2S 
Codonella occurs throughout the whole range of temperatures. The 
winter minimum and the decline during the maximum temperatures 
of summer, combined with the presence of vernal and autumnal, or 
late summer, pulses, indicate that the optimum conditions for this 
organism lie neither in winter nor in summer. The spring pulse 
was at temperatures of 60°-72°, and the autumnal one at a wider 
range of 57°-78°. Permanent increase in numbers does not begin 
(Table I.) until March 15 at 46°,and the permanent falling off is 
found on November 15 at 41°. The optimum temperatures in our 
waters thus lie near 60°-70°, and conditions favoring growth are 
limited to a range of 10°-15° upon either side of the optimum. 
This species readily escapes through the silk net on account of its 
small size and its motility, and such collections give at the best in- 
complete evidence of its seasonal distribution. The amplitude of its 
fluctuations is thus reduced, and owing to the irregularity of the 
error arising from leakage, the reduction is not proportionally distrib- 
uted throughout the year. Tests made of the loss of Codonella by 
leakage through the silk indicated that but one was retained to 
twenty-four foundin the filtrate. Codonella was counted in both the silk 
and filter-paper collections, with the result that in 1897 the totals for 
the year (omitting one date on which the filter collection contained 
an unusually large number of Codonella) showed one Codonella in the 
silk to twenty-five in the filter collection. In 1898, however, the 
ratio was one to four and a half. The error in the filter collection 
is large, but data seem to justify the conclusion that only a small 
proportion of the Codonella is retained within the silk net. The 
proportion for the whole period of collection by the two methods 
(August 3,’97, to March 28,’99) is one to seven, if one date on which 
aberrantly large numbers appear in the filter collections be omitted. 
This species is a typical planktont, and is apparently the same as 
C. lacustris Entz, by which name it is designated by European writers. 
Leidy’s name, however, has priority according to the accepted rules 
of nomenclature. It is an exceedingly variable organism, at least 
in the form, proportions, and size of the shell, in the degree of its con- 
striction, and in the foreign particles which fill its matrix. The rings 
or bands which ornament the orifice vary in their number, width, and 
relative proportions, and in the perfection of their development. 
The intergradation which these variants exhibit is sufficient to my 
mind to make their elevation to specific rank unjustifiable. 
