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 1s 
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. dzscoides 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 temperatune ol 75-9 -. 
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 
DONS eee ey is: 
