253 



American form described by Herrick as /. spinifer. A larger amount 

 of material exhibiting a fuller range of variation may, however, serve 

 to connect the two. 



Leptodora hyalina Lilljeborg. Average number, 3. This species 

 occurred in small numbers and somewhat irregularly in our collec- 

 tions of channel plankton in summer months. Our earliest record 

 was June 28 ; and the latest, August 30. It is our largest crustacean 

 planktont and a fairly active swimmer, and was often taken in our 

 tow-nets, which had a larger mouth and coarser mesh (No. 12) than 

 our plankton net. I took this species in great numbers in the upper 

 meter of water at midday in May- June in Lake Meredosia with a 

 seine of No. 000 silk. It may be that it is less abundant in the 

 channel than in the backwaters, and the small number in the plank- 

 ton collections from the channel may also be accounted for in part 

 by the escape of Leptodora from the small orifice (10 cm.) of the 

 plankton net, or to its negative rheotropism when stimulated by the 

 currents of the plankton pump. 



Macroihrix laticornis Jurine was found in the plankton in May 

 at 64-73, adventitious in flood waters. 



Moina micrura Kurz. Average number, 261 per m. 3 In 1897 

 it was much more abundant, averaging 5,106 in the more stable 

 conditions of that year. 



This is the most abundant of all our Cladocera, appearing in 

 great numbers in periods of stable low water during maximum 

 temperatures. It is exceedingly irregular in the extent of its devel- 

 opment in different years, the average numbers per m. 3 in 1894-1898 

 being respectively 21,844, 22,842, 188, 5,106, and 261. After mak- 

 ing allowances for the irregularity in the number and distribution 

 of the collections in the several years, it still remains apparent that 

 Moina is very uneven in its distribution. 



The seasonal distribution of Moina in channel plankton is con- 

 fined to July-September with the exception of 9 occurrences in 

 small mmibers in the last days of June and the early part of October. 

 The earliest record is June 19, in 1895, w r hen the very large number 

 of 329,448 per m. 3 were found, a degree of development which 

 implies a previous period of multiplication. The first records in 

 subsequent years were all later than this date in June or early in 

 July. After several recurrent pulses, each of 3 to 5 weeks ' duration, 

 the numbers decline to a very low level, and the species disappears 



