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CHAPTER X. 



FOSSIL MOLLUSCA (PALEOZOIC, OR PRIMARY). 

 BIVALVES AND UNIVALVES. 



It does not need much mental preparation to per- 

 ceive that the hard parts of those animals popularly 

 called " shell-fish " must have contributed very largely 

 to " fossil remains." They are so abundant, so widely 

 spread, so wonderfully adapted to almost every phy- 

 sical condition of the earth's surface — terrestrial, fresh- 

 water, brackish water, shallow and deep seas, — to cold, 

 temperate, and tropical regions alike — that it is not 

 surprising the geologist pays great attention to the 

 suggestions which fossil mollusca give him. More- 

 over, mollusca are, perhaps, among the most perma- 

 nent and stereotyped, and the least inclined to change, 

 of animal forms. The fossil fresh-water mussels which 

 flourished in the extensive lakes of the Old Red 

 Sandstone period {Anodonta Jtikesii) do not differ in 

 any important character from the Swan mussels 

 {Anodon), so abundant in English lakes and rivers at 

 the present time ; the Paliidina of the Wealden epoch, 



