Heart-Cockle and Fresh-water Cockle i 1 7 



The history of Driessensia in this country is very 

 singular. Geological evidence shows that in later 

 Tertiary times this species was abundant, not only 

 in Britain but over a good portion of Continental 

 Europe also. Then it vanished without leaving any 

 evidence why, but held its ground in the Aralo- 

 Caspian rivers, whence during the last one hundred 

 and fifty years it has been spreading once more over 

 Europe. In 1824 Mr. J. Sowerby discovered that it 

 had reached London. It was found in the Thames and 

 the Surrey Commercial Docks, and the very natural 

 conclusion was reached that it had come over from 

 the Baltic attached by its byssus to some of the 

 timber -laden vessels. It has since spread very 

 considerably about the country, and Ave have even 

 heard it whispered that a certain London water 

 company has occasionally supplied it to their 

 customers, wdio had only bargained for water. 



