232 



Herrick or C. acanthinus Ross, which appear to resemble C. rotunda 

 in some particulars, should be included here as forms or synonyms. 

 The genus is sadly in need of revision. 



The forms referred to C. rotunda were found in August, 1894, and 

 July-August, 1895, 16,536 per m. 3 appearing in the plankton on 

 July 18 of the latter year. 



Ceriodaphnia scitula Herrick. Average number, 1,539. This 

 species is closely related to the European C. quadrangula O. F. Mull., 

 if, indeed, it is not identical with it. It is not impossible that it is 

 the form imperfectly described by Say ('18) as Daphnia angulata. 

 In the absence of a critical monograph of the genus I use the name 

 applied in current American literature to this form. 



This is the most abundant species of the genus in our waters, 

 outnumbering all others by over sixfold in the totals of our records. 

 It is also one of the most important members of the Entomostraca 

 in the channel plankton (total of all records, 156,119), being ex- 

 ceeded in numbers only by Moina micrura (1,121,808), Bosmina 

 longirostris (381,598), Daphnia cucullata (237,444), and D. hyalina 

 (231,746). 



It occurs in all months of the year except January and February, 

 but in larger numbers and in more of the collections in May-Septem- 

 ber. Thus less than 6 per cent, (reduced to 2 per cent, if one col- 

 lection in the warm autumn of 1897 is omitted) of the individuals 

 and only 20 of the 79 occurrences are found outside of the May- 

 September period. Ceriodaphnia scitula is accordingly a summer 

 planktont in channel waters. It is found in each year, though in 

 varying numbers according to hydrographic and other conditions. 

 Thus in 1898 the vernal pulse in June attains the unsurpassed 

 amplitude of 55,800 per m. 3 , but declines in a fortnight and makes 

 no recovery during the disturbed hydrographic conditions of the 

 summer. In 1897, on the other hand, our records were too meager 

 to delineate fully the vernal pulse, and in the stable conditions of 

 the summer and autumn the species continued in numbers whose 

 totals exceed those of 1898 by 81 -fold. Similarly in 1896 the more 

 gradual changes in levels which attended the floods of that year 

 permitted a considerable development of Ceriodaphnia throughout 

 the summer. Stable hydrographic conditions thus conduce to 

 increase in Ceriodaphnia. The relations which I have shown to 

 exist between Bosmina and movement in river levels (see table on 



