29 
This wholesale destruction, together with the depreda- 
tions of hogs, which have exterminated whole shoals of 
Unios when the water in the brooks was low, and the sub- 
stances introduced into the water by manufacturing indus- 
tries, besides sewerage and refuse from factories, are rapid- 
ly causing the disappearance of animal life ate many of 
our water courses. 
It is probable that the existence of carbonate of lime in 
excess where mussels abound, influences the secretion 
that causes the growth of the pearls. In lime-stone regions, 
if the waters are polluted by products of decomposition 
that are acid, these unite with the lime and form other 
compounds, which are either precipitated or carried away 
with the impurities of the water. There can be no doubt 
that this cause would tend to decrease the amount of lime 
which the shell would receive, thus not only retarding the 
growth of the pearls, but often eventually leading to the 
extermination of the Unios themselves. At nearly all the 
marine pearl-fisheries, coral-banks abound; branches of 
coral frequently forming on the shells themselves, some- 
times three or four species on a single shell; and it may be 
that these have more or less influence on the development 
of the pearl in the shell. In Vermont, New Jersey and 
Ohio, where pearls were formerly found, a fine one is now 
rarely obtained. 
A unique method of collecting Unios is that practiced by 
the lumbermen, who, while sailing down the Canadian 
rivers on their rafts, collect Unios for food, by fastening 
bushes to the rear of the raft, so that when they pass 
through the mussel shoals, where the rivers are shallow, 
the bushes touch, the Unios close on the leaves and thin 
branches, holding them securely ; and at intervals the 
bushes are taken out and the Unios removed. 
In regard to these pearl excitements which appear from 
time to time, and as to where best to search for pearls, it 
may be well to state that it is advisable to search every 
