Chapter 45

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).

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."

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).

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.

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).

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.

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.

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."

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.

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.

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.

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.

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.

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.]