C 239 ) 



CHAPTER X. 



FOSSIL MOLLUSCA (PALAEOZOIC, 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 Jukesii] do not differ in 

 any important character from the Swan mussels 

 (Anodori)) so abundant in English lakes and rivers at 

 the present time ; the Palndina of the Wealden epoch, 



