UNIONIDiE AND MYTILID^. 87 



out the intruder at a more or less advanced stage ; so that 

 it will be found round and free^ perhaps hidden among 

 the fibres of the beard. 



The iridescent lining of our Unio is not so thick^ nor 

 that of our Mytilus so bright, as the oriental oyster, which 

 of course suggests a less rapid and pure secretion of the 

 fluid, and accounts for the inferior quahty and size of their 

 calculi. 



From this account it may be easily understood how the 

 Chinese manage to make pearls by keeping the shell-fish 

 alive, and introducing irritating substances through their 

 shells : they have even inserted images of baked clay, which, 

 in due time, have been covered with the enamel. 



The Unionid.e, or Fresh-water Mussels, are remark- 

 able for the large size of their tongue-shaped, compressed 

 foot, so peculiarly adapted for cutting their way rapidly 

 through the mud of the rivers in which they make their bed. 

 I remember, many years since, seeing a number of men, 

 women, and boys, with pots and pans, collecting specimens 

 of Anodon from the pond in the Green Park, opposite Pic- 

 cadilly. The water was all drawn off, and the people were 

 knee-deep in the mud, following the diagonal traces of the 

 active creatures^ retreat. It was a most exciting and really 



