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 curious fact. After the cylindrella has thus 

 voluntarily left the upper part of his shell, he builds a par- 

 tition across behind him. Often other inollusks 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 strepomatidse. 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 

 conch ologist, 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. Helix cooperi, of which a few are 

 still living in nooks and corners of the mountains. They 

 are of all sizes, degrees of variation, and ages, and lie 



