Marsh — Limnetic Crustacea of Green Lake, 221 



IAmnocalanus is not a very good genus to consider in connec- 

 tion with this discussion, because it does not often occur in any- 

 large numbers. It is significant, however, that in successive 

 hauls there were sometimes differences of from two to five hun- 

 dred per cent. On November 14, 1896, I found in a collection 

 made in the afternoon 56. In a collection made at about eight 

 o'clock the same evening, I found 200 in the upper two and one 

 half meters. In this case, curiously, the total number obtained 

 in the other hauls from the surface to twenty meters was only 106. 



An examination of the numbers of the other species as col- 

 lected at similar times shows the same variations. None of 

 them, however, seem to me to furnish such conclusive evidence 

 as we get from Diaptomus and G fluviatilis, because of the smaller 

 number involved. 



Thus my results are in harmony with those obtained by 

 Zacharias and France. Inasmuch as one certainly would not 

 question the accuracy of the work of the observers who have 

 come to different conclusions, the question arises whether 

 there is any way of explaining such differences I think a 

 critical examination of their work and the inferences derived 

 from it will show that such an explanation is possible. 



In the first place I would state my entire agreement with the 

 school of Hen sen, that only by an enumeration of individuals 

 can we get at exact results in plankton work. Volumetric deter- 

 minations have a value in a general way, and may be used even 

 in comparing different bodies of water, but only with a large 

 allowance for the possibilities of error. Many of the difficulties 

 in this method of work have been well pointed out by Ward 

 himself. (Ward, '95a, p. 256 ff.). Most important is the differ- 

 ence in the time of subsidence due to the differences in the char- 

 acter of the plankton at different times and places. Some kinds 

 of material will remain suspended for an almost indefinite 

 period. Consequently, the volumetric method would rarely be 

 sufficiently accurate to indicate even very considerable differ- 

 ences in horizontal distribution. There are, also, questions in 

 regard to the accuracy of any gravimetric method that has yet 

 been devised, although the amount of error by this method must 

 be much less than by the volumetric method. 



