to fire, and hence called the Igneous Rocks, and which are altered or metamorphosed by contact, no life remains have as yet been found. This absence is only negative testimony; but as investigation progresses, they may be found to be also fossiliferous-the fossils being of a low grade of existence. The system immediately overlying these metamorphic rocks is called the Laurentian, from being extensively developed in the Laurentide Mountains, in Canada, and was long supposed to be of the same nature. Of late years, however, both in Canada and in Ireland, a lowly organised fossil, 1. Astrea; 2. Turbinolia fungites; 3. Terebratula risca; 4. Leptæna lata; 5. Actinocrinites; 6. Euomphalus rugosus; 7. Asaphus de Buchii; 8. Asaphus tuberculatus ; 9. Calymene Blumenbachii; 10. Side view of Calymene while rolled up. the Eozoon canadense, or dawn of life animalcule,' has been discovered. It is the earliest trace of life we find, and seems to have grown in the Laurentian seas like a spreading bunch of coral. The Cambrian (formerly called the Grauwacke) yields a few sea-weeds, zoophytes, burrowing worms, and shrimp-like animals (crustacea), but all the life is of a very lowly type. The Silurian system-so *Contrary views are beginning to be entertained by some chemical geologists, but their theory does not affect our paleontological statements. -called because it is found extensively developed in certain counties in Wales, formerly the Siluria of the ancient Britons-owes its establishment and chief subsequent investigation to Sir R. I. Murchison, who, in his Siluria, has described and figured the various fossils peculiar to it. These are marine creatures, chiefly numerous species of zoophytes, or animals allied to the 'sea-pen' of our fishermen, corals, crinoids-a star-fish on a stem-various species of shell-fish, worms, crustacea, &c. There are also various marine plants, and seeds, and drifted fragments of other plants allied to our clubmosses are not unfrequent-pointing to rivers carrying down débris into a shallow estuary, though most of the animals may have lived in the deep sea. Some of the forms of animal life during the Silurian age are represented on the preceding page. These are-figs. I and 2, corals; 3 and 4, bivalveď molluscs (like the oyster); 5, a crinoid; 6, a single-valved mollusc (like a whelk); 7, 8, and 9, trilobites; and 10, the same as 9, but rolled up at rest. As some interest must attach to these early forms of life, we give a more particular account of some of them. THE TRILOBITES. The trilobite-so called from its three-lobed appearance-is a type of being extremely abundant in the seas of the Silurian and a few subsequent ages, yet long extinct in all its various species, and hardly represented by any existing animal, the only one approaching to it being the serolis. 'The trilobite was a true (that is, perfectly developed) crustacean, covered with shelly plates, terminating variously behind in a flexible extremity, and furnished with a headpiece composed of larger plates, and fitted with eyes of a very complicated structure. It is supposed by some to have made its way through the water by means of soft paddles, which have not been preserved; and by others merely to have sculled itself forward by the aid of its flexible extremity. Of its various organs, the most interesting is the eye, of which several specimens have been obtained in a very perfect state. This organ, according to fossil anatomists, is formed of 400 spherical lenses in separate compartments, on the surface of a cornea projecting conically upwards, so that the animal, in its usual place at the bottom of waters, could see everything around. As there are two eyes, one of the sides of each would have been useless, as it could only look across to meet the vision of the other; but on the inner side there are no lenses, that nothing may, in accordance with a principle observable throughout nature, be thrown away. It is found that in the Serolis, the surviving kindred animal, the eyes are constructed on exactly the same principle, except that they are not so high-a necessary difference, as the back of the serolis is lower, and presents less obstruction to the creature's vision. Philosophers have remarked with delighted surprise the *Page's Geology-Chambers's Educational Course. evidence afforded by the eye of the trilobite, that the air and light were generally the same in the early ages of the earth as now, and that the sea must have been as pure. If the water had been constantly turbid or chaotic, a creature destined to live at the bottom of the sea would have had no use for such delicate visual organs. 'With regard to the atmosphere,' says Dr Buckland, 'we infer that, had it differed materially from its actual condition, it might have so far affected the rays of light, that a corresponding difference from the eyes of existing crustaceans would have been found in the organs on which the impressions of such rays were then received. Regarding light itself also, we learn from the resemblance of these most ancient organisations to existing eyes, that the mutual relations of light to the eye, and of the eye to light, were the same at the time when crustaceans, endowed with the faculty of vision, were placed at the bottom of the primeval seas, as at the present moment.' CRINOIDEA. The crinoïdea, which reached their zenith in abundance of individuals and species during the subsequent age, and afterwards, like the trilobites, became extinct, were animals of a humble class, consisting generally of a stalk fixed at the lower end to the sea-bottom, and bearing at the other a cuplike body, with a mouth in the centre, and numerous tentacula or arms branching in all directions for the seizure of prey. The stalk and tentacula were composed of innumerable small plates of calcareous or bony substance, connected by a muscular integument, so as