle
of Wight be placed in the Middle Eocene, the only British
representatives of the Upper Eocene are the _Bembridge beds_.
These strata consist of limestones, clays, and marls, which have
for the most part been deposited in fresh or brackish water.
II. EOCENE BEDS OF THE PARIS BASIN.--The Eocene strata are very
well developed in the neighbourhood of Paris, where they occupy
a large area or basin scooped out of the Chalk. The beds of this
area are partly marine, partly freshwater in origin; and the
following table (after Sir Charles Lyell) shows their subdivisions
and their parallelism with the English series:--
GENERAL TABLE OF FRENCH EOCENE STRATA.
UPPER EOCENE.
_French Subdivisions._ _English Equivalents._
A. 1. Gypseous series of Mont 1. Bembridge series.
Montmartre.
A. 2. Calcaire silicieux, or 2. Osborne and Headon series.
Travertin Inférieur.
A. 3. Grès de Beauchamp, or 3. White sand and clay of
Sables Moyens. Barton Cliff, Hants.
MIDDLE EOCENE.
B. 1. Calcaire Grossier. 1. Bagshot and Bracklesham beds.
B. 2. Soissonnais Sands, or 2. Wanting.
Lits Coquilliers.
LOWER EOCENE.
C. 1. Argile de Londres at base 1. London clay.
of Hill of Cassel, near
Dunkirk.
C. 2. Argile plastique and 2. Plastic clay and sand with
lignite. lignite (Woolwich and Reading
series).
C. 3. Stables de Bracheux. 3. Thanet sands.
III. EOCENE STRATA OF THE UNITED STATES.--The lowest member of
the Eocene deposits of North America is the so-called "_Lignitic
Formation_," which is largely developed in Mississippi, Tennessee,
Arkansas, Wyoming, Utah, Colorado, and California, and sometimes
attains a thickness of several thousand feet. Stratigraphically,
this formation exhibits the interesting point that it graduates
downwards insensibly and conformably into the Cretaceous, whilst
it is succeeded _uncomformably_ by strata of Middle Eocene age.
Lithologically, the series consists principally of sands and
clays, with beds of lignite and coal, and its organic remains
show that it is principally of fresh-water origin with a partial
intermixture of marine beds. These marine strata of the "Lignitic
formation" are of special interest, as showing such a commingling
of Cretaceous and Tertiary types of life, that it is impossible
to draw any rigid line in this region between the Mesozoic and
Kainozoic systems. Thus the marine beds of the Lignitic series
contain such characteristic Cretaceous forms as _Inoceramus_
and _Ammonites_, along with a great number of Univalves of a
distinctly Tertiary type (Cones, Cowries, &c.) Upon the whole,
therefore, we must regard this series of deposits as affording a
kind of transition between the Cretaceous and the Eocene, holding
in some respects a position which may be compared with that held
by the Purbeck beds in Britain as regards the Jurassic and
Cretaceous.
The Middle Eocene of the United States is represented by the
_Claiborne_ and _Jackson_ beds. The _Claiborne series_ is extensively
developed at Claiborne, Alabama, and consists of sands, clays,
lignites, marls, and impure limestones, containing marine fossils
along with numerous plant-remains. The _Jackson series_ is
represented by lignitic clays and marls which occur at Jackson,
Mississippi. Amongst the more remarkable fossils of this series
are the teeth and bones of Cetaceans of the genus _Zeuglodon_.
Strata of Upper Eocene age occur in North America at Vicksburg,
Mississippi, and are known as the _Vicksburg series_. They consist
of lignites, clays, marls, and limestones. Freshwater deposits
of Eocene age are also largely developed in parts of the Rocky
Mountain region. The most remarkable fossils of these beds are
Mammals, of which a large number of species have been already
determined.
LIFE OF THE EOCENE PERIOD.
The fossils of the Eocene deposits are so numerous that nothing
more can be attempted here than to give a brief and general sketch
of the life of the period, special attention being directed to some
of the more prominent and interesting types, amongst which--as
throughout the Tertiary series--the Mammals hold the first place.
