278 THE AMERICAN MIDLAND NATURALIST 



dermis. No color or group of colors was found to be peculiar of any 

 given age. 



Observation on Sex-Correlative Coloration as associated with Epider- 

 mal Color. (106 shells). 



Greenish gold colors, (Olive Lake) appear to predominate in males. 

 Less of green and more of brown are to be found in females. 



VI. — Conclusions. 



1. In the species of Najades dealt with, there exists a wider 

 range of variation of epidermal color than that indicated by 

 standard specific descriptions. 



2. In practically all the species dealt with, a decided change of 

 epidi dermal color is observed going down stream from the head- 

 waters to the mouth. The usual tendency is for the primary epider- 

 mal, or ground color to darken from an Olive Green or Olive Gray 

 shade to Brownish or Buffy Colors, and this darkening is true in 

 part of all colors of the epidermis, whether due to inherent causes or 

 to those associated more witn environmental causes. Within the 

 limits of the material dealt with, it is further recognized that a darken- 

 ing of the epidermis may occur with advanced maturity or old-age, 

 which is quite independent of the geographical locality, but such 

 colors are always darkest in specimens from furthest down stream. 



3. The shells of L. Erie have in general lighter epidermal colors 

 than those of the Upper Ohio and Maumee Drainages. Lighter 

 yellows, browns and greens are more common among them, and 

 in this respect they resemble the smaller tributary streams of the 

 Upper Ohio Drainage. The shells of Conneaut and Chautauqua 

 Lakes have much the same relation as those of L. Erie. Other 

 conclusions, not however as completely substantiated as those 

 given above, but still sufficiently evident from the present data as 

 to deserve mention are, 



4. Each drainage leaves its own imprint on the shells collected 

 from it in the additional form of an associated or peculiar hue of 

 epidermal color, as has already been shown with regard to nacreous 

 and certain other physical characters of the shell. While the same 

 general hue may be present in different drainages, these may be 

 characteristically differentiated when necessary by the presence 

 of varying proportions of other colors. 



3. As a rule, the color distinctions may be carried so far as to 



