123 



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 found in 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. 



