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Here is a representation of the exterior of one of the numerous species, of which this genus is composed.
These shells are found of all sizes, from that of a few lines to nearly four feet in diameter; and above three hundred different species are said to have been observed.
The diagram here given represents the two sides of a species of crustaceous marine animal, which has
been wholly extinct from an early period in the formation of the crust of the globe; many ages may have elapsed since it ceased to exist. There are several species of the animal, which has been called Trilobite, from the body being composed of longitudinal divisions or lobes. It is found in the British isles, in Germany, and Sweden and specimens have been brought from North America. In some parts of Wales the slate is so full of fragments of the animal, that millions must have swarmed on the spot.
Another fossil animal which is very peculiar in its form is this, called the Lily Encrinite. It resembles
that flower upon its stalk, and still more so when the several parts of which the flower-like extremity is composed, are separated and spread out; specimens of it in this state are not unfrequently met with. That stalk is not a single piece, but consists of a number of distinct joints like those of the backbone, or like a necklace of beads, on which account the fossil has been sometimes called the Necklace-form Encrinite. The stalk is perforated through its whole length, and the joints, when separated, have figured surfaces such as are represented above in the circular bodies, the figure being different at different parts of the stalk. This family of radiated animals, which consists of many extinct genera and species, has not wholly disappeared, like the trilobite and ammonite; living representatives of it are still found in the seas of the West Indies, and a very perfect specimen may be seen in the Museum of the Geological Society: but the lily encrinite, that branch of the family, is not only wholly extinct, but has been so ever since the period when the New Red Sandstone was deposited. It appears to have had comparatively a short existence, for it has only been found in a limestone which occurs associated with the New Red Sandstone. It is met with abundantly in that particular limestone which occupies a great extent of country in Germany; but the fossil has never been seen in England, and that kind of limestone is not found in our island.
The remains of fishes occur in almost every stratum,
from the Old Red Sandstone up to the most recent deposits of fresh-water lakes. Fossil fish have been less accurately made out, as to the genera to which they belong, than any other kind of animal remains; because the natural history of fishes is not so far advanced as that of most other departments of zoology. The great French naturalist, Cuvier, began an extensive work on the subject; and, had he lived, much would have been done, for his master-genius threw light on every thing he touched. One of the most celebrated places for fossil fish is a hill near Verona in Italy, called Monte Bolca. Immense quantities have been found there in a very perfect state of preservation, as far as the form is concerned, but, as in most other cases, quite flattened and thin, so that they are like a painting, or engraving of a fish. These impressions are of rare occurrence, in comparison with the quantity of separate bones that are found in most strata: teeth of the shark are frequently met with, and sometimes of a size which shows them to have belonged to individuals of giant dimensions, such as are not now seen in any Ibid.
In our last lesson, we gave some examples of remarkable species of fossil-shells, corals, and crustacea; two of these, the trilobite and the lily encrinite, belonging to genera which became extinct after the deposit of the oldest secondary strata. In the extensive series of sand-stones lime-stones, and clays of the secondary rocks, from the coal measures up to, and including the chalk, the fossil remains of animals consist of a vast variety of shells, corals, sponges, and other marine productions of a similar description-of a
few kinds of crustacea, that is, animals having a crust or shell like that of the lobster or crab, a few kinds of fish, some great reptiles, and a few insects. No remains of land quadrupeds, or of the marine mammalia, or of birds, have yet been met with in chalk or any stratum under the chalk, except one supposed instance. Among the numerous animal remains that occur in the secondary strata, there is not a single species which has not been for many ages extinct; and even whole genera have totally ceased to exist.
The extinction of species is so important a fact in all that relates to the geological history of the earth, that we will, even at the risk of some repetition, endeavour, by a little popular explanation, to make clear what is meant by the term. Each particular kind or genus of animal usually consists of several individuals, which while they possess a common character or class of characters, have particular forms which distinguish them from each other; and such individuals constitute the species of a genus. The characters, by which geologists distinguish the relative ages of strata, in so far as animal remains are concerned, depend, not upon genus, but on the species; for while species have become extinct, one after the other in succession, the genera to which they belong have continued to exist from the period of the deposition of the oldest of the secondary strata to the present time. For example, the genus ostrea, or oyster, is found in the lime-stones which lie beneath the coal-measures; but not one of the many species of oyster, which are met with in almost all the strata from that lime-stone up to the chalk, is identical with any species of oyster inhabiting our present seas.
It is unnecessary for us to give the names of the marine remains, which are most abundant in the secondary strata, because even with the assistance of figures, they would convey to the general reader no clear idea of their peculiar forms, as distinguished from those of marine shells, corals, sponges, &c. now existing; but some of the marine reptiles are so extraordinary in point of form and size as to deserve a more particular notice. Of these monsters of the ancient seas, nine
different genera have already been found entombed in the secondary strata, and of some of the genera there are several species. They have been called saurians by geologists, from the resemblance they bear to the lizard tribe, saura being the Greek name for a lizard. A common green lizard is a tolerably good miniature representation of the general form of these reptiles : but a crocodile or alligator gives a still better idea of them. It must be remembered, however, that in speaking of the fossil remains of those animals, we mean only their skeletons or bones; the flesh is never converted into a fossil state. It very seldom happens, also, that the entire skeleton of any large animal is found, particularly in the strata that were deposited at the bottom of a sea, and for this reason-the bones in the living body are kept together by a cartilaginous substance or gristle, which after death putrefies, and then the several members fall asunder. Very often, too, we find only detached bones; and this may be accounted for by another circumstance attending the process of putrefaction. When that commences in a dead animal, a considerable quantity of gas is generated, which swells up the body, and, if that be in water, makes it so much lighter that it floats. In process of time the skin bursts, and the gradually loosened bones are scattered far apart. Such detached bones frequently constitute all the data, by which we are enabled to decide upon the nature of the animal; and the general reader may perhaps think that they are sufficiently scanty materials, considering the important conclusions which geologists sometimes draw from them. But the discoveries of philosophers, who have occupied themselves in comparing the anatomical structure of the lower animals with that of the human frame, and have created the interesting and beautiful department of science called Comparative Anatomy, have enabled them to establish certain fixed and invariable principles for our guidance in this curious branch of geological inquiry. This field of investigation has only been entered upon within a few years; but it has already yielded so rich a harvest, that it has established