VARIATION IN NACREOUS COLOR OF NAIADES 215 



IV.— Method. 



As previously indicated, the nacreous color of the Naiades does 

 not readily lend itself to the determination of any well defined 

 color pattern. The writer therefore confined his efforts to the 

 tabulation of the nacreous colors of each species of shell by means 

 of comparisons with the standard colors given in the Ridgeway 

 Color Nomenclature. Usually but one color was recorded — that 

 one most impressing the eye with its vividness and preponderance 

 in the nacre. Where the number of shells from a locality was 

 small, sometimes 2-3 colors were recorded, but only if they appeared 

 to be of equal tone in the shell. Such data was later useful in trac- 

 ing genetic relationship in the sequence of color change. The 

 color or colors judged most to match were then written in figuring 

 books opposite calculations previously made for the morphological 

 features of each shell. Shell were rejected when erosion of the 

 nacre was such that no definite determination of nacre color could 

 be made. As a rule, white as a color was not recognized when there 

 existed a fair suspicion that another color was the original one, 

 for scientifically speaking, white is a combination of colors, and 

 usually the writer was able to refer whitish shades to pearl blue 

 and closely similar colors. 



Where a large number of shells from one locality were concerned, 

 it was the usual practice to group all the shells of a closely similar 

 nacre color, and compare as a whole with the shades in Ridgeway. 

 By this means, a general or average hue was obtained, not accurate 

 of course for every shell, but very convenient in determining the 

 relative color of the shells at the locality to ones near it, above 

 or below in the particular body of water. As a rule these relative 

 colors were taken from a fairly large number of shells — 8 in most 

 cases, although lack of material often completed the use of smaller 

 numbers. Against such treatment, however, was the check of a 

 separate color comparison for each shell. 



By means of the method above described, it was possible to 

 determine for each species, a shade of nacre color peculiar to the 

 locality where the shell had been collected, and this being done 

 charts were prepared showing the sequence of color or color 

 changes passing down stream, or the distribution in different 

 parts of the same body of water. This data is presented in Plate 

 III. Even by this process of condensation, a very large number 

 of colors were obtained for each species, making it imperative to 



