These crustacea bear some resemblance, although a very remote one, to the common wood-louse, and, like that animal, they had the power of rolling themselves into a ball when attacked by an enemy. The eye of the trilobite is a most remarkable organ; and in that of one species, Phacops caudatus, not less than two hundred and fifty lenses have been discovered. This remarkable optical instrument indicates that these creatures lived under similar conditions to those which surround the crustacea of the present day.
At the period of the trilobites of the Silurian rocks, all the animals contemporaneous with them had the organs necessary for the preservation of life in the waters.
Next in order of time to the trilobite, the most singular animals inhabiting those ancient seas, whose remains have been preserved, are the Cephalopoda, possessing some traces of organs which belong to vertebrated animals. There are numerous arms for locomotion and prehension, arranged in a centre round the head, which is furnished with a pair of sharp, horny mandibles, embedded in powerful muscles. These prehensile arms are provided with a double row of suckers, by which the animal seized its prey. Of these cephalopodous animals there are many varieties, but all of them appear to be furnished with powers of rapid locomotion, and those with shells had an hydraulic arrangement for sinking themselves to any depth of the seas in which, without doubt, they reigned the tyrants.
Passing by without notice the numerous fishes, which appear to have exhibited a similar order of progression to the other animals, we must proceed to the more remarkable period when the dry land first began to appear.
All the animals found in the strata we have mentioned are such as would inhabit the seas; but we gradually arrive at distinct evidence of the separation of the land from the water, and the “green tree yielding seed” presents itself to our attention; not that the strata earlier than this are entirely destitute of any remains indicating vegetable growth, but those they exhibit are such as, in all probability, may be referred to marine plants.
Those plants, however, which are found in the carboniferous series are most of them distinguished by all the characteristics of those which grow upon the land; we, therefore, in the mutilated remains of vegetation left us in our coal-formations, read the history of our early world.
Then the reed-like calamite bowed its hollow and fragile stems over the edges of the lakes the tree-ferns grew luxuriantly in the shelter of the hills, and gave a wild beauty to the humid valleys; the lepidodendrons spread themselves in mighty forests along the plains, which they covered with their curious cones; whilst the sigillariæ extended their multitudinous branches, wreathing like serpents amongst the luxurious vegetation, and embraced, with their roots (stigmariæ), a most extensive space on every side.[234]
The seas and lakes of this period abounded with minute animals nearly allied to the coral animals, which are now so actively engaged in the formation of islands in the tropical and southern seas. During the ages which passed by without any remarkable disturbance of the surface of the earth, the many bands of mountain limestone were formed by the ceaseless activity of these minute architects. Encrinites (creatures in some respects resembling star-fish) existed in vast numbers in the oceans of this time; and the great variety of bivalve shells, and those of a spiral character, discovered in the rocks of this period, show the waters of the newer palæozoic period to have been instinct with life.
In the world then, as it does now, water acting on the dry land produced remarkable changes. We have evidence of extensive districts over which the most luxuriant vegetation must have spread for ages,—from the remains of plants in every state of decay,—which we find went to form our great coal-fields. These, by some changes in the relative levels of land and water, became covered with this fluid; and over this mass of decaying organic matter, sand and mud were for ages being deposited. At length, rising above the surface, it becomes covered with vegetation, which is, after a period, submerged; the same deposition of sand and mud again takes place, it is once more fitted for vegetable growth, and thus, cycle after cycle, we see the dry land and the water changing places with each other. This will be evident to every one who will carefully contemplate a section of one of the coal-fields of Great Britain. We find a stratum of coal lying upon a bed of under clay, and above it an extensive stratum of shale or sandstone, probably formed by the denudation of the neighbouring hills; and in this manner we have many strata of coal, shale, clay, ironstone, and sandstone alternating with each other; the coal-formations of the South Wales coal-field having the extraordinary thickness of 1500 feet. The lowest bed of this extensive series must at one time have been exposed as the surface of the country.
Ascending in the series, we have now formations of a more recent character, in which fishes of a higher order of organization, creeping and flying saurians, crocodiles and lizards, tortoises, serpents, and frogs, are found. The lias formations (a term corrupted from layers), consisting of strata in which an argillaceous character prevails, stand next in series. In these we have animals preserved in a fossil state, of a distinguishingly different character from those of the inferior strata. We meet with extended beds of pentacrinites, some inches in thickness; and their remains are often so very complete that every part of the skeleton can be made out, although so complicated that it cannot consist of less than 150,000 parts. In these formations we often find the curiously beautiful remains of the ammonites, of which a great variety have been discovered. Of the belemnites—animals furnished with the shell and the ink-bag of the cuttle-fish, with which it darkened the water to hide itself from enemies, numerous varieties have also been disentombed, with the ink-bag so well preserved, that the story of the remarkable fossil has been written with its own ink. In addition to these we find nautili; and sixty species of extinct fishes have been described by Agassiz from the lias of Lyme Regis alone.