THE PALEOZOIC ERA.

CHAPTER VIII.

THE LAURENTIAN, HURONIAN, CAMBRIAN, AND

SILURIAN LIFE-PERIODS.

The beginnings of life-Graphite a possible result of vegetation— Eozoön, the earliest animal form - Increase of molluscan and crustacean types-Lingulæ, graptolites, trilobites-The earliest plants-Scales of fishes, traces of vertebrata.

WHO shall say when life began? Can it even be determined whether a plant or an animal was the first organized being? We speak of the PALEOZOIC era provisionally, for, so far as our knowledge goes, it is the era of "ancient life," the earliest examples of which occur in the Laurentian rocks. As these, the oldest known of the stratified series, were deposited in water, denudation must have prevailed for an indefinite time, in order that they might be accumulated, and their parent rocks might have contained fossils of which every trace is lost. Mineral carbon (graphite) abounds in them, and is probably not less in the aggregate than

the vegetable carbon of the Carboniferous strata. It is quite conceivable that the Laurentian graphite, familiarly known as "black lead," is the residuum of plant life, completely metamorphosed by heat, pressure, and the long lapse of time; and the character of the iron ores in these rocks is suggestive of the influence of decomposing vegetable matter. Whether or not the Laurentian lands were covered with forest, of which there is yet no direct evidence, the sea appears to have been peopled by one form of animal life-named, from the great development of these rocks in North America, Eozoön Canadense. Here, at least, the microscope reveals a foraminifer-a member of the group of lowly creatures which contributed so much to the formation of the chalk-whose remains are arranged in laminæ consisting of carbonate and silicate of lime. By most of the ablest microscopists it is considered to be a veritable organism-this view being held by Dawson, Logan, Carpenter, Williamson, Parker, Gümbel, etc.; while Möbius has recently given reasons for the opinion that it is inorganic.* However this may be, it occurs in countless multitudes, and we cannot very well imagine that these seas were totally barren of life, especially when we can trace the accumulation of other rocks, rich in carbonate of lime, to the agency of foraminifera. The creature which heralded "the dawn of life" appears to have collected organic matter about its jelly-like

* A monograph on Eozoön is now being prepared by Carpenter and Dawson, in which further evidence for its organic character will be adduced.

protoplasm, and to have deposited it in the form of vast reefs, in a manner similar to that of the coral polyp; but its title to rank as an organism at all is necessarily indistinct, on account of its immense age and the defacing action of metamorphism. Principal Dawson has

FIG. 7.-Eozoon Canadense. 1. A section showing the layers, actual size. 2. The canals, magnified 100 diameters (after Dr. Carpenter). noticed, besides, worm-burrows and traces of another foraminifer (Archæospherina) in Laurentian rocks.

At present the position which should be assigned to the Huronian series is so doubtful, and the lifehistory of these rocks so little known, that nothing would be gained by a discussion of the subject; and we therefore pass on to the Cambrian period, rich in forms which, though zoologically low, are very distinct. The two curious fossils named Eophyton, the "dawn plant," and Oldhamia, after Dr. Oldham, may probably be regarded as the representatives of marine vegetation, if eophyton is not a land plant. Though some consider these coralline structures, there can be no doubt of their organic nature. Sponges of simple character, and sea

worms abound, and the Echinodermata are represented
by species of sea-lilies and star-fishes, and shell-fish by
the genus which gives its
name to the Lingula flags;
but the most important
feature of this period is the
appearance of the first arti-
culated animals-the crusta-
cean forms of water-fleas; and
the Trilobites related to the
king-crabs, no doubt the
most mighty and predatory
family of the time, living in
shallow waters, and becoming
entombed by millions in the
mud.

Rising hence to the Lower antiqua (natural size); Lower Silurian, we come among

Cambrian.

a few doubtful fucoids (sea-weeds), true sponges, a gigantic foraminifer (Stromatopora) allied perhaps to eozoön, corals and Graptolites, Brachiopods (lampshells), circular-chambered shells of nautiloid form; the straight Orthoceras, star-fishes, and swarms of trilobites. of higher structure, while some traces of fishes are found in detached bony scales. This was the culminating point of trilobite life, beyond which they become indistinct, and finally pass out of existence. But the ascent in the zoological scale does not stop with these first articulated forms. In the upper strata of the formation we have unquestionable sea-weeds, and an

N.

undoubted land plant (Lepidodendron), which subsequently becomes of such importance in the Devonian and Carboniferous periods, together with small bodies which Sir Joseph Hooker has identified as the seed vessels of a species of club-moss; besides a possible

FIG. 9.-Cambrian Trilobites. a. Paradoxides Bohemicus; b. Ellipsocephalus Hoff; c. Sao hirsuta; d. Conocoryphe Sultzeri; e. Agnostus rex; f. Head-shield of Conocoryphe Matthewi; g. Head-shield of Dikellocephalus Celticus; h. Tail-shield of Dikellocephalus Minnesotensis.

conifer and other plants in North America. The corals are now becoming more complex in structure, and the species more numerous, while the graptolites as a group are fading away. Among echinodermata, the "sea-lilies"