It is not uncommon, indeed, to speak of the Tertiary period as a
whole under the name of the "Age of Mammals," a title at least
as well deserved as that of "Age of Reptiles" applied to the
Mesozoic, or "Age of Molluscs" applied to the Palæozoic epoch.
As regards the _plants_ of the Eocene, the chief point to be
noticed is, that the conditions which had already set in with
the commencement of the Upper Cretaceous, are here continued,
and still further enforced. The _Cycads_ of the Secondary period,
if they have not totally disappeared, are exceedingly rare; and
the _Conifers_, losing the predominance which they enjoyed in the
Mesozoic, are now relegated to a subordinate though well-defined
place in the terrestrial vegetation. The great majority of the
Eocene plants are referable to the groups of the Angiospermous
Exogens and the Monocotyledons; and the vegetation of the period,
upon the whole, approximates closely to that now existing upon
the earth. The plants of the European Eocene are, however, in the
main most closely allied to forms which are now characteristic
of tropical or sub-tropical regions. Thus, in the London Clay
are found numerous fruits of Palms (_Napdites_, fig. 213), along
with various other plants, most of which indicate a warm climate
as prevailing in the south of England at the commencement of the
Eocene period. In the Eocene strata of North America occur numerous
plants belonging to existing types--such as Palms, Conifers,
the Magnolia, Cinnamon, Fig. Dog-wood, Maple, Hickory, Poplar,
Plane, &c. Taken as a whole, the Eocene flora of North America
is nearly related to that of the Miocene strata of Europe, as
well as to that now existing in the American area. We conclude,
therefore, that "the forests of the American Eocene resembled
those of the European Miocene, and even of modern America" (Dana).
[Illustration: Fig. 213.--_Napadites ellipticus_, the fruit of
a fossil Palm. London Clay, Isle of Sheppey.]
As regards the _animals_ of the Eocene period, the _Protozoans_
are represented by numerous _Foraminifera_, which reach here their
maximum of development, both as regards the size of individuals and
the number of generic types. Many of the Eocene Foraminifers are of
small size; but even these not uncommonly form whole rock-masses.
Thus, the so-called "Miliolite Limestone" of the Paris basin, largely
used as a building-stone, is almost wholly composed of the shells
of a small species of _Miliola_. The most remarkable, however, of
the many members of this group of animals which flourished in
Eocene times, are the "Nummulites" (_Nummulina_), so called from
their resemblance in shape to coins (Lat. _nummus_, a coin). The
Nummulites are amongst the largest of all known _Foraminifera_,
sometimes attaining a size of three inches in circumference;
and their internal structure is very complex (fig. 214). Many
species are known, and they are particularly characteristic of
the Middle and Upper of these periods--their place being sometimes
taken by _Orbitoides_, a form very similar to the Nummulite in
external appearance, but differing in its internal details. In
the Middle Eocene, the remains of Nummulites are found in vast
numbers in a very widely-spread and easily-recognised formation
known as the "Nummulitic Limestone" (fig. 10). According to Sir
Charles Lyell, "the Nummulitic Limestone of the Swiss Alps rises
to more than 10,000 feet above the level of the sea, and attains
here and in other mountain-chains a thickness of several thousand
feet. It may be said to play a far more conspicuous part than
any other Tertiary group in the solid framework of the earth's
crust, whether in Europe, Asia, or Africa. It occurs in Algeria
and Morocco, and has been traced from Egypt, where it was largely
quarried of old for the building of the Pyramids, into Asia Minor,
and across Persia by Bagdad to the mouths of the Indus. It has
been observed not only in Cutch, but in the mountain-ranges which
separate Scinde from Persia, and which form the passes leading
to Cabul; and it has been followed still further eastward into
India, as far as Eastern Bengal and the frontiers of China."
