THE OCEAN WORLD. 



geological periods, we may briefly state that they have been found in 

 every formation from the Silurian to the Tertiary. The species, at first 



m 





Fig. 22. Nummulites Kouaultf(d'Arclniic and J. Haime). 



very simple in their forms, begin to appear in increasing numbers in the 

 carboniferous formations. They become more numerous, and, at the 



Fig. 23. Siderolites calcitrapoides (Lamarck;. Natural size and magnified. 



same time, more complex in their forms, in the Cretaceous period ; they 

 are still more diversified, and appear to have multiplied much more 



