W
William Smith, the Father of English Geology, used to apologize for the study of palaeontology by claiming that “the search for a fossil is at least as rational a proceeding as the pursuit of a hare.” Those of us who are accustomed to take the field, armed with a hammer, in the search for “medals of creation” and from time to time have experienced the sporting enjoyment of bringing to light a rare or perfect specimen are quite prepared to support his claim. But the student of fossils needs the help of a text book to guide him to the literature on the subject, to help him with his identifications or to indicate that some of his finds are new and hitherto undescribed. European and American workers have long been provided with excellent books treating generally of fossils, but the illustrations have been quite naturally taken mainly from forms occurring in the Northern Hemisphere. Our own fossil forms both plants and animals are numerous, interesting and in many cases peculiar, but the literature concerning them is so widely scattered in variousscientific publications that a warm welcome should be given to this book of Mr. Chapman’s, in which the Australian evidence is brought together and summarised by one, whose training, long experience, and personal research qualify him to undertake the task. Especially will teachers and students of Geology and Palaeontology value such an undertaking. Workers in other countries who have only partial access to the Australian literature on the subject should also find this a valuable book of reference.
In the study of fossils we are concerned with the nature, evolution and distribution of the former inhabitants of the earth. The study of Palaeontology may be justified as a means of scientific discipline, for the contributions the subject makes to the increase of natural knowledge and the unfolding of panoramas of ancient life. It also provides perhaps the most positive evidence in the story of evolution. So, too, the student of the present day distribution of animals and plants finds the key to many a problem in zoo-geography in the records of past migrations yielded by the study of fossils in different lands. The stratigraphical geologist is of course principally concerned with two important aspects of the study of fossils.
The masterly generalisation of William Smith that strata can be identified by their fossil contents established by close study of the rocks and fossils of the British Oolites has been confirmed generally by subsequent work. The comparative study of the fossil contents of rocks in widely separated areas has proved to be the most valuable means by which thecorrelation of the rocks can be effected and their identity of age established. In some cases the recognition of a single fossil species in two areas separated, perhaps, by thousands of miles may suffice to demonstrate that the rocks are of the same age. For example, a graptolite such asPhyllograptus typusis found in many parts of the world, but has only a very restricted range in time. It has been found only in rocks of Lower Ordovician age. Its occurrence in Wales and in the rocks of Bendigo practically suffices to establish the identity in age of the rocks in these widely separated areas.
Generally, however, much closer study and a more detailed examination of a large number of the fossils of a rock series are required before the age of the rocks can be surely established and a safe correlation made with distant localities.
The stratigraphical generalisations to be made from the study of fossils however must be qualified by certain considerations. Among these are the fact that our knowledge of the life forms of a given geological period is necessarily incomplete, that the differences in the fossil contents of rocks may depend not only on differences of age but also in the conditions under which the organisms lived and the rocks were accumulated, and that forms of life originating in one area do not spread themselves immediately over the earth but migrate at velocities depending on their mode of life and the presence or absence of barriers to their progress.
Our incomplete knowledge of the forms living in remote geological periods arises partly from the factthat some forms had no permanent skeleton and were therefore incapable of preservation, partly to the obliteration of the skeletons of organisms through subsequent earth movements in the rocks or through the solvent action of water. Many land forms, too, probably disintegrated on the surface before deposits were formed over the area. Apart from these causes which determine that a full knowledge of the fossils from ancient rocks in particular, will never be acquired, our knowledge is incomplete by reason either of difficulty of access to certain areas or incomplete search. As a result of later discoveries earlier conclusions based on incomplete evidence as to the age of a rock series, have not infrequently been modified.
The study of the present distribution of animals and plants over the earth is a help in the attempt to decide how far the fossil differences in the sets of rocks are due to differences in the ages of the rocks or to differences in the conditions under which the organisms lived. The present, in this, as in many other geological problems, is the key to the past.
We know, for instance, that differences of climate largely control the geographical distribution of land animals and especially of land plants, and for that reason among others, fossil plants are generally less trustworthy guides to geological age than fossil animals.
