EIVEE GAEDENS; 
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, hut 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 Bomans 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 
48 
