402 
BULLETIN OF THE BUREAU OF FISHERIES. 
part of the river between Hastings and Red Wing, in Lake Pepin, in Lake St. Croix, 
and in the lower part of Lake Keokuk, hut very scarce in the river below Rock 
Island Rapids. In July it was entirely absent in the section between Davenport 
and Burlington. In some catches Melosira and Fragilaria make up more than 
80 per cent of the total amount of plankton Synedra delicatissima W. S., Stephano- 
discus niagarse E., and Cyclotella menenghiniana Breb. can be found in nearly all 
the samples, but do not constitute any considerable part of the total amount of 
plankton. 
Dr. Albert Mann, who has examined the diatoms in some samples of the collec- 
tions from different parts of the river, has come to the following conclusions: 
It is noteworthy that the range of species in all the gatherings is small as compared with the usual 
fresh- water diatom flora; also that they have a close resemblance to each other, although their geographi- 
cal range is considerable. It is interesting to find that several almost cosmopolitan fresh-water forms are 
absent; for example Navicula ( Stauroneis ) phoenicenteron, N. major, such as Surirellse as S. biseriata (E) 
Breb., S. splendida (E) K., S. cardinalis Kitt., and the almost universal Melosira varians Ag., unless the 
M. subflexilis K., sparingly found in Lake Keokuk, can be taken as a variety of that species. On the 
other hand, Melosira crenulata and its too close relative, M. granulata, are very abundant in nearly all 
samples. Neither of these species is at all frequent east of the Mississippi but appear from that river 
westward, and the latter of the two, M. granulata, formed vast beds of new fossil diatoms extending over 
the northwest part of the United States and running into Canada around Deadmans River, British 
Columbia. 
The blue-green algae form a great part of the plankton in Lake Pepin, in Lake 
St. Croix, and in the Mississippi River between Hastings and Rock Island Rapids. 
They were less abundant in Lake Keokuk and scarce in the river below Rock Island 
Rapids, where they were almost entirely absent during July. The principal forms 
of Cyanophycese found were as follows: Microcystis jlos-aguce (Wittr.) Kirchn., 
Clathrocystis ceruginosa (Kutz.) Henfrey, Aphanizomenen jlos-aguce (L) Ralfs., 
Anabcena jlos-aguce (Lyngb.) Breb., Anabcena spir aides Klebahn. In some samples 
these forms were as abundant as Melosira and Fragilaria; in others they were 
scarce. In Lake Keokuk in July there was only a little blue-green algas, except 
Lyngbya; in Lake Pepin, in Lake St. Croix, and in the river above Rock Island they 
were in excess in August and in September. Actinastrum hantzcMi L., Pediastrum 
duplex M., P. simplex R., Scenedesmus guadricauda B., and S. acuminatum Ch. were 
found in every sample, but were never numerous. Pediastrum duplex was usually 
more abundant than P. simplex. 
The Flagellata are chiefly represented by Platydorina caudata K., Plseodorina 
illinoisensis K., Eudorina elegans E., and Trachelmonas schauinslandii Lemm. The 
latter is more abundant in Lake Keokuk than in other parts of the river. Various 
species of Euglena — E. spirogyra, E. acus, and some others that could not be 
identified in the preserved material — occasionally occur in the samples. They are 
more abundant in the bays and sloughs than in the main channel. 
A few Volvox spermatosphara P. were found in many of the samples taken 
in the river and in the lakes. It is interesting to note that in the water-supply 
reservoir of the Fairport Biological Station Volvox occurred in such great abundance 
that in July a sample of water from the faucet in the laboratory room looked like 
a pure culture of this organism. The water in the reservoir is supplied from the 
Mississippi River, yet in the samples taken at the same time in the river one could 
hardly find more than two or three colonies of Volvox. 