In the distribution of marine animals at the present day we find that organisms of simple structure are generally more wide-spread and less susceptible to changes in their environment than are the more complex organisms with specialised structures. Hence we find, for instance, a fossil species of theForaminifera may persist unchanged through several geological periods, while a species of fossil fish has in general not only a short range in time but often a restricted geographical extent. If we consider the marine organisms found at the present day we find a number of free-swimming forms very widely distributed, while a large number are restricted either by reason of climate or of depth. Certain organisms are only to be found between high and low tide levels, others between low tide level and a depth of thirty fathoms, while many quite different forms live in deeper waters. If we confine our attention to shallow-water marine forms we note that certain forms are at the present day restricted to waters of a certain temperature. We find, therefore, a contrast between arctic and tropical faunas, while other types characterize temperate latitudes. Climatic and bathymetrical differences at the present day therefore lead to distinct differences in the distribution of certain organisms, while other forms, less sensitive to these factors, range widely and may be almost universally distributed. Similar conditions obtained in past geological times, and therefore in attempting to correlate the rocks of one area with those of another those fossils which are most wide-spread are often found to be the most valuable.
Attention should also be paid to the conditions under which the deposits accumulated, since it is clear that rocks may be formed at the same time in different areas and yet contain many distinct fossils by reason of climatic or bathymetrical differences. Among living marine organisms we find certain forms restricted to sandy or muddy sea-bottoms and othersto clear water, and these changes in the conditions of deposition of sediment have played their part in past geological periods in determining differences in the fossil faunas of rocks which were laid down simultaneously. We not infrequently find mudstones passing laterally into limestones, and this lithological change is always accompanied by a more or less notable change in the fossil contents of the two rock types. Such facts emphasize the close connection between stratigraphy and palaeontology, and indicate that the successful tracing out of the geological history of any area is only possible when the evidence of the stratigrapher is reinforced by that provided by the palaeontologist. The fact that species of animals and plants which have been developed in a particular area do not spread all over the world at once but migrate very slowly led Huxley many years ago to put forward his hypothesis of “homotaxis.” He agreed that when the order of succession of rocks and fossils has been made out in one area, this order and succession will be found to be generally similar in other areas. The deposits in two such contrasted areas are homotaxial, that is, show a similarity of order, but, he claimed, are not necessarily synchronous in their formation. In whatever parts of the world Carboniferous, Devonian and Silurian fossils may be found, the rocks with Carboniferous fossils will be found to overlie those with Devonian, and these in their turn rest upon those containing Silurian fossils. And yet Huxley maintained that if, say, Africa was the area in which faunas and floras originated, the migration of a Silurian fauna and flora might take place so slowlythat by the time it reached Britain the succeeding Devonian forms had developed in Africa, and when it reached North America, Devonian forms had reached Britain and Carboniferous forms had developed in Africa. If this were so a Devonian fauna and flora in Britain may have been contemporaneous with Silurian life in North America and with a Carboniferous fauna and flora in Africa.
This could only be true if the time taken for the migration of faunas and floras was so great as to transcend the boundaries between great geological periods. This does not appear to be the case, and Huxley’s idea in its extreme form has been generally abandoned. At the same time certain anomalies in the range in time of individual genera have been noted, and may possibly be explained on such lines. For instance, among the group of the graptolites, in Britain the genusBryograptusoccurs only in the Upper Cambrian and the genusLeptograptusonly in the Upper Ordovician rocks. In Victoria these two genera, together with typical Lower Ordovician forms, may be found near Lancefield preserved on a single slab of shale. In the same way, in a single quarry in Triassic rocks in New South Wales, a number of fossil fish have been found and described, some of which have been compared to Jurassic, others to Permian, and others to Carboniferous forms in the Northern Hemisphere.
Another point which the palaeontologist may occasionally find evidence for is the existence of “biological asylums,” areas which by means of land or other barriers may be for a long period separated from the main stream of evolution. We know thatthe present fauna and flora of Australia is largely of archaic aspect, as it includes a number of types which elsewhere have long ago become extinct or were never developed. This appears to be due to the long isolation of Australia and, as Professor Gregory happily puts it—its “development in a biological backwater.” We have some evidence that similar asylums have existed in past geological periods, with the result that in certain areas where uniform conditions prevailed for a long time or where isolation from competition prevented rapid evolution, some organisms which became extinct in other areas, persisted unchanged in the “asylum” into a younger geological period.
