28o THE AMERICAN MIDLAND NATURALIST 



2. Reaction of humic acid upon the yellow or red Fe2 03 of the 

 soil or of the water, resulting in its reduction to FeO, whence FeO 

 by interaction with CO2 of soil water or environment becomes 

 FeC03, a whitish or yellowish compound.* It was additionalUy 

 pointed out that iron is a part of the composition of the mussel 

 shell, and reasoning from the basis that it is known to be an im- 

 portant constituent of animal and vegetable pigments, it was 

 suggested that the deeper tints of nacreous color in the headwaters 

 was due by some similar process to the inclusion of greater amounts 

 of Fe203 in the shell, especially since the water in the tributaries 

 has a greater degree of oxygenation (due to greater rapidity of the 

 current), whence the transition from carbonate to ferric oxide might 

 be affected. In L. Erie, the nacreous colors of shells seemed more 

 closely related to the greater degree of alkalinity of the water. 



3. Further down stream, the reaction of the Humic acid upon 

 the ever increasing amount of lime may produce CO2. This or 

 other available CO2 may attact the iron oxides producing Fe2C03. 

 Under the conditions present, this latter compound may remain 

 stable, since oxygenation, (slower current) is less, light is less due 

 to increasing amounts of silt, and organic matter is more. Now it 

 is also known that organic matter at times may mask the red or 

 yellow iron oxides in clay, giving the latter a bluish, greenish or 

 bluish, greenish or even other colors. Provided then that Fe2C03 

 is the iron compound available downstream for mussel shells, and 

 that there is a greater inclusion of organic matter at such localities, 

 plausible explanation for the changes or fading out of nacreous 

 color is found. 



Much of the above may be made to apply to the problem of 

 epidermal color also when the following is born in mind. It has 

 been shown that the change in epidermal color is opposite to that 

 reported for nacreous color. Under the conditions, 2 groups of 

 factors acting separately or jointly may produce such an effect, 

 representing as they do natural and unnatural environments of 

 the shell. 



I. In connection with the natural group, it may be pointed out 

 that the amount of silt as well as the darkening of the epidermis 

 increases going down stream. Further, the mussel shell is subject 

 to constant erosion from CO2 or "other chemicals in the water 



*For a full discussion of Humic acid and its relation to iron compounds set 

 Pirsson, L. F. and Schuchert, C. S., "A Textbook of Geology." . 



