518 



CEXOZOIC ERA— AGE OF MAMMALS. 



Fishes. — The present relation between the three great orders of 

 Fishes — Teleosts, Ganoids, and Placoids — was first fairly established in 

 the Tertiary. Teleosts were first introduced in the Creta- 

 ceous, but only in the Tertiary did they become very 

 abundant. Ganoids, on the contrary, became fewer in 

 number ; they sank into their present subordinate position. 

 Among Placoids, the Hybodonts are gone, the Cestra- 

 cionts are few in number, but the Squalodonts reach their 



Figs. 884-888.— Tert r art Fisiies- 

 primigenius (after Agassiz). £ 

 megalodon, x \ (after Gibbes). 



Fig. 888. 



■Placoids: 884. Lamna elegans (after Agassiz). 885. Notidanus 

 S6. Carcharodon augnstidens (after Giblbes). 887. Carcharodor. 

 Teleost: 888. Clupea alta (after Leidy). 



maximum development, both in number and size. In the marine Ter- 

 tiary of the Atlantic border, both Eocene and Miocene, sharks' teeth 

 are found in immense numbers, and of very great size. Some of the 

 triangular teeth of the Carcliarodon megalodon (Fig. 887) are found 

 six and a half inches long and six inches broad at the base. The own- 