The broad generalizations that rocks may be identified by their fossil contents and that the testimony of the rocks demonstrates the general order of evolution from simple to complex forms, have only been placed on a surer footing by long continued investigations. The modifications produced by conditions of deposit, of climate and of natural barriers to migration, while introducing complexities into the problems of Palaeontology, are every year becoming better known; and when considered in connection with the variations in the characters of the rocks, provide valuable and interesting evidence towards the solution of the ultimate problems of geology and palaeontology, which include the tracing out of the evolution of the history of the earth from the most remote geological period to that point at which the geologist hands over his story to the archaeologist, the historian, and the geographer.
ERNEST W. SKEATS.
GENERAL PRINCIPLES.
NATURE AND USES OF FOSSILS.
Scope of Geology.—
T
The science of GEOLOGY, of which PALAEONTOLOGY or the study of fossils, forms a part, is concerned with the nature and structure of the earth, the physical forces that have shaped it, and the organic agencies that have helped to build it.
Nature of Fossils.—
The remains of animals and plants that formerly existed in the different periods of the history of the earth are spoken of as fossils. They are found, more or less plentifully, in such common rocks as clays, shales, sandstones, and limestones, all of which are comprised in the great series of Sedimentary Rocks (Fig. 1).
According to the surroundings of the organisms, whether they existed on land, in rivers, lakes, estuaries, or the sea, they are spoken of as belonging to terrestrial, fluviatile, lacustrine, estuarine, or marine deposits.
Fig. 1—Fossil Shells Embedded in Sandy Clay.About3/4nat. size. Of Cainozoic or Tertiary Age (Kalimnan Series).Grange Burn, near Hamilton, Victoria.(F.C. Coll.)(G = Glycimeris. L = Limopsis. N = Natica).
Fig. 1—Fossil Shells Embedded in Sandy Clay.About3/4nat. size. Of Cainozoic or Tertiary Age (Kalimnan Series).Grange Burn, near Hamilton, Victoria.(F.C. Coll.)(G = Glycimeris. L = Limopsis. N = Natica).
(F.C. Coll.)
Fig. 2—Tracks probably of Crustaceans (Phyllocarids).About3/4nat. size. Impression of a Slab of Upper Ordovician Shale. Diggers’ Rest, Victoria.(F.C. Coll.)
Fig. 2—Tracks probably of Crustaceans (Phyllocarids).About3/4nat. size. Impression of a Slab of Upper Ordovician Shale. Diggers’ Rest, Victoria.(F.C. Coll.)
(F.C. Coll.)
The name fossil, from the Latin ‘fodere’ to dig,—‘fossilis,’ dug out,—is applied to the remains of any animals or plants which have been buried either in sediments laid down in water, in materials gathered together by the wind on land as sand-dunes, in beds of volcanic ash, or in cave earths. But not only remains of organisms are thus called fossils, for the name is also applied to structures only indirectly connected with once living objects, such as rain-prints, ripple-marks, sun-cracks, and tracks or impressions of worms and insects (Fig. 2).
Preservation of Fossils.—
In ordinary terms, fossils are the durable parts of animals and plants which have resisted complete decay by being covered over with the deposits above-named. It is due, then, to the fact that they have been kept from the action of the air, with its destructive bacteria, that we are able to still find these relics of life in the past.
Petrifaction of Fossils.—
When organisms are covered by a tenacious mud, they sometimes undergo no further change. Very often, however, moisture containing mineral matter such as carbonate of lime or silica, percolates through the stratum which contains the fossils, and then they not only have their pores filled with the mineral, but their actual substance may also undergo a molecular change, whereby the original composition of the shell or the hard part is entirely altered. This tends almost invariably to harden the fossils still further, which change of condition is called petrifaction, or the making into stone.
Fig. 3. Thin Slice of Petrified or Silicified Wood in Tangential Section.Araucarioxylon Daintreei, Chapm. = Dadoxylon australe, Arber;× 28. Carbopermian: Newcastle, New South Wales.(Nat. Mus. Coll.)
Fig. 3. Thin Slice of Petrified or Silicified Wood in Tangential Section.Araucarioxylon Daintreei, Chapm. = Dadoxylon australe, Arber;× 28. Carbopermian: Newcastle, New South Wales.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Structure Preserved.—
Petrifaction does not necessarily destroy the structure of a fossil. For example, a piece of wood, which originally consisted of carbon, hydrogen, and nitrogen, may be entirely replaced by flint or silica: and yet the original structure of the wood may be so perfectly preserved that when a thin slice of the petrifaction is examined under a high power of the microscope, the tissues with their component cells are seen and easily recognised (Fig. 3).
