Imágenes de página
PDF
ePub
[ocr errors]

regulated by the elevation or depression of the spine, during the peculiar rotatory action of the body of Sharks. This action of the spine in raising and depressing the fin resembles that of a moveable mast, raising and lowering backwards the sail of a barge.

The common Dog-Fish, or Spine Shark, (Spinax Acanthias, Cuv.,) and the Centrina Vulgaris, have a horny elevator spine on each of their dorsal fins, but without teeth or hooks; similar small toothless horny spines have been found by Mr. Mantell in the chalk of Lewes. These dorsal spines had probably a farther use as offensive and defensive weapons against voracious fishes, or against larger and stronger individuals of their own species.*

The variety we find of fossil spines, from the Graywacke series to the Chalk inclusive, indicates the number of extinct genera and species of the family of Sharks, that occupied the waters throughout these early periods of time. Not less varied are the forms of palate bones and teeth, in the same formations that contain these spines; but as the cartilaginous skeletons to which they belonged have usually perished, and the teeth and spines are generally dispersed, it is chiefly by the aid of anatomical analogies, or from occasional juxtaposition in the same stratum, that their respective species can be ascertained.

Fossil Rays.

The Rays form the fourth family in the order Placoi

* Colonel Smith saw a captain of a vessel in Jamaica who received many severe cuts in the body from the spines of a Shark in Montego Bay. - (See Griffith's Cuvier.)

The Spines of Balistes and Silurus have not their base, like that of the spines of Sharks, simply imbedded in the flesh, and attached to strong muscles; but articulate with a bone beneath them. The Spine of Balistes also is kept erect by a second spine behind its base, acting like a bolt or wedge, which is simultaneously inserted, or withdrawn, by the same muscular mo. tion that raises or depresses the spine.

dians. Genera of this family abound among living fishes; but they have not been found fossil in any stratum older than the Lias; they occur also in the Jurassic limestone.

Throughout the tertiary formation they are very abundant; of one genus, Myliobates, there are seven known species; from these have been derived the palates that are so frequent in the London clay and crag. (See Pl. 27, B. Fig. 14.) The genus Trygon, and Torpedo, occur also in the Tertiary formations.

Conclusion.

In the facts before us, we have an uninterrupted series of evidence, derived from the family of Fishes, by which both bony and cartilaginous forms of this family, are shown to have prevailed during every period, from the first commencement of submarine life, unto the present hour. The similiarity of the teeth, and scales, and bones, of the earliest Sauroid Fishes of the coal formation (Megalichthys,) to those of the living Lepidosteus, and the correspondence of the teeth and bony spines of the only living Cestraciont in the family of Sharks, with the numerous extinct forms of that sub-family, which abound throughout the Carboniferous and Secondary formations, connect extreme points of this grand vertebrated division of the animal kingdom, by one unbroken chain, more uniform and continuous than has hitherto been discovered in the entire range of geological researches.

It results from the review here taken of the history of fossil Fishes, that this important class of vertebrated animals presented its actual gradations of structure amongst the earliest inhabitants of our planet; and has ever performed the same important functions in the general economy of nature, as those discharged by their living representatives in our modern seas, and lakes, and rivers. The great purpose of their existence seems at all times to have been, to

fill the waters with the largest possible amount of animal enjoyment.

The sterility and solitude which have sometimes been attributed to the depths of the ocean, exist only in the fictions of poetic fancy. The great mass of the water that covers nearly three-fourths of the globe is crowded with life, perhaps more abundantly than the air and the surface of the earth; and the bottom of the sea, within a certain depth, accessible to light, swarms with countless hosts of worms, and creeping things, which represent the kindred families of low degree which crawl upon the land.

The common object of creation seems ever to have been, the infinite multiplication of life. As the basis of animal nutrition is laid in the vegetable kingdom, the bed of the ocean is not less beautifully clothed with submarine vegetation, than the surface of the dry land with verdant herbs and stately forests. In both cases, the undue increase of herbivorous tribes is controlled by the restraining influence of those which are carnivorous; and the common result is, and ever has been, the greatest possible amount of animal enjoyment to the greatest number of individuals.

From no kingdom of nature does the doctrine of gradual Developement and Transmutation of species derive less support, than from the progression we have been tracing in the class of Fishes. The Sauroid Fishes occupy a higher place in the scale of organization, than the ordinary forms of bony Fishes; yet we find examples of Sauroids of the greatest magnitude, and in abundant numbers in the Carboniferous and Secondary formations, whilst they almost disappear and are replaced by less perfect forms in the Tertiary strata, and present only two genera among existing Fishes.

In this, as in many other cases, a kind of retrograde developement, from complex to simple forms, may be said to have taken place. As some of the more early Fishes united in a single species, points of organization which, at a

later period, are found distinct in separate families, these changes would seem to indicate in the class of Fishes, a process of Division and of Subtraction from more perfect, rather than of Addition to less perfect forms.

Among living Fishes, many parts in the organization of the Cartilaginous tribes, (e. g. the brain, the pancreas, and organs subservient to generation,) are of a higher order than the corresponding parts in the Bony tribes; yet we find the cartilaginous family of Squaloids co-existing with bony fishes in the Transition strata, and extending with them through all geological formations, unto the present time.

In no kingdom of nature, therefore, does it seem less possible to explain the successive changes of organization, disclosed by geology, without the direct interposition of repeated acts of Creation.

CHAPTER XV.

Proofs of Design in the Fossil Remains of Mollusks.*

SECTION I.

FOSSIL UNIVALVE AND BIVALVE SHELLS.

We are much limited in our means of obtaining information as to the anatomical structure of those numerous tribes of extinct animals which are comprehended under Cuvier's great division of Mollusks. Their soft and perishable bodies have almost wholly disappeared, and their external

*See note, p. 56.

shells, and, in a few cases, an internal apparatus of the nature of shell, form the only evidence of the former existence of the myriads of these creatures that occupied the ancient waters.

The enduring nature of the calcareous coverings which these animals had the power of secreting, has placed our knowledge of Fossil Shells almost on a footing with that of recent Conchology. But the plan of our present inquiry forbids us here to take more than a general review of the history and economy of the creatures by which they were constructed.

We find many and various forms, both of Univalve and Bivalve shells, mixed with numerous remains of Articulated and radiated animals, in the most ancient strata of the Transition period that contain any traces of organic life. Many of these shells agree so closely with existing species, that we may infer their functions to have been the same; and that they were inhabited by animals of form and habits similar to those which fabricate the living shells most nearly resembling them.*

All Turbinated and simple shells are constructed by Mollusks of a higher Order than the Conchifers, which construct Bivalves; the former have heads and eyes; the Conchifers, or constructors of bivalves, are without either of these important parts, and possess but a low degree of any other sense than touch, and taste. Thus the Mollusk, which occupies a Whelk, or a Limpet shell, is an animal of a higher Order than the Conchifer enclosed between the two valves of a Muscle or an Oyster-shell.

Lamarck has divided his Order of Trachelipods† into two

* See Mr. Broderip's Introduction to his Paper on some new species of Brachiopoda, Zool. Trans., vol, I., p. 141.

[ocr errors]

This name is derived from the position of the foot, or locomotive apparatus, on the lower surface of the neck, or of the anterior part of the body. By means of this organ Trachelipods crawl like the common garden snail (Helix aspersa.) This Helix offers also a familiar example of

« AnteriorContinuar »