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shells, the deeper the water they inhabit the more fragile they are, and the shallower the 

 water the thicker the shell is, no doubt the chances being that in the shallow water, from the 

 roughness of the sea at times, they would be more likely to be broken. This is easily seen in 

 the common buccinum of our own coast, which seems to be found at different depths in the 

 sea, and of great variety in the thickness of shell as well as shape. Another instance of the 

 variation, but in size only, we find in the full grown shells of the larger Fusus, but here we 

 have not the variation in the thickness of the shell, but merely the size, probably it never 

 comes into water influenced by the action of the waves but why should the mature shells and 

 animals not be always about the same size ? 



We see this prominently with the common mussel, which in some sheltered situations 

 grows to a large size, whilst on exposed rocks over which the waves break, they get clustered 

 in every water-washed hole and corner of the rock, many together, and of very small size, 

 evidently to resist the power of the water and prevent their being washed away, and in which 

 places they appear to live a long time, but do not apparently increase in size. Some of the 

 other kinds of shells, when full grown, vary much in size ; in some of the cowries this is 

 easily seen, as, when the lip once curls over, the shell, as well as the animal, has attained its 

 full growth, whilst in other closely allied kinds, when the lip turns over, all appear nearly about 

 the same size! How is this? In some kinds of. fishes also, maturity seems to have nothing to do 

 with the size of individuals for instance, mackerel and herrings do not vary very much when 

 mature ; but how different it is in the salmon, and halibut and other kinds, which, apparently 

 under certain unexplained circumstances, increase in size as long as they live. 



