Hie Era of Livertehrates 



sions, or chambers, the outermost of which con- 

 tains the animal. As growth proceeds chamber 

 after chamber is added, the occupant moving 

 forward and always residing in the last. The 

 few species of nautilus are the only living rep- 

 resentatives of a host of chambered shells that 

 started into existence toward the close of the 

 Cambrian period and soon acquired a prom- 

 inent position among the residents of the sea. 

 But the early cephalopods were by no means 

 all tightly coiled ; the shells of some formed an 

 open spiral, as if the shell of a nautilus were 

 uncoiled, while most were quite straight, like 

 the members of the genus Midoceras, which 

 comprises a few huge species six to twelve feet 

 in length. 



And now, most important of all, vertebrates 

 appear upon the scene, the first hint of the far- 

 off coming of man, but the story of this impor- 

 tant event and of the rise of the fishes may well 

 be reserved for another chapter, the more that 

 life does not progress from start to finish by 

 one continuous and even ascent, but has its ups 

 and downs. The highest species in any given 

 6 73 



