Vertical Distribution of Individual Species. 401 



ing the level below which the respective percentages of the 

 species were found and these were connected by lines. The dis- 

 tribution is based on assumption that the individuals of the 

 species were uniformly distributed throughout the 3 m. level in 

 which they were found. This assumption is peculiarly incorrect 

 for D. pitlicaria, since the species is limited to the region of the 

 thermocline. It is often confined within a space of 1 meter, 

 or even less, yet it often passes beyond these narrow lim- 

 its, as is indicated by the fact that not inconsiderable numbers 

 may be found in two or even three levels. While, therefore, the 

 diagram spreads out the distribution of the species during the 

 summer more than is correct, the general relations are well 

 enough indicated by its lines. It will be seen that in the latter 

 part of August more than 65 per cent, of the species was found 

 between 9 and 12 meters and that the species moved downward 

 during September as the thermocline moved down. In 

 October, after the breaking up of the thermocline, the 

 distribution was much more nearly equal. The center of popula- 

 tion rose rapidly and regularly from the latter part of September 

 to the middle of November, lying near 14 meters in late Septem- 

 ber and at 4 meters in the first part of November. After a 

 small fluctuation in the latter part of November, it rose once 

 more, and in the latter part of December lay about two meters 

 below the surface, where it remained during the early part of 

 the winter, until the decline in numbers came on in March or April. 

 If this diagram were reversed it would serve fairly well to indi- 

 cate the downward migration of the species in the spring. 



In Fig. 30 are given curves for the percentile distribution of 

 D. pulicaria for April 16-30, 1896, and August 16-31, 1895, 

 showing the extreme variation of its average distribution. 

 The diagram is similar to that described on p. 384. 



I have not found any other case recorded of a Daphnia which 

 in summer remains at or below the thermocline. At least one 

 other species of the genus has the same habit in this region. A 

 form which I have identified as D. longiremis Sars, belonging 

 to the cristata group, is regularly confined to the region be- 

 low the thermocline in some of the lakes of the Oconomowoc 

 system and in lake Geneva. 

 26 