Early Observers.—
Remains of animals buried in the rocks were known from the earliest times, and frequent references to these were made by the ancient Greek and Roman philosophers.
Xenophanes.—
Xenophanes, who lived B.C. 535, wrote of shells,fishes and seals which had become dried in mud, and were found inland and on the tops of the highest mountains. The presence of these buried shells and bones was ascribed by the ancients to a plastic force latent in the earth itself, while in some cases they were regarded as freaks of nature.
Leonardo da Vinci.—
In the sixteenth and seventeenth centuries Italian observers came to the fore in clearly demonstrating the true nature of fossils. This was no doubt due in part to the fact that the Italian coast affords a rich field of observation in this particular branch of science. The celebrated painter Leonardo da Vinci (early part of the sixteenth century), who carried out some engineering works in connection with canals in the north of Italy, showed that the mud brought down by rivers had penetrated into the interior of shells at a time when they were still at the bottom of the sea near the coast.
Steno.—
In 1669, Steno, a Danish physician residing in Italy, wrote a work on organic petrifactions which are found enclosed in solid rocks, and showed by his dissection of a shark which had been recently captured and by a comparison of its teeth with those found fossil in the cliffs, that they were identical. The same author also pointed out the resemblance between the shells discovered in the Italian strata and those living on the adjacent shores. It was not until the close of the eighteenth century, however, that the study of fossil remains received a decided impetus. It is curious to note that many of these laterauthors maintained the occurrence of a universal flood to account for the presence of fossil shells and bones on the dry land.
Fig. 4—William Smith (1769-1839.)“The Father of English Geology,” at the age of 69.(From Brit. Mus. Cat.)
Fig. 4—William Smith (1769-1839.)“The Father of English Geology,” at the age of 69.(From Brit. Mus. Cat.)
(From Brit. Mus. Cat.)
Fossils an Index to Age.—
A large part of the credit of showing how fossils are restricted to certain strata, and help to fix the succession and age of the beds, is due to the English geologist and surveyor, William Smith (Fig. 4). “The Father of English Geology,” as he has been called, published two works[1]in the early part of last century, in which he expressed his view of the value of fossils to the geologist and surveyor, and showed that there was a regular law of superposition of one bed upon another, and that strata could be identified at distant localities by their included fossils. Uponthis foundation the work of later geologists has been firmly established; and students of strata and of fossils work hand in hand.
[1]“Strata identified by Organised Fossils,” 1816-1819; and “Stratigraphical System of Organised Fossils,” 1817.
[1]“Strata identified by Organised Fossils,” 1816-1819; and “Stratigraphical System of Organised Fossils,” 1817.
Stratigraphy.—
That branch of geology which discusses the nature and relations of the various sediments of the earth’s crust, and the form in which they were laid down, is called Stratigraphy. From it we learn that in bygone times many of those places that are now occupied by dry land have been, often more than once, covered by the sea; and thus Tennyson’s lines are forcibly brought to mind—
“There where the long street roars hath beenThe stillness of the central sea.”
Elevated Sea-beds.—
A striking illustration in proof of this emergence of the land from the sea is the occurrence of marine shells similar to those now found living in the sea, in sea-cliffs sometimes many hundreds of feet above sea-level. When these upraised beds consist of shingle or sand with shore-loving shells, as limpets and mussels, they are spoken of as Raised Beaches. Elevated beaches are often found maintaining the same level along coast-lines for many miles, like those recorded by Darwin at Chili and Peru, or in the south of England (Fig. 5). They also occur intermittently along the Victorian coast, especially around the indents, where they have survived the wear and tear of tides along the coast line (Fig. 6). They are also a common feature, as a capping, on many coral islands which have undergone elevation.
Fig. 5—A Raised Beach at Black Rock, Brighton, England.(Original).
Fig. 5—A Raised Beach at Black Rock, Brighton, England.(Original).
(Original).
Fig. 6—Raised Beach (a) and Native Middens (b)Torquay, Victoria.(Original).
