VAST EXTENT OF ANCIENT STRATA 



most ancient vertebrate remains, yet disinterred 

 from the crust of the earth. 



Ancient, inconceivably ancient, as are these 

 Uj)per Silurian rocks, there are yet immense 

 thicknesses below them of stratified rock, con- 

 taining fossils in which no fish remains have 

 been discovered. We must not conclude that 

 the very curious-looking fishes of the Upper 

 Silurian are really the actual forefathers of all 

 later fish and of all vertebrate life. They just 

 happen to be preserved and dug up, but probably 

 soft-bodied fishes existed then and before that 

 time which had no bones inside and no hard 

 scales outside, and so have left no sign, in the 

 rocks, of their existence. The Upper Silurian 

 strata are, as you will see by looking at the Table 

 of Strata on p. 60, just halfway down in the 

 thicknesses of rocks, between the present river 

 gravels above and the Cambrian beds with the 

 oldest known fossils (certain Trilobites) below. 



We will revert to the Trilobites directly ; but 

 before leaving the extinct fishes I wish to men- 

 tion the great fossil sharks of the late Tertiaries 

 (Miocene and Pliocene). These we knoAV by 

 their teeth ; enormous shark's teeth are found 

 which are three times the length of the teeth 



263 