The shells of Nummulites have been found at an elevation of 16,500
feet above the level of the sea in Western Thibet; and the
distinguished and philosophical geologist just quoted, further
remarks, that "when we have once arrived at the conviction that
the Nummulitic formation occupies a middle and upper place in the
Eocene series, we are struck with the comparatively modern date to
which some of the greatest revolutions in the physical geography
of Europe, Asia, and Northern Africa must be referred. All the
mountain-chains--such as the Alps, Pyrenees, Carpathians, and
Himalayas--into the composition of whose central and loftiest parts
the Nummulitic strata enter bodily, could have had no existence
till after the Middle Eocene period. During that period, the
sea prevailed where these chains now rise; for Nummulites and
their accompanying Testacea were unquestionably inhabitants of
salt water."
[Illustration: Fig. 214.--_Nummulina loevigata_. Middle Eocene.]
The _Coelenterates_ of the Eocene are represented principally
by _Corals_, mostly of types identical with or nearly allied to
those now in existence. Perhaps the most characteristic group
of these is that of the _Turbinolidoe_, comprising a number of
simple "cup-corals," which probably lived in moderately deep
water. One of the forms belonging to this family is here figured
(fig. 215). Besides true Corals, the Eocene deposits have yielded
the remains of the "Sea-pens" (_Pennatulidoe_) and the branched
skeletons of the "Sea-shrubs" (_Gorgontidoe_).
The _Echinoderms_ are represented principally by Sea-urchins, and
demand nothing more than mention. It is to be observed, however,
that the great group of the Sea-lilies (_Crinoids_) is now verging
on extinction, and is but very feebly represented.
Amongst the _Mollusca_, the _Polyzoans_ and _Brachiopods_ also
require no special mention, beyond the fact that the latter are
greatly reduced in numbers, and belong principally to the existing
genera _Terebratula_ and _Rhynchonella_. The Bivalves
(_Lamellibranchs_) and the Univalves (_Gasteropods_) are exceedingly
numerous, and almost all the principal existing genera are now
represented; though less than five percent of the Eocene _species_
are identical with those now living. It is difficult to make any
selection from the many Bivalves which are known in deposits of
this age; but species of _Cardita, Crassatella, Leda, Cyrena, Mactra,
Cardium, Psammobia_, &c., may be mentioned as very characteristic.
The _Caradita planicosta_ here figured (fig. 216) is not only very
abundant in the Middle Eocene, but is very widely distributed,
ranging from Europe to the Pacific coast of North America. The
_Univalves_ of the Eocene are extremely numerous, and generally
beautifully preserved. The majority of them belong to that great
section of the _Gasteropods_ in which the mouth of the shell
is notched or produced into a canal (when the shell is said to be
"siphonostomatous")--this section including the carnivorous and
most highly-organized groups of the class. Not only is this the
case, but a large number of the Eocene Univalves belong to types
which now attain their maximum of development in the warmer regions
of the globe. Thus we find numerous species of Cones (_Conus_),
Volutes (_Voluta_), Cowries (_Cyproea_, fig. 218), Olives and
Rice-shells (_Oliva_), Mitre-shells (_Mitra_), Trumpet-shells
(_Triton_), Auger-shells (_Terebra_), and Fig-shells (_Pyrula_).
Along with these are many forms of _Pleurotoma, Rostellaria_,
Spindle-shells (_Fusus_), Dog-whelks (_Nassa_), _Murices_, and
many round-mouthed ("holostomatous") species, belonging to such
genera as _Turritella, Nerita, Natica, Scalaria_, &c. The genus
_Cerithium_ (fig. 219), most of the living forms of which are found
in warm regions, inhabiting fresh or brackish waters, undergoes a
vast development in the Eocene period, where it is represented
by an immense number of specific forms, some of which attain
very large dimensions. In the Eocene strata of the Paris basin
alone, nearly one hundred and fifty species of this genus have been
detected. The more strictly fresh-water deposits of the Eocene
period have also yielded numerous remains of Univalves such as
are now proper to rivers and lakes, together with the shells of
true Land-snails. Amongst these may be mentioned numerous species
of _Limnoea_ (fig. 220), _Physa_ (fig. 221), _Melania, Paludina,
Planorbis, Helix, Bulimus_, and _Cyclostoma_ (fig. 222).
