VARIATION IN EPIDERMAL COLOR OF NAJADES. 25 1 



points concerning the physical conditions are given in the following 

 'contrasting columns. 



LAK6 ERIE UPPER OHIO DRAINAGE 



Water colder than in Upper Ohio Waver warmer, but with greater 



but with more even regulation of extremes Sf temperature to face, 



temperature. Currents much less rapid Streams more rapid than current of 



than in streams; less agitated, except L. Erie; greater agitation, frequent 



by very moderate currents carrying falls and rapids, short stretches of 



but little sediment. Bottom of pebbles quiet pools. Rivers carry a load of 



or sand or mixture of these depending debris moving quickly over the bottom 



on region of lake, with coarser sedi- which consists of mud, glacial fill, 



ment derived from wear of land. cobbles. Food conditions {due to 



Temperature conditions favor a more extremes of temperature) , are less 



uniform production of food if in less stable, even if at times food is more 



abundance. Water is more highly abundant, 

 alkaline than that of Upper Ohio 

 Drainage. 



Walker, (i8) observes that L. Erie shells as a whole have brighter, 

 (clearer) colors than their fellows of the Upper Ohio Drainage, 

 are exceptionally polished, and otherwise characterized in dis- 

 tinction by their well developed lines of growth. 



IV. — Method. 



Befof-e attempting the study of the shells, the epidermis of each 

 was lightly scrubbed with a moderately stiff brush to remove any 

 sediment, etc., adhering to it. Care was taken not to injure the 

 epidermis in any way. Shells so badly eroded that a positive 

 determination of nacre color was impossible were ignored in further 

 study. The method pursued in the study of variation in epidermal 

 color was largely that used in the investigation of nacreous color. 

 (4). Here as with the nacreous colors, the epidermal colors did not 

 lend themselves to the determination of any well defined color 

 pattern. The problem is even more complicated in the latter, 

 since a large number of different colors may be represented in the 

 epidermis, due either to inherent causes or as the result of the 

 interaction, as we shall see is probable, with the environment. 

 To simplify matters to a stage where the problem could be grappled 

 with, at least two colors, obtained by comparison with the standard 

 colors of Ridge way (14), were recorded for each shell. Following 

 the clue cited from Marshall's paper, the first, (or primary), of these 

 two colors was that, which by its lighter hues, distribution and 

 relation to the other, (secondary) one taken was evidently the 



