VARIATION IN NACREOUS COLOR OF NAIADES 239 



to experienced collectors. With regard to the part iron plays in the 

 coloration of the shells of these animals, it is suggestively recalled 

 that it is the basis of many animal and plant pigments such as 

 haemoglobin, bilirubin, chlorophyll, etc. 



Speculations here may be unbridled as in other fields and the 

 only thing to qualify it is experimental proof. It may be noted 

 that in many organisms strong production of pigment has been 

 considered evidence of progressive fnetabolism and sometimes 

 associated with "femaleness" in particular. In other organisms 

 beside the Mollusca it has been also associated with amount of 

 oxygen present, and under the physical conditions the waters of 

 the Upper Ohio are as a whole better oxygenated than those of 

 L. Erie. Some of the observations bear out in part observations on 

 another group of the Mollusca, the Chitons, where Crozier, (i) 

 has found a more brilliant coloring of the soft parts associated with 

 the "female" condition, believing it merely to be the result of a 

 "metabolic accident." From the evidence given, there may be 

 reason to state that the nacreous "ground color" of shells, from 

 which all colors are produced by modification, is a Pearl Blue 

 or "whitish" hue. Dr. A. E. Ortman in an unpublished paper which 

 confirms observations of several other investigators, has shoAvn 

 that the shells in the headwaters of streams are usually smaller 

 and more compressed than the same species in the lower stretches, 

 where they have become more highly inflated. Some factor in the 

 environment may thus inhibit the full physical development of the 

 shell in the headwaters, but its racial metabolism, evident in 

 greater development down stream, may find an outlet in the 

 production of pigment, a variation hardly as harmful to it as 

 increase in size and inflation would be in the. swift streams of the 

 headwaters. Against the above we have the check of Koifoids 

 observation, (9), that the plankton elements on which these 

 creatures feed increase with the temperature, and are more abun- 

 dant in the lower stretches of the stream where there is more silt, 

 and where the shells are more highly inflated. Finally it must be 

 remembered that in so far as outside conditions are concerned, the 

 nacre throughout life is protected by the greater thickness of the 

 shell. 



In the preceding, endeavor has been made to consider the more 

 plausible factors responsible for variation in nacreous color among 

 the Najades. If a theory of "progressive metabilism" in organisms 



