RIVER GARDENS; 



interesting on account of its large size and agree- 

 ably tinted semi-transparent shells. The British 

 Pearl Mussel (Alasmodon margaritiferus} might 

 also he introduced. Pearls are occasionally found 

 in all the fresh-water Mussels, but most frequently 

 in the last-named species, being generally caused 

 by the accidental presence of some small irritating 

 substance within the shell, which cannot be ex- 

 pelled, and which is therefore coated with layers of 

 the same substance as that which forms the nacreous 

 lining of the shell itself. The artificial introduction 

 of such substances has been tried with success, but 

 not with sufficient certainty of result to be of any 

 commercial importance. The pearl fishery in some 

 of our British streams was, however, at one time 

 followed with such perseverance as, no doubt, 

 rendered the trade in them to some extent remune- 

 rative, and in the time of the Romans they were 

 sent to Italy in considerable quantities. Anti- 

 quarians have collected many interesting particulars 

 concerning this traffic, which we have only alluded 

 to en passant. There are many other varieties of 

 British fresh-water shells which might be intro- 

 duced with advantage into the Aquarium, but I 

 must refer those who would make them a pro- 

 minent feature in the tank, to the beautiful and 