[Illustration: Fig. 215.--_Turbinolia sulcata_, viewed from one
side, and from above. Eocene.]
[Illustration: Fig. 216.--_Cardita planicosta_. Middle Eocene.]
[Illustration: Fig. 217.--_Typhis tubifer_, a "siphonostomatous"
Univalve. Eocene.]
[Illustration: Fig. 218.--Cyproea elegans. Eocene.]
[Illustration: Fig. 219.--_Cerithium hexagonum_. Eocene.]
With regard to the _Cephalopods_, the chief point to be noticed
is, that all the beautiful and complex forms which peculiarly
characterised the Cretaceous period have here disappeared. We no
longer meet with a single example of the Turrilite, the Baculite,
the Hamite, the Scaphite, or the Ammonite. The only exception
to this statement is the occurrence of one species of Ammonite
in the so-called "Lignitic Formation" of North America; but the
beds containing this may possibly be rather referable to the
Cretaceous--and this exception does not affect the fact that
the _Ammonitidoe_, as a family, had become extinct before the
Eocene strata were deposited. The ancient genus _Nautilus_ still
survives, the sole representative of the once mighty order of the
Tetrabranchiate Cephalopods. In the order of the _Dibranchiates_,
we have a like phenomenon to observe in the total extinction
of the great family of the "Belemnites." No form referable to
this group has hitherto been found in any Tertiary stratum; but
the internal skeletons of Cuttle-fishes (such as _Belosepia_)
are not unknown.
[Illustration: Fig. 220.--_Limnoea pyramidalis_. Eocene.]
[Illustration: Fig. 221.--_Physa columnaris_. Eocene.]
[Illustration: Fig. 222.--_Cyclostoma Arnoudii_. Eocene.]
Remains of _Fishes_ are very abundant in strata of Eocene age,
especially in certain localities. The most famous depot for the
fossil fishes of this period is the limestone of Monte Bolca,
near Verona, which is interstratified with beds of volcanic ashes,
the whole being referable to the Middle Eocene. The fishes here
seem to have been suddenly destroyed by a volcanic eruption,
and are found in vast numbers. Agassiz has described over one
hundred and thirty species of Fishes from this locality, belonging
to seventy-seven genera. All the _species_ are extinct; but about
one-half of the _genera_ are represented by living forms. The
great majority of the Eocene Fishes belong to the order of the
"Bony Fishes" (_Teleosteans_), so that in the main the forms
of Fishes characterising the Eocene are similar to those which
predominate in existing seas. In addition to the above, a few
_Ganoids_ and a large number of _Placoids_ are known to occur
in the Eocene rocks. Amongst the latter are found numerous teeth
of true Sharks, such as _Otodus_ (fig. 224) and _Carcharodon_.
The pointed and serrated teeth of the latter sometimes attain
a length of over half a foot, indicating that these predaceous
fishes attained gigantic dimensions; and it is interesting to
note that teeth, in external appearance very similar to those
of the early Tertiary genus _Carcharodon_, have been dredged
from great depths during the recent expedition of the Challenger.
There also occur not uncommonly the flattened teeth of Rays (fig.
225), consisting of flat bony pieces placed close together, and
forming "a kind of mosaic pavement on both the upper and lower
jaws" (Owen).
[Illustration: Fig. 223.--_Rhombus minimus_, a small fossil Turbot
from the Eocene Tertiary, Monte Bolca.]
[Illustration: Fig. 224.--Tooth of _Otodus obliquus_. Eocene.]
[Illustration: Fig. 225.--Flattened dental plates of a Ray
(_Myliobatis Edwardsii_). Eocene.]
