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

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 Carboniferous 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. 27', 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 Philippi. The Cestracion Philippi, (Pl. 1, Fig. 18, and Pl. 274, A.) is the only living species in the family of Sharks that has flat tesselated 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, 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 sub-family Cestracion, and the smooth and sharp-edged cutting teeth of the Squaloids, or true Sharks, which commenced with the Cretaceous formations. They *See Pl. 27d. C. 3.

us to refer those very common, but little understood fossils, which have been called Ichthyodorulites, to extinct genera and species of the subfamily of Cestracionts. (See page 286). Several living species of the great family of Sharks have

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 27 C. 1.re 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.

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. 27. 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 deposits to the present era. (Pl. 27. 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 lancetshaped, 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 characterise the teeth of living Sharks. Not one species of the blunt-toothed Cestraciont family has yet been discovered in any Tertiary formation.

smooth horny spines connected with the dorsal fin. In the Cestracion Philippi 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 Ichthyodorulites: these hooks act as points of suspension and attachment, whereby the dorsal fin is connected with this bony spine, and its movements 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 further 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 Greywacke series to the Chalk inclusive, indi

* 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

cates 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.

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.

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.

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 shewn to have prevailed during every period, from the first commencement of submarine life, unto the present hour. The similarity 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