SHELLS IN LONDON "WATERS. 35 



be found in London waters, whatever may have been 

 the case formerly ; but the collector may procure 

 at least two species, Unio tumidus and pictorum 

 (PL III., figs. 1, 2), which belong to the same genus 

 as a pearl-bearing species, U. margaritifer (PI. IV., 

 fig. 3), formerly abundant in Great Britain, and 

 still to be obtained in some parts of the country. 

 In works which bear upon the subject, many rivers 

 are noticed as having been at one time the seats of 

 pearl fisheries ; and, as every one knows, Britain 

 nearly two thousand years ago was celebrated far and 

 wide for its pearls : — so much so, indeed, that, accord- 

 ing to trustworthy historians of that remote period, 

 we are indebted to these precious jewels for the first 

 hostile visit of the Romans to our shores, on which 

 occasion, to use the words of a humorous modern 

 writer, Cresar broke in upon the natives with con- 

 siderable energy. At the present day, we are in- 

 formed that but one pearl on the average is found 

 in every thirty shells ; and, as only one in about 

 ten is saleable, it requires the destruction of three 

 hundred shells to find that one gem. At the present 

 day, we are told, it is not unusual to find pearls in 

 the Teith and Tay worth from £1 to £'2 each. 



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