ee 
209 
American form described by Herrick as J. 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. 
Macrothrix laticornis Jurine was found in the plankton in May 
at 64°-73°, adventitious in flood waters. 
Moina micrura Kurz.—Average number, 261 per m.* 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.?* 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 
Mozina is very uneven in its distribution. 
The seasonal distribution of Moima in channel plankton is con- 
fined to July-September with the exception of 9 occurrences in 
small numbers in the last days of June and the early part of October. 
The earliest record is June 19, in 1895, when the very large number 
of 329,448 per m.* 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 
