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 rubens 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 zodids, 60. 
In the plankton this species was found attached to Alona affimis 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 Canthocamptus ; 
in January, attached to Brachtionus—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. 
