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appear for the most part related to genera now common in tropical seas, but are all of extinct species.

Family of Sharks.

As the family of Sharks is one of the most universally diffused and most voracious among modern Fishes, so there is no period in geological history in which many of its forms did not prevail.* Geologists are familiar with the occurrence of various kinds of large, and beautifully enamelled teeth, some of them resembling the external form of a contracted leech, (PI. 27% and 27':) these are commonly described by the name of Palate bones, or Palates. As these teeth are usually insulated, there is little evidence to indicate from what animals they have been derived.

In the same strata with thein are found large bony Spines, armed on one side with prickles, resembling hooked teeth, (see PI. 27d. C. 3, a.) These were long considered to be jaws, and true teeth; more recently they have been ascertained to be dorsal spines of Fishes, and from their supposed defensive office, like those of the genus Balistes and Silurus, have been named Ichthyodorulites.

M. Agassiz has at length referred all these bodies to extinct genera in the great family of Sharks, a family which he separates into three sub-families, each containing forms peculiar to certain geological epochs, and which change simultaneously with the other great changes in fossil remains.

The first and oldest sub-family, Cestracionts, beginning with the Transition strata, appears in eveiy subsequent formation, till the commencement of the Tertiary, and has only one living representative, viz. the Cestracion Phillippi, or Port Jackson Shark. (PI. 1. Fig. 18.) The second family, Hybodonts, beginning with the Muschel-kalk, and perhaps with the Coal formation, prevails throughout the Oolite series, and ceases at the commencement of the Chalk. The third family of " Squaloids," or true Sharks, commences with the Cretaceous formation, and extends through the Tertiary strata into the actual creation.*

* M. Agassiz has ascertained the existence of more than one hundred and fifty extinct species of fossil Fishes allied to this family.

* The character of the Cestracionts is marked by the presence of large polygonal obtuse enamelled teeth, covering the interior of the mouth with a kind of tessellated pavement. (PI. 27d. A. 1, 3, 4, and PI. 27d, B. 1, 2, 3, 4, 5.) In some species not less than sixty of these teeth occupied each jaw. They are rarely found connected together in a fossil state, in consequence of the perishable nature of the cartilaginous bones to which they were attached; hence the spines and teeth usually afford the only evidence of the former existence of these extinct fossil species. They are dispersed abundantly throughout all strata, from the Corboniferous series to the most recent Chalk.

In plate 27e, Figs, I, 2, represent a series of teeth of the genus Acrodus, in the family of Cestracionts, from the lias of Somersetshire; and PI. 27f, a series of teeth of the genus Ptychodus, in the same family, a genus which occurs abundantly and exclusively in the Chalk formation.

In the section PI. 1, Fig. 19 represents a tooth of Psammodus, and Fig. 19', a tooth of Orodus, from the Carboniferous limestone; and Fig. 18', a recent tooth of the Cestracion Phillippi. The Ceslracion Phillippi, (PI. 1, Fig. 18, and PI. 27d, A.) is the only living species in the family of Sharks that has flat tessellated teeth, and enables us to refer numerous fossil teeth of similar construction to the same family. As the small anterior cutting teeth (PI. 27d, A. Figs. 1. 2. 5.) in this species, present a character of true Sharks, which has not been found in any of the fossil Cestracionts, we have in this dentition of a living species, the only known link that connects the nearly extinct family of Cestracionts with the true Sharks or Squaloids.

The second division of the family of Sharks, Hybodonts, commencing probably with the Coal formation, prevailed during the deposition of all the Secondary strata beneath the Chalk; the teeth of this division possess intermediate characters between the blunt polygonal crushing teeth of the subfamily Cestracion, and the smooth and sharp-edged cutting teeth of the Squaloids, or true Sharks, which commenced with the Cretaceous formations. They are distinguished from those of true Sharks by being plicated, both on the external and internal surface of the enamel. (See Plate 27d. B. Figs. 8, 9, 10.) Plate 27dC. 1". represents a rare example of a series of teeth of Hybodus rcticnlatus, still adhering to the cartilaginous jaw bones, from the Lias of Lyme Regis. Striated teeth of this family abound in the Stonesfield slate and in the Wealden formation.

Fossil Spines, or Ichthyodorulites.*

The bony spines of the dorsal fins of the Port Jackson Shark (Pi. 1. Fig. 18.) throw important light on the history of fossil Spines; and enable us to refer those very common, but little understood fossils, which have been called Ichthyodorulites, to extinct genera and species of the sub-family of Cestracionts. (See page 218.) Several living species of the great family of Sharks have smooth horny spines connected with the dorsal fin. In the Cestracion Phillippi alone, (PI. 1, Fig. 18,) we find a bony spine armed on its concave side with tooth-like hooks, or prickles, similar to those that occur in fossil Ichthyodorulites: these hooks act as points of suspension and attachment, whereby the dorsal fin is connected with this bony spine, and its movements

Another genus in the sub-family of Hybodonts, is the Onchus, found in the Lias at Lyme Regis; the teeth of this genus are represented, PL 2711, B. 6, 7.

In the third, or Squaloid division of fossils of this family, we have the character of true Sharks; these appear for the first time in the Cretaceous formations, and extend through all the Tertiary deposites to the present era. (PI. 27d, B. 11, 12, 13.) In this division the surface of the teeth is always smooth on the outer side, and sometimes plicated on the inner side, as it is also in certain living species; the teeth are often flat and lancet-shaped, with a sharp cutting border, which, in many species, is serrated with minute teeth. Species of this Squaloid family alone, abound in all strata of the Tertiary formation.

The greater strength, and flattened condition of the teeth of the families of Sharks (Cestracionts and Hybodonts,). that prevailed in the Transition and Secondary formations beneath the Chalk, had relation, most probably, to their office of crushing the hard coverings of the Crustacea, and of the bony enamelled scales of the Fishes, which formed their food. As soon as Fishes of the Cretaceous and Tertiary formations assumed the softer scales of modern Fishes, the teeth of the Squaloid sub-family assumed the sharp and cutting edges that characterize the teeth of living Sharks. Not one species of the blunt-toothed Cestraciont family has yet been discovered in any Tertiary formation.

• See PI. 27". C. 3.

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 Placoidians. 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.

* 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 Silnrus 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 motion that raises or depresses the spine.

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 PI. 27d, 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

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