to be capable of bending in all directions, and likewise, as some suppose, covered with a gelatinous coating. The bones of the stalk, perforated for an internal canal, are of different form in different species, some being round, and some angular, and at intervals there are some of greater thickness, all being beautifully marked and nicely adjusted to each other. In the accompanying drawing of a crinoïdean (the Encrinites moniliformis, or necklace-shaped encrinite), the stalk is abridged to much less than the usual length, for the Encrinites moniliformis. sake of convenience, and the arms are represented as closed. As many as 26,000 bones have been reckoned to go to the composition of a single animal of this kind; and some of the family are supposed to have had many more. The bottom of a sea, filled with a number of such animals, yielding to its every current and impulse, and each spreading about its far-reaching arms for prey, must have been a striking sightt—a vast field of tulips, waving in the wind, being the only idea we can form at all approaching to it. Fragments and single bones of the crinoïdea are found in vast quantities in early rocks, forming in some places the principal portion of masses a hundred and twenty feet thick; and marble mantel-pieces, in which these fragments appear in all attitudes and forms, are common in this country. The single wheel-like bones of the stalk are also gathered in abundance on some sea-beaches, and strung up as beads. In the northern parts of England they are called St Cuthbert's beads, and connected with a popular superstition. 'On a rock by Lindisfarn Saint Cuthbert sits, and toils to frame The sea-born beads that bear his name.'-Marmion. THE CEPHALOPODS. In these early ages, as in the present day, the mollusca (shellfish) formed a conspicuous portion of animated nature. The species, however, have been repeatedly changed in the course of time. The most abundant order in the early ages was that of Brachiopoda, a set of creatures living in bivalve shells, and possessing two organs, somewhat like arms, with which to catch prey. They are supposed to have been 'the scavengers of ancient seas, living upon such fragments of animal matter as found their way to the great depths.' But the most remarkable molluscs of those ages were the Cephalopoda, an order occupying univalve shells, and so high in organisation as usually to possess an internal bony skeleton. Of this order there are still representatives in our seas; but in ancient times, they seem to have been far more abundant both in species and in individuals, acting then as the butchers of the marine animal world, to restrain within due bounds the redundant life of which nature is ever so prodigal. The most remarkable species of the early ages were those termed nautili and ammonites. The existing nautilus has enabled geologists to arrive at a very clear understanding of the economy of the cephalopoda of ancient times, of which the shells are now almost the sole remains. The cephalopoda possessed a body resembling a closed bag, containing a heart, stomach, and other organs, and furnished with a head and prominent eyes, as also a number of long arms or tentacula, which at once served for the locomotion of the animal, and for the seizure of its prey. The arms were each provided with a double row of suckers, which enabled it to take a firm hold of smaller animals, and convey them to its mouth, which was armed with a pair of strong horny mandibles or nippers, not unlike the beak of a parrot. The cephalopod lodged in a shell, straight or curved, consisting of a series of air-chambers, terminating in an outer one which was more particularly the residence of the animal. It formed these chambers one after another in the course of its life, according as they were needed for the purpose to which nature had destined them. This purpose was to enable the animal to float in the water. The reader may be reminded that the principle on which floating in water depends is, that more water must be displaced than would weigh the same as the article or object displacing it. A lifeboat is made incapable of sinking, by having empty copper boxes distributed within its structure, these, with the wood, displacing more water than is equal to the weight of the whole vessel. Now, the airchambers in the ammonite or nautilus are like the copper boxes of the life-boat; they displace a certain quantity of water. But the creature required to be able to rise and sink in the water at pleasure; therefore something more was needed. The end is supposed to have been served in two ways. Down through the centre or side of the series of air-chambers, but not communicating with them, there was an elastic pipe, called the siphuncle (represented in No. 2), the upper extremity of which was connected with the cavity of the animal's heart. This cavity was in general filled with a dense fluid, which partly filled the siphuncle, the remainder being occupied by air. 1. Ammonites obtusus; 2. Section of Ammonites obtusus, shewing the interior It may easily be seen how this arrangement acted. Whenever the animal, for any reason, whether to escape danger or in search of prey, wished to sink, it contracted itself into the outer chamber, thus pressing the fluid of its heart into the siphuncle, and reducing the space occupied by the air, at the same time that the gravity of its body was increased by its displacing less water. Accordingly, being then heavier than the surrounding medium, it sunk. When, again, it wished to rise, it had only to dilate its body and arms, and allow the air in the siphuncle to expand to its usual space, when, becoming lighter than the surrounding element, it necessarily ascended. Some species of this order of molluscs were also provided with a bag containing an inky secretion, which they could express upon occasion, so as to No. II. 9 |
