, VARIATION IN EPIDERMAL COLOR IN NAJADES. 283 



environment in determining what the epidermal color shall be. 

 In the case of brighter hues of nacreous colors in females, such a 

 finding with regard to the epidermal color may also be safely re- 

 garded as a "metabolic accident." 



VIII. — Relative Variation in Epidermal Color in 

 Species Dealt With. 



The shells were so unevenly distributed with regard to localities 

 that it was impossible to determine those places where the great- 

 est ^nount of variation in epidermal color took place. Some idea 

 may be obtained from PI. Ill, when such is studied from the stand- 

 point of any great body of water a^ a whole. In an effort to make a 

 partially balanced determination of the relative variability of 

 epidermal color among them, a rough and arbitrary comparison 

 was taken by dividing the number of "relative colors" observed 

 in each shell by the number of that species examined. From 

 this data, it seems that within" the limits of this investi- 

 gation that the larger number of shells is apparently associated 

 with less variation in epidermal color. At the same time the results 

 are hardly fair for those species represented by a small number of 

 specimens. Results from this method show the relative varia- 

 bility of the shells to be as indicated in the following table. The 

 small numerals following the name of each species indicate its 

 order in range of variation of nacreous color similarly determined. 

 (4). and from it a convenient comparison of the relative variability 

 of epidermis and nacre may be taken. 

 Species No. Relative Colors taken 



1. Eurynia recta (i) 45 



2. Proptera alata (2) 42 



3. Anodont. ferussacianus (6) 45 



4. Paraptera fragilis (3) 33 



5. Anodonta grandis (7) 54 



6. Symphynota costata (11) 31 



7. Lampsilis ovata (9) 62 



8. Amblema plicata (5) 59 



9. P. obliquum coccin. (4) 59 



10. Lampsilis luteola (12) 63 



11. Fusconaja flava (8) 42 



12. Elliptic dilatatus (10) 65 



If, in view of difficulties the reader will comprehend were en- 

 countered in organizing this type of data, 2, (or in some cases 3) 

 ranks in the above table on Relative Variation of Color is allowed 



