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Fishes of the Chalk Formation.

The next and most remarkable of all changes in the character of Fishes, takes place at the commencement of the Cretaceous formations. Genera of the first and second orders (Placoidean and Ganoidean,) which had prevailed exclusively in all formations till the termination of the Oolitic series, ceased suddenly, and were replaced by genera of new orders (Ctenoidean and Cycloidean,) then for the first time introduced. Nearly two-thirds of the latter also are now extinct; but these approach nearer to Fishes of the tertiary series, than to those which had preceded the formation of the Chalk.

Comparing the Fishes of the Chalk with those of the elder Tertiary formation of Monte Bolca, we find not one species, and but few genera, that are common to both.*

Fishes of the Tertiary Formations.

As soon as we enter on the Tertiary strata, another change

* It has been already stated, that the remarkable deposite of fossil Fishes at Engi, in the Canton of Glaris, are referred by M. Agassiz to the lower portion of the Cretaceous system.

Many genera of these are identical with, and others closely approximate to, the fishes of the Inferior chalk (Pläner kalk) of Bohemia, and of the Chalk of Westphalia (see Leonhard and Bronn. Neues Jahrbuch, 1834.) Although the mineral character of the slate of Glaris presents, as we have before stated, an appearance of high antiquity, its age is probably the same as that of the Gault, or Speeton clay of England. This alteration of character is consistent with the changes that have given an air of higher antiquity than belongs to them, to most of the Secondary and Tertiary formations in the Alps.

The Fishes of the Upper chalk are best known by the numerous and splendid examples discovered at Lewes by Mr. Mantell, and figured in his works. These Fishes are in an unexampled state of perfection; in the abdominal cavities of one species (Macropoma) the stomach, and coprolites are preserved entire, in their natural place.

takes place in the character of fossil Fishes, not less striking

than that in fossil Shells.

The fishes of Monte Bolca are of the Eocene period, and are well known by the figures engraved in the Ittiolitologia Veronese, of Volta; and in Knorr. About one-half of these fishes belong to extinct genera, and not one is identical with any existing species; they are all marine, and the greater number approach most nearly to forms now living within the tropics.*

To this first period of the Tertiary formations belong also the Fishes of the London clay; many of the species found in Sheppy, though not identical with those of Monte Bolca, are closely allied to them. The Fishes of Libanus also are of this era. The Fishes in the gypsum of Mont Martre are referred to the same period by M. Agassiz, who differs from Cuvier, in attributing them all to extinct genera.

The Fishes of Oeningen have, by all writers, been referred to a very recent local lacustrine deposite. M. Agassiz assigns them to the second period of the Tertiary formations, coeval with the Molasse of Switzerland and the sandstone of Fontainbleau. Of seventeen extinct species, one only is of an extra-European genus, and all belong to existing genera.

The gypsum of Aix contains some species referable to one of the extinct genera of Mont Martre, but the greatest part are of existing genera. M. Agassiz considers the age of this formation as nearly coinciding with that of the Oeningen deposites.

The Fishes of the Crag of Norfolk, and the superior Sub-apennine formation, as far as they are yet known,

*M. Agassiz has re-arranged these Fishes under 127 Species, all extinct, and 77 Genera. Of these Genera 38 are extinct, and 39 still living; the latter present 81 fossil species at Monte Bolca, and the former 46 species. These 39 living Genera appear for the first time in this forma tion.

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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, (Pl. 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 them are found large bony Spines, armed on one side with prickles, resembling hooked teeth, (see Pl. 274. 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 every subsequent formation, till the commencement of the Tertiary, and has only one living representative, viz. the Cestracion Phillippi, or Port Jackson Shark. (Pl. 1. Fig. 18.) The second

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

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.*

* 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. (Pl. 274. A. 1, 3, 4, and Pl. 27a, 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 27, Figs, 1, 2, represent a series of teeth of the genus Acrodus, in the family of Cestracionts, from the lias of Somersetshire; and Pl. 27o, 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 Pl. 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 Cestracion Phillippi, (Pl. 1, Fig. 18, and Pl. 27a, 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 (Pl. 274, 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, Hy bodonts, 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 27. B. Figs. 8, 9, 10.) Plate 274 C. 1re. represents a rare example of a series of teeth of Hybodus reticulatus, 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 (Pl. 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, (Pl. 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 Iehthyodorulites: 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. 27a, 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. (Pl. 274, 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 Pl. 27d, C, 3,

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