16 THE NAUTILUS. 



erosion of shells to the presence of C0 2 in the water. This is 

 also confirmed in a way from the interpretation of geologic data, 

 which gives evidence of the solvent power of ' ' carbonic acid. ' ' 

 Not only is C0 2 being continually liberated in nature in other 

 ways, but there is hardly any doubt but that the interaction of 

 humic acid often present in streams with lime may also produce 

 C0 2 . Thus the observation of Shrubsole, whose shells were 

 collected from a drainage containing a diversified material, may 

 plausibly fit in here. Of course the fact must never be excluded 

 that coarser material carried along by the current also plays a 

 part in the erosion of shells, but the consequences of such a 

 factor may be intensified by the chemical reactions which already 

 may have taken place. Most of the eroded shells I have ex- 

 amined come from streams having an abundance of gravel. 

 Again, it is probable that in some cases an abundance of lime 

 in a stream may neutralize the humic acid before the latter can 

 produce any marked effect. 



Later on, some evidence will be presented in support of 

 March's contention to the effect that high CaC0 3 content of the 

 water somehow inhibits absorption of material, preventing the 

 shell from becoming as thick as it might. This, however, is 

 only a phase of the well-established principle that living cells 

 are able to control the absorption of substances used in their 

 metabolism. Since it is admitted that the lime of shells comes 

 from the water in which they live, there is reason to think there 

 may be some correlation positive or negative between the 

 amount of lime present and the thickness of the shells. Several 

 investigators have indicated their probable attack of this prob- 

 lem, but so far there does not seem to be any published results. 



Having already secured data on the thickness of the shell and 

 reduced it to a convenient factor, (the thickness just superior 

 to the pallial line directly beneath the umbo, divided by the 

 height), I found a publication of the U. S. Geological Survey 

 (8) which fortunately gave analyses of the water at the same 

 or what seem to be reasonably adjacent points to where my 

 material had been collected. All the localities concerned col- 

 lecting, and points where analysis of water was taken, are indi- 

 cated in the data which appear to correlate for my conclusions 

 in the table. 