In the class of the _Reptiles_, the disappearance of the
characteristic Mesozoic types is as marked a phenomenon as the
introduction of new forms. The Ichthyosaurs, the Plesiosaurs,
the Pterosaurs, and the Mosasaurs of the Mesozoic, find no
representatives in the Eocene Tertiary; and the same is true of the
Deinosaurs, if we except a few remains from the doubtfully-situated
"Lignitic formation" of the United States, On the other hand, all
the modern orders of Reptiles are known to have existed during
the Eocene period. The _Chelonians_ are represented by true marine
Turtles, by "Terrapins" (_Emydidoe_), and by "Soft Tortoises"
(_Trionycidoe_). The order of the Snakes and Serpents (_Ophidia_)
makes its appearance here, for the first time under several
forms--all of which, however, are referable to the non-venomous
group of the "Constricting Serpents" (_Boidoe_). The oldest of
these is the _Paloeophis toliapicus_ of the London Clay of Sheppey,
first made known to science by the researches of Professor Owen.
The nearly-allied _Paloeophis typhoeus_ of the Eocene beds of
Bracklesham appears to have been a Boa-constrictor-like Snake
of about twenty feet in length. Similar Python-like Snakes
(_Paloeophis, Dinophis_, &c.) have been described from the Eocene
deposits of the United States. True Lizards (_Lacertilians_)
are found in some abundance in the Eocene deposits,--some being
small terrestrial forms, like the common European lizards of the
present day; whilst others equal or exceed the living Monitors
in size. Lastly, the modern order of the _Crocodilia_ is largely
represented in Eocene times, by species belonging to all the
existing genera, together with others referable to extinct types.
As pointed out by Owen, it is an interesting fact that in the
Eocene rocks of the south-west of England, there occur fossil
remains of all the three living types of Crocodilians--namely, the
Gavials, the true Crocodiles, and the Alligators (fig. 226)--though
at the present day these forms are all geographically restricted
in their range, and are never associated together.
[Illustration: Fig. 226.--Upper jaw of Alligator. Eocene Tertiary,
Isle of Wight.]
Almost all the existing orders of _Birds_, if not all, are
represented in the Eocene deposits by remains often very closely
allied to existing types. Thus, amongst the Swimming Birds
(_Natatores_) we find examples of forms allied to the living
Pelicans and Mergansers; amongst the Waders (_Grallatores_) we
have birds resembling the Ibis (the _Numenius gypsorum_ of the
Paris basin); amongst the Running Birds (_Cursores_) we meet with
the great _Gastornis Parisiensis_, which equalled the African
Ostrich in height, and the still more gigantic _Dasornis
Londinensis_; remains of a Partridge represent the Scratching
Birds (_Rasores_); the American Eocene has yielded the bones of
one of the Climbing Birds (_Scansores_), apparently referable
to the Woodpeckers; the _Protornis Glarisiensis_ of the Eocene
Schists of Glaris is the oldest known example of the Perching
Birds (_Insessores_); and the Birds of Prey (_Raptores_) are
represented by Vultures, Owls, and Hawks. The toothed Birds of
the Upper Cretaceous are no longer known to exist; but Professor
Owen has recently described from the London Clay the skull of a very
remarkable Bird, in which there is, at any rate, an approximation
to the structure of _Ichthyornis_ and _Hesperornis_. The bird
in question has been named the _Odontopteryx totiapicus_, its
generic title being derived from the very remarkable characters
of its jaws. In this singular form (fig. 227) the margins of
both jaws are furnished with tooth-like denticulations, which
differ from true teeth in being actually portions of the bony
substance of the jaw itself, with which they are continuous, and
which were probably encased by extensions of the horny sheath
of the bill. These tooth-like processes are of two sizes, the
larger ones being comparable to canines; and they are all directed
forwards, and have a triangular or compressed conical form. From
a careful consideration of all the discovered remains of this
bird, Professor Owen concludes that "_Odontopteryx_ was a
warm-blooded feathered biped, with wings; and further, that it
was web-footed and a fish-eater, and that in the catching of
its slippery prey it was assisted by this Pterosauroid armature
of its jaws." Upon the whole, _Odontopteryx_ would appear to be
most nearly related to the family of the Geese (_Anserinoe_)
or Ducks (_Anatidoe_); but the extension of the bony substance
of the jaws into tooth-like processes is an entirely unique
character, in which it stands quite alone.