Fig. 6—Raised Beach (a) and Native Middens (b)Torquay, Victoria.(Original).
(Original).
Fig. 7—Marine Fossils (Orthis flabellulum, Sowerby.)About nat. size. In Volcanic Tuff of Ordovician Age. From the Summit of Snowdon, North Wales, at an elevation of 3571 feet above sea level.(F.C. Coll.)
Fig. 7—Marine Fossils (Orthis flabellulum, Sowerby.)About nat. size. In Volcanic Tuff of Ordovician Age. From the Summit of Snowdon, North Wales, at an elevation of 3571 feet above sea level.
(F.C. Coll.)
Sea-beds far from the Present Coast.—
Marine beds of deeper water origin may be found not only close to the coast-line, but frequently on the tops of inland hills some miles from the sea-coast. Their included sea-shells and other organic remains are often found covered by fine sediment forming extensive beds; and they may frequently occur in the position in which they lived and died (Fig. 7). Although it is well known that sea-birds carry shell-fish for some distance inland, yet this would not account for more than a few isolated examples.
Raised Beaches as Distinct from Middens.—
Again, it may be argued that the primitive inhabitants of countries bordering the coast were in the habit of piling up the empty shells of the edible molluscs used by them for food: but these “kitchen middens” are easily distinguished from fossil deposits like shelly beaches, by the absence of stratified layers; and, further, by the shells being confined to edible species, as the Cockle (Cardium), the Blood-cockle (Arca), the Mussel (Mytilus), and the Oyster (Ostrea) (Fig. 8).
Fig. 8—Remains of Edible Shell Fish(Kitchen-midden—native, mirrn-yong)in Sand Dunes near Spring Creek, Torquay, Victoria.(Original).
Fig. 8—Remains of Edible Shell Fish(Kitchen-midden—native, mirrn-yong)in Sand Dunes near Spring Creek, Torquay, Victoria.(Original).
(Original).
Fig. 9—Part of a Submerged Forestseen at low water on the Cheshire coast at Leasowe, England.(From Seward’s “Fossil Plants”)
Fig. 9—Part of a Submerged Forestseen at low water on the Cheshire coast at Leasowe, England.(From Seward’s “Fossil Plants”)
(From Seward’s “Fossil Plants”)
Submerged Forests.—
Evidence of change in the coast-line is shown by the occurrence of submerged forest-land, known as “fossil forests,” which consist of the stumps of trees still embedded in the black, loamy soil. Such forests,when of comparatively recent age, are found near the existing coast-line, and may sometimes extend for a considerable distance out to sea (Fig. 9).
From the foregoing we learn that:—
1.—Fossils afford data of the various Changes that have taken place in past times in the Relative Positions of Land and Water.
Changes of Climate in the Past.—
At the present day we find special groups of animals (fauna), and plants (flora), restricted to tropical climates; and others, conversely, to the arctic regions. Cycads and tree-ferns, for example, seem to flourish best in warm or sub-tropical countries: yet in past times they were abundant in northern Europe in what are now temperate and arctic regions, as in Yorkshire, Spitzbergen, and Northern Siberia, where indeed at one time they formed the principal flora.
The rein-deer and musk-sheep, now to be found only in the arctic regions, once lived in the South of England, France and Germany. The dwarf willow (Salix polaris) and an arctic moss (Hypnum turgescens), now restricted to the same cold region, occur fossil in the South of England.
In Southern Australia and in New Zealand, the marine shells which lived during the earlier and middle Tertiary times belong to genera and species which are indicative of a warmer climate than that now prevailing; this ancient fauna being like that met with in dredging around the northern coasts of Australia (Fig. 10).
Fig. 10—A Fossil Shell (Pecten murrayanus, Tate).Of Oligocene to Lower Pliocene Age in Southern Australia; closely allied to, if not identical with, a species living off the coast of Queensland. About nat. size.(F.C. Coll.)
Fig. 10—A Fossil Shell (Pecten murrayanus, Tate).Of Oligocene to Lower Pliocene Age in Southern Australia; closely allied to, if not identical with, a species living off the coast of Queensland. About nat. size.(F.C. Coll.)
(F.C. Coll.)
From the above evidence we may say that:—
2.—Fossils teach us that in Former Times the Climate of certain parts of the earth’s surface was Different from that now existing.
