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Even then its hard parts will probably find their way to the bottom. At the bottom the remains will soon be covered by the always dropping sediment. They are on the way to become fossils. Some land animals also might, after death, get carried by a river to the lake or ocean, and find their way to the bottom, where they, too, will become fossils. Or they may die on the banks of the lake or ocean and their bodies may get buried in the soft mud of the shores. Or, again, they are often trodden in the mire about salt springs or submerged in quicksands. It is obvious that aquatic animals are far more likely to be preserved as fossils than land animals. This inference is strikingly proved by fossil remains. Of all the thousands and thousands of kinds of extinct insects, mostly land animals, comparatively few

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FIG. 244.-A fossil brachiopod (Spirifer
cameratus Morton).
Coal measures,
St. Joseph, Mo.

If now this

It is obvious that any portion of the earth's surface covered by stratified rocks must have been at some time under water, the bottom of a lake or ocean. portion shows a series of layers or strata of different kinds of sedimentary rocks, it is evident that it must have been under water several times, or at least under different conditions. It is also evident that fossils found in this portion of the earth will contain remains of only those animals which were living at the various times this portion of the earth was under water. Of the animals which lived on it when it was land, there will be no trace, except, possibly, a few land or fresh-water forms which might be swept into the

sea or might be preserved in the mud of small ponds. That is, instead of finding in the stratified rocks of any portion of the earth remains of all the animals which have lived on that portion since the earth began, we shall find, at best, only remains of a few kinds of those animals which have lived on this portion of the earth when it was covered by the ocean or by a great lake.

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339. Geological epochs and their animals. Thus, the great body of fossil remains of animals reveal only a broken and incomplete history of the animal life of the past. But

FIG. 245.—A Pterodactyl or flying reptile (Rhamphorhynchus gemmingi), Jurassic

of Bavaria. - After ZITTEL.

the record, so far as it goes, is an absolutely truthful one, and when

the many deposits of fossils in all parts of the different continents are examined and compared, it is possible to state numerous general truths in regard to past life and the succession of animals in time. The science of extinct life is known as paleontology.

The study of Paleontology has revealed much of the history of the earth and its inhabitants from the first rise of the land from the sea till the present era. This whole stretch of time-how long nobody knows-is divided into eras or ages; these ages usually into lesser divisions called periods, and the periods into shorter lengths of time called epochs. Each epoch is more or less sharply distinguished from every other by the different species of animals and plants which lived while its rocks were being deposited. In the earth's crust, where it has not been distorted by foldings and breaks, the oldest stratified rocks lie at the bottom of the series, and the newest at the top. The fossils found in the lowest or oldest rocks represent, therefore, the oldest or earliest animals, those in the upper or newest rocks

the newest or latest animals. An examination of a whole series of strata and their fossils shows that what we call the most specialized or most highly organized animals did not exist in the earliest epochs of the earth's history, but

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FIG. 246.-An Ostracoderm (Pterichyodes milleri), Lower Devonian of Scotland.After TRAQUAIR. (The jointed appendage on the head is not a limb.)

that the animals of these epochs were all of the simpler or lower kinds. For example, in the earlier stratified rocks there are no fossil remains of the backboned or vertebrate animals. When the vertebrates do appear, through several geological epochs they are fishes only, members of the lowest group of backboned animals. More than this, they represent generalized types of fishes which lack many of

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FIG. 247.-An Arthrodire (Coccosteus decipiens), Lower Devonian of Scotland.-After WOODWARD.

the special adaptations to marine life that modern fishes show. For this reason, they bear a greater resemblance to the earlier reptiles than do the fishes of to-day. In other words, they were a generalized type, showing the beginnings of characters of their own and other types. It is always through generalized types that great classes of animals approach each other.

In a later epoch the batrachians or amphibians appeared; in a still later period, the reptiles; and last of all, the birds and the mammals, the last being the highest of the backboned animals. On the opposite page is shown a table giving the names and succession of the various geological periods, and indicating briefly some of the kinds of animals

FIG. 248.-A Crossopterygian fish (Osteolepis macrolepidotus), Devonian of Scotland.From Zittel, after PAUDER.

living in each. In each of these divisions of geological time some one class of animals was especially numerous in species, and was evidently the dominant group of animals through that period. The different ages are therefore spoken of in terms of the prevailing life. Thus, the Silurian Age is known as the age or era of invertebrates; the Devonian, as the age of fishes. In the same way we have the

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FIG. 249. Cladoselache fyleri (Newberry).-After DEAN, from Devonian rocks in Ohio. The most primitive of known sharks.

Reptilian Age, the Mammalian Age, according to the great class of animals predominating at that time. Of course, in each of the later epochs there lived animals representing the principal classes or groups in all of the preceding ones, as well as the animals of that particular group which may have first appeared in this epoch, or was its dominant group.

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