362 THE AMERICAN NATURALIST [Vol.LVI 



from the length measurements of the entire material. 

 The young of the year (obtained in 1919) are referred to 

 as the 1919^ class ; those of the previous year as the 1919, 

 class, and so forth. The 1919^ year-class of the Notropis 

 atherinoides is further divided into three subclasses, A, 

 B and C, named in the direct order of hatching, hence in 



the indirect order of size. The year-classes for both spe- 

 cies are indicated on the graphs forming Figs. 1 and 2. 

 The s\Tnbols on the curve for the 19198 class of each spe- 

 cies indicate the sex predominant among the representa- 

 tives of each size. 



Ill 



A series of water temperatures appear unavailable, but 

 in the case of such a shallow, nearly enclosed lagoon the 

 air temperatures of the region may safely be substituted. 

 Hence the CUmatological Data (Illinois Division, 1918 

 and 1919) for Chicago were used in constructing Fig. 3; 

 the temperatures given for each week were obtained by 

 averaging the daily means. 



On the temperature chart there are indicated the 

 periods of development for each of the two species as ob- 



