BY MOLECULAR COALESCENCE. 99 
This arrangement is best demonstrated by putting the 
entire valve of a small mussel into hydrochloric acid, so 
feeble that the too sudden escape of the carbonic acid will 
not break the membrane into fragments, but raise it entire. 
Fig. 6. 
The most external membrane of the shell of this mollusc 
does not become calcified, nor does it receive an addition 
to its entire surface, but only grows at its free border. It 
is of a horny structure, presenting distinct markings. But 
