IN A SNAILERY. 31 
of their long, slender shells as they grow old. The aban- 
doned portion speedily becomes dead, and cracks off upon 
the least injury. The sign of a perfect adult shell in these 
species, therefore, is that it is broken! Mr. Thomas Bland, 
the distinguished student of West Indian conchology, dis- 
covered this enrious fact. After the cylindrella has thus 
voluntarily left the upper part of his shell, he builds a par- 
tition across behind him. Often other mollusks are driven 
to a similar expedient by accident or the decay of extreme 
age. This is called decortication, and is almost always to 
be seen in the beaks of the larger unios or fresh-water mus- 
sels of our inland rivers. The spiral shells most likely to 
be thus affected are those that live in swift running water, 
where the bottom is rocky—such as the members of the 
families viviparidse and strepomatide. The latter are rare- 
ly seen otherwise than dreadfully broken. 
Another curious thing is to be noticed in this connection: 
whole species sometimes suddenly die out. Not only a 
conchologist, but others, travelling through certain parts 
of our western territories, are. struck by the prodigious 
quantities of dead white snail-shells scattered over the 
ground. These are the //elix cooper, of which a few are 
stil] living in nooks and corners of the mountains. They 
are of all sizes, degrees of variation, and ages, and lie 
