126 



with that of Amphileptus. The species appear in November or De- 

 cember and continue through March in temperatures below. 50, but 

 the numbers retained by the silk net are too small to trace their sea- 

 sonal routine. Their seasonal distribution in the plankton "coincides 

 with the period of greatest access of sewage and bacterial increase in 

 the river at Havana. Roux ('01) finds this genus well represented in 

 the fauna of swamps, and most abundant in October and March. 



Loxodes rostrum Ehrbg. was identified but once March 22, 1897, 

 at 44. 



Nassula ntbens Perty occurred July 30, 1897, at 84. 



Opercularia articulata Goldf. This species is parasitic upon 

 aquatic Coleoptera. In the plankton of June 28, 1897, eleven 

 colonies or fragments of a colony were found, the largest with 115 

 zooids. 



Opercularia nutans (Ehrbg.). Average number of zooids, 60. 

 In the plankton this species was found attached to Alona afjinis in 

 January, 1898, and to Cyclops in April and August. 



Opercularia not specifically determined were found free in the 

 plankton in June and July ; in November, attached to Canihocamptus ; 

 in January, attached to Brachionus and even to the eggs of this 

 species. An unidentified form was also found upon Cyclops. 



Ophryoglena atra Lieberk. Five irregular occurrences of this 

 species in small numbers were recorded in 1899 from January to the 

 middle of March. 



Paramecium spp. Average number, 41. Paramecium was 

 found 18 times in the plankton. Two of these instances were in May 

 and August at temperatures of 64 and 79, and the remainder were 

 between November 20 and March 30 at temperatures below 48. 

 Most of the occurrences are in midwinter at minimum temperatures 

 under the ice. P. aurelia (O. F. Mull.) has been found in the river 

 waters (Hempel, '99), but not all taken in the plankton belong to this 

 species. Specific determinations are not easily made with accuracy 

 in preserved plankton material. In our plankton, Paramecium is 

 present principally during the period of greatest contamination by 

 sewage. 



Plagiopyla nasuta Stein*. Average number, 1,181,000 during 

 the winter of 1898-99 from November 29 to March 28. This species 

 was not recognized in the plankton of previous winters. It reaches 

 a pulse of 11,520,000 on January 3, 1899, at 32.2 under the ice. 



