240 OUR COMMON BRITISH FOSSILS. 



and the Planorbis of the Eocene, are so like species 

 of the same genera now abundant in any of our 

 streams and ponds, that the least informed student 

 would identify the relationship at once. 



Perhaps this wonderful persistence of type in 

 fresh-water mollusca is to be found in the fact that 

 fresh-water conditions experience less change in phy- 

 sical environment than any other. The water of the 

 Old Red Sandstone lakes may have been exposed 



Fig. 22S.^Orthonota pamlleia. Fig. 226. — Dlinxhisoma gracilis. 



to warmer conditions than those of Great Britain now, 

 but in the deeper parts the bivalve mollusca would 

 find almost similar conditions that modern fresh-water 

 bivalves would be able to select, if they chose to 

 do so. 



The composition of the shells of bivalve mollusca 

 is not always the same. Indeed, we may somewhat 

 definitely separate them into two classes, according to 

 the chemical composition of their shells — calcific 



