254 THE AMERICAN MIDLAND NATURALIST 



Ortmann's discovery that the sex of the animal is readily deter- 

 minable from the structure of the gills. In associating epidermal 

 color with the estimated age of the animal, the latter was determined 

 by counting the number of winter rings on the shell. As I have 

 given elsewhere some discussion of the accuracy and inaccuracy 

 of the results attending this method, (4), it need only be stated 

 that the conclusion staken from the Upper Ohio shells werechecked 

 by similar ones from the L. Erie specimens, where this method of 

 estimating the age is less objectionable. Moreover, the conclusions 

 are so general in character as not to be readily affected by mistakes 

 in the age of a very small minority. Pressure of these observations, 

 and also the fact that as a whole few differences could be observed 

 in the texture of the epidermis of shells, lead me to disregard the 

 latter character altogether. 



V. — Results. 



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

 parallel columns: 



(a.) Descriptive material concerning epidermal color as taken 

 from Simpson. 



(b.) The equivalent in the writers 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. 

 Additionally the writer has endeavored to supplement this by 

 the use of such generalized color terms as he could command. 

 For the sake of greater clarity there are also given latterly in the 

 Ridgeway column the peculiar shades of the L. Erie shells, although 

 these are by the convenient and arbitrary arrangement adopted, 

 but varieties of the hues given in the tables dealing with "Dis- 

 tribution of Colors as a Whole," in which effort is made to throw 

 light upon the first problem stated. There then follow tables 

 giving the relative distribution of epidermal colors in the Upper 

 Ohio Drainage as a whole ; and separately, the component drainages. 

 The same is done for L. Erie and its various collecting stations. 

 Additionally, remarks largely in explanation of PI. Ill, (chart 

 illustrating sequence of. epidermal color changes), and on the 

 prevalence and quality of the rays of the epidermis. The dis- 



