57 



in our plankton than that species, though this does not seem to be 

 the case in some European waters. 



Meridian circulare Ag. has appeared but four times in winter 

 planktons, from December to March, and seems to_ be adventi- 

 tious. 



Navicula iridis 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. brebissonii Kiitz. 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. 



Nitzschia amphioxys (Ehrbg.) Kutz. 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. 



Rhizosolenia eriensis 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. 



