A PRETTY LAND-SHELL. i?7 



oyster or other bivalves ; but it has com- 

 pletely disappeared in most land and fresh- 

 water snails, as well as among many marine 

 species. The fact of its occurrence in the 

 cyclostoma would thus be quite inexplic- 

 able if we were compelled to regard it as 

 a descendant of the other lung-breathing 

 mollusks. So far as I know, all naturalists 

 have till lately always so regarded it ; but 

 there can be very little doubt, with the new 

 light cast upon the question by Darwinism, 

 that they are wrong. There exists in all our 

 ponds and rivers another snail, not breathing 

 by means of lungs, but provided with gills, 

 known as paludina. This paludina has a 

 door to its shell, like the cyclostoma ; and so, 

 indeed, have all its allies. Now, strange as 

 it sounds to say so, it is pretty certain that 

 we must really class this lung-breathing 

 cyclostoma among the gill-breathers, because 

 of its close resemblance to the paludina. It 

 is, in fact, one of these gill-breathing pond- 

 snails which has taken to living on dry land, 



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