100 



With but few exceptions the seasonal distribution exhibited by 

 the hyaline form was very similar in time and numbers to that of the 

 brown form. Both occurred more frequently and in larger numbers 

 in the warmer months, and irregularly and in small numbers in the 

 colder waters. Both entered in larger numbers with flood waters. 

 The differences though slight are suggestive. The hyaline form was 

 less frequent than the brown both in occurrences and numbers dur- 

 ing cold weather, and summer floods sometimes brought a rela- 

 tively larger number of the hyaline type. These are conditions 

 that might be expected if the latter is only the young (that is, the 

 daughter organism occupying the new shell after fission of the oc- 

 cupant of the old) of Arcella discoides. In warmer months food is 

 more abundant and, presumably, fission more frequent. For this 

 reason the young individuals abound at that time. Owing to the 

 difference in the specific gravity of the two, the hyaline type is 

 more readily transported by flood waters. Though not con- 

 clusive, the data here presented seem to favor the view that the 

 hyaline form is only a stage in the life history of the individual 

 Arcella discoides. 



The species A. artocrea Leidy and A. polypora Penard occur also 

 in our waters, but were included with A. discoides in the enumera- 

 tion. Typical representatives of these species are not, however, 

 present in any numbers 



Arcella mitrata Leidy was found but once on Aug. 1, 1895, in 

 small numbers, at 78.5. 



Arcella stellata Perty. Under this designation are included only 

 those individuals which have well-defined prolongations on the 

 margin of the shell. Only a single occurrence in small numbers 

 (48 per cubic meter) was recorded for the typical A . stellata July 

 29, 1895, at a temperature of 75.5. 



Arcella vulgaris Ehrbg. Average number, 1,098. This species 

 is somewhat more abundant than A . discoides, but occurred in fewer 

 collections. It is a somewhat common planktont, whose seasonal 

 distribution exhibits some irregularities attributable in part, as in 

 the case of other members of the genus, to flood conditions. It was 

 found in 61 of the 180 collections examined, and in approximately 

 one third of those made in each year, excepting in 1894, when it 

 was not recorded, and in 1898, in which year it was found in about 



