338 STRUCTURE OF THE VERTEBRATES 



Mesozoic Time 



Cenozoic Time 



A new line is considered as beginning when fossils of un- 

 doubted relationships have been found in n particular period. 

 Evidently the group had more primitive ancestry, and as re- 

 search continues in the fossil bearing beds the origin of lines is 

 being steadily pushed farther back in history. The sharks, for 

 example, are listed as having arisen in the Silurian Period al- 

 though their ancestry must go back into the Ordovician; for 

 their rise to dominance is synchronous with that of the armored 

 fish (Antiarchs and Arthrodires) . These are undoubted fish, al- 

 though gill-arch jaws are not present. 



