218 FOSSIL SHARKS. 



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 famihar with the 

 occurrence of various kinds of large, and beautifully ena- 

 melled teeth, some of them resembling the external form 

 of a contracted leech, (PL 27% and 27^:) these are com- 

 monly 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 deriv^ed. 



In the same strata with them are found large bony Spines, 

 armed on one side with prickles, resembling hooked teeth, 

 (see PI. 27^ C. 3, a.) These were long considered to be 

 jaws, and true teeth ; more recently they have been ascer- 

 tained to be dorsal spines of Fishes, and from their sup- 

 posed defensive office, like those of the genus Balistes and 

 Silurus, have been named Ichthyodorulites. 



M. Agassiz has at length referred all these bodies to ex- 

 tinct genera in the great family of Sharks, a family which 

 he separates into three sub-families, each containing forms 

 pecuUar to certain geological epochs, and which change 

 simultaneously with the other great changes in fossil re- 

 mains. 



The first and oldest sub-family, Cestracionts, beginning 

 with the Transition strata, appears in eveiy subsequent for- 

 mation, 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 



* M. Agassiz has ascertained the existence of more than one hundred and 

 fifty extinct spccice of fossil Fishes allied to this family. 



