« AnteriorContinuar »
singular and robust proportions. Although the head is small, the naked jaws (covered with enamel instead of skin) are lined with a double row of formidable teeth, the outer ones being thickly set and fringing the enamelled edges of the mouth, but the inner ones wider apart, and at least twenty times the size of the others. The scales on the body of this fish, and the bones, are so like what is seen in reptiles, that they were, when first discovered, supposed to belong to some large saurian; and the scales might indeed have served for the defensive armour of a crocodile five times as large as the fish. Not less than five species of this remarkable genus have been already determined from the beds of the old red sandstone period, and eight from the carboniferous rocks, all of them exhibiting more or less distinctly the peculiarly massive and robust character of the family.
But the great reptilian fish were not the only inhabitants of the sea during this period, nor were they even the only ones of large size and possessed of great strength and voracity. Not less than sixty species belonging to various genera, all nearly allied to the shark tribe and some of them of very large proportions, are indicated by the remains of teeth discovered in various localities in the limestones, sandstones, and shales of the carboniferous series; and thirty-three species have been determined from fragments of fins and detached vertebræ from the same beds. Now, as there are no more than seven species of shark-like animals determined from the fossils of the old red sandstone, even including two which may be identical with some of the other five, it seems that a great and important
change had taken place in the introduction of a large number of species of this class, which was very imperfectly represented at first, but which continued important for a very long time, and still forms a group peforming a distinct part in the economy of creation.
Fragments of placoid fishes, whose remains, although consisting only of teeth and bony fins, are thus abundant in species, are in some places very common.* They were not, like those of the former class (the ganoids), securely encased in enamelled armour, but were covered at intervals with small detached plates, which could scarcely serve the purpose of defence. It is probable that this was little needed, and that the animal depended chiefly on its extreme swiftness of motion both to obtain its prey and escape from its enemies; while the perfect apparatus of teeth (the commonest fossil remains of these fishes) indicates beyond doubt its ordinary habits; and the bony rays (also very frequently met with) attest the provision that was made to enable the animal to turn itself on its back and seize its prey when overtaken, with a rapidity and precision of which we are scarcely able perhaps to form an idea.
The numerous rays, or bony spines, called Icthyodorulites, so often found fossil in these and newer strata, seem to be identical with the bony spine with which the Port Jackson shark is provided,
Among the most remarkable localities are several in the carboniferous limestone of Bristol, and others in the same rock in the neighbourhood of Armagh.
+ Ixovs (ichthys), a fish; dopv (doru), a spear; Xɩoç (lithos), a stone: these fossils being spear-like projections from the back and belly of a shark-like fish, supporting fins, and serving probably also as weapons.
and which being moveable, and attached to a fin, enables the animal to turn itself readily on its back while swimming. These spines are variously marked according to the species or genus to which they belong. They will be described at greater length in a future chapter when treating of lias fossils.
See then the great and striking change that had supervened towards the close of this carboniferous period. The corals and the encrinites remained with little alteration of general form; the trilobites were nearly extinct, and seem but scantily replaced by other crustaceans; the Brachiopoda had assumed new forms, which some of them retained long afterwards, and which are even handed down to the present day; the ordinary bivalve and univalve shells were gradually increasing; and the prevailing Cephalopoda, retaining up to this period the elongated straight form of orthoceratites, were also developed in the spiral form seen in goniatites, and afterwards continued in ammonites, a form better fitted perhaps for the altered conditions of the sea and the greater stir of life that was about to succeed. But the fishes present the newest and the most striking appearances. The minute, but probably fierce and voracious species, which first marked the introduction of this class of animals, had been succeeded by a comparatively clumsy and awkward race, coarse feeders, of small size, and indifferent swimmers, but covered either with strong plaited armour or with fine coats of mail, and apparently very abundantly distributed. These lasted for a time, but then gave
* Some Terebratulæ of the carboniferous period are exceedingly like oolitic species, and some of them closely resemble species still existing.
way to the advance of other and higher groups. Innumerable sharks of all sizes, and perhaps of many forms, rapid and powerful swimmers, fiercely and insatiably carnivorous, were associated with huge monstrous fishes, more resembling reptiles than any of their own class at the present day, and incredibly powerful and voracious. The fishes at this time had attained, it would seem, their maximum of development in point of vigour, and in some respects (though in some respects only, and by analogy) in structure; and it is not a little interesting to find, that, at this point, so far as we can tell, the true reptiles were actually introduced (the remains of that class being indicated in the coal-measures, and actually found in the magnesian limestone associated with carboniferous species of fishes).
The reptiles thus appearing were not, however, members of that group through which the passage from sauroid fishes to true saurians takes place, but belonged to a higher form, and to a complicated type of that form. It seems clear, therefore, that, while progression and a general advance in point of organization is in one sense a method observed by nature, still there is not such a regular gradation that an animal of lower organization can be supposed to be employed as the agent in introducing a higher group; this view, however plausible, not being borne out by observation, but, on the other hand, being distinctly contradicted by the results of geological and palæontological investigation.
THE CLOSE OF THE FIRST EPOCH OF CREATION. THE MAGNESIAN LIMESTONE, OR PERMIAN SYSTEM OF DEPOSITS.
AFTER the coal-measures had been deposited in the creeks and at the mouths of rivers, and probably very near the land anciently existing, a great change seems to have taken place in the northern hemisphere in the relative level of the land and sea bottom, so that a quantity of coarse gravelly matter, apparently the debris of some sandy or granitic rock, was more or less abundantly deposited. This sand sometimes reposes conformably and evenly on the upper coal grits, which pass into it: at other times the upper surface of these latter beds has previously undergone much wearing and grinding away, and occasionally the lower beds, originally horizontal, have been tilted up at various angles to the horizon, and the upper ones removed before the newer sandstones were placed upon them.
Notwithstanding the turmoil and agitation which marks this movement in some districts, it is yet certain that the disturbances began by small and comparatively unimportant changes of elevation, occurring at intervals and at distant points, so that the general aspect of submarine life, as known by the fossils found embedded, was scarcely so far altered as to require the introduction of any distinct groups.