202 



Noteus quadricornis Ehrbg. Average number, 19. This is a 

 rare species in the plankton, being found in 1895 and 1896 in July at 

 maximum temperatures, and in 1898, on April 12, at 52, and on 

 November 8, at 46. 



Notholca longispina Kell. This species, which has been found 

 in the summer plankton of many European and American waters, 

 especially our Great Lakes, was noted but once in the Illinois in 

 January, 1895 (Hempel, '99). It seems to prefer cooler and purer 

 waters. 



Notholca striata Ehrbg. Average number, 437, including varie- 

 ties. This is a winter planktont in our waters, appearing in 1897 on 

 November 30, at 34, reaching a maximum of 10,840 March 22 

 (Table I.), at 51, and disappearing April 19; at 52. It reappears 

 the following autumn on November 1, at 45, and attains a maxi- 

 mum of 4,000 March 21, at 37. In previous years the occurrences 

 all lie within the limits of November 1 and April 24 with the excep- 

 tion of two records in 1895 September 5 and October 15, at 74 and 

 56. The spring maximum in 1896 (7,778) was on April 10, at 

 52, and in 1897 (4,260) on March 22, at 43. In each year but a 

 single pulse, that of March-April, is indicated. Minor fluctuations 

 during the winter (Table I.) are in some cases attributable to flood 

 agencies. 



The temperature limits of this species are quite definitely estab- 

 lished. The species reappears in autumn when 45 is reached, and 

 declines rapidly in the spring after 50 is passed and is but rarely 

 found above 60. It attains its greatest numbers late in winter or 

 early in spring in the face of flood conditions, though the numbers 

 attained in the channel waters are never very large. 



Empty loricas have been found in the plankton after the decline 

 of the species in April, and females with a single egg were noted in 

 small numbers in 1895 during the rise of the pulse. 



I follow the suggestion of Weber ('98) that N. striata should 

 include as varieties the following: N. labis Gosse, N. jugosa Gosse, 

 and N. acuminata Gosse. Examination of many individuals in the 

 plankton proves beyond a doubt the great variability of the organ- 

 ism whose seasonal occurrence we have traced. It varies in the 

 length of the posterior spine, in the proportions of the lorica, and 

 in the development of the striae and the anterior spines. Of a total 

 of 81 ,227 of Notholca striata in this wider sense, 68,887 were referred 



