265 



Stable hydrographic conditions appear to favor the increase in 

 C. bicuspidatus, as is seen in the large pulse of November 15, 1897 

 (3 ,560) , and the slight pulse (240) during declining levels in February, 

 1899. 



The vernal development of 1898 (Table I.) is distinctly pulse-like, 

 and there are traces elsewhere of similar phenomena, but in general 

 the numbers of C. bicuspidatus are too small to exhibit clearly the 

 phenomenon of recurrent pulses. 



Of the totals of all individuals recorded in 1894-1899 I find that 

 37 per cent, are males, 16 per cent, egg-bearing females, and 47 per 

 cent, females without eggs. Immature forms and nauplii were not 

 distinguished from those of other species. With the exception of 

 a few stragglers, the egg-bearing females were limited principally 

 to March-May. In exceptional cases the males greatly outnum- 

 bered the females, as on November 15, 1897, when the ratio was 

 2,820 to 680. 



Though apparently widely distributed, this species does not 

 appear frequently among the planktonts reported from European 

 lakes. Scourfield ( '98) reports it as a common species in the waters 

 of Epping Forest throughout the year with the exception of a period 

 of absence or depression in July-August, and Scott ( '99) finds it in 

 shore collections made in various months of the year in Scottish 

 lakes, and more abundantly in the warmer months. It has been 

 reported in the potamoplankton in Europe only by Rossinski ('92) 

 from the Moskwa, by Zernow ('01) from the Schoschma, and by 

 Volk ('03) from but one of seven localities in the Elbe at Ham- 

 burg. 



In American waters, on the other hand, C. bicuspidatus is more 

 abundant, and in the Great Lakes it forms a very important part 

 of the plankton. Forbes ('82) finds it (as C. thomasi] to be the 

 dominant Cyclops in the summer plankton of Lake Michigan and 

 ('90) also abundant in that of Lake Superior. Marsh ('93 and '95) 

 finds it in the summer plankton of the Great Lakes, near Charlevoix, 

 in Lake St. Clair, the Detroit River, and Lake Erie, but only rarely 

 and in small numbers in the smaller bodies of water in Wisconsin 

 and Michigan. E. B. Forbes ('97) extends its recorded range to 

 Massachusetts and to the lakes and rivers of Wyoming, and states 

 that it is widely distributed in America and occurs in large ponds 

 and rivers. Brewer ( '98) reports it in the vernal plankton of deep 



