195 



genus, serves to distinguish this form, and in the absence of proof 

 of its occurrence in jbrms of urceolaris as here defined I prefer 

 to leave variabilis as a separate species. In any event it is closely 

 related to the urceolaris group, and may ultimately be found to 

 belong within its seasonal range of variation. Seligo ('00) has 

 suggested that B. angularis is also a variety of urceolaris, but I do 

 not so regard it. The averages of the different forms in the several 

 years are given in the table on the next page, which also includes B. 

 variabilis. The discussion of the different varieties follows : 



Brachionus urceolaris Ehrbg., type. Average number of in- 

 dividuals, 18. The type form was not abundant in any year, and 

 its appearances were sporadic. It was recorded in February, June, 

 and July. It includes less than one per cent, of the individuals 

 referred to this species. 



Brachionus urceolaris var. rubens Ehrbg. Average number of 

 individuals, 244; of. eggs, 41. This variety was more abundant 

 during the stable conditions of 1897 (5,290 and 1,976) and the low- 

 water years of 1894 and 1895. It includes over 99 per cent, of all 

 the individuals referred to this species. 



It is apparently the winter form of the species. This appears 

 clearly in its seasonal distribution in the later years, but in 1894 

 and 1895 it was found in summer months and in large num- 

 bers. It is thus capable of development in the whole range of 

 temperatures. 



The pulses recorded in the table on page 193 are in the main 

 composed of this variety. It is quite abundant during the summer 

 of 1894, attaining a pulse of 181,764 on August 15 at 84, disappear- 

 ing in September, and not reappearing until the April collection. It 

 attains a pulse of 324,254 on June 19 at 80, declines in July, then 

 occurs sporadically until the following February. It then continues 

 till June 6, with a pulse of 8,398 on April 17 at 66. An isolated 

 occurrence of 10,000 in July is the only record in the summer of 

 1896. It is in the November-December plankton of 1896 and the 

 March-May plankton of 1897, and attains a pulse of only 6,400 on 

 April 2 7 , at 60. It does not reappear until the 14th of the following 

 September, in whose stable conditions a pulse of 121,200 on the 21st, 

 at 71, is found. It disappears October 5, and is irregularly present 

 from January to April, with larger numbers in the latter part of 

 the period. It is not found in 1898 (Table I.) from May 1 to Decem- 



(14) 



