364 THE AMERICAN NATURALIST [Vol.LVI 



ably corresponds with the cold period in April (see Fig. 

 3). The scales of the medium-sized specimens (subclass 

 B) show on the average a narrower nuclear area suggest- 

 ing slackened growth. It is presumed that these individ- 

 uals passed through their early development toward the 

 end of this cold period. The scales of the smallest spec- 

 imens, those of subclass C, show no such nuclear area 

 of weak concentrated circuli. These fishes supposedly 

 developed during the warm weather of May. 



The data on the developmental period of these two 

 species for the preceding breeding season (1918) are less 

 complete than those for 1919, yet not wholly lacking. 

 Lepomis incisor, at least, bred during the corresponding 

 weeks in both years (but in less abundance in 1918 than in 

 1919). 



A comparison of the available observational data with 

 the temperature chart (Fig. 3) indicates that, on the 

 average, the developmental period for Notropis ather- 

 inoides was colder in 1919 than in 1918, whereas these 

 temperature relations were distinctly reversed in the case 

 of Lepomis incisor, and furthermore, that the tempera- 

 ture was distinctly higher at the beginning and toward the 

 close of the 1919 breeding season for the Notropis, than 

 during the middle of this period. 



IV 



These differences in the developmental temperature ap- 

 pear to be correlated with variations in the number of 

 segments in the case of both fishes. Comparisons will 

 first be made between the two year groups of Notropis 

 atherinoides, then between the same year groups of 

 Lepomis incisor, and finally between the three subclasses 

 into which the 1919 brood of the Notropis has been di- 

 vided. 



The vertebraB in the 1919,, class of Notropis atherinoides 

 are sufficiently more numerous on the average than those 

 of the 19198 class to shift the modal number from 41 to 

 42, the average from 41.41 (±0.04)' to 41.74 (+0.015). 