[Illustration: Fig. 227.--Skull of _Odontopteryx toliapicus restored.
(After Owen.)]
The known _Mammals_ of the Mesozoic period, as we have seen,
are all of small size; and with one not unequivocal exception,
they appear to be referable to the order of the Pouched Quadrupeds
(_Marsupials_), almost the lowest group of the whole class of
the Mammalia. In the Eocene rocks, on the other hand, numerous
remains of Quadrupeds have been brought to light, representing
most of the great Mammalian orders now in existence upon the
earth, and in many cases indicating animals of very considerable
dimensions. We are, in fact, in a position to assert that the
majority of the great groups of Quadrupeds with which we are
familiar at the present day were already in existence in the
Eocene period, and that their ancient root-stocks were even in
this early time separated by most of the fundamental differences
of structure which distinguish their living representatives.
At the same time, there are some amongst the Eocene quadrupeds
which have a "generalised" character, and which may be regarded
as structural types standing midway between groups now sharply
separated from one another.
The order of the _Marsupials_--including the existing Kangaroos,
Wombats, Opossums, Phalangers, &c.--is poorly represented in
deposits of Eocene age. The most celebrated example of this group
is the _Didelphys gypsorum_ of the Gypseous beds of Montmartre,
near Paris, an Opossum very nearly allied to the living Opossums
of North and South America.
No member of the _Edenates_ (Sloths, Ant-eaters, and Armadillos)
has hitherto been detected in any Eocene deposit. The aquatic order
of the _Sirenians_ (Dugongs and Manatees), with their fish-like
bodies and tails, paddle-shaped forelimbs, and wholly deficient
hind-limbs, are represented in strata of this age by remains of
the ancient "Sea-Cows," to which the name of _Halitherium_ has
been applied. Nearly allied to the preceding is the likewise aquatic
order of the Whales and Dolphins (_Cetaceans_), in which the body
is also fish-like, the hind-limbs are wanting, the fore-limbs are
converted into powerful "flippers" or swimming-paddles, and the
terminal extremity of the body is furnished with a horizontal,
tail-fin. Many existing Cetaceans (such as the Whalebone Whales)
have no true teeth; but others (Dolphins, Porpoises, Sperm Whales)
possess simple conical teeth. In strata of Eocene age, however, we
find a singular group of Whales, constituting the genus _Zeuglodon
(fig. 228), in which the teeth differed from those of all existing
forms in being of two kinds,--the front ones being conical incisors,
whilst the back teeth or molars have serrated triangular crowns,
and are inserted in the jaw by two roots. Each molar (fig. 228,
A) looks as if it were composed of two separate teeth united on
one side by their crowns; and it is this peculiarity which is
expressed by the generic name (Gr. _zeugle_, a yoke; _odous_,
tooth). The best-known species of the genus is the _Zeuglodon
cetoides_ of Owen, which attained a length of seventy feet. Remains
of these gigantic Whales are very common in the "Jackson Beds" of
the Southern United States. So common are they that, according
to Dana, "the large vertebræ, some of them a foot and a half
long and a foot in diameter, were formerly so abundant over the
country, in Alabama, that they were used for making walls, or
were burned to rid the fields of them."
[Illustration: Fig. 228.--_Zeuglodon cetoides_. A, Molar tooth of
the natural size; B, Vertebra, reduced in size. From the Middle
Eocene of the United States. (After Lyell.)]
The great and important order of the Hoofed Quadrupeds (_Ungulata_)
is represented in the Eocene by examples of both of its two principal
sections--namely, those with an uneven number of toes (one or three)
on the foot (_Perissodactyle Ungulates_), and those with an even
number of toes (two or four) to each foot (_Artiodactyle Ungulates_).
Amongst the Odd-toed Ungulates, the living family of the Tapirs
(_Tapirdoe_) is represented by the genus _Coryphodon_ of Owen.
Nearly related to the preceding are the species of _Paloeotherium_,
which have a historical interest as being amongst the first of
the Tertiary Mammals investigated by the illustrious Cuvier.
