LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
409 
The stems of the water plants were covered with many filamentous algas, such 
as CEdogonium, Spirogyra, and Stigeoclonium. The Copepoda were abundant 
here, hut the Cladocera scarce. All stems of Potamogeton were covered with 
Hydra sp. Besides the Rotifera that usually occurred in plankton samples, the 
following species were found amid water plants : 
Monostyla cornuta O.F.M O 
lunaris E O 
quadridentata E O 
pyriformis D O 
Euchlanis dilatata E 0 
Metopidia acuminata E 0 
Colurus uncinatus E O 
deflexus G 0 
Diaschiza gibba E 0 
Diglena forcipata E O 
Lecane arcula O 
The plankton of Lake St. Croix is the same as in Lake Pepin (Table 23), 
excepting that Cyclotella menenghiniana Bl., generally present at other stations, 
here disappears and Melosira crenulata K. is replaced by M. granulata (E) R. and 
M. spiralis E. 
The plankton of Lake Keokuk is not so uniform as that in Lake Pepin, the upper 
and lower parts of the lake differing one from another not only in the amount of 
plankton, but in its composition. (See Table 23.) Roughly speaking, the upper 
part of the lake is richer in Rotifera, whereas in the lower part the diatoms and the 
blue-green algse are more abundant. 
In July the blue-green algse were not so abundant in Lake Keokuk as were 
diatoms. During this time Melosira crenulata K. was the principal form found in 
the plankton, and in the lower part of the lake it made up almost 80 per cent of 
the total mass in the sample at some stations. Among the blue-green algse the 
filaments of Lyngbya very often occurred in the samples. This alga was very com- 
mon in the upper part of the lake, where all trunks of the submerged trees on the 
islands were covered with a thick layer of this organism. Lyngbya was also very 
often found between the leaves of Lenina and was carried downstream by the 
drifting Lemna groups. 
The Rotifera were very numerous in Lake Keokuk, especially in its upper 
part, and consisted of representatives of Brachionus, Asplanchna, Noteus, Notops, 
Anursea, and Pedalion. Among the species of Brachionus, B. pala E. was the 
most numerous and was represented by the varieties amphiceros, dorcas, and spi- 
nosus. Brachionus pala amphiceros E. was represented by various forms, beginning 
from the almost spineless organisms to the forms with extremely long spines. At 
many stations there were found also the males of Brachionus. 
Both forms of Brachionus angularis, the typical spineless Brachionus and B. 
angularis caudatus B. and D., were also present. * The caudal spines of this species 
are subject to wide fluctuation, and all variations between the two extreme forms 
were observed in the samples taken from Keokuk Lake. A great part of B. pala E. 
and B. angularis G. was infected with a sporozoon, Ascosporidium asperosporum 
Zach. 
Pedalion mirum H. was found at nearly all stations in the lower part of the 
lake but did not occur in the upper part. 
The Cladocera population was mainly represented by Moina rectirostris L. and 
M. hrachiata I. Both species were more numerous in the lower part of the lake, 
