a7 
in our plankton than that species, though this does not seem to be 
the case in some European waters. 
Meridion circulare Ag. has appeared but four times in winter 
planktons, from December to March, and seems to be adventi- 
tious. 
Navicula tridis Ehrbg.*—Average number, 297,307. Appears 
at irregular intervals, often with flood waters and in the colder 
months. It seems to be adventitious. 
Navicula spp.*—Average number, 8,569,038. About twice as 
abundant in 1897. Under this head I have included a number of 
species of Navicula, and, possibly, even species of genera resembling 
Navicula. The individuals are all of small size, and are principally of 
the type of the smaller forms of N. brebtssomi Kutz. and N. gracilis 
-Ehrbg. They are quite abundant in collections from Quiver Creek 
and Spoon River. Their greater abundance in 1898 as compared with 
1897 may be caused by the greater movement in river levels in the 
former year (85.6 ft.) as compared with that of the latter (55.5 ft.). 
This feature of the distribution of these forms suggests that they 
are adventitious in the plankton. This view is further supported 
by the fact that some, though not all, of their apparent pulses 
appear with flood waters; for example, the pulse of 64,000,000 on 
May 17, 1898. There are indications, independent of floods, of 
pulses in April-May and November—December, which may, how- 
ever, be simply reflections of pulses in the normal habitat of these 
diatoms—the shores and bottom of the river and its tributaries. 
They are represented in the plankton at all seasons, and the diver- 
gence in numbers is at no time so marked as it is in typical plank- 
ton diatoms, such as Asterionella. 
Nutzschia amphioxys (Ehrbg.) Kitz. appeared several times in 
winter collections, and N. sigmoidea (Nitzsch) W. Sm. is adventi- 
tious in small numbers in flood waters. Several species of Pleu- 
rosigma appear at irregular intervals throughout the year in both 
flood waters and stable conditions and are apparently adventitious, 
appearing in relatively small numbers. 
Rhizosolemia ertensis H. L. Smith was noted on a few occasions 
in winter planktons. Its exceeding transparency and the abun- 
dance of silt and debris at the times of its occurrence so obscure it 
that it may have escaped detection in many instances. 
