3H 



ESSENTIALS OF BIOLOGY 



and Gasteropods since the earliest known times and these are the 

 main molluscan types. They exhibit persistence and mediocrity. 

 We shall study the episodial Cephalopods presently. 



Tertuxry 



TertLary 



CretojceoiLs 



Jurcussix: 



TrixLsstc 



PermLcun 



CarboTuJkrovLS 



DevoTucuru 



Siluricurh 



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d> 



o 







I 

 5;: 



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to 

 u 



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1 1 



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gl 



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r 



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 z- 



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Fig. 43. — Periods and Affinities of the Chordate Animals. 



The dotted lines suggest the affinities. 



via. The Brachiopods, Polyzoa, Annelids, etc. A mixed 

 group of animals related to each other in ways that are difficult to 

 trace. They, also, have always been an animal type that is per- 

 sistent and of mediocre density. 