Several species of _Paloeothere_ are known, varying greatly in
size, the smallest being little bigger than a hare, whilst the
largest must have equalled a good-sized horse in its dimensions. The
species of _Paloeotherium_ appear to have agreed with the existing
Tapirs in possessing a lengthened and flexible nose, which formed
a short proboscis or trunk (fig. 229), suitable as an instrument
for stripping off the foliage of trees--the characters of the
molar teeth showing them to have been strictly herbivorous in
their habits. They differ, however, from the Tapirs, amongst
other characters, in the fact that both the fore and the hind
feet possessed three toes each; whereas in the latter there are
four toes on each fore-foot, and the hind-feet alone are three-toed.
The remains of _Paloeotheria_ have been found in such abundance in
certain localities as to show that these animals roamed in great
herds over the fertile plains of France and the south of England
during the later portion of the Eocene period. The accompanying
illustration (fig. 229) represents the notion which the great
Cuvier was induced by his researches to form as to the outward
appearance of _Paloeotherium magnum_. Recent discoveries, however,
have rendered it probable that this restoration is in some important
respects inaccurate. Instead of being bulky, massive, and more
or less resembling the living Tapirs in form, it would rather
appear that _Paloeotherium magnum_ was in reality a slender,
graceful, and long-necked animal, more closely resembling in
general figure a Llama, or certain of the Antelopes.
[Illustration: Fig. 229.--Outline of _Paloeotherium magnum_,
restored. Upper Eocene, Europe. (After Cuvier.)]
The singular genus _Anchitherium_ forms a kind of transition
between the _Paloeotheria_ and the true Horses (_Equidoe_). The
Horse (fig. 230, D) possesses but one fully-developed toe to
each foot, this being terminated by a single broad hoof, and
representing the _middle_ toe--the _third_ of the typical
five-fingered or five-toed limb of Quadrupeds in general. In
addition, however, to this fully-developed toe, each foot in the
horse carries two rudimentary toes which are concealed beneath the
skin, and are known as the "splint-bones." These are respectively
the _second_ and _fourth_ toes, in an aborted condition; and the
first and fifth toes are wholly wanting. In _Hipparion_ (fig.
230, C), the foot is essentially like that of the modern Horses,
except that the second and fourth toes no longer are mere
"splint-bones," hidden beneath the skin; but have now little
hoofs, and hang freely, but uselessly, by the side of the great
middle toe, not being sufficiently developed to reach the ground.
In _Anchitherium_, again (fig. 230, B), the foot is three-toed,
like that of _Hipparion_; but the two lateral toes (the second
and fourth) are so far developed that they now reach the ground.
The _first_ digit (thumb or great toe) is still wanting; as also
is the _fifth_ digit (little finger or little toe). Lastly, the
Eocene rocks have yielded in North America the remains of a small
Equine quadruped, to which Marsh has given the name of _Orohippus_.
In this singular form--which was not larger than a fox--the foot
(fig. 230, A) carries _four_ toes, all of which are hoofed and touch
the ground, but of which the _third_ toe is still the largest. The
_first_ toe (thumb or great toe) is still wanting; but in this
ancient representative of the Horses, the _fifth_ or "little"
toe appears for the first time. As all the above-mentioned forms
succeed one another in point of time, it may be regarded as probable
that we shall yet be able to point, with some certainty, to some
still older example of the _Equidoe_, in which the first digit
is developed, and the foot assumes its typical five-fingered
condition.
[Illustration: Fig. 230.--Skeleton of the foot in various forms
belonging to the family of the _Equidoe_. A, Foot of _Orohippus_,
Eocene; B, Foot of _Anchitherium</>, Upper Eocene and Lower Miocene;
C, Foot of _Hipparion_, Upper Miocene and Pliocene: D, Foot of
Horse (_Equus_), Pliocene and Recent. The figures indicate the
numbers of the digits in the typical five-fingered hand of Mammals.
(After Marsh.)]