Fossils as Guides to Age of Strata.—
In passing from fossil deposits of fairly recent origin to those of older date, we find the proportion of living species gradually diminish, being replaced by forms now extinct. After this the genera themselves are replaced by more ancient types, and if we penetrate still deeper into the series of geological strata, even families and orders of animals and plants give place to others entirely unknown at the present day.
From this we conclude that:—
3.—Fossil Types, or Guide Fossils, are of great value in indicating the Relative Age of Geological Formations.
Gradual Evolution of Life-forms from Lower to Higher Types.—
As a general rule the various types of animals and plants become simpler in organisation as we descend the geological scale. For example, in the oldest rocks the animals are confined to the groups of Foraminifera, Sponges, Corals, Graptolites, Shell-fish and Trilobites, all back-boneless animals: whilst it was not until the Devonian period that the primitive fishes appeared as a well-defined group; and in the next formation, the Carboniferous Series, the first traces of the Batrachians (Frog-like animals) and Reptiles are found. Birds do not appear, so far as their remains are known, until near the close of the Jurassic; whilst Mammals are sparsely represented by Monotremes and Marsupials in the Triassic and Jurassic, becoming more abundant in Cainozoic times, and by the Eutheria (Higher Mammals) from the commencement of the Eocene period.
It is clear from the above and other facts in the geological distribution of animal types that:—
4.—The Geological Record supports in the main the Doctrine of Evolution from Simpler to more Complex types; and fossils throw much light upon the Ancestry of Animals and Plants now found Living.
THE CLASSIFICATION OF FOSSIL ANIMALS AND PLANTS.
A
An elementary knowledge of the principles underlying the classification of animals and plants is essential to the beginner in the study of fossils.
The Naming of Animals.—
In order to make a clearly understood reference to an animal, or the remains of one, it is as necessary to give it a name as it is in the case of a person or a place. Before the time of Linnaeus (1707-1778), it was the custom to refer, for example, to a shell, in Latin[2]as “the little spiral shell, with cross markings and tubercles, like a ram’s horn;” or to a worm as “the rounded worm with an elevated back.” Improvements in this cumbersome method of naming were made by several of the earlier authors by shortening the description; but no strict rule was established until the tenth edition of Linnaeus’ “Systema Naturae” (1758), when that author instituted his binomial nomenclature by giving eachform enumerated both a generic and specific name. In plain words, this method takes certain life-forms closely related, but differing in minute particulars, and places them together in a genus or kindred group. Thus the true dogs belong to the genusCanis, but since this group also includes wolves, jackals, and foxes, the various canine animals are respectively designated by a specific name; thus the dog (Canis familiaris), the dingo (C. dingo), the wolf (C. lupus), the jackal (C. aureus), and the fox (C. vulpes). The generic name is placed first. Allied genera are grouped in families, (for example, Canidae), these into orders (ex. Carnivora), the orders into classes (ex. Mammalia), and the classes into phyla or subkingdoms (ex. Vertebrata).
[2]The Latin description was used more commonly than it is at present, as a universal scientific language.
[2]The Latin description was used more commonly than it is at present, as a universal scientific language.
Plants are classified in much the same way, with the exception that families and orders are, by some authors, regarded as of equal value, or even reversed in value; and instead of the term phylum the name series is used.
Classification of the Animal Kingdom.
Classification of Animal Kingdom.
The first seven groups of the above classification are back-boneless animals or Invertebrata; the eighth division alone comprising the animals with a vertebra or backbone.
Characters of the Several Phyla.—
In the first group are placed those animals which, when living, consist of only one cell, or a series of similar cells, but where the cells were never combined to form tissues having special functions, as in the higher groups.
PROTOZOA.—
TheAmoebaof freshwater ponds is an example of such, but owing to its skin or cortex being soft, and its consequent inability to be preserved, it does not concern us here. There are, however, certain marine animals of this simple type of the Protozoa which secrete carbonate of lime to form a chambered shell (Foraminifera); or silica to form a netted and concentrically coated shell held together with radial rods (Radiolaria); and both of these types are found abundantly as fossils. They are mainly microscopic, except in the case of the nummulites and a few other kinds of foraminifera, which are occasionally as large as a crown piece.
