234 



Ceriodaphnia scitula appears but once in the records of European 

 plankton, Scourfield ( '98) finding it in the waters of Epping Forest 

 in September. The closely related C. quadrangula as well as the 

 other species have been frequently recorded by European investi- 

 gators both in the littoral and the limnetic fauna, but they appear 

 to be less generally found there than the other dominant Cladocera 

 of our waters. 



It does not appear in the plankton of our Great Lakes (Forbes '82 

 and '90, Birge '95), or in that of Lake Mendota (Birge '95 and '97), 

 or .Green Lake (Marsh '97), but Herrick ('84) reports it as the most 

 abundant species in Minnesota, and Fordyce ('00) finds it in 

 Nebraska in shallow waters. A revision of the genus is needed 

 before the seasonal distribution of the various species can be worked 

 out on a basis that will make satisfactory discussions of the literature 

 possible. 



Chydorus sphcericus O. F. Mull. Average number, 422, of which 

 26 are egg-bearing females, and 6 are immature, the remainder, 390, 

 being females in which the ova were not prominent. 



The identification of species of Chydorus is attended by consider- 

 able uncertainty. Comparison with named specimens from Europe 

 supplied by Prof. G. O. Sars, leaves no doubt that C. sph&ricus is 

 common in our waters, and it is apparently the dominant species. 

 It is probable that several other species, as, for example, C. globosus 

 Baird and C. c&latus Schoedler, occur sparingly in our waters and 

 have been included with C. sphczricus in my enumerations. The 

 difficulties which attend the attempt to assign every individual to 

 one of the several species of Chydorus can be appreciated only by 

 one who makes the effort. The problem of their specific validity 

 should be solved by a statistical analysis of the range of varia- 

 tion. 



The seasonal distribution of Chydorus sphcericus in channel 

 waters is in its general outlines very characteristic and well defined. 

 The following table, which gives the average number of Chydorus 

 per m. 3 for each month of our collections, shows clearly that it is a 

 vernal planktont, and that there is a slight tendency toward an 

 autumnal pulse in September, when vernal temperatures return. 

 The number for November (222) would probably be considerably 

 reduced if more than one collection had been taken in that month 

 in 1896. Omitting this year, the average for November falls to 



