284 THE AMERICAN MIDLAND NATURALIST 



for inaccuracy of color determination, it will be seen that variability 

 in epidermal color is closely associated with that of nacreous color. 



Sources of Error. 



The Ridgeway Color Nomenclature was used with careful con- 

 sideration of the directions given in it. It may well be urged that 

 the sense of color is so varied in its development among humanity 

 that results of this kind may not have the same significance for all 

 interested in such problems. But the same criticism could be applied 

 to the ornithologist who uses the Nomenclature continually. The 

 writers confidence in his own observations is largely based on the 

 fact that U. S. Army Tests have shown his vision to be normal in 

 every way. 



It is also true that at times, the mussels migrate from place to 

 place in the same stream, and probably from the rivers into the 

 tributaries. Where a small number of shells were used in making 

 comparisons this might have some effect on the results obtained, 

 but as the evidence of most observers is that migration is com- 

 paratively rare among them, this can hardly have any effect on the 

 general impressions this paper hopes to convey, indeed, they might 

 be held to account for discrepancies which will be observed here 

 and there. Finally, pollution of streams by sewage, industrial 

 waste, erosion by gravel and water etc., may be so extensive as to 

 produce a color in some cases thoroughly unnatural to the animal. 

 A check which covered most of these cases was the primary color 

 taken which part being usually buried in the mud, would be more 

 immune to such influences, and apt to show its truest tints. 



Washington and Jefferson College, 

 Washington, Pa. 



IX. — LITERATURE CITED. 



1. Grier, N. M. "Morphological Features of Certain Mussel Shells of 

 L. Erie Compared with Corresponding Ones of the Upper Drainage." Annals 

 Carnegie Museum, 1920. 



2. Ibid. "On the Erosion and Thickness of the Shells of the Fresh Water 

 Mussels." Nautilus, 1920. 



3. Ibid. "Sexual Dimorphism and Some of its Correlations in the Shells 

 of Certain Species of Najades." Amer. Mid. Nat. Vol. 6, 1920. p. 165. 



4. Ibid. "Variation Tn Nacreous Color of Certain Species of Najades 

 Inhabiting the Upper Ohio Drainage and their Corresponding Ones in L. 

 Erie." Amer. Mid. Nat. 1920. • - 