COELENTERATA.—
The second group, the Coelenterata, shows a decided advance in organisation, for the body is multicellular, and provided with a body-cavity which serves for circulation and digestion. The important divisions of this group, in which the organisms have hard parts capable of being fossilised, are the limy and flinty Sponges, the Corals, and allied groups, as well as the delicate Graptolites which often cover the surface of the older slates with their serrated, linear forms, resembling pieces of fret-saws.
ECHINODERMATA.—
The third group, Echinodermata, comprises the Sea-lilies (Crinoids), Starfishes and Sea-urchins, besides a few other less important types; and all these mentioned are found living at the present day. Their bodies are arranged in a radial manner, the skin being strengthened by spicules and hardened by limy deposits ultimately forming plates. They have a digestive canal and a circulatory system, and are thus one remove higher than the preceding group.
VERMES.—
The fourth group, Vermes (Worms), are animals with a bilateral or two-sided body, which is sometimes divided into segments, but without jointed appendages. Those which concern the student of fossils are the tube-making worms, the errant or wandering worms which form casts like the lob-worm, and the burrowing kinds whose crypts or dwellings become filled with solid material derived from the surrounding mud.
MOLLUSCOIDEA.—
Group five, the Molluscoidea, contains two types; the Flustras or Sea-mats (Polyzoa) and the Lamp-shells (Brachiopoda). They are at first sight totally unlike; for the first-named are colonies of compound animals, and the second are simple, and enclosed between two valves. They show in common, however, a bilateral symmetry. The mouth is furnished with fine tentacles, or with spirally rolled hair-like or ciliated processes.
MOLLUSCA.—
The sixth group, the Mollusca, includes all shell-fish. They are soft-bodied, bilaterally symmetrical animals, without definite segments. The shells, on account of being formed of carbonate of lime on an organic basis, are often found preserved in fossiliferous strata.
ARTHROPODA.—
The seventh group, the Arthropoda, or joint-footed animals, are distinguished by their segmented, lateral limbs, and by having a body composed of a series of segments or somites. The body and appendages are usually protected by a horny covering, the ‘exoskeleton.’ The group of the Trilobites played an important part in the first era of the formation of the earth’s crust; whilst the other groups were more sparsely represented in earlier geological times, but became more and more predominant until the present day.
VERTEBRATA.—
The great group of the Vertebrata comes last, with its chief characteristic of the backbone structure, which advances in complexity from the Fishes to the Higher Mammals.
A Simplified Classification of the Vegetable Kingdom.
Characters of the Plant Series.
THALLOPHYTA.—
The first series, the Thallophytes, are simple unicellular plants, and occupy the same position in the vegetable kingdom as the Protozoa do in the animal kingdom. Fossil remains of these organisms seem to be fairly well distributed throughout the entire geological series, but, owing to the soft structure of the fronds in most of the types, it is often a matter of doubt whether we are dealing with a true thallophyte or not. Many of the so-called sea-weeds (fucoids) may be only trails or markings left by other organisms, as shell-fish and crustaceans.
BRYOPHYTA.—
The second series, the Bryophytes or moss plants, are represented in the fossil state by a few unimportant examples.
PTERIDOPHYTA.—
The third series, the Pteridophytes, includes the Ferns found from the Devonian up to the present day, Horse-tails and allied forms, likeEquisetites, and the Club-mosses andLepidodendronof the Carboniferous period in various parts of the world.
PTERIDOSPERMEAE.—
The fourth series, the Pteridospermeae, comprises some of the earliest seed-bearing plants, asAlethopterisandNeuropteris. They occur in rocks of Upper Palaeozoic age as far as known.
GYMNOSPERMEAE.
The fifth series, the Gymnospermeae, contains the most important types of plants found fossil, especially those of the primary and secondary rocks: they were more abundant, with the exception of the Coniferae, in the earlier than in the more recent geological periods.
ANGIOSPERMEAE.—
The sixth series, the Angiospermeae, comprises all the Flowering Trees and Plants forming the bulk of the flora now living, and is divided into the kinds having single or double seed-leaves (Monocotyledones the Dicotyledones respectively). This important group came into existence towards the close of the Cretaceous period simultaneously with the higher mammals, and increased in abundance until modern times.
THE GEOLOGICAL EPOCHS: AND THE TIME RANGE OF FOSSILS.
Superposition of Strata.—