Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/224

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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 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

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 Liss at Lyme Regis; the teeth of this genus are represented, Pl. 27d. 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. 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.