Passing on to the Even-toed or _Artiodactyle Ungulates_, no
representative of the _Hippotamus_ seems yet to have existed, but
there are several forms (_Choeropotamus, Hyopotamus_, &c.) more
or less closely allied to the Pigs (_Suida_); and the singular
group of the _Anoplotheridoe_ may be regarded as forming a kind of
transition between the Swine and the Ruminants. The _Anoplotheria_
(fig. 231) were slender in form, the largest not exceeding a
donkey in size, with long tails, and having the feet terminated
by two hoofed toes each, sometimes with a pair of small accessory
hoofs as well. The teeth exhibit the peculiarity that they are
arranged in a continuous series, without any gap or interval
between the molars and the canines; and the back teeth, like
those of all the Ungulates, are adapted for grinding vegetable
food, their crowns resembling in form those of the true Ruminants.
The genera _Dichobune_ and _Xiphodon_, of the Middle and Upper
Eocene, are closely related to _Anoplotherium_, but are more
slender and deer-like in form. No example of the great Ruminant
group of the Ungulate Quadrupeds has as yet been detected in
deposits of Eocene age.
[Illustration: Fig. 231.--_Anoplotherium commune_. Eocene Tertiary,
France. (After Cuvier.)]
Whilst true Ruminants appear to be unknown, the Eocene strata
of North America have yielded to the researches of Professor
Marsh examples of an extraordinary group (_Dinocerata_), which
may be considered as in some respects intermediate between the
Ungulates and the Proboscideans. In _Dinoceras_ itself (fig.
232) we have a large animal, equal in dimensions to the living
Elephants, which it further resembles in the structure of the
massive limbs, except that there are only four toes to each foot.
The upper jaw was devoid of front teeth, but there were two very
large canine teeth, in the form of tusks directed perpendicularly
downwards; and there was also a series of six small molars on each.
Each upper jaw-bone carried a bony projection, which was probably
of the nature of a "horn-core," and was originally sheathed in
horn. Two similar, but smaller, horn-cores are carried on the
nasal bones; and two much larger projections, also probably of
the nature of horn-cores, were carried upon the forehead. We may
thus infer that _Dinoceras_ possessed three pairs of horns, all
of which resembled the horns of the Sheep and Oxen in consisting
of a central bony "core," surrounded by a horny sheath. The nose
was not prolonged into a proboscis or "trunk," as in the existing
Elephants; and the tail was short and slender. Many forms of
the _Dinocerata_ are known; but all these singular and gigantic
quadrupeds appear to have been confined to the North American
continent, and to be restricted to the Eocene period.
[Illustration: Fig. 232.--Skull of _Dinoceras mirabilis_, greatly
reduced. Eocene, North America. (After Marsh.)]
The important order of the Elephants (_Proboscidea_) is also not
known to have come into existence during the Eocene period. On the
other hand, the great order of the Beasts of Prey (_Carnivora_)
is represented in Eocene strata by several forms belonging to
different types. Thus the _Ardocyon_ presents us with an Eocene
Carnivore more or less closely allied to the existing Racoons;
the _Paloeonyctis_ appears to be related to the recent Civet-cats;
the genus _Hyoenodon_ is in some respects comparable to the living
Hyænas; and the _Canis Parisiensis_ of the gypsum-bearing beds
of Montmartre may perhaps be allied to the Foxes.
[Illustration: Fig. 233.--Portion of the skeleton of _Vespertilio
Parisienis_. Eocene Tertiary, France.]
The order of the Bats (_Cheiroptera_) is represented in Eocene
strata of the Paris basin (Gypseous series of Montmartre) by
the _Vespertilio Parisiensis_ (fig. 233), an insect-eating Bat
very similar to some of the existing European forms. Lastly, the
Eocene deposits have yielded more or less satisfactory evidence
of the existence in Europe at this period of examples of the
orders of the Gnawing Mammals (_Rodentia_), the Insect-eating
Mammals (_Insectivora_), and the Monkeys (_Quadrumana_).[24]
[Footnote 24: A short list of the more important works relating to
the Eocene rocks and fossils will be given after all the Tertiary
deposits have been treated of.]