220 BONY SPINES OF SHARKS. 



Fossil Spines, or Ichihyodorulites.* 



The bony spines of the dorsal fins of the Port Jackson 

 Shark (PI. 1. Fig. 18.) throw important hght on the history 

 of fossil Spines; and enable us to refer those very comnaon. 

 but little understood fossils, which have been called Ichthyo- 

 dorulites^ 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 hony spine armed on it? 

 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 Hybodoats, is the Onchus, found 

 in the Lias at Lyme Regis; the teeth of this genus are represented, PI. 27'', 

 6. 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. 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 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 Ter- 

 tiary 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 bencatli the Chalk, had relation, most probably, 

 to their office of crushing tiie hard coverings of the Crustacea, and of the 

 bony enamelled scales of the Fishes, which formed their food. As soon a« 

 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 ciiaracterize the teeth of living Sharks. Not one 

 species of the hlunttoothcd Cestraciont family lias yet been discovered ia 

 :iny Tertiary formation^ 



* See PI. 27". C. 3. 



