2l6 THE AMERICAN MIDLAND NATURALIST 



simplify further in order that the evidence for the relative preva- 

 lence of different nacreous colors in those species where more than 

 one was described, might be rendered more intelligible for report. 

 Just as the systematist for roughly descriptive purposes has picked 

 out a number of the more prominent nacreous colors of each 

 species, the writer, largely following Simpson's Descriptive Cata- 

 logue of the Naiades(i5), chose from the previously prepared charts, 

 the 5-15 "leading colors" in the nacreous color of each species, to 

 which the large majority of the rest could be assigned. Percentages 

 of these leading colors were then calculated for each species in the 

 bodies of water, drainages, groups of drainages in which they were 

 found, as best seemed to throw light on the problems to be attacked. 

 While all recorded shades in a large number of the species would 

 not conform to this treatment, they represent percentages in the 

 extreme minority. These may be inferred to exist in those species 

 where the table on "Distribution of Colors as a Whole," does not 

 add up to 100%. Strictly speaking, even this comparatively large 

 number of "leading colors" could have been condensed to a smaller 

 number, but a larger number was necessary in order that certain 

 close distinction in the color of nacre for purposes of the investiga- 

 tion might be made, for example, between the color of shells of a 

 river and those of its tributaries. In the discussion of any particular 

 body of water, however, when the latter is considered by itself, 

 the leading colors given represent my reduction to lowest terms, 

 of the colors represented in it. 



V. — Results. 



Each species is dealt with separately, there first being given in 

 parallel columns, 



(a) Descriptive material concerning the nacreous color as taken 

 from Simpson (15). 



(b) The equivalent in the writer's opinion of the Simpson colors 

 in terms of the Ridgeway Color Nomenclature. 



It is felt that by this arrangement, and the inferences to be 

 drawn from the names of the Ridgeway Colors themselves, it 

 will be possible for the reader to sufficiently understand the 

 terminology used as to convey the principles this paper hopes 

 to make clear. For the sake of even greater clarity, there are also 

 given latterly in the Ridgeway column, the peculiar shades of the 

 Lake Erie shells, although these are, by the convenient arbitracy 



