VARIATION IN EPIDERMAL COLOR OF NAJADES. 279 



say tentatively at least that certain shades of epidermal color are 

 characteristic even of different parts of a given locality. 



6. The rays of the epidermis disappear with age and have in 

 the limits of the material worked with, their widest distribution 

 is specimens from small rivers and creeks. 



7. With regard to the relation of epidermal color to the esti- 

 mated age of the animals, it was found that no one color or group 

 of colors was peculiar to a given age of the animals, except the 

 yellowish or grayish colors of early youth, or the deep browns and 

 blacks of old age and advanced maturity. 



8. The epidermis of most species shows clearly defined sex- 

 correlative coloration. 



VII. — Suggestions as to Causes of Facts. 



Introductory remarks embody the writers comments on the 

 first of these conclusions. In view of the evidence presented, the 

 most plausible explanation of the second would seem to be found 

 in the physical and chemical conditions under which the shells 

 live. A summary of the more outstanding physical and chemical 

 conditions in the Upper Ohio Drainage and L. Erie has been 

 previously given. 



A physical condition which may seem closely related to the problem 

 of epidermal color is the warmer temperature of the water in the 

 former, for it has been shown that the shells from the Upper Ohio 

 possess more pigment, pigment is the result of chemical reactions, 

 and the degree of chemical reaction in general is increased by heat. 



It is readily comprehended that the problem of epidermal color is 

 a more complicated one than that of nacreous color. The epidermis, 

 protectory in function, is in direct contact with the environment, 

 and is the recipient of all chemical and physical forces involved 

 whereas the nacre, while probably the subject of all forces acting 

 through solution, is probably interacted upon by relatively few 

 physical forces. In a previous paper it was shown that the tints 

 of nacreous color lighten going down stream in the Upper Ohio 

 Drainage, and that the nacre of L. Erie shells possesses lighter hues 

 than those of the former. Suggested causes for these phenomena 

 were, 



I . Presence of humic acid in the headwaters of streams, which 

 with a greater amount of available light due to less amount of 

 ^ilt there, affords favorable ccuditions for the production of pigment. 



