240 THE AMERICAN MIDLAND NATURALIST 



be held accountable in any way for variation in the nacreous 

 color of Najades, a factor of possible connection is that of the age 

 of the mussels. In obtaining any light upon the relation of the 

 nacreous color and the age of the animal, a difficulty which presents 

 itself is the accurate determination of the latter. Isely* who has 

 probably made the most extensive study of the growth of Fresh 

 Water Mussels, states that ^ordinarily the prominent rings of 

 the shell are presumably winter rings, (delayed growth), and 

 therefore each may represent a year of the animals life. However, 

 rings may develop as the result of unfavorable conditions such as 

 water shrinkage, temporary stranding, etc., and at any time of 

 the year. Moreover, the rate of growth is variable for individuals 

 of a single species in the same stream apparently depending on 

 conditions of food, oxygen etc. . . . Growth may also slow down 

 after sexual maturity. All these unfavorable conditions probably 

 act to some degree on shells in the Upper Ohio Drainage. However 

 at the time the color data vvas recorded, the writer estimated the 

 age of these and those from L. Erie by counting the rings of greatest 

 prominence, which Isely indicates are presumably the winter rest 

 rings. Here it may be remarked as already shown elsewhere, (3), 

 that the shells of L. Erie are under more stable conditions, and 

 greater confidence may be placed in such a method of estimating 

 the age of them. This latter fact was the basis of a check in the 

 following procedure. 



There was only the task of rearranging the different recorded 

 colors by classes of the estimated age, and this being done to see 

 whether association was evident between the recorded hues and the 

 estimated age. As insufficient space prevents reproduction of the 

 tabulations thus secured, it can only be stated that all of the colors 

 of all species were found to be dijJtributed through all ages rather 

 than being peculiar of certain ones. Certain generalizations are 

 worthy of note,t that as a rule deeper colors in all species fade 



* Isely, F. B. "Experimental Study of Growth and Migration of Fresh 

 Water Mussels" Bureau of Fisheries Document, 792 (1914) 



t Thus with age, (using terminology of Simpson) ; in Fusconaia, the 

 percentage of whites and salmons increase, rose tints decrease; Amblema, 

 the bluish colors tend toward lilac shades; salmon, straw, and white increase 

 in Elliptio, purples become lighter as also in Proptera and Paraptera; the 

 reds fade out in Pleurohema and the salmons and buff in Symphnota, when 

 the pinkish hues become more prominent. Bluish white increases in Anodonta 



